CfP‐ADMIN‐AWP19‐ENR‐01

Scientific proposal for Enabling Research project (max 10 pages, excluding title page)

Title Routes to High Gain for Inertial Fusion Energy

Principal Investigator Peter A Norreys, Professor of Inertial Fusion Science, University of Oxford Beneficiary CCFE (United Kingdom)

Project 24 months duration

Abstract

The award of the two EUROfusion Enabling Research grants “Towards Inertial Fusion Energy (ToIFE)” from 2014‐2018 enabled our consortium to make significant progress in achieving a fundamental understanding of the physics needed to demonstrate the viability of laser‐driven fusion as an alternative, complementary road towards sustainable, clean and secure energy source. We plan to build on these achievements by further integrating our activities by undertaking a series of collaborative experiments on existing European facilities to qualify new diagnostics, instruments and techniques, in preparation for deployment on the new PETAL/LMJ facility in Bordeaux early in the next decade. We also plan to support these investigations by state of the art theoretical and computational modelling. Our proposal has three main objectives: a) Studying fundamental materials properties and laser‐plasma interactions to acquire new insights into basic physics for ignition on MJ‐scale facilities, including the development of new X‐ray, particle and optical diagnostics on the Vulcan laser facility, in preparation for commissioning on the PETAL/LMJ facility at CEA; b) critically evaluating advanced alternative schemes for the high gain target designs that are required for inertial fusion energy, including the exciting new auxiliary heating approach that was developed under the ToIFE project; c) developing key IFE innovative materials, lasers and target fabrication technologies. The two previously awarded ToIFE grants improved the co‐ordination between the European partners, as indicated by the growing number of collaborative studies, and enabled us to identify more clearly the commonalities between Inertial and Magnetic Confinement Fusion activities, thus providing a route to reinforce links between the two communities.

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Introduction. This new “Routes to High Gain for Inertial Fusion Energy” proposal here builds upon the insights and strong collaborations established under the 2014‐2018 “Towards Inertial Fusion Energy (ToIFE)” project1, albeit it with a rotation of the project co‐ordinator from France (S. Jacquemot) to the United Kingdom (P.A. Norreys). From the commencement of the ToIFE project in 2014, thanks to EUROfusion financial support, substantial quantities of high quality experimental data related to inertial confinement fusion experiments at the National Ignition Facility and the Laboratory for Laser Energetics have been made available to the consortium, either through the published literature or through active collaborative contacts with American colleagues. From this data, along with the pioneering work undertaken within the ToIFE grants, we have significantly advanced our scientific understanding of the physics of thermonuclear ignition and have identified a number of critical issues that must be addressed in order to achieve a burning hot‐spot, including the implosion energetics, the pusher adiabat, tamping effects, and extending the inertial confinement time. The new approaches, described here in this proposal, have a new focus on the Vulcan laser facility and the PETAL/LMJ facility in Bordeaux that will enable European leadership for some of these topical questions into the next decade.

Objectives and expected outcomes. We propose here to build upon our recent successes to reinforce three main axes: a) Studying fundamental materials properties and laser‐plasma interactions to acquire new insights into basic physics for ignition on MJ‐scale facilities, including the development of new X‐ray, particle and optical diagnostics on the Vulcan laser facility, in preparation for commissioning on the PETAL/LMJ facility at CEA; b) critically evaluating advanced alternative schemes for the high gain target designs that are required for inertial fusion energy, including the exciting new auxiliary heating approach that was developed under the ToIFE project; c) developing key IFE innovative materials, lasers and target fabrication technologies The two previously awarded ToIFE grants improved the co‐ordination between the European partners, as indicated by the growing number of collaborative studies1, and enabled us to identify more clearly the commonalities between Inertial and Magnetic Confinement Fusion activities, thus providing a route to reinforce links between the two communities.

Description and methodology. Axis A. Studying fundamental material properties and laser‐plasma interaction mechanisms The implosion energetics still need to be optimised, of course, in order to achieve a burning hot spot and, beyond that, ignition. In particular, it is necessary to increase the implosion kinetic energy that is available so that there is more thermal energy in the hot spot at stagnation, which is a key limitation at this point in time. This can be done by either enhancing the drive coupling energy or by optimization of the capsule design, e.g. by deploying different ablator materials, studying the variations in mass, along with their dependence on the target thickness. Similarly, the reduction in the pusher adiabat – i.e. the degree of departure of the pressure of the compressed fuel from the ideal Fermi degenerate pressure ‐ can be achieved by controlling both the fast electron and the hard X‐ray preheat, primarily arising from laser‐ plasma instabilities, as well as Rayleigh‐Taylor induced mixing, the latter by reducing the coasting time of the implosion. The increase of the effective disassembly time can be achieved by improvements to the implosion symmetry. Certainly, the recent successes on the National Ignition Facility, such as the

1 The “Towards Inertial Fusion Energy (ToIFE)” project, which started in 2014 with the award of the two EUROfusion Enabling Research grants, was aimed at achieving a fundamental understanding of the physics needed to demonstrate the viability of laser‐driven fusion as an alternative road towards sustainable, clean and secure energy sources. The projects have allowed significant progress to be made in the field of laser‐ plasma interaction physics and inertial fusion energy science, as shown on the pages of the ToIFE website (http://web.luli.polytechnique.fr/IFE‐KiT/ToIFE.htm), which details the major results from the start of the project in 2014 until the latest reporting stage, at the end of 2017.

