ESS Instrument Construction Proposal <‌RESPECT‌>

2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015

Name Aﬃliation Main proposer Prof. Peter Böni Physik Department, Prof. Christian Pﬂeiderer Technische Universität München Dr. Robert Georgii FRM II, Technische Universität München Co-proposers Jonas Kindervater Physik Department, Technische Universität München ESS coordinator Dr. Melissa Sharp ESS

Note: All proposals received by ESS will be included as Expressions of Interest for In-kind contributions. ESS will use this information for planning purposes and the proposer or aﬃliated organization is not obligated to materially contribute to the project. The following table is used to track the ESS internal distribution of the submitted proposal.

Name Aﬃliation Document Ken Andersen ESS reviewer Distribution Dimitri Argyriou, Oliver Kirstein, Arno Hiess, Robert Connatser, Sindra Petersson Årsköld, Richard Hall-Wilton, Phillip Bentley, Iain Sutton, Thomas Gahl, relevant STAP

European Spallation Source ESS AB Visiting address: ESS, Tunavägen 24 P.O. Box 176 SE-221 00 Lund SWEDEN 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015

www.esss.se

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EXECUTIVE SUMMARY

We propose the construction of a REsonance SPin-echo spECtrometer for exTreme studies, RE- SPECT, that is ideally suited for the exploration of non-dispersive processes such as diffusion, crystallisation, slow dynamics, tunneling processes, crystal electric field excitations or spin fluctuations. The estimated cost of construction of RESPECT is around 8.8 Million Euros. The following aspects characterise RESPECT as a world-wide unique spectrometer:

• RESPECT represents a high-resolution spin-echo spectrometer at a spallation source based on the longitudinal neutron resonance spin-echo (LNRSE) technique.

• The energy and momentum resolution of RESPECT will be equivalent to conventional state of the art neutron spin-echo instruments reaching spin-echo times up to 1 µsec.

• RESPECT uses Longitudinal Neutron Resonance spin-ﬂipper for Larmor labelling, replacing the precession coils in conventional state-of-the-art neutron spin-echo spectrometers.

• The energy densities needed in RESPECT to correct the eﬀects of precession ﬁeld inhomogeneities are at least one order of magnitude below those needed for precession coils in conventional state-of-the-art neutron spin-echo spectrometers.

• The dynamic range of RESPECT reaches up to eight orders of magnitude by means of the so-called ﬁeld-subtraction method, without need for changes of instrument components or instrument conﬁgurations.

• The length of RESPECT is 37 m. Its layout is equivalent to conventional neutron spin-echo spectrometers.

• RESPECT can be set up in a conventional experimental hall and does not require zero ﬁeld shielding or a screened room.

• RESPECT will fully exploit the long pulse structure of the ESS.

• The LNRSE spin ﬂipper allow easy use of focussing neutron guides, permitting gains of intensity of up to three orders of magnitude as compared with conventional neutron spin echo instruments.

• The estimated cost of an additional MIEZE-1 Option for spin-echo spectroscopy at small momentum transfers under depolarising conditions, such as the largest steady-state magnetic ﬁelds currently accessible on a laboratory scale, or hydrogenated samples, will be 1.2 Million Euros.

• The estimated cost of an additional MIEZE-2 Option, which converts RESPECT into a wide- angle spin-echo spectrometer, will be 5 Million Euros.

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TABLE OF CONTENTS

EXECUTIVE SUMMARY 3

INSTRUMENT PROPOSAL 6

1 Scientific Case 6 1.1 Key Scientific Drivers ...... 6 1.1.1 From Grand Challenges to Materials ...... 6 1.1.2 From Materials to Neutron Spectroscopy ...... 9 1.2 Implications for ESS Instrument Suite ...... 13 1.2.1 Required Instrumentation and Infrastructure ...... 13 1.2.2 ESS Instrumentation under Construction ...... 14 1.2.3 Justification for a LNRSE spectrometer ...... 15 1.3 Expected User Community ...... 16

2 General Instrument Concept and Performance 16 2.1 Longitudinal Neutron Resonance Spin-Echo Spectroscopy ...... 17 2.2 Future Options ...... 18 2.2.1 Spin-Echo Spectroscopy under Depolarising Conditions (MIEZE-1) ...... 18 2.2.2 Wide-Angle Spin-Echo Spectroscopy (MIEZE-2) ...... 19 2.2.3 Inelastic Spin-Echo Spectroscopy and Larmor Diﬀraction ...... 20

3 Instrument Concept of RESPECT 20 3.1 Overall Layout of the LNRSE Spectrometer ...... 20 3.2 Primary Spectrometer ...... 22 3.2.1 Chopper System ...... 22 3.2.2 Bender 1 and shutter section ...... 22 3.2.3 Guide System ...... 23 3.2.4 Polarizing cavities ...... 24 3.2.5 Bender 2 ...... 24 3.2.6 Collimation stage ...... 25 3.2.7 Focusing guides ...... 25 3.2.8 LNRSE coil system ...... 26 3.3 Secondary Spectrometer ...... 26 3.3.1 LNRSE coil system ...... 26 3.3.2 Analyzer ...... 26 3.3.3 Detector ...... 26 3.4 Future Options ...... 26

4 Comparison of RESPECT with Existing Instruments 28

5 Technical Maturity 29 5.1 LNRSE Spectrometer ...... 29 5.2 Future Options ...... 29 5.3 Synergies within ESS Instrument Suite ...... 30 5.3.1 Instrument Speciﬁc Aspects ...... 30 5.3.2 General Infrastructure ...... 30

6 Costing 31 6.1 LNRSE Spectrometer ...... 31 6.2 Future Options ...... 33

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LIST OF ABBREVIATIONS 34

A Simulations of Performance for RESPECT 41 A.1 Mean brilliance of the cold pancake moderator of ESS ...... 41 A.2 Beam properties ...... 43 A.3 Homogeneity of ﬁeld integrals ...... 47

B Proof of principle experiments 49

C First Scientiﬁc Highlights 57

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INSTRUMENT PROPOSAL

1 Scientiﬁc Case

The scientific mission of the ESS are major advances in the Grand Challenges faced by human mankind. The perhaps most important technical aspect to achieve these advances is undoubtedly the progress made in material specific questions. The same materials classes are thereby pivotal for different grand challenges. The presentation of the key scientific drivers of our proposal begins therefore in section 1.1.1 with an account of the materials classes associated to different grand challenges. Starting then from each materials class in section 1.1.2, we illustrate the importance of neutron spectroscopy with ultra-high energy and momentum resolution and, in particular, the outstanding role played by direct measurements of the intermediate scattering function, and how they allow to resolve these material specific questions and advance the frontiers of science. Following a short account in section 1.3 of the diverse user community active in this area, as well as new user communities for which the types of experimental studies addressed here will become essential in the future, the presentation turns to the implications of the key science drivers for the ESS instrument suite in section 1.2. In view of the required instrumentation and infrastructure, specified in 1.2.1, and the ESS instrumentation under development and review in 1.2.2, the justification for a neutron spin-echo instrument based on the longitudinal resonance spin-echo technique is presented in section 1.2.3.

1.1 Key Scientific Drivers 1.1.1 From Grand Challenges to Materials In the light of the rapid growth of world population, as well as increasing average age and living stan- dards Humanity faces Grand Challenges of supply and sustainability. To address the issues associated with these Grand Challenges it is intuitive and helpful to distinguish the following areas: (i) Energy, (ii) Environment, (iii) Health, (iv) Agriculture, (v) Mobility, and (vi) Digital Society. As the binding element of these areas there is great need for major advances in materials specific questions. In the following the strong link between the grand challenges and some of the most prominent questions in specific materials classes are illustrated in terms of selected examples.

Energy Technological progress pursued to overcome the energy crisis yields several facets: increase of renewable energy sources, new concepts for energy transportation, reduction of waste energy, efficient energy storage. Major efforts are, for instance, dedicated to the development of fuel cells, where unresolved questions concern the detailed mechanism controlling diffusion of hydrogen in solids. Another prominent area of research pursues novel energy sources such as solar cells or exploitation of sea-based-stored methane. The detailed structural dynamics, e.g., in recently discovered clathrate hydrates, is here key. Conversion of solar energy as another potential route forward concerns major advances in solar cells or catalytic converters, where the slow dynamics and diffusive processes at the interfaces are topics of ongoing research. Another important example is the conversion of waste heat into electrical power using novel thermoelectric materials. Optimisation of the ratio of thermal to electrical transport properties based on the precise microscopic understanding of lattice and electronic excitations and their mutual coupling are here center stage. Similarly, great efforts are made to advance magnetic cooling methods based on high spin-entropy materials, in which a magnetic field is used for entropy reduction of a suitably designed spin system. Major challenges concern the detailed magneto-elastic and magnet-electronic coupling, which require high resolution studies of the low lying dynamics. Advances in efficient energy transportation may be achieved with both conventional and unconventional superconductors. For instance, the pinning of superconducting flux lines by defects and

6(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 the associated mechanisms of flux lattice melting require an in-depth understanding of the associated low lying excitations. Intense research is also dedicated to the superconductive pairing mechanism in conventional and unconventional superconductors which requires high resolution spectroscopic studies of the fluctuation spectra of both lattice and spin degrees of freedom. These superconductors are important for efficient and fast switches in high power electrical devices. For improved energy storage, e.g., in the context of electro-mobility, Li-batteries are being developed, where the Li migration and diffusion raise questions of the underlying slow dynamical processes. Another approach in this area concerns hydrogen storage in intermetallic compounds for which hydrogen dynamics in solids is of central importance [1, 2, 3]. In particular the latter, when combined with improved catalytic hydrogen generation by sun light represents a major step forward in the development of carbon-neutral energy options.

Environment The massive increase of use of carbon-based fuels and the associated production of green house gases arguably represents the most pressing environmental challenge. The situation is aggravated by additional sources of green-house gases such as the production of Portland cement, which adds another 5% through the decarbonisation of limestone, kiln fuel combustion and exhaust gases of machines used for processing. In turn, efforts to reduce the release of green-house gases concern either of elegantly storing them or processing them further. Scientifically these activities require the detailed understanding of how to store CO2 and related gases, e.g., in solids, as well as catalytic processes at custom designed interfaces to handle the emission of pollutants such as Dioxin, NOx, SO2. At the same time, these environmental challenges motivate the search for materials suitable as a replacement, where, for instance, typical questions of the microscopic mechanism controlling the materials properties have to be resolved. A different aspect of increasing pollution concerns atmospheric particulate matter such as aerosols, i.e., microscopic solid or liquid matter suspended in the Earth’s atmosphere. Nucleating cloud formation fine dust along with pollen, bacteria and fungal spores effectively control the overall energy balance of the atmosphere. Yet, on the molecular level the importance of slow dynamic processes in the sense of both collective excitations and highly stochastic processes affecting the nature of the binding forces involved in condensation and crystallisation are a topic of active research. As an additional spin-off these interfacial processes are also of interest for crop resistance to changing weather conditions as noted also below in the context of the agricultural challenges.

Health The enormous progress achieved in recent decades in the understanding of complex diseases has been made possible through increasingly detailed microscopic studies. Here the investigation of slow molecular processes underlying bacterial and viral infections and in cell modification, e.g., in cancer, diabetes and Alzheimer’s, are now center stage. This permits, for the first time, to move away from the traditional trial-and-error search of new pharmaceuticals going towards their specific microscopic design. An important line of work focusses on modifications of enzymatic, stereo-, regio-, or chemo-selectivity as well as changes of functionality through variations of local environment and solvents. Moreover, drug design and the development of bio-inspired enzymes targets now metastable protein states for docking of active structural elements. Another major area of research on health-related challenges addresses methods of controlled local drug delivery, such as their slow release from polymer matrices. Likewise great efforts are dedicated to improve the long-term storage of pharmaceuticals and the preservation of structure and functionality of biomolecules, which requires an understanding of fast protein dynamics. Health-related challenges are finally also concerned with research in hard condensed matter, where new materials are designed for implants and health care devices. Important questions are here related to the fabrication of extremely tough and light-weight materials akin those used in automobile and aircraft design, which are, yet, also non-toxic and bio-compatible. This makes the precise properties of strongly heterogeneous interfaces between living, soft and hard condensed matter a topic of central importance.

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Agriculture The rapidly growing demands in quality and quantity of world food supply pose a multifaceted set of agricultural challenges, comprising of the development of alternative food supplies such as algae, production of bio-fuels, or the consequences of genetic modifications, say, for drought resistance. Concerning disease resistance of plants and animals pressing scientific questions to those already described above. They define in particular a large demand for microscopic studies of the dynamical properties polymers, protein and enzymes as well as complex systems such as bio-membranes. The associated questions originate in typical soft and hard matter systems such as complex fluids or fertilisers, but include also strongly heterogeneous materials with a view of molecular details of plant metabolism. The precise dynamical properties of surfaces and interfaces under varying functional aspects urgently require clarification, for instance regarding surface nucleation of ice on plants, which reflects sensitively vibrations of the hydration shell and catalyser as well as their interplay.

Mobility Mobility for work and pleasure is not only a matter of energy, but also a question of finding low-cost, light-weight materials that are superstrong and extremely tough. Poised at the border between conventional crystalline solids and quasicrystalline materials so-called complex metallic alloys, with extremely large unit cells and very subtle superstructure appeal with precisely these characteristics. Yet, slow dynamical processes at the heart of crystallisation when forming complex metallic alloys and the low lying structural dynamics are unresolved. In fact, crystallisation even in simple binary and ternary metallic melts as model systems have not been fully understood. Another major area of interest in the broader context of mobility are the microscopic nature of friction and lubrication. Here molecular interactions and binding, which are deeply rooted in the microscopic dynamical properties of surfaces and interfaces, await clarification. In particular modification of these properties and the interplay of materials with novel mechanical properties amenable for vehicle design with typical complex soft matter systems is of great interest. These challenges even include aerodynamic and hydrodynamic interfaces as atomic layers modify effective friction and the formation of laminar and turbulent flow.

