Neutron Spectroscopy Studies of Heating Effects in Fusion Plasmas

Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 861

BY HANS HENRIKSSON

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Till mina föräldrar Ingrid och Alf This thesis is based on the summary and the following four papers, which are referred to in the text by the Roman numerals I to IV:

I Neutron emission study of DT plasmas heated with tritium neutral beams, H. Henriksson, L. Ballabio, S. Conroy, G. Ericsson, G. Gorini, A. Hjalmarsson, J. Källne, M. Tardocchi, Rev. Sci. Instr., 72, No 1 (2001) 832

II Neutron emission from JET DT plasmas with RF heating on minority hydrogen, H. Henriksson, S. Conroy, G. Ericsson, G. Gorini, A. Hjalmarsson, J. Källne, M. Tardocchi and M. Weiszflog, Plasma Phys. Control. Fusion, 44 (2002) 1253-1261

III Systematic spectral features in the neutron emission from NB heated JET DT plasmas, H. Henriksson, S. Conroy, G. Ericsson, G. Gorini, A. Hjalmarsson, J. Källne, M. Tardocchi and M. Weiszflog. In manuscript.

IV Synergetic RF and NB heating effects in JET DT plasmas studied with neutron emission spectroscopy, H. Henriksson, S. Conroy, G. Ericsson, G. Gorini, A. Hjalmarsson, J. Källne, M. Tardocchi and M. Weiszflog. In manuscript.

I acknowledge permission from the American Institute of Physics (paper I) and the Institute of Physics (IOP) Publishing Limited, Bristol (paper II). The home pages of these journals can be found at:

Review of Scientific instruments: http://ojps.aip.org/rsio Plasma physics and controlled fusion: http://www.iop.org/journals/ppcf

iv Contents

Contents...... v

Preface ...... vii

Glossary...... 1

1 Introduction to fusion plasma research...... 3

1.1 Fusion power for mankind ...... 3 1.2 Fusion reactor concepts...... 6 1.3 A magnetic bottle ...... 6 1.4 The tokamak configuration ...... 7 1.5 Plasma heating externally and internally...... 9 1.6 The JET and ITER reactors...... 11

2 Plasma diagnostics...... 12

3 Neutron experiments...... 14

3.1 The discovery of the neutron...... 14 3.2 Neutron sources and detection ...... 14 3.3 Fusion neutrons ...... 15

4 Information from the neutron emission ...... 17

5 The MPR neutron spectrometer ...... 21

5.1 MPR principles...... 21

Contents v 5.2 The MPR data acquisition system...... 24 5.3 Background suppression and correction...... 26

6 MPR development work and operation ...... 27

6.1 Absolute energy calibration...... 27 6.2 Operation at JET...... 29 6.3 New developments ...... 30

7 Analysis of fusion experiments...... 31

7.1 Basic and auxiliary heating ...... 31 7.2 Experimental aspects of neutron measurements ...... 36 7.3 Analysis model for neutron spectra...... 37

8 Results and discussion...... 40

9 Conclusion and outlook...... 45

10 Synopsis of attached papers...... 46

Acknowledgements...... 48

References ...... 50

vi Contents Preface

Utan tvivel är man inte riktigt klok. Tage Danielsson, Swedish quotation freely translated: “Without doubt one is not quite sane”

This thesis deals with work performed at the world's largest nuclear fusion experimental facility, the Joint European Torus (JET), and the analysis of data obtained from the magnetic proton recoil (MPR) neutron spectrometer. The MPR was installed in 1996 at JET for studies of the first full deuterium-tritium experimental campaign (DTE1) of 1997 aimed at obtaining as high a fusion power as possible. JET produced record high fusion power (16 MW) and the correspondingly high neutron production rate (6·1018 neutrons / s) was taken full advantage of with the MPR in terms of record high data collection rates. The MPR has since been the main instrument for the experiments carried out at JET by the fusion neutron group at the Department of Neutron Research (INF) of Uppsala University. Subsequently, the MPR has operated continuously taking data for plasmas using deuterium only. I joined the INF fusion neutron group in 1998 to complete my Diploma thesis for the MSc degree in Engineering Physics. In January 1999 I was admitted to the Advanced Instrumentation and Measurements (AIM) graduate research education by which I have financed my PhD studies. The fusion neutron group has changed from being a group of about 8 people concentrating on one diagnostic when I joined, to become a group of over 14 persons involved in many different diagnostics and aspects of neutron research connected to fusion. I have mainly studied data obtained with the MPR from the DTE1 campaign. This includes analysis of data from our extensive neutron emission spectroscopy (NES) database, background subtraction and data collection. The method of analysing the obtained spectra involved running several simulation codes developed specifically for the study of the JET neutron emission. A spectral fit of calculated emission to data obtained results typically in a good description of the fusion plasma conditions at hand. To be able to interpret and compare the results I developed some computer codes for extracting and analysing JET data. Part of my work has