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demonstration of a fusion output energy that is more than twice the total kinetic energy of the imploding fuel with unprecedented yields and stagnation pressures, show that increased capsule performance can indeed be achieved through the mitigation of mix in both the hotspot and the compressed DT fuel [S. Le Pape et al., Phys. Rev. Lett. 120, 245003 (2018)]. A.1 Atomic physics In most capsule designs deployed until now on the National Ignition Facility, mid‐Z elements have been used as dopants (Ge or Si / Cu) to the ablator material (plastic / high density carbon / beryllium) to increase the preheat shielding and to image the shell in‐flight. In the ToIFE grants, we concentrated on the atomic physics associated with this problem, to great success. A series of indirectly‐ driven near‐LTE opacity experiments were performed on the LULI2000 laser facility. Thanks to an inventive twin‐hohlraum target concept and a dual‐channel spectrometer developed at LIDyL, XUV and x‐ray photo‐ absorption spectra of mid‐Z elements (copper and nickel) have been, for the first time, simultaneously recorded [M. Dozières et al., HEDP 17, 231 (2015); F. Thais IFSA2017]. A final campaign on the ELFIE laser facility of the three‐year X‐ray and XUV emission spectroscopy program is being conducted in 2018 on germanium and its analysis, together with the one of the 2nd experiment, postponed to the end of 2017 due to a laser breakdown, will be carried out in order to complement the LULI2000 data and further benchmark the codes. The LiDyL High Energy Density Matter group in Saclay recently had some insight into the relative contributions of electric‐dipole and magnetic‐dipole transitions in highly ionized plasmas. The approach involves the Flexible Atomic Code, using either a detailed or "statistical" approach dealing the transitions within the Unresolved Transition Array formalism. The Wigner Research Centre for Physics has experience in plasma diagnostics, either in light scattering or in x‐ray spectroscopy. Tasks 2019‐2020 – (CEA/LULI, CEA/LIDyL and Wigner Institute). We intend to complete the data analysis of the emission experiments conducted on the ELFIE laser facility in 2019 [ATOM1]. We will also analyze the contribution of various transition types to the radiative processes in low‐electronic‐ density tungsten plasmas present in the divertor of tokamaks. Such plasmas being out of thermodynamic equilibrium, this objective requires to solve large systems of kinetic equations. Among other results, this will help in determining the radiative losses in the tokamak divertor, which may have a strong influence on its operation [ATOM2]. At present a new spectrometer (based on conically bent von Hamos arrangement) is being designed now at the Wigner Institute which allows simultaneous observation of K‐shell lines from different microdots. These will allow time dependent detection of radiation transport from different emitters in different depths [ATOM3]. A.2 Mix studies We now turn our attention to a new challenge – that of high resolution X‐ray imaging of the shell at stagnation to better understand mix and hot spot dynamics. The radiography source must have sufficient spatial and temporal resolution to resolve the growth of high modes of the Rayleigh‐Taylor instability (𝑚2𝜋𝑟/𝜆. One expects modes up to ~300 (λ ~20 μm) to have significant growth during the acceleration phase of the implosion, along with the surface roughness of capsules where the dominant unstable mode is ~150. Similarly during the deceleration phase, very short wavelength perturbations grow at the internal fuel‐ablator interface, corresponding to mode numbers up to ~2000 (λ ~ 2 μm). The radiography sources must also have sufficient flux to overcome the bremsstrahlung radiation emission from the implosion as well as high enough energy to penetrate the dense plasma. X‐ray betatron emission from high intensity driven laser wakefield accelerators is a very promising candidate for these studies. The first advantage of betatron emission is that it is emitted into a narrow cone angle. One can therefore use this property to increase the signal relative to the noise from the background bremsstrahlung radiation emission from the hot spot, which is isotropic by nature, by placing the detector at a suitable distance from the capsule. The second advantage is that betatron X‐rays have a demonstrated ultra‐fine spatial resolution of  1 μm that is ideal for direct phase contrast imaging of the growth of the high‐order modes. The small spatial scale results from the small excursions that the accelerated electrons undergo at the back of the blow‐out accelerating region of the plasma wakefield. The third advantage is that betatron X‐rays can be optimised to work at >25 keV. Finally, if a moderate‐ energy, ultrashort (20‐30 fs) driver pulse from e.g. a Ti:Sa laser is used to generate the X‐rays, the betatron emission duration is limited to sub‐10 fs, providing a means to freeze the motion of the compressed fuel on time‐scales matched to the expected spatial resolution. Very similar properties to betatron X‐rays, albeit with energy tuneable, quasi‐monochromatic spectral distribution are offered by laser‐driven