Digital Society Massive data processing and storage have radically changed daily life in recent decades. With the demands continuously increasing the technological limits of conventional technologies are expected to reach their ultimate limits in the very near future. These challenges ahead a main thrust of activities is placed on the search for novel electronic materials with new and unexpected functionalities and clarification of their inner workings. In the area of hard condensed matter this includes a remarkable wide range of new ordering phenomena, such as new classes of superconducting materials, new forms of magnetic or ferroelectric order with non-trivial topological characteristics and even new forms of electronic order such as the topological insulators, Dirac and Weyl metals. Detailed microscopic exploration of the dynamical properties characterising these materials and further examples, for instance in the area of quantum magnetism, provide deep fundamental insights into equally perplexing and facinating collective phenomena of quantum matter. Intimately associated with the search for new materials properties of conventional hard matter systems are great efforts to develop new operational concepts for information technology. In the simplest scenario these may be novel spintronics devices with much improved characteristics. However, also radically new concepts are possible. For instance, when exploiting quantum entangled states for so-called quantum computing much enhanced and faster algorithms are possible. Such quantum bits have been realised on a proof-of-concept level by means of molecular nano magnets. For future developments determination of the low-lying eigenstates of these magnetic systems are essential. An entirely different line of effort is, finally, dedicated to bio-inspired computing systems. Exploiting liquid-ion gated interfaces analogies with the human brain synaptic data processing is pursued. For these efforts to make progress detailed studies of slow microscopic dynamics at the interfaces will be essential.

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1.1.2 From Materials to Neutron Spectroscopy Neutrons can decipher vital molecular mechanisms through non-destructive and non-invasive structural and dynamic analysis of complex assemblies. In the following we turn to the wide range of diﬀerent material systems for which ultra-high resolution neutron spectroscopy down to micro-second timescales is essential. In many cases direct measurements of the intermediate scattering function are extremely important, as they permit direct comparison with theoretical simulations. Taken together, this provides a strong motivation of neutron spin-echo as an important component of the ESS instrument suite.

Polymers Great eﬀorts are currently dedicated to studies of polymer systems and polymer mixtures to identify the link between functionalities such as anisotropic moduli, molecular mobility and self-healing mechanisms with topological characteristics such as branching points, docking sites and fundamental building blocks [4, 5, 6, 7]. For the same reasons custom-designed interactions of polymers with nano-particles and the role of the containment interfaces in geometrically constraint systems such as micro-sized pores and gaps receive great interest. The necessary measurements require systematic substitution of small structural units and detection of processes at intermediate momentum transfers with reasonable momentum resolution and very high energy resolution.

Reactive Surfaces and Interfaces Time-dependent surface processes and reactions on the mil- lisecond to second timescale are important in a range of soft materials. Connecting these rather slow processes with fast dynamical characteristics on the molecular level is necessary to optimise phase transitions, such as the swelling of polymer hydrogel films, which can be tuned for use as nano-scale switches in miniaturised sensors. Further, liquid-liquid interfaces such as oil-water, are of significant interest for emulsion technologies, where environmentally friendly mixtures of biosurfactants are being studied to identify most active compounds in complex mixtures. To relate the slow functional response with the fast microscopic processes requires studies of thin film systems at high energy resolution and small momentum transfer, ideally without need for deuteration.

Composite Materials Nanoparticles in composite polymers or fibrous materials provide enhanced strength or flexibility, controlled by the dispersion and aggregation. For instance, Kevlar body ar- mour fabric is reinforced by a colloidal suspension of silica nanoparticles that stiffens reversibly under impact due to shear-thickening without impairing mobility. High resolution spectroscopy is required to detect how polymer segmental dynamics, e.g., in grafted silica-polymer composites, is coupled to the aggregation of the nanoparticles causing the solidification of the material.

Food Science The texture and appearance of many natural and processed foods arises from colloidal structural elements, controlling in particular how they are formed or how they are digested. Similar procedures are used for advanced packaging materials using antimicrobial surfaces, to obtain microp- orous and biodegradable materials. In this area careful labelling allows to distinguish in microscopic studies diﬀerent components in milk, tannin precipitates in wine, and protein-lipid nanoparticles for controlled release. Next generation advances will become possible when tracking the interplay of the diﬀerent components in high-energy and momentum resolution measurements.

Complex Fluids Operational characteristics of complex fluids, e.g., micro-emulsions such as mixtures of water with oil, surfactants, nano-particles or protein and polymer macro-molecules [8, 9, 10] yield great technological potential [11, 12, 13, 14, 15]. In particular the properties of highly het- erogenous systems containing certain proteins touch on questions pursued in biophysics and the life sciences [16, 17, 18]. Again both, the interplay of different fluid components with each other or with containment interfaces require studies relating fairly fast processes at large momentum transfers to slow dynamics on large scales.

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Macromolecular Systems While x-ray techniques establish basic structural information, they can neither determine the positions nor the dynamical characteristics of hydrogen atoms, which are crucial in many biochemical processes. Determination of these hydrogen positions by neutron crystallography for enzymatic processes, protein-ligand interactions or proton transport across membranes represents essential input when designing better inhibitors for use as pharmaceuticals and for engineering in- dustrial enzymes. The large, multi-component complexes involved in the biological function of most macromolecules are often diﬃcult to crystallise. However, such structures can be elucidated in solution. The ability to discern separate components and subunits and their motions, is essential for understanding the intricate interactions between biological macromolecules in solution. In either case, i.e., large marcomolecules as well as multi-component complexes in solution studies of the slow dynamical properties are essential to unravel the operational details of their functionalities.

Biomolecules The dynamical properties of biological macromolecules are pivotal to their function. For example, the changes in the vibrational dynamics of dihydrofolate reductase (an important target of antimicrobial and cancer drugs) aﬀect sensitively the binding free energy with the drug methotrex- ate. Other examples include water and lipid dynamics at biological interfaces, and large amplitude domain motions in proteins. It is thereby essential to follow the dynamic changes of biological macromolecules in response to external stimuli, where studies of the internal protein dynamics over many length- and time-scales, from very fast dynamics on the atomic scale to the slow domain of motions taking place over much longer distances are required.

Biological membranes Biological membranes constitute soft interfaces that mediate or regulate many cellular functions between organelles and the cytosol, as well as between the cells and their environment. The lipid matrix contains or anchors a wide range of membrane proteins, carrying out speciﬁc tasks such as transport across the membrane and signalling between cells. Of great interest are the molecular interactions and transfer processes across biological membranes, such as how natural antibiotics penetrate the outer membrane of a bacterium, how cholesterol is transferred across a membrane, or the toxic interaction of ozone with lipids lining the lung alveoli. Yet, the dynamics of biological membranes are largely unexplored, requiring measurements of lipid molecular vibrations at the picosecond time-scale, and collective motions, rotations and diﬀusion at much longer time-scales.

Living Cells The cytoplasm of a living cell forms a stable intracellular fluid of thousands of proteins, together with nucleic acids, metabolites and signalling substances. At the onset of protein condensation diseases such as cataract, sickle cell disease and others the proteins become unstable. In recent years, it has become clear that the common denominator of many neurodegenerative diseases is uncontrolled protein aggregation. The diffusion of the proteins strongly influences processes such as signal transmission or protein reactions. Combined with modern colloid theories for hydrodynamic interactions the internal protein dynamics and overall diffusion in a crowded environment have to be entangled for the quantitative understanding of protein diffusion and its role in a variety of biological processes.

Catalytic Interfaces For bespoke catalytic reactions tailored surfaces and interfaces are created involving hard and soft matter systems. At the heart of catalysis are then multistep (diﬀusive) processes in combination with stochastic collective excitations which require element speciﬁc tracking. To understand the individual steps time-resolved measurements are required down to the microsecond scale at small momentum transfers.

Pharmaceuticals The development of new pharmaceuticals depends critically on the degree of polymorphism and stability of the active ingredient in the presence of other polymorphs. Dissolution problems and low bioavailability arise when the lowest free energy polymorph is selected, whereas the

10(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 highest energy polymorph may yield high solubility that is accompanied by high toxicity and a very short lifetime. Determination of the precise location of the hydrogen atoms and their dynamics in molecular drugs allows to map out the energy landscape more accurately. In turn, high resolution spatial and time correlations are essential for better evaluation of the polymorphs to be exploited in pharmaceuticals.

Fuel Cells Fuel cells receive great interest for they combine efficient energy conversion with a clean energy carrier, notably hydrogen. The most prominent challenge in the development of fuel cells is to optimise the energy efficiency. This requires a better understanding of the relationship between proton diffusion and crystal structure. The necessary experimental studies concern measurements of hydrogen diffusion at high resolution in space and time.

Gas Storage Materials The interest in gas storage materials concerns reversible hydrogen storage, encapsulation of small molecules, such as alkanes, and trapping of harmful gases, such as carbon dioxide. Typical examples include hydrides, hydrates, clathrates, metal-organic frameworks and nan- otubes. For instance, understanding how clathrates form, coexist, transform and decompose under particular conditions of temperature and pressure is crucial to their technological use. In situ neutron diﬀraction allows to monitor hydrogenation and dehydrogenation reactions and site preferences, whereas measurements of the characteristic rotational spectrum of hydrogen models are uniquely able to identify the amount of un-bound hydrogen in a sample and to reveal if it is present as dihydrogen. Also of interest is carbon dioxide sequestration and understanding the implications for climate change of uncontrolled release of methane and carbon dioxide from undersea clathrate beds. For improved studies high-resolution neutron spectroscopy is required avoiding constraints imposed by the presence of hydrogen.

Battery Materials Many batteries rely on ion exchange and/or the ionic conductivity of lithium and hydrogen. This makes them prime candidates for neutron scattering studies in operation to visualise working batteries under charge and discharge conditions. Quasi-elastic neutron scattering could be used for diﬀusion measurements on the weakly scattering lithium ions, probing dynamic properties, especially in lithium-ion battery materials thus making it possible to study the diﬀusion mechanism directly.

Thermoelectric Materials Broad use waste-heat recovery devices and electrically driven refriger- ation require cheaper and environmentally friendly thermoelectric materials to be eﬃcient. Excellent performance requires low thermal conductivity and high electrical conductivity [26]. As many thermoelectric materials exhibit cage-like host structures, one important approach is to load these cages with heavy ions scattering heat-transporting acoustic phonons. For the tailored design of thermoelectric materials high energy resolution studies of low-lying phonon anomalies are essential. With current eﬀorts moving towards nano-structured materials with increasingly larger unit cells high resolution experiments of the broad spectrum of very low-lying modes is required.

Photovoltaics Great efforts are currently put into polymer-based photovoltaics to improve the efficiency of converting sunlight into electricity. Given their low manufacturing cost, these systems are potentially serious rivals to silicon-based cells. At the heart of the efficiency is the composition and morphology at the interfaces, where optimised performances depends on the structure of the active layer, notably the uniform distribution of the active component. An important open challenge is the molecular dynamics in organic photovoltaic devices and its relationship for the overall performance. For studies of this kind very high energy resolution is obligatory.

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Glass Formation Polymeric low-molecular glasses, watery mixtures of methanol, glycerol, ethylene, and glycol as well as complex ternary, binary and even elemental metallic melts, all of which may be supercooled dramatically to obtain deep insights [19, 20, 21, 22, 23]. Identification of different relaxation channels, such as α-, β-, and γ-processes in metallic glasses in the same experiment require investigations over a wide momentum range, as well as the need for a dynamic range covering fast and slow processes simultaneously represent a kew scientific driver. Further, the microscopic processes of the crystallisation of complex metallic alloys with up to 103 atoms per unit cell in comparison to compounds at the border of quasicrystalline order require also an extremely wide parameter range [24, 25]. Here, the interplay of magnetic and structural properties forming spin-glass order have not been addressed at all.

Molecular Magnets Molecular magnets are a new classes of nano-scale magnetic materials based on multifunctional molecules aimed for spintronics as well as biological and medical applications. Molecular magnets will be used to facilitate the separation, purification and concentration of different biomolecules encased within magnetic molecules from the bulk solution through the application of a magnetic field. Molecular magnets reveal highly unusual spin dynamics with distinct low lying eigenstates that require very careful experiments for identification.

Superconductors The origin of unconventional superconductivity in copper oxide-, rare earth- and iron-based materials and its relationship to magnetism remains one of the great scientific enigmas of the 21st century. Many observations challenge the well-established BCS theory of phonon-driven superconductivity and help to benchmark theoretical models evoking magnetism. Great progress has been made in understanding the excitation spectra. However, studies at very high energy resolution will be required to distinguish between different scenarios. Moreover, the flux pinning and flux lattice melting are still microscopically fairly unexplored awaiting high resolution spectroscopic studies as a function of small momentum transfers.

Magnetic Materials Research in magnetic materials is exceptionally diverse and spectroscopic studies are a primary tool in their studies. A prototypical example are second order phase transitions as a means to explore the validity of the present day understanding of critical phenomena on very large length and time scales [26]. A more recent example concerns the spin glass transition, where hidden processes in reentrant spin glasses [27] and Berry phase contributions in chiral spin glasses have revived current interest [28, 29]. Last but not least, an important example concerns skyrmion lattices identified recently in chiral magnets. It has been shown theoretically that this form of magnetic order represents a fluctuation-induced state in a finite magnetic field [30, 31]. All of these phenomena comprise of microscopic dynamics touching on the limit of characteristic meso-scale textures, where very small energy scales become important. Their exploration requires very high-resolution measurements through a wide range of momentum transfers with high momentum resolution.

Heterostructures In magnetically ordered systems the dynamic properties of thin magnetic films, e.g., during field driven magnetisation reversals, constitute an important area of research. An exciting opportunity for future studies are microwave driven spin excitations, and the search of microwave driven melting and condensation of topological forms of magnetic order in bulk compounds [32]. Functional aspects of magnetic materials concern multiferroic compounds as well as systems with coupled order parameters (say ferromagnetic and superconducting) [33, 34]. For clarification of these phenomena high-resolution spectroscopy under large magnetic fields with a large dynamic range and good momentum resolution are necessary.