Preface vii been devoted to data collection and the overall development of monitoring and control systems for the MPR. The JET experiments have been concentrated on D-plasmas after DTE1, a situation for which the MPR is not optimised. Nevertheless, interesting results on the performance of the MPR in D-plasmas have been presented in several diploma theses based on these data. I was also responsible for the data acquisition software since 1998. This includes updating and writing codes for new or replaced data collection electronics. The thesis summary is divided in the following sections. Chapter 1 gives an introduction to fusion and plasma physics research, and chapter 2 deals with plasma diagnostic techniques. Chapter 3 presents neutron research and specifically neutron diagnostics while chapter 4 illustrates why neutron measurements are useful and important for fusion experiments. The MPR instrument is introduced in chapter 5 with a basic overview of the technique followed by important modifications related to calibration, maintenance, and normal operation in chapter 6. Chapter 7 presents the analysis of experimental work performed at JET, and the analysis of data obtained from the MPR. The analysis model is described with examples of simulated spectra. Some results from my analysis of data are shown in Chapter 8 followed by concluding remarks in chapter 9 with an outlook of the field of neutron emission spectroscopy and fusion neutron diagnostics. Chapter 10, finally, gives a summary of the attached papers.

viii Preface Glossary

ADC Analog to digital converter AIM Advanced Instrumentation and Measurements AKN Alpha knock-on neutron BES Beam emission spectroscopy CXRS Charge exchange recombination spectroscopy DAQ Data acquisition DDN Neutrons from dd-reactions DEMO Demonstration electricity generating fusion power plant DTE1 Deuterium-tritium experimental campaign at JET DTN Neutrons from dt-reactions ECE Electron cyclotron emission ET Epithermal FPS Computer code calculating fusion product spectra HE High energy ICF Inertial confinement fusion ICRH Ion cyclotron resonance heating ITER International thermonuclear experimental reactor JET / JET-EP Joint European Torus / JET enhanced performance LHCD Lower hybrid current drive LMJ ‘Laser Mégajoule’ (laser facility for ICF in France) MCF Magnetic confinement fusion MPR / MPRu Magnetic proton recoil / MPR upgrade NB Neutral beam (injected for heating of the fusion plasma) NES Neutron emission spectroscopy NIF National ignition facility (for ICF in the USA) NPA Neutral particle analyser RF Radio frequency (frequency range for auxiliary heating on ions by means of ICRH)

Glossary 1 RTN Residual tritium neutrons SNAP Spectroscopic neutron analysis program ST Supra-thermal TBN Triton burn-up neutron TOFOR Time of flight optimised for rate TRANSP Transport analysis code for fusion plasmas TTE Trace tritium experiments at JET W7-X WENDELSTEIN 7-X (proposed stellarator in Germany) XCS X-ray crystal spectroscopy ZETA Zero Energy Toroidal Assembly µCF Muon catalysed fusion

2 Glossary 1 Introduction to fusion plasma research

ITER, "the way" in Latin (ITER home page: http://www.iter.org/)

1.1 Fusion power for mankind The development of thermonuclear fusion for use as a never-ceasing power source is a challenging field of research. Over 50 years of fusion studies have led to a variety of reactor concepts and significant progress in the fields of plasma physics and nuclear engineering. There is an enormous potential for a fusion-powered reactor. Fuel can be taken from the sea and would last for thousands of years with no radioactive fuel waste produced. The reactor would be a closed system concerning radioactive elements. The fuel would be injected continuously into the reactor furnace giving a stable source of energy with little environmental impact, even in case of accidents. Nuclear fusion of two light nuclei releases energy, Q, from their binding energy that could be used in a power reactor. The difference between the binding energy of the light nuclei and the product nucleus gives a surplus released as kinetic energy. For nuclei to fuse, they must overcome the repulsion from the so-called Coulomb barrier. This barrier increases with their positive nuclear charges why light nuclei with low proton number, Z, are preferable. Some of the most important fusion reactions of light nuclei are summarised in Table 1-1 [1] where also the Q-values are stated. Both exothermic (Q > 0) and endothermic (Q < 0) reactions are shown. The favourable reactions for fusion are the exothermic ones, like the dt-reaction (reaction #2a in Table 1-1), i.e., when the two hydrogen isotopes deuterons (d) and tritons (t) fuse, 2H and 3H, respectively, releasing energy carried by a neutron and an Į-particle (4He). The dd-reactions (#1a and #1b) can also contribute to fusion, but these have about a hundred times lower reaction cross section, ı. The reactions involving electromagnetic radiation (e.g., #1c and #2b) are less probable and will not be considered further. Tritium is radioactive, with a half-life of t1/2 = 12.3 years; it therefore does not naturally occur and has to be produced. This can be solved by the use of