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Thomson/Compton backscatter sources. By tuning the photon energy to make use of selective absorption, this feature can be used to enhance the absorption contrast of the compressed fuel or will lead to sharper phase contrast features. However, these sources require at least one ultra‐short laser pulse for driving a quasi‐mono‐energetic electron bunch. Betatron emission has been extensively studied at 100‐TW‐class, 30‐fs Ti:Sa lasers whose pulses are ideal for driving GeV‐scale, mrad divergence electron beams via laser‐ wakefield acceleration in the nonlinear blowout regime. Here, the source is well‐characterized and optimization strategies are well known, which typically yields <10 fs, few‐mrad‐divergence X‐ray bursts with 10’s of keV photon energy and approx. 109 photons/shot, all from a 1‐2 µm source spot. However, this regime of electron acceleration is not accessible with typical kJ‐class, ps‐duration petawatt facilities available at inertial fusion research laboratories, such as PETAL/LMJ. Alternative approaches have thus to be considered, such as self‐modulated wakefield acceleration (F. Albert et al, Phys. Rev. Lett. 118, 134801 (2017)). Synergetic to the development of betatron emission, phase contrast imaging based on laser‐ driven Talbot‐Lau deflectometer is a compelling diagnostic to achieve the micrometric spatial resolution required for mix measurements (M.P. Valdivia et al, Rev. Sci. Instrum. 87, 11D501 (2016)). Tasks 2019‐2020 (MPQ, CEA/CELIA, CEA/LULI, CCFE/RAL, CCFE/Oxford, IST). Since such betatron sources are very attractive back‐lighters, we propose a two‐pronged approach: The first is a study of betatron source properties with ps, kJ PW drive lasers, in a yet unknown experimental regime. However, first experiments in that realm are soon to be carried out by a group from Imperial College London at the Vulcan‐PW system at Rutherford Appleton laboratory, and the results will serve as valuable input for both our theoretical and experimental studies. Since this first experiment will focus on a gas jet as an electron source, we plan to follow up with a second study in this proposal using a plasma plume from a solid surface as the target in 2019 [MIX1]. The second task for 2020 is that we will study betatron and Thomson X‐ray generation on our 2‐PW Ti:Sa laser system ATLAS‐3000 in Garching, in order to map out the source characteristics in another yet uncharted driver/interaction pulse regime. Being able to drive high‐quality electron bunches from classical laser‐wakefield acceleration, this approach will provide solid experimental data for true high‐brilliance X‐ray sources (high spectral purity, small divergence, few‐fs duration). We plan to characterize the X‐ray source temporally by mapping out the driving electron pulse duration via CTR spectroscopy or Faraday rotation, spectrally by single‐hit spectroscopy in a TimePIX detector array and spatially via knife‐edge diffraction and phase‐contrast imaging. This research will establish whether extending PETAL duration to 10’s of ps (compared to 600 fs actually) could significantly boost the imaging capabilities [MIX2]. In parallel, the development of a phase‐ contrast imaging a Talbot‐Lau interferometer which provides electron density gradients as well as information on attenuation, material composition and small‐angle scatter, will be pursued on LULI2000; it allows studying RT growth rates and will be possibly fielded in the MPQ experiments [MIX3]. Finally in synergy with wetted foams design (see Section B.3), there is still the idea of mitigating Rayleigh Taylor imprint with foams, as has been done previously in planar geometry (B. Delorme et al, PoP 23, 042701 (2016)).We are collaborating with the University of Rochester on this subject and will undertake numerical simulations in support of these experiments [MIX4]. A.3 Role of magnetic fields Magnetic fields in MJ‐irradiated targets can affect a wide range of parameters, e.g. heat flow, instability thresholds, Rayleigh‐Taylor growth rates, etc. Measurements of magnetic fields in these targets is, however, far from straightforward. The development of new experimental techniques to measure and characterise magnetic field generation and growth in order to enable predictive capabilities via fluid and kinetic computer codes is of clear benefit to both the inertial and magnetic fusion communities. A promising approach is that of proton radiography, i.e. the deflection of protons from a secondary source by self‐generated magnetic fields. There are several well‐developed methods of accelerating ions from these secondary sources, including target normal sheath acceleration (TNSA) and shock wave acceleration (SWA). TNSA spectra are known to be broadband in energy. In contrast, experiments investigating SWA have produced “quasi‐mono‐energetic" ion spectra with an energy spread of 4%. Of particular interest are the experimental results of Haberberger et al. [Nature Phys. 8, 95 (2012)]. They found that propagating a short pulse from a 10.6 µm CO2 laser through a critical density plasma to drive a collisionless electrostatic shock can accelerate ions an order of magnitude beyond that expected by a simple hole‐boring model. Theoretical work carried out by Fiuza et al. [Phys. Rev. Lett. 109, 215001 (2012)] provided an explanation