Quantum Matter Over the last century the description of (soft and hard) condensed matter systems has become deeply routed in the notion of elementary excitations and particle-like states. How-

12(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 ever, in many areas of science increasing evidence suggests an abundance of fluctuations on all scales. This paves the way to a dramatic change of paradigm, suggesting strongly that fluctuations are not only spectator, but are at the heart of fundamentally new forms of order and functionalities. For instance, in geometrically frustrated materials such as spin ice, the emergence of magnetic monopoles has been proposed [35, 36]. Another major area of interest are materials forming spin liquid phases, i.e., properties in which quantum entanglement boosts the effects of quantum fluctuations to prevent conventional long-range order. Further, zero temperature phase transition driven by quantum fluctuations (also referred to as quantum phase transitions), are characterised by excitation spectra with emergent quantum statistics and symmetries [37, 38]. Even though quantum phase transitions that are driven by magnetic fields, such as the transverse field Ising transition, are believed to provide text-book examples of quantum criticality, the actual quantum critical slowing-down so far has not been demonstrated in any system (see e.g. [39]). Here the coupling to the nuclear spin system represents an avenue to completely new physics [40]. In a broader context all of these examples represent important corner stones for theoretical developments towards an understanding of high-Tc supercoductivity. Last but not least, magnetic field driven Bose-Einstein condensation of magnons represents another major area awaiting ultra-high resolution spectroscopic studies over a large range of momentum transfers [41, 42, 43].

1.2 Implications for ESS Instrument Suite 1.2.1 Required Instrumentation and Infrastructure The scientific drivers of our proposal summarised above require, in principle, the full range of neutron spectroscopy. However, the selection of problems and materials we have emphasised here, demonstrates in particular the need for the highest possible energy resolution combined with medium and high momentum resolution. These high resolution studies should be able to access a wide range of energies up to several ten meV at momentum transfers from zero up to large values. This is the parameter range characteristic of neutron-spin echo instruments. An important instrumentation requirement that cannot be emphasised enough is based on the fact, that scientific progress for essentially all of the topics listed above results from a very close interplay between theory and experiment. In particular for complex large scale materials systems the primary tool are molecular dynamics (MD) simulations. These simulations predict directly real- time correlation functions, corresponding to the intermediate scattering function. In turn, comparison of data obtained with conventional spectrometers (time of flight, triple axis and backscattering) by default is always more complex involving additional analysis and assumptions. We therefore believe that it is very important to include in the ESS instrument suite a beam-line that provides directly the intermediate scattering function. Another important aspect for progress on the scientific issues described above is the necessary infrastructure. Concerning soft-matter systems extreme conditions such as variable humidity, pressure, variations of PH, electrical stimuli, or the possibility to operate equipment for pump probe studies of various types. Further, for studies that are typically carried out in the context of hard matter systems, the possibility for extreme conditions, notably ultra-low temperatures down to the low milli-Kelvin range as well as high pressure cells are important. Of increasing importance in recent years is the possibility to study the effects of large magnetic fields. While the need for large fields is somewhat obvious in magnetic materials and superconductors, there has been growing appreciation, that they offer also a very powerful tool to generate contrast in typical soft matter and biological systems. The characteristic field values necessary, notably the need to exceed ∼ 10 Teslas, are thereby perfectly compatible with standard magnet systems. However, it has so far been exceedingly difficult to combine large magnetic fields with neutron spin echo spectroscopy.

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Figure 1: Overview of neutron-based techniques and complimentary methods for the exploration of different length and time scales. Note the wide parameter range accessible by Neutron Spin-Echo Spectroscopy and the large overlap with time of flight and backscattering. In comparison to this depiction, the time of flight instruments VOR, CSPEC and CAMEA in the ESS instrument suite offer even better energy resolution. Plot taken from ESS technical design report [44].

1.2.2 ESS Instrumentation under Construction A detailed comparison of the conventional experimental methods, as presented in the ESS technical design report, with different neutron scattering instrumentation is shown in Fig. 1. Instrumentation for the investigation of structure comprise of diffractometers, reflectometers, small angle scattering and imaging beam lines. A wide dynamic range at large momentum transfers is typically covered by means of Time-of-Flight (ToF), triple-axis (TAS) and backscattering (BS) spectrometers. At ESS so far a total of nine beam-lines for structural studies have been selected, notably LOKI, ODIN, ESTIA, HEIMDAL, FREIA, DREAM, NMX, SKADI and BEER. For these instruments modular upgrades enabling energy analysis have so far not been considered, but may be possible in some cases, by means of the neutron resonance techniques described in our proposal. Regarding the suite of instruments for studies of dynamical processes selected so far for ESS, the three chopper spectrometers VOR, CSPEC and CAMEA will allow to cover a wide (Q, ω) regime. For a first screening of the dynamic properties VOR is ideally suited, using a very broad thermal wavelength

14(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 band. This is followed by the direct ToF spectrometer CSPEC. With a true small angle scattering option planned, CSPEC oﬀers high energy resolution for a wide range of momentum transfers covering a wide parameter range in soft and hard matter systems as described above. In fact, in comparison with the energy and momentum range anticipated in the ESS technical design report, shown in Fig. 1, VOR and CSPEC extend to even smaller energies into the area characteristic of typical backscattering instruments. Being typical ToF instruments, VOR and CSPEC are particularly suited for the study of non- dispersive excitations. In contrast, the indirect ToF spectrometer CAMEA with its highly innovative multiple analyser banks oﬀers continuous angular and energy analysis for studies of dispersive excitations as normally carried out by triple-axis spectroscopy. However, when taken together, the very large parameter regime accessible with neutron spin-echo spectroscopy is so far not covered at all by the ESS instrument suite.

1.2.3 Justification for a LNRSE spectrometer Since Mezei’s celebrated proposal in 1972 [45] to encode tiny energy transfers in neutron scattering by the Larmor precession of the neutron spin in carefully designed magnetic fields, neutron spin-echo spectroscopy (NSE) has become an extremely well-developed technique. Scientific challenges addressed at world-leading instruments, such as IN11 or IN15, reflect intimately the power of NSE in studies of non-dispersive ultra-slow dynamics, notably relaxation, and diffusion processes on large scales and thus small momentum transfers, as well as tunnelling or crystal electric field excitations. In turn, scientific activities addressed by NSE cover all disciplines in soft and hard condensed matter. In contrast, many of the most pressing scientific issues require measurements with the resolution characteristic of NSE spectroscopy, however, frequently under more demanding conditions such as (i) large momentum transfers, (ii) strict zero magnetic field, (iii) very large magnetic fields, (iv) strongly depolarising samples, (v) strong incoherent scatterers (notably materials containing hydrogen), (vi) spherical polarisation analysis of the dynamical properties, or (vii) very small sample quantities. These requirements represent the main reasons why NSE is not used more extensively in areas such as crystallography, material science, condensed matter magnetism, and superconductivity. To address the parameter range of the scientific drivers described above a high-resolution neutron spin echo instrument is necessary. On the one hand a spin echo instrument would greatly extend the parameter range accessible with the ESS instrument suite. On the other hand, a NSE instrument would uniquely provide the intermediate scattering function directly, which will be very important for a wide range of scientific problems. As technical realisation of a neutron spin-echo instrument we propose to built a longitudinal neutron resonance spin-echo (LNRSE) spectrometer for the following reasons:

• It has been widely argued, that neutron resonant spin echo (NRSE) cannot reach equally large spin echo-times as conventional spin echo. Measurements we have performed recently unam- biguously establish that this statement is wrong (see appendix for a summary and data). The longitudinal version of NRSE not only overcomes previous limitations, but has the potential to cover a much larger parameter range then conventional spin echo.

• The true limitation of conventional spin echo spectroscopy to reach significantly larger spin echo times consist in the very large energy densities needed in the correction coils, that are used to compensate inhomogeneities of the main precession solenoids. In comparison, the energy densities required to correct the effects of precession field inhomogeneities in the LNRSE method we advertise for fundamental geometric reasons are at least one order of magnitude smaller. This implies massive cost savings both in construction and operation. It also suggests strongly, that a much wider parameter range (notably spin-echo times much longer than a microsecond) may be possible in the future.

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• The much simpler conceptual approach used for Larmor labelling in LNRSE, notably spin ﬂipper at the beginning an end of the precession section, implies an open geometry. In turn, it is easy to install neutron guides or other beam shaping equipment (see details further below).

• The dynamical range of the LNRSE method may be tuned very elegantly with the ﬁeld integral- subtraction method (a small solenoid between the spin ﬂipper), where we have already achieved eight orders of magnitude in our proof-of-principle tests.

• It has long been known, that NRSE spin ﬂipper yield a very wide variety of additional measurement methods. An important examples is the MIEZE-1 option described below, which allows to perform spin-echo studies at small momentum transfers under strongly depolarising conditions such as very large magnetic ﬁelds. An LNRSE instrument may be upgraded at comparatively low cost to include these additional options.

• We have recently demonstrated that NRSE techniques have a great potential for use as add- on modules. Considering the nine instruments for structure determination we see considerable potential for such modular features adding energy resolution to these beam-lines. Construction of a LNRSE beam line at the ESS will cultivate the necessary technical radio-frequency know-how as a strategic option for the future.

• Despite being a ultra-high resolution technique the LNRSE method is fairly insensitive to external magnetic fields and other disturbances. In particular, it can be set up in a conventional experimental hall and does not require zero field shielding or a screened room. This implies great additional flexibility for potential sample environment.

1.3 Expected User Community Conventional NSE spectroscopy is well-established in the soft matter community, reflecting a large fraction of the activities at world-leading instruments such as IN11 or IN15. It is also an important tool in studies of glassy transitions, diffusion, tunnelling processes or fluctuations in spin systems. In addition, the list given above illustrates several additional areas in which NSE spectroscopy has not been used widely but may become very important. NSE instruments available internationally exhibit over-load factors of the order 2 to 3. This implies that scientific proposals for NSE-quality studies overall are at the border of not receiving sufficient beam time at existing instruments for the parameter regime (and intensity) of these instruments. Given that the instrument we propose will offer much improved performance (higher intensity and more flexible access to the entire parameter range) we anticipate that the user community of the proposed beam-line will be much broader comprising of a diverse range of different areas. In particular, the proposed beam-line will allow to address for the first time some of the most pressing scientific issues across a wide range of topics. Keeping additionally in mind, that the instrument concept we propose offers additional options, we anticipate an even wider user community.

2 General Instrument Concept and Performance

Neutron spin echo techniques diﬀer from traditional neutron scattering methods as the measurements yield the Intermediate Scattering Function S(q, τ). Thus they provide direct access to information on the lifetime of excitations in the sample. The unprecedented energy resolution of NSE is made possible by decoupling the resolution function of the instrument from the wavelength spread of the incident neutrons. The energy transfer of the neutrons is encoded in the polarization of the neutrons and not in the change of the wavelength of the scattered neutrons. Three variants of neutron spin echo spectrometers have so far been been built: Neutron Spin Echo (NSE) [45], Neutron Resonance Spin Echo (NRSE) [46, 47], and the MIEZE technique in the transverse ﬁeld geometry [48, 49], where

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polarizer solenoidLNRSELNRSE sample solenoidLNRSELNRSE analyzer B

rf-coil rf-coil rf-coil rf-coil detector

Figure 2: Schematic of the proposed spectrometer in LNRSE configuration: The static fields are printed in blue and the guide field in red. Note that in contrast to classical NRSE the orientation of the static B0-fields is parallel to the optical axis of the instrument. The solenoids between the LNRSE coils can be used to tune the spin echo times towards zero. Note that the length of the LNRSE coils ist typical much shorter as the equivalent coils of length L1 in a classical NSE instrument.

MIEZE is the abbreviation for Modulation of IntEnsity with Zero Effort. The resolution and parameter range of these three types of spectrometers is limited due to various technical constraints. For the instrument RESPECT we propose as a baseline to use a resonant spin echo technique with a longitudinal field geometry throughout the beam line, i.e., Longitudinal NRSE (LNRSE) similarly as first proposed by Häußler et al. [50], which combines the advantages of both NSE and NRSE in one spectrometer. On the one hand, this combination allows to profit from the experience and knowledge of instrument design gathered over 40 years using NSE and NRSE. On the other hand LNRSE allows to use the same correction techniques as the ones established in classical NSE and therefore to achieve at least the same energy resolution as classical NSE spectrometers. The technique of resonant spin flips employed in LNRSE provides the possibility to readily use the spectrometer in a MIEZE-1 configuration and thus to study samples under depolarising conditions. Therefore we propose a later optional upgrade which will allow for spin echo measurements in depolarising samples (containing hydrogen, being ferromagnetic) and extreme sample environments. Another optional upgrade using a MIEZE-2 configuration would provide similar functionalities as the wide angle NSE-spectrometer WASP to be realized at the ILL [51].

2.1 Longitudinal Neutron Resonance Spin-Echo Spectroscopy

In contrast to conventional NRSE the longitudinal setup is based on static fields B0 which are parallel to the wavevector ki of the incident neutrons. This mimics the highly symmetric cylindrical field geometry of classical NSE. The radio frequency fields are installed perpendicular to ki as in conventional NRSE. A schematic setup is shown in Fig. 2. The precession region is defined by two π/2-flippers at the beginning and the end of the first and second spectrometer arm, respectively. Each arm contains two LNRSE coils and a solenoid in between them. Along the flight path between the two π/2-flippers there is a longitudinal guide field to maintain the polarisation of the neutrons. If required, the precession can be reversed by a π-flipper in front of the sample. The static B0-coils consist of main and auxiliary coils in Helmholtz-configuration where the auxiliary coils are producing an antiparallel field with respect to the main precession field B0. These auxiliary coils are used as in classical NSE to improve the field homogeneity at the entrance and exit of the coils and moreover to homogenise the magnetic field in the region of the radio frequency coils. A dedicated LNRSE setup [52] has been realised at the instrument RESEDA at MLZ (see also [53, 54]). First test measurements demonstrate that up to an effective field integral of 0.160 Tm no field corrections are required. The usefulness of the standard correction coil technique was demonstrated in an earlier test experiment at IN11 [50]. In this experiment, the second conventional NSE arm was replaced by a LNRSE arm to compare directly the magnitude of the required correction fields, revealing that much smaller correction currents are needed in the LNRSE arm. Therefore much less material from the Fresnel coils is in the beam and the demands for their cooling are tremendously reduced. The smaller correction currents are caused by two intrinsic properties of LNRSE: i) due to

17(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 the spin flip in the center of the LNRSE coil most field inhomogeneities at the entrance of the coil are canceled by the similar field inhomogeneities at the exit of the coil. ii) the B0 coils are typically 10 times shorter than NSE coils reducing the field corrections accordingly. Moreover, the fabrication of short Helmholtz-coils is of minor technical challenge when compared with the fabrication of field- optimized coils for NSE. In contrast to the conventional transverse NRSE technique, the cylindrically symmetric LNRSE configuration allows to guide the polarization of the neutrons through the whole spectrometer and no spin rotations are required, reducing the effort to maintain the polarization. In addition, no bulky mu-metal shielding is required. Therefore, maintaining the polarization of neutrons with large wavelength λ is facilitated. The neutrons with large λ are particularly important as the resolution of the NSE-techniques increases with λ3. The spin flip of the neutrons in the LNRSE coils allows to subtract efficiently field integrals by applying a magnetic field between the coils using an additional solenoid (see Fig. 2). This solenoid enhances the flexibility of the NRSE setup when compared with the NSE-technique as the spin echo times can be tuned continuously towards zero [53, 55] thus extending the range of accessible spin echo times by several orders of magnitude. The additional solenoid can be easily integrated into the instrument and does not destroy the guide-field as it is co-aligned. Combining the high resolution of LNRSE with effective field integral subtraction allows to cover 8 orders of magnitude in one single set-up.