Introduction to fusion plasma research 3 lithium in the walls in a future reactor, which would breed tritium inside the reactor, according to reactions #9 and #10 in Table 1-1 [2]. One branch of the dd-reaction (#1b) also produces tritons, which in a secondary reaction can generate dt reactions. Many of the reactions produce neutrons and can give information about the fuel in the reaction. For mono-energetic dd or dt fuel, the produced neutrons are mono-energetic, but when the fuel is heated, also the neutron emission changes. Neutron measurements are therefore useful and important in fusion plasma research.

Table 1-1 Summary of fusion reactions between light nuclei and their Q- values.

# Branching Reaction Q1) ratio [MeV]

1a 0.5 d + d → 3He + n 3.27 1b 0.5 d + d → t + p 4.03 1c 10-5 d + d → 4He + γ 23.6

2a 1.0 d + t → 4He + n 17.59 2b 7·10-5 d + t → 5He + γ 16.6

3 t + t → 4He + 2n 11.3 4 d + p → 3He + γ 5.5 5 t + p → 3He + n -0.76

6a 1.0 d + 3He → 4He + p 18.35 6b 3·10-5 d + 3He → 5Li + γ 16.5

7a 0.59 t + 3He → 4He + p + n 12.1 7b 0.41 t + 3He → 4He + d 14.3

8 3He + 3He → 4He + 2p+γ 12.7 9 n + 6Li → 4He + t 4.8 10 n + 7Li → 4He + t + n -2.5

1) Taken from Refs [1] and [2].

The dt-reaction has the highest reaction cross-section of the reactions presented in Table 1-1 for reactor relevant conditions. For this reaction, ı increases with energy up to a peak value of 5 barns at about 100 keV. The

4 Introduction to fusion plasma research rate of the fusion reactions is given by the product of ı and the relative velocity between the reactants, v. Integrating over the distribution functions of the reactants one obtains the reactivity, <ıv>, plotted in Fig. 1-1 [3] versus temperature under the assumption that the reactants have Maxwellian velocity distributions. The optimal temperature for thermonuclear fusion is of the order of 108 K, or about 10 keV if it is converted into energy units, eV (1 eV = 1.6·10-19 J), by the Boltzmann constant, k = 8.617·10-5 eV/K. At that temperature, the dt-fuel is almost fully ionised with electrons and ions disassociated forming a plasma, the fourth state of matter. The word ‘plasma’ (from Greek, meaning form or mould) was used by Irving Langmuir for this physical phenomenon for the first time in 1927 when he compared the red and white corpuscles in blood plasma with the ions and electrons in an ionised gas in a glow discharge [4].

Fig. 1-1. The fusion reactivity, <ıv>, for reactions between light nuclei as function of plasma temperature. Adapted from [3].

Introduction to fusion plasma research 5 Many questions remain to be answered. Is it possible to heat the plasma to the required temperatures? What device can contain such a hot state of matter for a time long enough for the fusion reactions to take place? How can we measure the heating efficiency, and how can we verify that fusion reactions have occurred? Can internal heating from product nuclei be used and is it sufficient to provide a continuous thermonuclear burn?