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for Haberberger et al.'s results and placed strong constraints on the systems that are able to support SWA 1/2 of this form. They found that the optimum target size is: L = /2 × (mi/me) , where  is the wavelength of the laser driving the shock and any generated plasma must be close to the critical density. For the Vulcan system, this corresponds to a target width in the range of 25‐ 100 µm. Additional work (conducted within the Oxford group based on an extensive simulation campaign utilising the PRISM suite of hydrodynamics and spectroscopic modelling, and particle‐in‐cell simulations using the OSIRIS code) has corroborated the predictions of Fiuza et al.'s theory and allows us to predict the shape and energy of the ion spectra that can be produced, e.g. on the Vulcan laser facility at the Rutherford Appleton Laboratory.

Tasks 2019‐2020 (CCFE/Oxford, CCFE/RAL, CEA/CELIA, IST) Our particle‐in‐cell modelling suggests that the conditions are appropriate for driving SWA that will match with Fiuza et al.'s theory with

a critical Mach number of Mcr  1.7. Thus we expect to generate narrowband ion beams with energy on the order of 50 MeV. Ions at these energies offer exciting new diagnostic capabilities for this IFE programme because they would be able to penetrate the hohlraum during an indirect‐ drive implosion and offer an unprecedented level of detail (due to their mono‐energetic properties) for direct drive implosions. A proposal by the Oxford, RAL, IST and CELIA groups has been approved by the STFC Facility Access Panel for the Vulcan 100 TW laser facility for scheduling in 2019. Our task in 2019 is to realise this experiment [MAGFIELDS1]. In 2020, our task is to complete the analysis of this experiment and to extend these results to PETAL/LMJ laser facility [MAGFIELDS2]. A.4 Electromagnetic pulse (EMP) mitigation One of the effects often accompanying the laser‐target interaction at high laser intensities, as expected on the PETAL/LMJ facility, is the generation of strong electromagnetic pulses (EMP) with frequencies in the range of tens of MHz to few GHz. Such pulses may interfere with the electronic systems inside and outside of the experimental chamber up to destroying them, and hence pose a threat to the safe and reliable operation of high‐intensity laser facilities. Similar EMP pulses will likely arise in tokamaks with ELM disruptions with the associated fast particle generation. Understanding these processes will therefore be of great benefit to both the inertial and magnetic confinement communities. It is well‐established that the intensity and thus the related detrimental effects of EMP fields scale directly with laser energy and with intensity. For this reason research on EMPs and on their mitigation is very important for the Fast Ignition approach to ICF. It is thus of great importance to properly characterize such pulses, develop predictive models and work out mitigation strategies. Considerable progress has been made in this area in recent years, with EMP measurements being performed at the Eclipse facility (CELIA, Bordeaux), the ABC facility (ENEA, Frascati), the PALS facility (IPP.CR, Prague), and the Vulcan PW (RAL, Didcot), for a variety of thick and thin targets and a range of laser conditions. However, there still remain some unresolved issues. First of all the set of the laser/target configuration for which the information on the EMP generation was recorded remains quite small. Secondly, the configurations of the E and B fields recorded in each experiment are very limited, which is due to the fact that there are not so many EMP probes available and high‐quality EMP measurements require expensive electromagnetic shielding for oscilloscopes. As a consequence it is difficult to perform convincing cross‐checks of competing hypotheses on the mechanisms of EMP generation, such as electromagnetic emission originating from the neutralization current with the target support acting as an antenna; an emission in the charge separation phase; the interaction of ejected electrons with the chamber walls; the photoionization or fields due to accelerated charged particles and their direct deposition on probes and cables. As for the neutralization current and target charge measurements, in the case of short‐pulse facilities they are limited to a very specific target configuration that allows for a direct measurement of the current. Tasks 2019‐2020 (ENEA, CCFE/RAL, CCFE/York, IPPLM, IPP.CR). We have identified four tasks for the consortium in this forthcoming period. The beam‐time would be obtained either through a Laserlab grant or as an in‐kind contribution from such a facility. (1) We will perform at least one collaborative, multi‐probe, high‐statistics, high‐bandwidth experiment dedicated directly to EMP studies at a European PW short‐pulse facility. In these efforts attempts will be made to study the correlation between the EMP generation and slow and fast escaping electrons and ions, and obtain information on the neutralization current and the target charge. Care would be applied to systematize the methodology of EMP measurements [EMP1] (2) To perform at least one collaborative, multi‐probe, 5 | Scientific Proposal CfP‐ADMIN‐AWP19‐ENR‐01