The main advantages of LNRSE may be summarised as follows: • The LNRSE technique displays massively reduced ﬁeld inhomogeneities.

• Deterioration of the resolution due to the eﬀects of beam divergence can be compensated using standard NSE correction techniques.

• Using the longitudinal ﬁeld geometry, the polarization can be guided adiabatically through the instrument, making magnetic shielding dispensable.

• The longitudinal ﬁeld coils reduce the amount of material in the beam path when compared with traditional NRSE. Therefore, small angle neutron scattering and absorption are reduced.

• The longitudinal ﬁeld geometry allows for the ﬁeld integral subtraction method. In turn, the dynamic range of the spectrometer is massively increased, in particular towards very small spin echo times ( ps).

• The NRSE concept allows for removing the second spectrometer arm with minimal eﬀort yielding a MIEZE-type spectrometer. The MIEZE-mode is particularly useful for the investigation of depolarising samples and samples in large magnetic ﬁelds as explained in the next paragraph.

2.2 Future Options 2.2.1 Spin-Echo Spectroscopy under Depolarising Conditions (MIEZE-1) A MIEZE spectrometer requires only the primary arm of a (L)NRSE spectrometer (see Fig. 3). The advantage of this method is that all manipulations of the neutron spin are performed before the sample position. Therefore, effects of the sample on the polarization of the beam do not affect the performance of the instrument. Hence, even depolarising samples and samples exposed to magnetic fields can be investigated. Even spherical polarization analysis can be implemented easily. Moreover, absorption and small angle neutron scattering are reduced when compared with LNRSE. However, because the MIEZE-method is sensitive to differences in the path length, measurements to date are restricted to small samples and/or samples with a special shape [49]. MIEZE is currently in operation at two beam lines at the FRM II, namely MIRA and RESEDA. Its usefulness has been benchmarked in high magnetic fields up to 17 T [52] without reduction in contrast.

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2 SD polarizer solenoidLNRSELNRSE analyzer B

rf-coil rf-coil sample detector

Figure 3: Schematic of RESPECT in the MIEZE-1 conﬁguration. MIEZE-1 allows for investigating depolarising samples and samples in large magnetic ﬁelds in a small angle geometry. L2 is the distance from the last LNRSE coil to the detector and can be determined from the MIEZE equation L2 = L / ω2 − 1 with ω being the two RF frequencies in the two coils. The MIEZE-time (equivalent 1 ω1 1,2 hLSDωM to the NSE-time) is then given by τ = 3 with ω = 2 × (ω − ω ) and v the velocity of the M mnv M 2 1 neutrons.

polarizer solenoidLNRSELNRSE sample B LNRSE

longi. guide ﬁeld analyzer multiple detectors

Figure 4: Schematic of RESPECT in the MIEZE-2 configuration. MIEZE-2 allows for simultaneous investigation of a wide q-range. The polarization is preserved through the sample position and the back precession is started by a large window LNRSE coil behind the sample. In contrast to MIEZE-1 measurements under depolarizing conditions are not straight forward to perform. However path length differences in the sample due not influence the measurements.

MIEZE times up to 15 ns have been reached in this high magnetic field. Furthermore, MIEZE times up to 105 ns were reached at RESEDA using neutrons with λ = 20 Å(see Fig. 25) in zero magnetic field. The measurements had to be conducted in the direct beam because the neutron flux at 20 Å was extremely small due to the characteristics of the cold source at the FRM II.

2.2.2 Wide-Angle Spin-Echo Spectroscopy (MIEZE-2) The restrictions of MIEZE-1with respect to path length diﬀerences will be resolved by installation of a third LNRSE coil after the sample. Here, no polarizer before the sample is required. After the sample, a wide angle RF-spin-ﬂipper followed by a polarizer will be used (not shown in Fig. 5) combined with the large area detector from the MIEZE-1 set-up covering an angular range of approximately 40◦. Larger solid angles can be covered by moving the detector closer to the sample. This technique is called MIEZE.2. However, in this case non-depolarizing samples must be used or an additional polarizer is required near the sample similarly as in standard NSE.

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2.2.3 Inelastic Spin-Echo Spectroscopy and Larmor Diffraction For the investigation of excitations with dispersion and for Larmor diffraction it is required to tilt the field boundaries of the precession region. This task can be ideally performed by means of NRSE with B0 coils in a transverse field configuration (TNRSE). The most famous instrument is the polarized thermal triple-axis spectrometer TRISP at FRM-II where the two precession arms are mounted before and after the sample. Typically, lifetimes of the order of 1 ns [56] and d spacings ∆d/d ' 10−6 [57, 58] can be resolved. Using the intense flux of cold neutrons at ESS, these limits can be extended by at least one orders of magnitude. Inelastic measurements using the MIEZE-I technique have very recently been conducted at the beam line MIRA at FRM-II on the isotropic Heisenberg ferromagnet EuS and the helimagnet MnSi. Using MIRA, the regime of 10 ns can be reached. There is a possibility to simply exchange at RESPECT the arms for LNRSE by TNRSE and to do inelastic spin echo with spin echo times larger than on MIRA. Because of the extremely wide dynamic range of RESPECT, the damped oscillations from scattering by excitations can be directly measured. Presently, we are developing a technique to perform inelastic neutron scattering and Larmor diffraction in the LNRSE configuration at RESEDA at the FRM-II.

3 Instrument Concept of RESPECT

3.1 Overall Layout of the LNRSE Spectrometer This paragraph outlines the design of the versatile instrument RESPECT (Fig. 5) that implements the LRNSE technique using state of the art technology for each component. The design goal is utilizing a wide wavelength band of approximately 2 – 22 Å in order to achieve spin-echo times in the sub-ps to µs range with the excellent flux provided by the pancake moderator. The overall length of the instrument is approximately 37 m. To provide flexibility for bulky sample environment it is possible to move the sample table further away from the primary precession region as well as to move the secondary precession arm further away from the sample position. The focusing device can be extended from the end of the precession region to the position LE closer to the sample. The neutrons are transported using a neutron guide with a small cross section 40 mm × 40 mm that yields an efficient and homogenous illumination of the sample avoiding an illumination of the sample environment (Fig. 6). For more details see Appendix A.2. The flux (cm−2 s−1 Å−1) of RESPECT is compared with the NSE beam lines IN15 at the ILL and ESSENSE at ESS in Fig. 7. See Appendix A.1 for details. It is clearly seen that RESPECT yields a signigicantly higher flux than its competitors for a comparable divergence of the beam at the sample in particular at short wavelength. For the simulations, the brilliance data from P. Willendrup and K. Andersen from 10-Feb-2015 was used. The black and red lines indicate the results of simulations for ESSENSE with and without beam losses (due to absorption) taken into account, respectively. In the following the individual components of the guide system are discussed in more detail. Because RESPECT is less sensitive to magnetic fields than the existing NSE instruments, it is favorable placing RESPECT in one of the short guide halls housing conventional instruments As we intend using large magnetic fields a placement in the hall for low field environment does not appear reasonable, even when compensated magnets are used. Further, it is important to place the beam line at a side of a guide bundle to allow for enough space to move the second LRNSE arm to large scattering angles or to install in the future a wide angle option MIEZE-2. In the following the instrument concept will be more detailed following the direction from the moderator to the detector.

20(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 additional focusing 3 -resolution. For more details see Q Figure 5: Layoutguide of elements the can LNRSE be spectrometer installedto RESPECT. between For interrupt the the the 30 investigation line-of-sight of meter fromused small position the to samples and moderator with reduce the to a the sample. volume the divergencetext. A of sample of shutter a two the is few times. beam installed mm if before The it chopper two collimators 1. is (vertical required The / for second horizontal) achieving bender before high is the spin used ﬁrst echo RF-coil times are and/or improved

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Figure 6: The figure compares the distribution of the neutrons on an area 3 cm × 3 cm and the divergence at the sample position of ESSENSE (width/height: a = 80 mm) and RESPECT (a = 40 mm). The beam of RESPECT is nicely confined to the sample area and exhibits a homogenous divergence distribution. By inserting collimators upstream of the precession coils, the divergence of RESPECT can be varied between 0.1◦ and 1.8◦ / 0.9◦. Note the increase in flux of about a factor of 3 for RESPECT.

3.2 Primary Spectrometer 3.2.1 Chopper System According to the scientific case, one of the major applications of RESPECT are investigations of dynamic processes in soft matter and in magnetism at very high resolution. This raises the question of using one, two, three or more frames of the pulse from the ESS moderator, which then defines the total length of the instrument. One frame (4λ ' 20 Å ) is not feasible because the instrument length would have to be restricted to less than ' 16 m provoking serious problems with background (shielding) and problems with space to operate RESPECT. Similarly, two frames would yield a too short instrument creating serious background problems. In contrast, using more than three frames would decrease the integrated intensity over the anticipated wavelength window of the frames and hamper the operation of RESPECT at large wavelengths where the brilliance of cold moderators is small. Therefore, we propose using three frames leading to bandwidths of approximately 6.75 Å thus covering a wavelength range from about 2 Å to 22.5 Å (Fig.8). Using the design of standard ESS chopper systems leads to an instrument length of 37 m. The three wavelength bands are defined by choppers 1 and 2, which are adjustable via their phase shift. They are positioned 6.5 m and 15.5 m downstream of the moderator (Fig. 5). In principle, the pulse is defined by the time structure of the moderator, however, we propose the installation of chopper 1 to define a well-defined beam by cutting the tails of the ESS pulse. The resolution δλ τ = λ αLλ with α = 252µs/Å/m, decreases from 5% at 6 Å to 1.6% at 16 Å which are reasonable numbers for applications as described in the scientific case. The wide frame at large wavelengths compensates for the intensity losses due to the decreasing flux at large λ. Furthermore due to the small beam size and the compact chopper system the pulses will be chopped in a very clean way. There is also no need for a frame overlap chopper, thus nearly the full intensity per pulse can be delivered to the sample.

3.2.2 Bender 1 and shutter section

The bender has a length Lbend = 3.96 m, a radius of curvature Rb = 140.34 m, and comprises four channels. The walls are coated with supermirror m = 4.5 leading to a large deﬂection angle ψ = 1.6◦ and a low critical wavelength λ∗ = 1.51 Å. The small λ∗ leads to an excellent transmission for λ ≥ 2 Å, which is matched very well to the down-stream guide system that uses coatings m = 2. Of course,

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Figure 7: The figure compares the flux at the sample position of RESPECT with the spectrometers ESSENSE and IN15 [59]. The solid lines represent McStas calculations performed during the course of the optimization of RESPECT. The simulated values for ESSENSE are in good agrement with the flux quoted in Table 1 of Ref. [59]. The direct calculation of the flux (green star) at λ = 6 Å based solely on the brilliance of the pancake moderator, phase space considerations and the transmission functions of the various components is in perfect agreement with McStas (Appendix A.1. by increasing the number of channels and m, the transmission at small λ could be improved further. The line-of-sight is interrupted a couple of meters away from the biological shielding, i.e. much quicker than by using a curved guide. Moreover, the 4 channels lead to a very homogeneous beam. The use of metallic substrates (made from Cu or Al) for the body of the bender and Si-wafers for the blades will reduce the flux of high energy neutrons and γ radiation, leading to cost savings in shielding. The exit of the bender may be equipped with a thin Al window to decouple the vacuum (or 4He environment) of the bender from the first chopper and the straight guides. These details have to be discussed with ESS.

3.2.3 Guide System The proposed guide system with a cross section 40 mm × 40 mm (Fig. 5) is well adapted to a sample and moderator size of 30 mm × 30 mm yielding a maximum vertical and horizontal divergence of 0.92◦ (0.94◦)and 1.8◦ (1.0◦), respectively. The numbers in brackets give the corresponding divergences for the ESSENSE spectrometer (Fig. 6). The compact phase space of the neutrons allows not only for high-resolution experiments but also experiments requiring a high intensity at a reduced resolution, i.e. short spin-echo times. For more details see appendix A.2. The guide system is designed for neutron wavelengths λ ≥ 2 Å. The small cross section of the guide is particularly beneﬁcial for Larmor precession techniques such as LNRSE by the following reasons:

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Neutron wavelength range 2 A˚ — 8.75 A˚ Neutron wavelength range 8.75 A˚ — 15.5 A˚ Neutron wavelength range 15.5 A˚ — 22.25 A˚ 36.0 36.0 36.0

15.5 15.5 15.5

Distance (m) 6.5 Distance (m) 6.5 Distance (m) 6.5 0.0 0.0 0.0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100120140160180 0 40 80 120 160 200 240 Time of flight (ms) Time of flight (ms) Time of flight (ms)

Figure 8: The three wavelength bands, which will be used by the instrument. The opening times for the PSC and the wavelength frame selection chopper are 11.41 ms and 34.03 ms, respectively.

• small background from neutron source

• compact phase space of transported neutrons (adapted to pancake moderator)

• line-of-sight-distance is reduced

• the choppers become smaller and therefore more precise and signiﬁcantly cheaper

• small critical wavelength (λ = 2 Å)

• reduction of illumination of sample environment

• implementation of focusing elements facilitated

• cheaper guide system, reduced costs for shielding

• polarization of incident neutron beam simple and cheap

• complexity of spin manipulation devices and requirements on homogeneity of magnetic ﬁelds are strongly reduced thus allowing better resolution

• external stray ﬁelds are less detrimental.