1.2 Fusion reactor concepts Most fusion experiments today aim at creating a hot plasma state. One exception is the so-called muon (µ) catalysed fusion, first predicted in 1947 [5], where the electron in the hydrogen nucleus is replaced by a muon with about a 200 times greater mass. This results in a drastically shorter distance between the nuclei in the atom and an increased probability for fusion. The first observed muon catalysed reaction was the pdµ-reaction (basically reaction #4 in Table 1-1) in 1956 [6]. The muon has a very short lifetime -6 (t½ = 2.2·10 s) so it has to be produced continuously. Within its lifetime it must stick to a nucleus (e.g., a triton or a deuteron), create a molecule (e.g., dtµ) which then fuses and a new cycle can start when the muon is released [7]. Various ideas of so-called cold fusion have also been published; the most recent [8] concerned fusion reactions from sonoluminescence in deuterated bubbles radiated with neutrons. However, Saltmarsh and Shapira questioned the results immediately [9] after repeating the experiment. The confinement of a hot plasma state can be achieved in different ways. In the sun, the gravitational force keeps the fusion process confined with a very long time scale because of its size; this is not an option on earth. A possible solution is to heat the dt fuel very fast with laser beams or energetic heavy ion irradiation, using inertia to keep the target together long enough for fusion reactions to occur. With a high repetition rate of what is effectively small H-bomb implosions a reactor configuration might be built. This technique is known as inertial confinement fusion and two major facilities, NIF in the US [10], and LMJ in France [11] are planned to be fully operational by the end of 2008. However, the most successful technique is to contain the plasma in a magnetic bottle, referred to as magnetic confinement fusion.

1.3 A magnetic bottle The early theory of plasma physics was established in the 1930’s, even if the combination of plasma physics and thermonuclear research was not considered at that time. Separately from this, Atkinson and Houtermans

6 Introduction to fusion plasma research considered the idea of thermonuclear reactions as a source of energy as early as in 1928 [12]. An early device utilised the already known ‘pinch’ effect from a magnetic field and was suggested as a device for compressing the plasma. Thonemann and Thomson proposed a toroidal vessel in the UK, in which an induced current sent through would create a poloidal magnetic field that pinched the plasma inside towards the centre. Strong instabilities arose in the plasma confinement and to cure this a small toroidal magnetic field was introduced with the ZETA machine. In 1958 Thonemann published the first article on results from ZETA indicating that thermonuclear fusion had occurred [13]. The results turned out to be too optimistic when new studies based on data from neutron diagnostics disproved that thermonuclear fusion had taken place. Neutrons were produced, but not from thermal reactions in the plasma [14]. The US astrophysicist Spitzer suggested in 1951 the ‘Stellarator’ configuration as a means to mimic the power generation of the stars [15], [16]. A stellarator uses magnetic coils formed in such a way that they create a helical magnetic field around the toroidal vessel. No inductive current is needed for this device. One such machine was the model-C stellarator in the US and the present Japanese Large Helical Device. A new stellarator (W7-X) is planned for operation in 2010 in Greifswald, Germany. The model-C stellarator was later converted into a ‘tokamak’ because of promising results from Russia and the tokamak is today the configuration that has achieved the highest fusion power output so far.

1.4 The tokamak configuration The tokamak configuration was invented in Russia as an improved toroidal pinch machine where the difference is that the toroidal magnetic field is stronger and dominates over the poloidal field generated by the transformer action. The helical magnetic field that resulted from the two fields gave better confinement than in earlier machines. This configuration, suggested in 1950 by Sakharov and Tamm, is sketched in Fig. 1-2a (original sketch by Sakharov) produced very promising results published in 1969 [17]. The worlds largest tokamak of today, JET, is shown in Fig. 1-2b [18] together with the proposed ITER [19] tokamak (Fig. 1-2c). The cutouts shown of JET and ITER are to scale.

Introduction to fusion plasma research 7 Fig. 1-2. Magnetic confinement fusion devices based on the tokamak configuration. A sketch of the tokamak configuration drawn by the inventor, Sakharov, in 1950 (a) [17], the JET tokamak (b) [18] and the ITER reactor design (c) [19]. Note that figures (b) and (c) are to scale.