high‐bandwidth experiment at sub‐ns, kJ‐class PALS facility. The mechanism of EMP generation at long‐pulse facilities is different than at short pulse laser facilities and is not yet fully understood [EMP2] (3) To advance development of new diagnostics: to improve the design of the inductive return current probe developed at PALS so that it could be used at other high energy facilities as well; to extend the features of optical probes for a cross‐calibration with conductive probes at GHz frequencies; to advance the design of X‐loop B‐dot probes constructed at IPPLM that allow for a measurement of two components of the B field simultaneously. Furthermore, to improve techniques for using conductive probes inside the experimental chamber in spite of the possible effects of ionizing radiation [EMP3]. (4) To test various EMP mitigation schemes such as EMP dumps and advanced target supports, and to design and develop plasma and particle diagnostics and measurement techniques with improved EMP rejection and robustness [EMP4]. To this end it is planned that at least one experimental session would be conducted at the 10TW short pulse facility in IPPLM, devoted to EMP mitigation. To perform tailored theoretical‐numerical modeling of physics phenomena related to possible EMP sources and thus to the related field distributions, for the purpose of defining effective strategies to control and minimizing them. A.5 Laser‐Plasma Interaction (LPI) Analysis of the state‐of‐the‐art of IFE‐relevant LPI physics clearly shows that energy transfer from laser to plasma still remains a major issue and that controlling, or avoiding laser‐ plasma instabilities altogether, is key to the future success of most ICF approaches. Crossed‐beam energy transfer (CBET) is for instance reducing, in directly‐driven implosion campaigns, the ablation pressure by transferring – through Stimulated Brillouin Scattering (SBS) ‐ laser energy from incoming beams to outgoing ones, consequently increasing the total scattered energy. Mitigation measures (in the spatial and spectral domains) are of course studied but building predictive LPI capabilities remains essential, even if it is a key challenge due to the disparate scales involved. The energy exchange between crossing laser beams, CBET, is essentially due to the plasma density perturbation induced by the ponderomotive force of overlapping beams. The resonant growth of plasma waves together with the onset of energy transfer starts therefore rapidly from the induced plasma waves. The onset of stimulated scattering inside each individual beam, starting from lower noise, has yet been disregarded in the context of CBET. However, the competition between CBET and backscattering from noise will be of importance at higher plasma densities relevant to the “shock ignition” concept. So far, simulation studies for shock ignition have been limited to relatively simple configuration using particle‐in‐ cell codes, for which, due to high computational expense, multi‐speckle beam configurations cannot be considered. Tasks 2019‐2020 (CEA/CPhT, CCFE/RAL, CEA/CELIA/CNRS, CEA/LULI) Our tasks for this proposal are (1) studies using realistic smoothed laser beams will allow a more systematic study of SBS and CBET together in steep density profiles, with the code HARMONY and the validated version of the PCGO model [LPI1]. (2) While CBET has been found to cause additional band‐width [PRL 118, 055002 (2017)], the laser band‐width properties will be an important part of the studies in order to understand how to control and to limit LPI instability processes to acceptable levels in backscattering and CBET [LPI2]. (3) While SBS and SRS processes represent serious problems for laser fusion, they have potential utility as beneficial applications for the amplification of ps‐ and sub‐ ps seed laser pulses from both ion‐acoustic oscillations (SBS‐amplifier) and electron‐plasma waves (SRS‐amplifier). Yet the experimental success using these schemes is limited; partial success has been achieved for SBS amplification. Both PIC codes and wave‐coupling codes are available in the collaboration, and a new hybrid real valued, complex ray optics model based upon a laser adaptive grid, is under development at CELIA/CNRS. By using their complementary features, the optimized regimes for laser plasma amplifiers can be identified [LPI3]. Plasma amplifiers offer a route to overcome the technological limitations of current laser systems based on Chirped Pulse Amplification (linked to low damage threshold of optical components) to deliver pulses above a few petawatts. They rest on energy transfer between laser pulses inside a plasma, induced by parametric instabilities, as CBET, and, thus, their study with the help of identical numerical codes can contribute to deepen our understanding of this latter process. Tasks 2019‐2020 (CCFE/Oxford, CCFE/RAL, IST, CEA/CELIA, CEA/CPhT) – We have an active collaboration with the University of Rochester (Prof Dustin Froula and Dr Dan Haberberger) to