3.2.4 Polarizing cavities Polarizing cavities are one of the most efficient means to polarize neutrons over a wide range of wavelengths. However, if the taper angle of the polarizing blades becomes comparable to the critical angle of reflection of the neutrons for the spin down neutrons (m ' 0.68) also the good neutrons are reflected away from the beam. These are actually the most valuable neutrons with a small divergence. Therefore, two cavities with = 0.35◦ and 1.9◦ for the wavelength ranges 2 Å ≤ λ ≤ 10 Å and 8 Å ≤ λ will be used, respectively. The polarizing coatings are made from FeSi-supermirror with m = 4. The polarization of the neutrons has been calculated using McStas assuming a sample size 30 mm × 30 mm. The results shown in Fig. 9 demonstrate a reasonably high polarization 92% ≤ P ≤ 98% is achieved. To achieve polarizations P > 99% double V-cavites [60] may be installed. Note that for simulating the flux of polarized neutrons at the sample position (green solid line in Fig. 7) a long cavity for the complete range of wavelengths 2 Å ≤ λ ≤ 30 Å was assumed. Therefore, the flux at large λ is underestimated.

3.2.5 Bender 2 Bender 2 has the same geometry as bender 1. It guarantees that all neutrons have to be reﬂected at least two times before reaching the sample. In contrast to bender 2, the guide body and the dividing blades are manufactured from glass.

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Figure 9: The figure shows the polarization of the short and the long cavity for small and large wavelength ranges, respectively. Effects of multireflections of the neutrons within the Si-wafers, which are coated on both sides, and absorption in Si and FeSi are taken into account.

3.2.6 Collimation stage The vertical and horizontal divergence of the neutron beam at the sample position of RESPECT without collimations are 0.92◦ and 1.8◦, respectively (Fig. 6). As shown in the Appendix A.3, a large divergence is detrimental for the resolution of Larmor precession techniques. For example, without correction coils (a special feature of LNRSE) one may achieve TNSE ' 1 µs if neutron beams with a divergence less than 0.2◦ are used (Fig. 20). Of course, by means of corrections coils larger divergencies can be used (Appendix A.3). To deﬁne the divergence of the beam at the sample position, two translation stages providing vertical and horizontal collimations between 10 min and 60 min are provided [61]. Of course, the use of collimations will ”switch-oﬀ” the neutron guide following the collimators. If focusing guides are installed between the exit of the precession region at 30 m and LE, the reduction of the divergence of the beam will lead to an improved focusing of the guide. Experience shows that a beam size below 1 mm can be achieved.

3.2.7 Focusing guides For measurements on samples with a size below 30 mm, parabolical focusing devices will be introduced bridging the flight path between the last precession coil and the position LE (Fig. 5), which may be as close as 80 mm from the sample [62]. According to Liouville’s theorem and experience, a reduction of the flux at the sample position increases roughly proportional to the area of the beam. Therefore, reducing the beam size by a factor of 3 to 1 cm × 1 cm will provide a gain of almost an order of magnitude. If the resolution conditions allow, one may end up (for example at λ = 6 Å) with a polarized neutron flux at the sample of F > 5 · 109 cm−2s−1 neutrons indeed a very intense beam [63, 64]. The path length diverences are presently worked out by the authors of the proposal.

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3.2.8 LNRSE coil system The proposed RESPECT spectrometer will include four LNRSE coils. Two of them in the primary spectrometer. The static B0 coils consist of normal conducting, water cooled windings designed to produce a magnetic field of 0.17 T. With a coil distance of L1 = 3 m, employing π-flips the effective field integral produced by one spectrometer arm will be J = 0.17 T · 3 m · 2 = 1.02 Tm yielding spin echo times τNSE = 180 ns and τ = 1.4 µs at λ = 10 Å and λ = 20 Å, respectively. The radio frequency coils will be operated at ≈ 5 MHz. The q-range in the LNRSE configuration will extend to at least 4.5 Å−1 depending on the maximal available scattering angle that is given by possible restrictions imposed by neighboring beam lines. For details concerning field integrals in LNRSE see Appendix A.3.

3.3 Secondary Spectrometer The secondary spectrometer will be very similar to a classical NSE-spectrometer the major diﬀerence being the replacement of the solenoids for NSE by two RF-coils for LNRSE. In the following we make a few remarks concerning the layout of the components.

3.3.1 LNRSE coil system The coil system for the secondary arm is almost identical to the system in the primary arm. To allow for a large detector with the dimensions of aproximately 30 cm × 30 cm the RF- and B0 coils are enlarged. This applies in particular for the second RF-coil as indicated in Fig. 5.

3.3.2 Analyzer A large area polarizing analyzer as used for example for the beam line JNSE at FRM2 will be installed. The blades have the dimensions of typically 30 cm × 30 cm and are coated with remanent supermirror FeCoV/TiNx. This type of coating is particularly useful for NSE because large magnetising fields for the supermirror coatings are detrimental for the operation of NSE [65]. Because LNRSE is less sensitive to magnetic fields, we will consider using more advanced coatings Fe/Si with m = 5.5, a reflectivity of 70% at m = 5.5, and a polarization P > 99% (radial analyzer of POLANO at JPARC, Ef = 27 meV). The newest generation of coatings can be magnetised in fields as small as 20 mT. To respect the symmetry of LNRSE, the coatings will be magnetized in a longitudinal field.

3.3.3 Detector We consider an identical detector system as in the ESSENSE proposal [66]. This detector will have dimensions of 30 cm × 30 cm and with a moderate (2D) position resolution (better than 3 cm or 1 inch), high eﬃciency and high maximum count rate (better than 5 KHz/cm2). A robust possible solution would consist of an array of 100-200 front end counting tubes (3He) with square cross-section or an array of linear position sensitive counting tubes.

3.4 Future Options There are two MIEZE-conﬁgurations proposed as options. For both conﬁgurations the second LNRSE arm is removed.

MIEZE-1 Maximum spin echo times τM = 90 ns at λ = 10 Å and τM = 720 ns at λ = 20 Å will be available in the small angle regime. At the sample position, magnetic fields as large as 17 T can be applied (Fig. 25). Moreover, depolarizing samples can be used. For MIEZE-1 a polarizer is inserted before the sample (see Fig. 10). The magnetising field will be along the beam direction (longitudinal) to respect the symmetry of the field configuration of the beam

26(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 line. To reduce the disturbance of the ﬁelds of the precession coils, the magnetic ﬁelds to magnetise the coatings of the cavity are self-shielded. Of course, the polarizer can also be used to perform neutron scattering experiments on depolarising samples using the LNRSE or MIEZE-2 set-ups for arbitrary angles and/or to conduct spherical polarisation analysis.

Figure 10: For the longitudinal MIEZE option, the secondary precession coils are removed and a large detector is placed in the forward direction. In addition, a short multi-channel polarizing cavity is inserted between the last RF-coil in the primary arm and the sample.

A fast detector system is needed in order to detect the high frequency intensity variations. This detector system will consist of an array of 32 CASCADE detectors (25 × 25 cm2) for the small angle MIEZE setup. The CASCADE detector concept of CDT we propose to use for the MIEZE instrument option is a self-sufficient detector system comprised of a detector front end and an integrated detector readout system with on board histogramming electronics. The front end consists of 10 layers of GEM foils with B10 coating. A mixture of 85 %Ar and 15 %Co gas is used as counting gas and electron getter for avoiding sparking, respectively. Each of the foils allows two readout 128 by 128 pixels simultaneously. The signals of the different foils are added up in order to reach a detection efficiency of 50–60 % for a neutron wavelength of 5 Å. In the MIEZE mode, the 128 by 128 pixels of the ten foils can be counted in a histogram in time for each pixel separately. Together with the phase information from the radio frequency (RF) current this allows the detection of the MIEZE modulation in each pixel. CASCADE detectors have been successfully commissioned at the instruments RESEDA and MIRA at the FRM II. The correct working parameters together with the optimum mixing parameters for the detector gas were determined and together with the resonant circuits have been used already in several experiments detecting small angle neutron scattering. The parameters for the MIEZE mode including the detection of the time signal of every GEM foil have been verified in experiments at RESEDA [67]. The operation of a MIEZE spectrometer with a pulsed neutron beam was already tested [68] and the necessary adaption in the sweeping of the amplitude of the RF-fields has already been developed (see Appendix) . It is planned to base data evaluation on the existing ILL code library for NSE instruments as used at the ILL and the FRM-II. This code also includes already the necessary adaptation for pulsed beams. A recently developed software package [69] for graphical data evaluation based on this ILL library could be adopted to the ESS software suite by DSM.

MIEZE-2 For experiments at large scattering angles and if path-length differences become an issue [70], the MIEZE-2 configuration will be used. Here, no polarizer before the sample is required. After the sample, a wide angle RF-spin-flipper followed by a polarizer will be used combined with the large area detector from the MIEZE-1 set-up covering an angular range of approximately 40◦. Larger solid angles can be covered by moving the detector closer to the sample. MIEZE-2 will provide similar functionalities as the wide angle NSE-spectrometer WASP to be realized at the ILL [51]. However, the complexity of our set-up is significantly reduced. Measurements

27(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 on depolarizing samples can be performed by translating the polarizer of the MIEZE-1 set-up into the beam before the sample (Fig. 11). For the MIEZE-2 option a similar CACADE system as for MIEZE-1 can be used but with a much more relaxed spatial resolution in the oder of 3 cm or 1 inch. As the system needs to cover a much larger solid angle we will need 32 additional detectors for covering the whole solid angle provided by the wide angle coil.

Figure 11: The longitudinal MIEZE-2 option is realized by replacing the secondary LNRSE-arm by a wide-angle RF-coil followed by a wide-angle analyzer. A time-resolved wide-angle detector, for example CASCADE, monitors the time-dependent signal. This set-up is ideally suited for the investigation of non-depolarizing samples. For the investigation of small samples, parabolic focusing elements can be installed in the primary arm.

4 Comparison of RESPECT with Existing Instruments

In Fig. 12 it shown how the LNRSE technique integrates within the disfferent spectroscopic techniques. There are several NSE instrument based at reactors, like IN11(C) and IN15 at the ILL, J-NSE at the MLZ (FRM II), NSE at NIST, MUSES at LLB and NSE at Tokai. NSE at SNS and LAMOR at ISIS are instruments running at pulsed neutron sources. J-NSE and NSE at NIST are clones of the original ILL IN15 design, which is currently upgraded. Therefore, the leading NSE instruments at reactors, like the newly designed IN15 at the ILL with a field integral of 1 Tm will offer 600 ns time resolution in standard operation, which can be extend to the µs regime and serves as the benchmark for NSE and NRSE instruments. A wide-angle NSE project currently being realized at the ILL is WASP. It will provide a comparable field integral as IN11 of the order of 0.27 Tm. This should allow to reach a time resolution of 100 ns at around 15 Å and a maximal q of 3.3 Å−1. It is planned to start with 90 degree detector coverage and should have 50 times the count rate of IN11(C). The NRSE technique in its transverse implementation is used at ZETA at the ILL, at TRISP, MIRA and RESEDA at the MLZ (FRM-II). Only two instruments with MIEZE-option are existing at a reactor source (RESEDA and MIRA). One further NSE and one NRSE instrument is being build at JPARC BL06 (Vin rose). These instruments will operate in a wavelength band between 3 Å< λ< 20 Å. The NRSE instrument will operate in a q-range of 0.2Å−1 < q < 20Å−1 and provide up to 100 ns time resolution. The MIEZE instrument will have full detector coverage over 90 degrees in 2θ and can measure in a q-range of 0.02Å−1 < q < 3.5Å−1 [71].

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-1 -1 (a) Q (Å ) Q (Å ) (b) 10 1 0.1 0.01 0.001 0.0001 10 1 0.1 0.01 0.001 0.0001 4 -4 4 -4 10 10 10 10 Inel. Inel. 2 X-ray VUV- Raman -2 2 X-ray VUV- Raman -2 10 Chopper FEL scatt. 10 10 FEL scatt. 10 UT3 UT3 0 0 0 0 10 TAS Brillouin 10 Time 10 Brillouin 10 Time

(meV) (meV) -2 scatt. 2 -2 LNRSE scatt. 2

10 Backscattering 10 (ps) 10 10 (ps)

-4 SR 4 -4 SR 4

µ Photon µ Photon

Energy 10 NSE 10 Energy 10 10 correl. correl. -6 Diel. spec. 6 -6 Diel. spec. 6 10 10 10 10 X-ray correl. spec. X-ray correl. spec.

NMR NMR -8 8 -8 8 10 10 10 10 -1 0 1 2 3 4 -1 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 10 10

Length (Å) Length (Å)

Figure 12: Dynamic range accessible with diﬀerent microscopic probes in studies of soft and hard condensed matter systems. (a) Depiction of the regimes of conventional neutron scattering techniques. (b) Depiction of the regime of longitudinal neutron resonance spin-echo (LNRSE). (see also [53, 54])

5 Technical Maturity

5.1 LNRSE Spectrometer The baseline version of RESPECT will use existing technology based on 40 years experience in NSE instrumentation (see also [53, 54]). The main critical components of the instrument are:

Magnetic coil system We have based our estimates of the magnetic coils on the experience gathered in the context of the new design of IN15. In turn, the technological risks involved are low. In our concept neither the necessary field integral of 1 Tm will have to be exceeded nor the requirements of the magnetic field homogeneity. In fact, as an inherent advantage of the LNRSE technique the high geometric symmetry and the much shorter size of the B0 field results in much better self-correction of field inhomogeneities (see Appendix A3). This implies strong requirements for field correction as compared with conventional NSE. Moreover, standard NSE correction coils may be used, but at much reduced current densities. In combination this ensures that the design parameters are well- controlled and any additional complexities of superconducting solenoids, required at conventional NSE spectrometers to reach the high required current and power densities, may be avoided. In addition we note, that we do not require a large µ-metal housing of the instrument.

RF coils The RF coils, which will be required for RESPECT, need to operate at frequencies up to 6 MHz. To demonstrate that these frequencies may be reached we have successfully performed tests of resonant spin ﬂips up to 1.7 MHz at RESEDA as described in the appendix. In addition we have shown over the past two weeks, that the coils can be operated at 4 MHz, i.e. there are no doubts that the high frequencies can be reached since we are still far below the currents and frequencies used modern communication devices. Moreover, the larger power consumption is also in the range of present-day power ampliﬁers available commercially. The adaptation of the amplitude control of the RF current and feedback loop required for operation at a pulsed neutron source are currently being developed in the framework of NMI3 project FP7b (see Appendix A).