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further explore the concept of Raman amplification in plasma into the non‐linear regime. Integrated experiments are planned for 2019 to test the concept at the few Joule level. We plan to contribute to these landmark studies at the Laboratory for Laser Energetics by computational and theoretical support and by participating in the integrated campaigns [LPI4]. Also, the exploration of novel plasma amplification processes described above requires multi‐scale computational models capable of bridging the different scales and of including the relevant physics at high powers, long propagation lengths, and multi‐dimensions. This topic will involve the generalisation of collision and ionisation algorithms to take advantage of novel reduced algorithms in the OSIRIS framework, and the exploration and benchmark of these algorithms in plasma amplification processes [LPI5]. Axis B. Critically evaluating advanced alternative schemes for fusion energy The fast ignition scheme for inertial fusion presents the opportunity to achieve higher efficiency fusion while using more relaxed driver and symmetry requirements. Fast ignition promises these enhancements by separating the compression phase from the ignition phase. The scheme allows much lower implosion velocities. This means that more fusion fuel can be compressed for the same drive energy, thereby increasing the achieved gain, once the 10 keV ignition temperatures have been achieved in a hot spot created on the side of the dense fuel, principally by collisional heating. Although fast ignition shows great potential, it is still a relatively new scheme compared with the central hot spot approaches which have received much more attention and funding, and is therefore not as thoroughly explored. For example, the most recent results in cone‐guided fast ignition suggest that frequency conversion of the petawatt laser beam to its second (or higher) harmonic is necessary in order to reduce the irradiance (I2) incident into the cone. This will reduce the fast electron beam divergence and increase the coupling of fast electron energy to the compressed fuel. B.1 Electron fast ignition Integrated modelling of the disparate time and length scales in typical fast‐ ignition relevant experiments is a major challenge. We have access to a state‐of‐the‐art ICF modelling suite based on the Odin (and Hyades) radiation hydrodynamics code and EPOCH particle in cell codes. Odin has recently undergone major development and by the beginning of 2019 should be able to simulate direct drive implosions (Hyades provides a backup code in case of delays in Odin development). Odin pre‐pulse simulation combined with the modelling of the high‐intensity interaction using the particle in cell code EPOCH can be used to simulate high‐intensity laser matter interactions and so enables the simulation of experiments relevant to alternative schemes such as the generation and transport of fast electrons in fast ignition and the use of auxiliary heating of compressed targets. We have made progress in fast electron beam divergence control using novel structured targets [A.P.L. Robinson et al., Plasma Phys. Control. Fusion 57 064004 (2015); A.P.L. Robinson et al., Physics of Plasmas 22, 043118 (2015)] and Lancaster has recently performed experiments with these targets to assess their performance (results still being analysed) – further experiments are planned. Odin enables realistic modelling of structured collimating targets in fast ignition scenarios, where the degradation of these targets due to hydrodynamics effects (shocks, ablation) may determine their viability.

Tasks 2019‐2020 (CCFE/York, CCFE/RAL, CEA/LULI) Comparing simulations results to experimental data is greatly simplified if synthetic diagnostic output is simulated by the code. We will develop short‐pulse synthetic diagnostics in EPOCH suitable for experiments relevant to alternative schemes. We will include bremsstrahlung and Cu K‐alpha emission in EPOCH, useful tools for diagnosing fast electron transport [ELECTRONFI1]. The bremsstrahlung diagnostic will be based on similar routines to the synchrotron emission routines implemented in EPOCH by Ridgers [C.P. Ridgers et al, J. Comp. Phys., 260, 273 (2014)] and Ridgers also has experience implementing Cu K‐ alpha diagnostics into Vlasov codes [A.G.R. Thomas et al, New J. Phys. 15, 015017 (2013)]. EPOCH and ODIN simulations will be used to interpret existing fast‐electron resistive guiding experiments using structured collimators on Vulcan and plan future experiments to demonstrate resistive guiding as well as investigate the durability of these targets in realistic implosions [ELECTRONFI2]. Note that in current experiments, unlike a full scale fast ignition facility, the timescale of the short‐ pulse interaction is sufficiently short that hydrodynamic expansion does not occur during this interaction and so complicated in‐line coupling of EPOCH and ODIN is not required.

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B. 2 Ion‐driven fast ignition Despite the substantial progress carried out on ion fast ignition (IFI) over the last years [J.C. Fernandez et al., Nucl. Fusion 54, 054006 (2014)], there are still many physical aspects not well known and a point design for ion driven fast ignition has not been proposed so far. Proton acceleration in hollow cones by the TNSA mechanism has demonstrated most of the requirements for a suitable ion driven fast ignition scheme: high conversion efficiencies [C.M. Brenner et al., Appl. Phys. Lett. 104, 081123 (2014)], appropriate proton energies [S. Hatchett et al., Physics of Plasmas 7, 2076 (2000)], multi‐ picosecond laser pulse lengths [K. Mima et al., recent proton generation experiments at ILE, Osaka, Japan] and experimental proof of intense proton beam focusing to the required 30 – 40 μm spot diameters [T. Bartal et al., Nature Phys. 8, 139 (2012)]. It is still pending to demonstrate the fulfilment of all these achievements simultaneously in a` single experiment on multi‐PW, multi‐ps facilities. The generation of quasi‐monoenergetic ion beams is in a similar condition: the beam requirements have been demonstrated in a number of different experiments, but not simultaneously. The advantages of quasi‐monoenergetic ions are higher coupling efficiency with the imploded fuel and simpler target design. These advantages are balanced by the higher focusing requirements if the ion source is placed far from the imploded core to avoid using re‐entrant cones [J.C. Fernandez et al., Phys. Plasmas 24, 056702 (2017)]. The possibility of generating quasi‐monoenergetic ion beams with kJ energies in multi‐ps pulses has still to be demonstrated. We intend to assess the IFI potential as an alternative ignition scheme by means of integrated calculations and experiments (see Section A.3 above). Integrated calculations will include from multi‐dimensional PIC simulations for the ion source to hybrid modelling of ion transport and energy deposition. Tasks 2019–2020 (CIEMAT/UPM/ETSIAE, CCFE/RAL, IPPLM) In a first step, a comprehensive characterisation and optimization of laser‐driven beams of protons and heavier ions (C and V ions) will be carried out by PIC simulations [IONFI1]. They will include on‐line dynamic ionization of the target and the ion beam during the whole acceleration process. Simulations will be performed using the PIC code EPOCH [T.D. Arber et al., Plasma Phys. Control. Fusion 57, 113001 (2015)] as well by that developed by the IPPLM. In a second step, the ion distribution function in radius, momentum and time will be used by the Fokker‐Planck (FP) code described in [J.J. Honrubia and M. Murakami, Phys. Plasmas 22, 012703 (2015)] to compute the ion energy deposition in the imploded fuel. The BPS stopping power model [L.S. Brown et al., Phys. Rep. 410, 237 (2005)] will be included in the FP code because it has been recently validated in experiments [Cayzak et al. Nature Com. 8,15693 (2017)]. In addition, resistive fields generated by the ion beams will be considered in the FP simulations because they may be important in zones with intermediate or heavy materials, such as the cone tip [IONFI2]. Experiments will be proposed to the European facilities, such as VULCAN, PETAL and PHELIX [IONFI3]. B. 3 Auxiliary heating It has been suggested that the advantages of low‐convergence ratio wetted foam implosions [R.E. Olson et al., Phys. Rev. Lett. 117, 245001 (2016)] are combined with the crossing beam concept [N. Ratan et al. Phys. Rev E. 95, 013211 (2017)] to increase the available fusion yield – the auxiliary heating concept. The understanding and control of beam‐plasma instabilities and plasma turbulence has applications not only to auxiliary heating, but also to ELM control in magnetic confinement. The concept of auxiliary heating was inspired by the magnetic fusion community’s approach to ion heating in tokamak plasmas and was undertaken as a direct result of the ToIFE grant. Tasks 2019‐2020 (CCFE/Oxford, CCFE/York, IST, CEA/CPhT, CEA/CELIA). The near one‐ dimensional performances of wetted foam implosions naturally lend themselves to scalable, predictable fusion output. Advantage is taken of the increased hot‐spot core diameter that has been observed for these implosions on the National Ignition Facility. The hot‐spot core diameter is consistently a factor of two larger than typical shots with cryogenic ice layers. This results from the much larger initial vapour pressure in the centre of the capsule, one that provides more mass for the hot spot at peak compression. We plan to design a low‐convergence ratio, sub‐ignition‐ scale direct‐drive wetted foam implosion platform for experimental tests on PETAL/LMJ in the 2020’s [AUX1]. We will also design experiments for the Vulcan laser facility to test the onset and development of turbulence induced by beam‐plasma instabilities from petawatt class laser pulses that propagate in orthogonal directions. A proposal will be submitted to STFC’s Facility Access Panel for scheduling in 2020 [AUX2].