5.2 Future Options We have performed extensive instrument test with the LRNSE version of RESEDA at the FRM II providing sound evidence that also the MIEZE options of RESPECT will work to the design. We

29(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 showed that MIEZE works for spin echo times up to 100 ns and under large magnetic ﬁelds up to 17 T. In FexCr1−x, in MnSi and Fe we performed ﬁrst measurements obtaining new results.

5.3 Synergies within ESS Instrument Suite 5.3.1 Instrument Speciﬁc Aspects The required components for RESPECT are largely based on state of the art technology that is already in use at various beam lines world-wide. A list of the essential components reads as follows:

• neutron transport system: bender, guides, and focusing devices are used at existing spallation sources

• beam polarization: polarizing cavities of the required type are extensively used at NIST and at FRM II

• static ﬁeld coils: in operation at the RESEDA spectrometer at FRM II

• RF-spin ﬂipper coils: NRSE instruments at FRM II (TRISP, MIRA, RESEDA), ILL and LLB.

• Fresnel coils: in operation at all NSE-spectrometers, RESPECT requires power densities that are typically an order of magnitude lower

• electronic equipment: RF-generators and ampliﬁers are commercially available

• magnetic shielding: no µ-metal shielding required

• neutron detection: it is planned to use classical 3He detectors . For the MIEZE options CAS- CADE detectors are planned. They are in operation at FRM II (MIRA, RESEDA) and other facilities since many years.

• software: Programs to analyse NRSE and MIEZE data are available.

• regular choppers: The demands on the choppers are comparable or even relaxed compared to those used for typical chopper spectrometers to be built at ESS. The design will be based on the technology available from the technical groups at ESS.

• shielding: The requirements on the shielding will be relaxed as a neutron beam with a small cross section of 40 mm × 40 mm is extracted and the line of sight is eﬀectively interrupted within the biological shielding by a bender.

Of course, during the design phase of the instrument new technical developments will be taken into account whenever possible. This concerns in particular the development of large area detectors with excellent time and spatial resolution and innovative methods for beam extraction using Montel optics [72].

5.3.2 General Infrastructure The infrastructure and supporting facilities required to address the key scientific drivers correspond to the standard sample environment characteristic of the different research areas listed above. For the soft-matter community this concerns on-site laboratories for the preparation of volatile and sensitive samples and the necessary infrastructure for handling such samples at the instrument. For studies of glassy and crystallographic transitions furnaces are required, that provide the necessary high temperatures at low signal background. Unlike in NSE spectroscopy, magnetic stray fields of current carrying leads in these furnaces are of minor concern for the proposed beamline.

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Studies of both electronic and nuclear quantum fluctuations in magnetic materials and superconductors require standard cryogenic sample environment (dilution refrigerator temperatures) and large magnetic fields. The magnetic fields may be applied perpendicular to the neutron beam with a split-pair magnet or along the neutron beam, in small angle scattering configurations. In a recent project funded by the German federal ministry of research and education (BMBF, project number 05K10WOC) one of us (CP) compiled design criteria for actively shielded superconducting magnet systems for neutron scattering. As a main result this study showed, that a liquid He-cooled superconducting magnet system with zero boil-off may be purchased from commercial suppliers at reasonable costs, offering 17 T maximum field in symmetric mode (15 T in asymmetric mode). This magnet system would be perfectly suitable for the instrument we propose. The same BMBF study also established, that actively shielded superconducting magnet systems for magnetic fields up to 25 T may become available on a time-scale between five and ten years, but at considerably higher costs. It is thereby important to note, that stray field reduction for the operation of RESPECT is not a very serious precondition, but advantageous. In fact, in a proof-of-principle study at RESEDA we recently demonstrated that LNRSE-MIEZE can be combined with an unshielded 17 T magnet [52].

6 Costing

6.1 LNRSE Spectrometer The costing is calculated on the LNRSE version of RESPECT as the baseline. This instrument will deliver the same time resolution as ESSENCE with a signiﬁcant larger ﬂux. The total costs for this will be less than 9 million Euro (see Table 3).

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Table 3: Cost estimate for the instrument

Main component sub components costs Estimate based on

Collimation 2 Choppers 350.000 e 2 Bender 353.000 e P. Böni Guides and guide ﬁelds 213.000 e P. Böni Focusing guides 143.000 e P. Böni

First arm LRNSE coils 350.000 e RESEDA components Adjustment 200.000 e MIRA and RESEDA components Power supply B0 106.000 e Heinzinger, Rosenheim, Germany RF-Ampliﬁer 34.000 e MIRA and RESEDA components additional coils (like Fresnel) 250.000 e MIRA and RESEDA components

Sample table Mechanics 280.000 e Huber, Prien, Germany

Second arm LNRSE Coils 350.000 e RESEDA components Power supply B0 106.000 e Heinzinger, Rosenheim, Germany RF-Ampliﬁer 34.000 e MIRA and RESEDA components Adjustment 200.000 e MIRA and RESEDA components

Analyser 332.000 e P. Böni Detectors 490.000 e

Shielding 2.000.000 e Support for the guides 380.000 e MIRA and RESEDA components Vacuum container for guides 300.000 e MIRA and RESEDA components

Divers Computers 100.000 e MIRA and RESEDA components Vakuum pumps 100.000 e MIRA and RESEDA components Hutch 200.000 e MIRA and RESEDA components

Man power 16 man years 2.000.000 e

8.871.000 e

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6.2 Future Options For additional 1.25 Million Euro a MIEZE-1 option can be added (See in detail Tab. 4) giving the possibility to measure depolarizing samples and in strong magnetic ﬁelds. Adding a wide angle option based on a MIEZE-2 design will cost additional 5 million Euro opening the possibility to measure spin echo with a wide range of q and τ in parallel. The costs are summarized in Tab. 5.

Table 4: Cost estimate for the additional MIEZE-1 option

Main component sub components costs Estimate based on

MIEZE-1 Polarisier 250.000 e P. Böni Detectors 1.000.000 e CDT, Heidelberg, Germany 1.250.000 e

Table 5: Cost estimate for the additional wide angle MIEZE-2 option

Main component sub components costs Estimate based on

wide angle MIEZE-2 Polarizers 1.400.000 e P. Böni Guide ﬁelds 100.000 e Detectors 3.500.000 e CDT, Heidelberg, Germany 5.000.000 e

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LIST OF ABBREVIATIONS Abbreviation Explanation of abbreviation TAS Triple Axis Spectrometer NSE Neutron Spin Echo NRSE Neutron Resonant Spin Echo LNRSE Longitudinal Neutron Resonant Spin Echo MIEZE-1 Modulation of Intensity Emerging from Zero Eﬀort: All spin manipulations are performed before the sample. MIEZE-2 Modulation of Intensity Emerging from Zero Eﬀort: There is an additional RF-coil behind the sample position allowing a "WASP"-type operation of the spectrometer.

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[43] T. Giamarchi, C. Rüegg, and O. Tchernyshyov, “Bose-Einstein condensation in magnetic insulators,” Nature Physics, vol. 4, pp. 198–204, MAR 2008.

[44] ESS technical design report.

37(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015

[45] F. Mezei, “Neutron spin echo: A new concept in polarized thermal neutron techniques,” Zeitschrift für Physik A, vol. 255, no. 2, pp. 146–160, 1972.

[46] R. Golub and R. Gähler, “A neutron resonance spin echo spectrometer for quasi-elastic and inelastic scattering,” Physics Letters A, vol. 123, no. 1, pp. 43 – 48, 1987.

[47] R. Gähler and R. Golub, “Neutron resonance spin echo, bootstrap method for increasing the eﬀective magnetic ﬁeld,” J. Phys. France, vol. 49, no. 7, pp. 1195–1202, 1988.

[48] W. Besenböck, R. Gähler, P. Hank, R. Kahn, M. Köppe, C. H. D. Novion, W. Petry, and J. Wut- tke, “First scattering experiment on mieze: A fourier transform time-of-ﬂight spectrometer using resonance coils,” Journal for Neutron Research, vol. 7, no. 1, pp. 65–74, 1998.

[49] R. Georgii, G. Brandl, N. Arend, W. Häußler, A. Tischendorf, C. Pﬂeiderer, P. Böni, and J. Lal, “Turn-key module for neutron scattering with sub-micro-ev resolution,” Applied Physcis Letters, vol. 98, p. 073505, 2011.

[50] W. Häußler, U. Schmidt, G. Ehlers, and F. Mezei, “Neutron resonance spin echo using spin echo correction coils,” Chemical Physics, vol. 292, no. 2, pp. 501 – 510, 2003. Quasielastic Neutron Scattering of Structural Dynamics in Condensed Matter.

[51] ILL, “Instrument layout of WASP: https://www.ill.eu/instruments-support/instruments- groups/instruments/wasp/description/instrument-layout/.”

[52] J. Kindervater, N. Martin, W. Häußler, M. Krautloher, C. Fuchs, S. Mühlbauer, J. A. Lim, E. Blackburn, P. Böni, and C. Pﬂeiderer, “Neutron spin echo spectroscopy under 17T magnetic ﬁeld at RESEDA,” ArXiv e-prints, June 2014.

[53] J. Kindervater, W. Häußler, S. Säubert, F. Haslbeck, C. Pﬂeiderer, P. Böni, Versatile High- Resolution Spectrometer with Extrem Dynamic Range, preprint (2015). This paper reports proof- of principle LNRSE-MIEZE measurements at RESEDA covering nearly eight orders of magnitude in dynamic range with MIEZE times reaching 100 ns.

[54] J. Kindervater, M. Krautloher, W. Häußler, The new longitudinal NRSE setup at RESEDA, preprint (2015). This paper presents a pedagogical introduction to LNRSE and reports experimental tests demonstrating the virtues of the LNRSE technique.

[55] W. Häußler and U. Schmidt, “Eﬀective ﬁeld integral subtraction by the combination of spin echo and resonance spin echo,” Phys. Chem. Chem. Phys., vol. 7, pp. 1245–1249, 2005.

[56] T. Keller, P. Aynajian, K. Habicht, L. Boeri, S. Bose, and B. Keimer, “Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy,” Physical review letters, vol. 96, no. 22, p. 225501, 2006.

[57] C. Pﬂeiderer, P. Böni, T. Keller, U. K. Rößler, and A. Rosch, “Non fermi liquid metal without quantum criticality,” Science, vol. 316, pp. 1871–1874, June 2007.

[58] P. Niklowitz, C. Pﬂeiderer, T. Keller, M. Vojta, Y.-K. Huang, and J. Mydosh, “Parasitic small- moment antiferromagnetism and nonlinear coupling of hidden order and antiferromagnetism in uru 2 si 2 observed by larmor diﬀraction,” Physical review letters, vol. 104, no. 10, p. 106406, 2010.

[59] ESSENSE proposal from Jan. 8, 2015.

[60] P. Böni, W. Münzer, and A. Ostermann, “Instrumentation with polarized neutrons,” Physica B: Condensed Matter, vol. 404, pp. 2620–2623, 2009.

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[61] A. Komarek, P. Böni, and M. Braden, “Parabolic versus elliptic focusing - optimization of the focusing design of a cold triple-axis neutron spectrometer by monte-carlo simulations,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 647, no. 1, pp. 63 – 72, 2011.

[62] T. Adams, G. Brandl, A. Chacon, J. Wagner, M. Rahn, S. Mühlbauer, R. Georgii, C. Pﬂeiderer, and P. Böni, “Versatile module for experiments with focussing neutron guides,” Applied Physics Letters, vol. 105, no. 12, p. 123505, 2014.

[63] T. Hils, P. Böni, and J. Stahn, “Focusing parabolic guide for very small samples,” Physica B, vol. 350, pp. 166 –168, 2004.

[64] N. Kardjilov, P. Böni, A. Hilger, M. Strobl, and W. Treimer, “Characterization of a focusing parabolic guide using neutron radiography method,” NIM A, vol. 542, pp. 248 – 252, 2005.

[65] C. Pappas, G. Kali, P. Böni, R. Kischnik, L. Mertens, P. Granz, and F. Mezei, “Performance of the multidetector nse spectrometer span at bensc,” Physica B: Condensed Matter, vol. 267, pp. 285–288, 1999.

[66] ESSENSE proposal from Jan. 8, 2015.

[67] W. Häußler, P. Böni, M. Klein, C. J. Schmidt, U. Schmidt, F. Groitl, and J. Kindervater, “Detec- tion of high frequency intensity oscillations at RESEDA using the CASCADE detector,” Review of Scientiﬁc Instruments, vol. 82, no. 4, p. 045101, 2011.

[68] G. Brandl, J. Lal, J. Carpenter, L. Crow, L. Robertson, R. Georgii, P. Böni, and M. Bleuel, “Tests of modulated intensity small angle scattering in time of ﬂight mode,” Nuclear Instruments and Methods in Physics Research Section A, vol. 667, pp. 1–4, 2012.

[69] T. Weber, G. Brandl, R. Georgii, and P. Böni, “An Open-Source Software Package for Data Treat- ment in a MIEZE Experiment,” in INTERNATIONAL WORKSHOP ON NEUTRON OPTICS AND DETECTORS (NOP&D 2013) (Ioﬀe, A, ed.), vol. 528 of Journal of Physics Conference Series, 2014.

[70] G. Brandl, R. Georgii, W. Häußler, S. Mühlbauer, and P. Böni, “Large scales–long times: Adding high energy resolution to SANS,” Nuclear Instruments and Methods in Physics Research A, vol. 654, pp. 394 – 398, 2011.

[71] “H. Eno et al., MLF annual report , 2012.”

[72] S. Weichselbaumer, G. Brandl, R. Georgii, J. Stahn, T. Panzner, et al., “Montel optics: Tailoring phase-space in neutron beam extraction,” ArXiv e-prints, 2014.

[73] P. Böni, “High Intensity Neutron Beams for Small Samples,” vol. 502 of Journal of Physics Conference Series, p. 012047, 2014.

[74] C. Zeyen and P. Rem, “Optimal larmor precession magnetic ﬁeld shapes: application to neutron spin echo three-axis spectrometry,” Measurement Science and Technology, vol. 7, no. 5, p. 782, 1996.

[75] F. Mezei, The principles of neutron spin echo. Springer, 1980.

[76] M. Krautloher, Implementation of a LNRSE option into RESEDA / MLZ. Master Thesis, Ludwig-Maximilians-Universität München (LMU), 2014.

39(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015

[77] J. Kindervater, N. Martin, W. Häußler, M. Krautloher, C. Fuchs, S. Mühlbauer, J. A. Lim, E. Blackburn, P. Böni, and C. Pﬂeiderer, “Neutron spin echo spectroscopy under 17 T magnetic ﬁeld at RESEDA,” EPJ Web of Conferences, vol. 83, no. 03008, 2015.

[78] S. Säubert, J. Kindervater, J. Wagner, W. Häußler, O. Holderer, A. Bauer, S. M. Shapiro, C. Pﬂei- derer, and P. Böni. This paper presents a combination of NRSE, MIEZE and susceptibility data measuered on diﬀerent FeCr samples.

[79] J. Kindervater, A. Bauer, W. Häußler, N. Martin, S. Mühlbauer, F. Haslbeck, M. Garst, P. Böni, and C. Pﬂeiderer. This paper reports SANS and spin echo measurements of the paramagnetic to helical transition in MnSi under applied magnetif ﬁeld.

[80] J. Kindervater, W. Häußler, S. Sauebert, and P. Böni. This paper reports MIEZE measurements at the Curie point of iron.

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A Simulations of Performance for RESPECT

A.1 Mean brilliance of the cold pancake moderator of ESS The brilliance of the moderators of ESS is available from the home page of McStas (http://mcstas.org/download/share/ESS_moderator_July_2014.tgz). Fig. 13 reproduces the flux of the cold pancake moderator that has the dimensions 30 mm (vertical) × 320 mm (horizontal). It is a simple task to estimate the neutron flux at the sample position of RESPECT using the expression [73]: F = ηtot · ηpol · ∆λ · Ω · Ψ. (1) Based on the following parameters at λ = 6 Å a flux Ψ = 6.98 · 108 cm−2s−1 is obtained:

Figure 13: Brilliance Ψ versus λ of the cold pancake moderator of ESS. Ψ is largest around λ = 3 Å, i.e. an important wavelength for the investigation of single-crystalline samples.

• Brilliance at 6 Å: Ψ = 4.683 · 1012 cm−2s−1Å−1sterad−1 (provided by ESS)

• beam size 40 mm × 40 mm

• divergence at sample: 0.92◦ × 1.77◦ = 4.96 · 10−4 rad (see Fig. 16 below).

• wavelength band: 4λ = 1 Å

• eﬃciency of transport: ηtot = 60%

• eﬃciency of polarization: ηpol = 50% (50% of the neutrons are lost) The transport eﬃciency is obtained under the following assumptions:

• transmission of polarizing cavity: 80% (for the correctly polarized neutrons)

• eﬃciency of extraction of beam: 85%

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• transmission of benders: 94% each

• The absorption of neutrons by Al, air, and He are neglected. For example, the transmission of 4 cm Al is 83.1%.

The calculated brilliance for λ = 6 Å is shown as a star in Fig. 7 and reproduces the value of the simulation very well. In conclusion, the guide system for RESPECT provides a large brilliance transfer and is well adapted to the science to be performed at the instrument. Moreover, the loss mechanisms of the guide system are clearly identiﬁed.

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A.2 Beam properties

The small height of the pan cake moderator, Hpk = 30 mm is well adapted to neutron guides with a small vertical dimensions. Guides with a large entrance height are seriously under-illuminated leading to an under-illumination of the neutron guide and therefore to a dilution of phase space. Note that the phase space inside a neutron guide is given by the product of the cross section of the neutron guide multiplied with the solid angle (Ωguide of the transported neutrons. Ωguide is typically proportional to the square of the index of the supermirror, m, and the wavelength λ. A typicall reﬂectivity of a supermirror is given in Fig. 14. We have performed Monte-Carlo simulations using McStas, in

Figure 14: The reflectivity of supermirrors decreases approximately linear with increasing index m which is proportional to the momentum transfer as given by the labels at the top of the figure. order to optimize the guide system and to obtain information on the divergence of the neutrons at the sample position. Fig. 15 shows a comparison of the divergence for quadratic neutron guides with cross sections 80 mm × 80 mm as proposed for ESSENSE and 40 mm × 40 mm as proposed for RESPECT as obtained for a sample size 30 mm × 30 mm. The vertical divergence for the 80 mm guide is very inhomogenous in the vertical direction due to the under-illumination of the guide. Particularly disturbing is the dip around 00 divergence which leads to a lowering of the intensity for highly collimated beams. A quantitative analysis of the divergence is shown in Fig. 16. Gaussian parametrizations to the guide configurations are given in the tables. If the 40 mm guide was terminated at the same position as the 80 mm guide, the vertical and horizontal divergencies are reduced by about a factor of two. As shown in Fig. 7 the flux is, however, only reduced by approximately a factor of two because the phase space density is not significantly reduced by the 40 mm guide. Of course, a small divergence is beneficial for achieving large spin echo times and a low background at the sample position. Because of the LNRSE concept, however, it is possible to extend the guides through-out the spin precesson regions thus increasing the vertical divergence by a factor of two, i.e. to a similar value as for the 80 mm guide while the vertical divergence is increased to 1.80, i.e. approximately 80% larger than for 80 mm. At the same time, the flux is increased by about a factor of four (Fig. 7) at small wavelengths and an order of magnitude near the flux maximum around 3 Å. The concept of RESPECT forsees translating various collimations into the beam before the Larmor precession region thus allowing to vary the horizontal and vertical divergence independently between 0 10 min and 60 min. Of course, collimations αvert and αhoriz < 1 will essentially eliminate the effect

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Figure 15: The vertical divergence is very inhomogenous and homogenous for the 80 mm and 40 mm guide system, respectively, while the horizontal divergencies are homogeneous. The 40 mm guide provides a very compact phase space at the sample which is beneficial for neutron spin echo. By means of the additional 3 m long guide, the horizontal divergence can be increased by more than a factor of three leading to significant flux gains.

Figure 16: The vertical divergence of the beam (left) at the sample position of RESPECT (Hvert = 40 mm) is very homogeneous in contrast to the 80 mm large guide proposed for ESSENSE. By inserting a guide throughout the precession region the divergence and therefore the flux can be increased. The horizontal divergence is almost independent from the width of the neutron guide because the moderator has a width 80 mm. The extension of the neutron guide through-out the precession region, the divergence and therefore the flux can be increased significantly. A supermirror coating with m = 6 (instead of the proposed m = 2) improves the transport of neutrons with short wavelengths (not shown). The divergence is not significantly affected.

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Figure 17: The beam size at the sample positions exceeds the dimensions of the sample if the neutron guide is large (80 mm) and not extended through-out the precession region. The RESPECT guide ”focuses” the neutrons nicely on a sample size of approximately 30 mm × 30 mm. For the simulations, a flat spectrum was used. The number provide the integrated flux over a beam area of 30 mm × 30 mm. of the guide between the RF-coils. If a parabolic focusing tube is used between the last RF-coil and the sample, the insertion of the collimators allows tuning the size of the neutron beam at the sample to below 1 mm × 1 mm. Finally, we discuss the results of varying the dimensions of the neutron guide in the limits 30 mm≤ a ≤ 80 mm. Fig. 17 shows the evolvment of the beam size with a and the effect of adding a neutron guide in the precession region of the first LNRSE-arm. The results show that the neutrons are nicely contained within a beam area of 30 mm × 30 mm if a beam size of 40 mm is used and the guide is extende through the precession region. Increasing the supermirror coating from m = 2 to 6 does not change the beam profile. The intensity is doubled when going from 40 mm to 80 mm (not a factor of four, as might be expected). Extending the guides by three meters leads to significant increases in flux. Note that the flux for the extended 40 mm guide is only about 20% smaller than for the 80 mm guide. The 30 mm guide is not large enough to fully illuminate the sample in the vertical direction. The almost 50% difference in flux favor a solution with the 40 mm guide. This conclusion is further corraborated by the results shown in Figs. 17 and 18. Fig. 18 shows the essentially expected result that the intensity at the sample position of RESPECT increases linearly (and not quadratically) with the dimensions of the guide. The black and the red curves represent the results for the 80 mm and 40 mm guide, respectively. For these simulations we have removed the polarizers and any beam deflecting devices. If the guides are extended by 3 m towards the sample, the intensity increases dramatically. The advantage of using a 40 mm guide is the lowering of the critical wavelength to smaller values (Fig. 18). The benefit of choosing coatings m = 6 will further lower the critical wavelength and may be considered as a valuable option for RESPECT.

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Figure 18: The intensity at the sample position increases proportional to the width of the guide as the guide is fully illuminated in the horizontal plane (left). Extending the neutron guide throughout the precession region leads to large gains in intensity (right).

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A.3 Homogeneity of field integrals In order to achieve large spin echo times the spin precession has to be conducuted very homogeneously. Here, LNRSE as realized within the concept of RESPECT offers tremendous advantages when compared with conventional NSE because field corrections for non-divergent beams are essentially zero. Fig. 19 shows a solenoid as used for NSE (left hand side) and a pair of LNRSE coils for LNRSE on the right hand side. The colors indicate the distribution of the field amplitudes as calculated with COMSOL. For a solenoid as used for NSE, the field integral for neutrons with a divergence ψ = 0 is given by

r2 J(r) ' B0Lsol 1 + , (2) 2RLsol where Lsol and R are the length and radius of the solenoid, respectively, and r the distance of the neutron trajectory from the axis of the solenoid. As the neutrons encounter the same field gradients at the entrance and the exit of the solenoid, the inhomogeneities have to be corrected for, for example by means of Fresnel or Pythagoras coils. In NRSE, the spin precession is performed in a low field region between two RF-spin flippers. Because the neutron spin is flipped by π between the entrance and exit at each RF-spin flipper, the effect of the field gradients on the neutron polarization cancels. For properly manufactured Helmholtz- coils no corrections required. The situation is more involved for divergent beams. Here, the concept

Figure 19: On the left is shown a typical solenoid as used for NSE. In LNRSE, the precession region is confined between two RF-spin flipper coils. The B0 fields are produced by means of Helmholtz coils. Compensation coils reduce the stray fields at the exit and entrance of the coils. The red and blue arrows indicate the spin directions at the entrance and exit of the B0 fields. In LNRSE, the fringe fields are compensated because the neutrons undergo a spin flip in the RF-coil that is positioned at the centre of the B0-coils (not shown). of solenoids with an Optimal Field Shape (OFS) as introduced by Zeyen [74] can be applied to correct for the influence of a finite divergence in NSE. We estimate the influence of the divergence of the neutron beam using the following model for the precession regions of RESPECT. The path length L, relevant for precession is given by

L(0) α2 L(α) = ' 1 + (3) cos α 2 in the small angle approximation, where L(0) is the path length for neutrons with α = 0. α designates the angle between the symmetry axis of the precession arm and the trajectory of the neutron. The neutrons attain a precession angle φ given by γ m φ = n n BLλ, (4) h

47(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015

−27 −34 where γn = 183.25MHz/T, mn = 1.675 · 10 kg, and h = 6.626 · 10 Js. By combining Eqs. (3) and (4) one obtains for the phase difference of beams with an inclination ±α when compared with beams along the axis (α = 0) α2 ∆φ = 4.632 · 104B[T] L[m] λ[A] [rad2]. (5) 2 For example, for J = BL = 1.02 Tm (the maximum field integral anticipated for RESPECT), λ = 3 Å (flux maximum of Pancake moderator at ESS) and α = ±0.2◦ one obtains ∆φ = ±48◦ corresponding to a polarization P (48◦) = cos(48◦) = 0.67 (Fig. 20). For a uniform distribution of precession angles φ between ±∆φ the average polarization is given by

φ 1 Z sin ∆φ P = cos(φ)dφ = . (6) tot 2∆φ ∆φ −φ Therefore, neutrons with a divergence ψ = 2α = 0.4◦ assume an average polarization of 89%. These results are in agreement with the results quoted by Mezei [75] p. 19 Based on the above equations we have calculated the maximum divergence versus λ that the ◦ neutrons are allowed to have to guarantee a polarization Ptot = 90% (∆φ = ±45 ) and 64% (∆φ = ◦ ±90 ), respectively, if the polarization of the incident neutrons is Pi = 1.

Figure 20: left: An increasing inclination α increasing the ﬂight path leading to an additional phase of the spin of the neutrons thus to a decreasing polarization. right: Divergence versus wavelength for Ptot = 90% and 64% are shown as a blue and a green line, respectively. The ﬁeld integral is 1.02 Tm yielding spin-echo times of 1 µs.

From Fig. 20 we conclude that it is possible to conduct experiments using a neutron wavelength ◦ λ = 20 Å, a field integral J = 1.02 Tm and a divergence of 0.2 , yielding Ptot = 64% without using correction coils! Therefore, spin echo times of 1 µs can be reached. Of course, it is possible to correct for the effects of divergent beams using Fresnel coils or Pythagoras coils, similarly as for the non-divergent beams in NSE. According to Krautloher [76], the field integral of a pair of RF-spin flipper coils with a separation of L(0) is approximately given by r2 L J(r) = B L(0) 1 + · coil (7) 0 2RL(0) L(0) where Lcoil is the length of the B0 coil. When comparing this expression with the expression for NSE it is immediately seen that the power densities are much smaller because Lcoil L(0) As shown by Fig. 20 the required corrections are indeed very small.