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Axis C. Developing key IFE innovative materials In the future nuclear fusion plants, and particularly in Inertial Fusion Reactors under almost non‐ protective hypothesis, extreme irradiation conditions of the materials in the first wall and optics are expected. The first wall of IFE systems will receive ions from hundreds of keV to few MeV (D,T, He, …..) from the target explosion with very high instantaneous fluxes of particles. Associated with each burning shot, very short and high intense X‐rays pulses are also expected that, when using very low pressure residual gas protection, conduct to the formation of plasma discharged on the wall in time of microseconds. Similar problems envisioned for divertor in magnetic fusion can also be extreme through events of ELMs with plasma discharge where repetitive high peaks of flux could appear. Progress have been achieved in effect of electronic excitation by irradiation and a satisfactory description of some of the intricate physical mechanisms involved in electronic excitation is available (Permanent modifications in silica produced by ion‐induced high electronic excitation: experiments and atomistic simulations, A. Rivera et al., Scientific Reports 7(1):10641, 2017). However, a full picture has not been established yet; in particular, for electronic damage in dielectrics where inter‐band transitions, hole formation, charge localization, exciton formation with subsequent radiative and non‐radiative decay routes and phonon interaction play interconnected roles that require a sophisticated time‐dependent quantum mechanical approach to depict the physical reality. Contrary to elastic collisions processes of damage, electronic damage is not completed and experimental and theoretical work is needed. Tasks 2019‐2020 (CIEMAT/UPM, CIEMAT/UNED) Research will be performed to define more clearly the 3D results of neutronic calculations for reactor blanket concepts, including thermo‐ hydraulic calculations, tritium diffusion and plant balance. The CIEMAT/UNED group are world experts in development of precision tools for neutronic‐activation calculations. Our goal here is the development of new computational tools that will be applied to reactor designs based upon high gain concepts described in Axes B [MAT1]. A study of electronic damage produced by high‐ energy ions irradiation in different dielectric materials, in particular ceramics and optics, by using laser pulses of femtosecond (fs) to solve in‐situ the evolution of the system. The diagnosis will be conducted by: Structural characterization using X‐ray diffraction; morphological with AFM, SEM, TEM; optical properties of the samples by measures in‐situ of reflectivity and transmittance techniques. Finally, we will compare the effect of high electronic excitation due to high‐energy ions with that of laser pulses to generate a valuable experimental database. Resources available at CMAM (ions) in the Universidad Autónoma de Madrid and the Universidad Complutense de Madrid (high intensity pulsed lasers) will be used [MAT2]. The development of SiC coatings on steel to increase working temperature in the cooling system was already proposed in HiPER project, anticorrosion material and minimizing the T permeation. An analysis of these coatings for First Wall materials will be also undertaken [MAT3]. Work plan Milestones No Title Description Expected date 1 MANAG joint meetings and revision of IFE roadmap 31/12/2020 2 ATOM1 complete the data analysis of the XUV time resolved 31/12/2019 bromine experiments 3 ATOM2 analysis of radiative processes in low‐electronic‐density 31/12/2020 tungsten 4 ATOM3 construction and deployment of von‐hamos X‐ray 31/12/2020 spectrometer 5 MIX1 characterisation of X‐ray betatron emission from plasma 31/12/2019 plume 6 MIX2 betatron and Thomson X‐ray generation 2‐PW Ti:Sa laser 31/12/2020 system ATLAS‐3000 in Garching 7 MIX3 development of Talbot Lau interferometer 31/12/2020 8 MIX4 simulations for foam target RT imprint mitigation 31/12/2020 9 MAGFIELDS1 realise shock wave acceleration experiment on Vulcan 31/12/2019

9 | Scientific Proposal CfP‐ADMIN‐AWP19‐ENR‐01

10 MAGFIELDS2 design shock wave acceleration experiment for PETAL/LMJ 31/12/2020 11 EMP1 study the correlation with slow and fast escaping charged 31/12/2019 particles for petawatt‐class interactions 12 EMP2 study the correlation with slow and fast escaping charged 31/12/2020 particles for ns, terawatt‐class interactions 13 EMP3 advance EMP measurement methods: inductive return 31/12/2019 current probe, optical probes, X‐loop B‐dot 14 EMP4 Develop EMP mitigation schemes with support of 31/12/2020 experimental and numerical results. 15 LPI1 LPI studies using realistic smoothed laser beams 31/12/2019 16 LPI2 LPI studies of laser band‐width properties 31/12/2020 17 LPI3 PIC computations of SRS and SBS to optimise plasma 31/12/2020 amplifiers 18 LPI4 simulation and experiment support for Rochester 31/12/2019 experiments 19 LPI5 collisional effects in OSIRIS for plasma amplifiers 31/12/2020 20 ELECTRONFI1 synthetic diagnostic implementation in the particle in cell 31/12/2019 code EPOCH 21 ELECTRONFI2 simulation study of fast electron transport in structured 31/12/2020 collimating target to interpret experimental data and determine the durability of these targets in implosions. 22 IONFI1 modelling of ion acceleration using EPOCH 31/12/2019 23 IONFI2 modelling of ion stopping using Vlasov‐Fokker‐Planck 31/12/2020 models 24 AUX1 design a low‐convergence ratio, sub‐ignition‐scale direct‐ 31/12/2020 drive wetted foam implosion for PETAL/LMJ 25 AUX2 design of turbulence experiments using crossed relativistic 31/12/2020 electron beams 26 MAT1 improvement of computational tools for the evaluation of 31/12/2019 IFE reactors based upon high gain targets 27 MAT2 mechanisms of electronic excitation in materials: radiation 31/12/2020 damage 28 MAT3 nanostructured materials as coatings for first wall and 31/12/2020 cooling systems Deliverables Year Title Description 2020 MANAG Delivery of the revised IFE roadmap 2020 Axis A Report on the study of fundamental materials properties and laser‐plasma interactions to acquire new insights into basic physics for ignition on MJ‐scale facilities, including the development of new X‐ray, particle and optical diagnostics on the Vulcan laser facility, in preparation for commissioning on the PETAL/LMJ facility at CEA; 2020 Axis B Report on the critical evaluation of advanced alternative schemes for the high gain target designs that are required for inertial fusion energy, including the exciting new auxiliary heating approach that was developed under the ToIFE project; 2020 Axis C Report on the developments in key IFE innovative materials, lasers and target fabrication technologies

10 | Scientific Proposal