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B Proof of principle experiments

In preparation of this proposal we performed a series of test experiments to demonstrate the working principle and the advantages of the longitudinal NRSE technique. For these measurements a longitudinal NRSE setup specifically designed for the beamline RESEDA at MLZ has been put into operation. The equipment of the primary spectrometer arm to operate RESEDA in a MIEZE mode was put together in early 2014. Since March 2015 the secondary arm was set up with the longitudinal NRSE coils. Fig. 21 shows a photograph of the primary and secondary spectrometer arm. The significantly increased homogeneity of the longitudinal NRSE coils allowed to operate the spectrometer at much higher frequencies achieving right away much higher field integrals. Fig. 22 shows the test setup which was used to determine the parameters for the rf circuit when using the standard rf-coils of RESEDA at higher frequencies and to electronically test new rf-coil designs. The existing standard rf-coils have so been used up to 1.75 MHz in the NRSE setup. New coils have been tested electronically up to 4 MHz, allowing to apply the required magnetic field amplitudes. Fig. 23 shows the performance of the newly assembled LNRSE setup at RESEDA for frequencies in the range from 35 kHz to 1.75 MHz. The corresponding very large effective field integrals up to 240 mTm could be achieved without the use of correction coils. The resolution of the LNRSE setup is shown in Fig. 24 for different scattering angles and neutron wavelengths. The decreased polarization for small and large spin echo times is well understood and will be compensated in tests planned for the near future. The resolution of the longitudinal MIEZE-1 option which was already realized before the LNRSE option at RESEDA is shown in Fig. 21. First tests with very strong magnetic fields up to 17 T demonstrate the potential of MIEZE-1 to perform experiments with spin echo resolution under extreme conditions. In Fig. 26 the resolution of the setup under magnetic field is shown. Furthermore the feasibility to modulate the amplitude of the rf-field according to the wavelength band of a pulse from the spallation source could be shown in a proof of principle experiment at the Reactor Institute Delft. The results are shown in Fig. 27.

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Figure 21: The longitudinal NRSE setup realized at RESEDA. The ﬁgure shows the primary and secondary spectrometer arm. Neutrons path the instrument from left to right. For more details see Ref. [53] and [54].

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Figure 22: Setup to electronically test the RF-coils used in the LNRSE setup. Panel (a) shows a frequency generator and a RF-ampliﬁer, panel (b) a oscilloscope and capacities connected to a RF- coil. Panel (c) shows the standard RF-coil as used at RESEDA up to 1.75 MHz. In panel (d) a new RF-coil is shown. This coil is electronically tested and allows to apply oscillating magnetic ﬁelds up to 4 MHz with the required amplitude.

51(61) 2014/2015 Instrument Construction Round Proposal Revision Date 4/16/2015 2.0 0.8 15% kHz

62 mTm

1.5 240

1750

0.7 =

f eff 60 (a.u.)

(a.u.) J (mT)

1.0 0 RF sym B B 0.6 B 58 0.5 0.5 (f) (l) (r) 0 0.8 48 25% kHz

2 ﬁeld in one spectrometer arm is 0.7 46 0 179mTm

1307

B =

f eff (a.u.)

J (a.u.) (mT)

0.6 0 RF 1 sym B 44 B B 0.5 flip. In the second row (g) to (l) scans of the 42 (e) (k) (q) π 0 field was set to the resonance obtained in the first 1.5 0 kHz 34%

0.7 B 32 877

120mTm

1.0 f =

0.6 eff (a.u.)

J (a.u.) 30 (mT)

0 RF sym B resonance scans, were the B B 0.5 0.5 0 28 B (j) (d) (p) 0.4 0 24 1.0 kHz 59%

0.7 85mTm

622

f 22 eff J (a.u.)

(a.u.) (mT)

0.6 0 0.5 RF sym B B B 20 0.5 (i) (c) (o) 18 0 0.7 kHz

0.10 62% 3 mTm

0.6 f =

eff J 2 (a.u.)

(a.u.) (mT)

0 0.5 RF sym 0.05 B B B 1 -ﬂip. At the central maximum each resonant coil is set to a π 0.4 in both RF-coils of one spectrometer arm are shown. The (b) (h) (n) 0 0 RF 3 B kHz

58% 60 mTm

0.6 5

is set to a =

2 f =

eff a.u.) J 40 -3 RF (a.u.)

(mT) 0.5

1 B 0 (10

sym B B RF B 20 0.4 0 (a) (g) (m)

8 7 6 5 0.3 7 6 5 8 6 4 2

c/s) (10 I

c/s) (10 I c/s) (10 I

3 3 3 row. Panel (m) toto (r) 240 mTm. show spin For more echo details groups see measured Ref. at [53]. different RF-frequencies. The effective field integral accumulated varies from 5 mTm amplitude of the rf field Figure 23: Characterization ofshow the resonant resonant RF-flips flips and from thechanged corresponding 35 while kHz spin echos to measured 1.7 with MHz. the LNRSE Panel setup (a) at to RESEDA. The (f) panels show typical

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(a) 1.0 2θ = 0

2θ = 20°

2θ = 40°

)/S(q,0) 0.5 τ S(q,

λ = 8.3 Å 0

(b) 1.0 λ = 8.3 Å

λ = 10.6 Å

)/S(q,0) 0.5 τ S(q,

2θ = 0 0 0.1 1 10 τ NRSE (ns)

Figure 24: Resolution of the longitudinal NRSE setup at RESEDA. In panel (a) the resolution for diﬀerent scattering angles 2Θ = 0 − 40◦ is shown. Panel (b) shows the resolution for diﬀerent neutron wavelength 8.3 Å and 10.6 Å. The decrease of the resolution towards small spin echo times is well understood and can be explained by a misalignment between the coils. As the full LNRSE setup at RESEDA is only realized since March 2015 this will be done in a next step. The same holds for the reduced resolution towards higher spin echo time. For more details see Ref. [53].

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(a) (c) 1.0 B = 17 T λ = 8.3 Å B = 0 20 I

15 (a.u.) 0.5 10 contrast ~60% contrast 5 τ = 1.1 ns λ = 10.5 Å 0 MIEZE 0 (b) (d) 1.0 ~20% contrast B = 0 λ = 21.7 Å B = 0 60 I = 21.7 Å

λ (c/4000s) 50 0.5

contrast 40

τMIEZE = 101.5 ns 0 30 -6 -4 -2 0 2 0 4 8 12 10 10 10 10 10

τMIEZE (ns) time channel

Figure 25: Resolution of the longitudinal MIEZE-1 setup realized at RESEDA for different neutron wavelength and under strong magnetic fields. The MIEZE-1 resolution for λ = 8.3 Å and 21.7 Å are shown in (a) and (b), respectively. Panel (c) shows the MIEZE contrast versus the 16 time channels recorded by the CASCADE detector at λ = 10.5 Å in a magnetic field of 17 T. In panel (d) a MIEZE-1 echo at a MIEZE time of τMIEZE = 101.5 ns is shown. The echo was measured in zero magnetic field and at a neutron wavelength of λ = 21.7 Å. For more details see [53, 54, 52].

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Figure 26: Resolution of the longitudinal MIEZE-1 setup at RESEDA under strong magnetic ﬁelds up to 17 T. (a) Primary spectrometer arm with the LNRSE setup and the unshielded 17 T magnet. (b) Resolution of the MIEZE setup with 17 T applied to the sample position measured in the direct beam. (c) MIEZE echo measured in 17 T. For more details see Ref. [77]

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(a) 6

4 (a.u.)

RF B 2

0 (b) 1.0

0.5 Flipping efficiency 108 kHz 191 kHz 265 kHz 0 0 2 4 6 8

λ (Å)

Figure 27: Flipping efficiency of one transversal NRSE coil with wavelength adapted rf-amplitude modulation. In panel (a) the modulation of the rf-amplitude ist shown versus neutron wavelength. The shape is adjusted to the neutron wavelength and reflects the expected 1/λ characteristics. Panel (b) shows the flipping efficiency of the NRSE coil for different wavelength of the pulse. In the region from 2.5 Å to 7 Å very good results for the flipping efficiency are realized. For smaller and larger wavelength the statistics becomes worse due to low neutron flux and decreased analyzer efficiency. Below 2.5 Å the rf-amplitude is not well adjusted due to the rf circuit used, which is optimized for larger wavelength. These data proof the feasibility of the rf-amplitude modulation. The measurement has been performed at the Reactor Institute Delft (RID) in the framework of the NMI3 project II JRA 19.3. The pulse length is 20 ms and the width of each time channel 40 µs.

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C First Scientiﬁc Highlights

In this section we present the first scientific highlights measured with the LNRSE setup presented in the previous section. Due to time constraints the LNRSE setup was only used within one measurement up to now, investigating the diffusion rate of standard PEP polymer. The results shown in Fig. 28 are in good agreement with those measured on a classical NSE instrument. The longitudinal MIEZE-1 option has been engaged in several studies up to now. Our first example is the study of the relaxation rate in the reentrant spin-glass FexCr1−x as function of compositional tuning, shown in Fig. 29. The ferromagnetic phase for small Fe concentrations makes classical NSE studies very difficult. Another example is a spin echo study of the critical fluctuations at the helimagnetic transition of MnSi as function of magnetic field. The first results are shown in Fig. 30. The last example using the longitudinal MIEZE-1 option is a study of the classical isotropic ferromagnet Iron. This is a paradigm for the investigation of critical fluctuations and dynamical scaling theory. Our results allows a direct comparison between MIEZE-1 and classical NSE. The results are shown in Fig. 31.

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(a) -1 1.0 q = 0.05 Å -1 q = 0.11 Å -1 q = 0.18 Å )/S(q,0)

τ 0.5 S(q,

PEP d-decane 0 2 3 4 5 6 2 3 4 5 6 2 3 4 5 0.1 1 10

τ (ns)

(b) 1.0 RESEDA JNSE

0.5 (1/ns)

0 t Diffusion: 2 t0 = D0q 2 D0 = 20.1Å /ns 0 0 0.05 0.10 0.15 0.20 0.25 -1 q (Å )

Figure 28: Diffusion rate of PEP deuterated decane measured with the LNRSE setup at RESEDA. In panel (a) the intermediate scattering function normalized to the resolution is shown for different scattering vectors q. Panel (b) shows the q dependence of the relaxation rate t0 measured at RESEDA compared to data measured at JNSE. The solid line is a fit to the data considering a simple diffusion model. For more details see Ref. [53]

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(a) Burke et al.

(refs within)

300

Fe Cr T

x 1-x

200

Schmakat

et al.

T T (K)

100 FM T

AFM

0.3 0.2 0.1 0.0

Fe-rich Cr

Fe content x

(b) 1.0

-1 q = 0.039 Å

)/S(Q,0) T = 12.0 K τ T = 15.0 K 0.5 T = 18.9 K S(Q, T = 24.2 K T = 28.1 K T = 34.0 K T = 40.0 K Fe17.5Cr82.5 T = 101.0 K 0 -5 -4 -3 -2 -1 0 1 10 10 10 10 10 10 10 τ MIEZE (ns)

40 eV) µ (

20 T = 24.2 K T = 28.1 K T = 34.0 K

0.04 0.06 0.08 0.10 -1 Q (Å )

Figure 29: Spin echo study of the reentrant spin glass FexCr1−x. Panel (a) shows the temperature versus concentration phase diagram of FexCr1−x. In panel (b) the normalized intermediate scattering function as function of temperature measured on a ﬁxed scattering vector q is shown. Panel (c) shows the q dependence of the relaxation rate for diﬀerent temperatures. These data were measured using the longitudinal MIEZE-1 setup at RESEDA. For more details see Ref. [78].

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(a) 2nd FP H||<110> 0.4 TCP conical st (T) 1 int MnSi (b) (c) H 0.2 0 µ SkX 1st (d) (e) 0 helical FDPM 28 30 32 T (K)

(b) 1.0 (c) 3 1.0 MnSi B || <110> || n

I 28.61 K 2 (10 28.77 K -3

28.81 K c/mon) eV) )/S(q,0) 0.5 µ τ 28.85 K ( 0.5 28.96 K Γ S(q, 29.15 K 1

MnSi B = 186.8 mT B = 186.8 mT 0 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 0 0 0.1 1 28 29 30 31

τ MIEZE (ns) T (K)

(d) 1.0 (e) 3 3.33 K 10 28.57 K 28.79 K

I 28.99 K 2 (10 29.09 K -3

29.20 K c/mon) eV)

0.5 µ 29.28 K ( 5 Γ contrast 29.69 K 29.89 K 1 30.05 K MnSi 30.10 K MnSi B = 0 30.25 K B = 0 0 2 3 4 5 6 7 8 9 2 3 4 0 0 0.1 1 28 30 T (K) τ MIEZE (ns)

Figure 30: Magnetic phase diagram of MnSi and MIEZE-1 study of the critical fluctuations at the transition into the paramagnetic regime measured as function of magnetic field. Panel (a) shows the temperature versus magnetic field phase diagram of MnSi. The blue lines indicate the path along which the MIEZE-1 measurements are performed. Panel (b) and (d) show the normalized intermediate scattering function for different temperatures in 168.8 mT and 0 mT, respectively. The resulting linewidth of the critical fluctuations as function of temperature are shown in panel (c) and (d). The blue data points are the scattering intensity. For more details see Ref. [79].

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(a) -1 150 Fe 0.012 Å -1 0.020 Å -1 0.029 Å 100 -1 0.038 Å c/mon) -1 -6 0.046 Å (10

I 50

0 1020 1030 1040 1050

T (°C)

(b) 1.0 q = 0 q = 0.009 q = 0.027 q = 0.011 q = 0.030 q = 0.013 q = 0.032

)/S(q,0) q = 0.016 q = 0.034

τ 0.5 q = 0.018 q = 0.037

S(q, Fe q = 0.020 q = 0.039 q = 0.023 λ = 8.0 Å q = 0.042 q = 0.025 q = 0.043 T +0.5 K C 0 -5 -4 -3 -2 -1 0 10 10 10 10 10 10 τ MIEZE (ns)

100 eV) µ (

Γ 10

λ = 8.0 Å Tc Mezei 1 6 7 8 9 2 3 4 5 6 7 8 9 2 3 0.01 0.1 -1 q (Å )

Figure 31: Critical fluctuations at the Curie point of iron. In panel (a) the temperature dependence of the critical scattering from iron for different scattering vectors q is shown. Panel (b) shows typical normalized intermediate scattering functions at TC measured with the longitudinal MIEZE-1 option at RESEDA. Panel (c) displays the q-dependence of the resulting linewidth Γ. The results are in very good agreement with those of a classical NSE study by Mezei [26]. The solid red line reflects the expected q dependence of Γ as calculated in dynamical scaling theory. For more details see Ref. [80].

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ESS Instrument Construction Proposal &lt;‌RESPECT‌&gt;

ESS Instrument Construction Proposal <‌RESPECT‌>