8 Introduction to fusion plasma research 1.5 Plasma heating externally and internally When the plasma is hot enough, internal heating from Į-particles can provide the means for a self-sustained burn. To reach this stage the plasma has to be heated externally. In the tokamak the induced current through the resistive plasma medium provides one source of heating, the so-called ‘Ohmic’ heating, PΩ. This heating scheme is inefficient at higher temperatures because of decreasing resistivity and other methods are needed. Auxiliary heating, PAUX, is provided by injecting electromagnetic wave power into the plasma in the radio frequency (RF) range or by injecting a beam of energetic neutral particles (NB). The reactivity of thermonuclear fusion for a number of reactions is shown in Fig. 1-1. The produced power from the dt-reaction (per unit volume) is [2]

= σ pnnvQFdt (1)

where nd and nt are the densities of the fuel ions and Q is the energy released per reaction (see Table 1-1, #2a). The produced Į-particles in the fusion reaction between deuterium and tritium carry some of the energy released

n2 m pnnvE==σσ vn Q (2) ααdt + 4 mmα n which can be used to heat the plasma internally. Here nd = nt, each equal to half the ion (or electron) density (ni = ne = n). This power may be enough for a self-sustained fusion burn if the power losses, ploss, are balanced. The energy stored in the plasma, W, together with the energy confinement time, ĲE , gives [2]

WnT33()nnnT++ p ≈≈edt = (3) loss τττ E 2 EE

(where T is the plasma temperature). Radiation losses from bremsstrahlung,

= 2 pcZnnTbr eff e Z (4)

3 would at most account for pbr § 0.015 MW/m << pĮ for a reference plasma 19 -3 of ne = nZ = n = 5·10 m , T § 25 keV, and an effective charge, Zeff, (impurity level) of Zeff=1.5 (Zeff = 1 for a pure DT-plasma); however, for lower temperatures, T < 5 keV, pbr dominates over pĮ, but the absolute power level is very low as compared to ploss [2]. The needed total external power per unit volume, varies according to the power balance

Introduction to fusion plasma research 9 =− pppTOT loss α 3nT n2 (5) ppΩ +=−σ vEα AUX τ E 4

The different power contributions are shown as function of temperature in 19 -3 Fig. 1-3 for a reference plasma of n = 5·10 m and ĲE = 3.5 s. It is here shown that the auxiliary heating is needed initially, when the plasma temperature is low. A crossing point where the internal heating balances the losses, without any auxiliary heating applied, can be found as shown for T > 16 keV in Fig. 1-3. The ratio between PF and the external power required

P = F (6) QF PTOT indicates a figure of merit for the experiment (Fig. 1-3). Break-even is reached for QF = 1 and ‘ignition’ is reached for QF → .

10 ] 3 Q 0.2 F P α 8 Q

0.15 F P -value loss 6

0.1 4

0.05 2 P AUX Power (per unit volume) [MW/m unit volume) (per Power 0 0 0 5 10 15 20 25 T [keV]

Fig. 1-3. Internal (pĮ, circles) and auxiliary (pAUX, diamonds) input power together with output losses (ploss, triangles) as function of temperature (assuming 19 -3 Ti = Te = T) for a DT fusion plasma with the reference values, n = 5·10 m , ĲE = 3.5 s, and reactivities taken from Ref. [20]. The QF-value (crosses) is shown on the right axis. The lines are only guides for the eye.

10 Introduction to fusion plasma research 1.6 The JET and ITER reactors The JET fusion experiment produced its first plasma in 1983 after 10 years of planning and construction [21]. The Culham site, south of Oxford in England was chosen in 1977 and the construction work began in 1979. JET is a tokamak with a major radius of 3 m and an overall plasma volume of almost 90 m3. This should be compared to the proposed ITER, with a major radius of 6.2 m, and a plasma volume of about 850 m3 [19]. JET is equipped with three auxiliary heating systems; the NB injection system consisting of 16 beam lines, the ion cyclotron resonance heating (ICRH) and the lower hybrid current drive (LHCD) heating. The NB power, PNB, available to be injected in the plasma is about 22 MW. The installed ICRH power possible to launch is 32 MW, while the LHCD system can deliver about 6 MW. The Ohmic heating accounts for up to 3 MW of power to the plasma. The combined auxiliary heating typically reaches 25 MW at best. More auxiliary heating is to be installed at JET as part of an upgrade of the JET facility within the JET enhanced performance (EP) program that is underway. A large number of experiments have been carried out at JET, both concerning technology and physics; among them are the preliminary tritium experiments in 1991 and the first main deuterium tritium campaign (DTE1) of 1997, when the world record in produced fusion power was achieved with PF = 16.1 MW. The record in total fusion energy produced during one discharge was also set with W = 21.7 MJ, for a plasma discharge generating about 4.5 MW of fusion power over a period of almost 5 s. The goal for JET is QF = 1, and it was almost reached during DTE1 with QF = 0.65 during fusion power increase, which gives a calculated QF of 0.9 for steady state conditions [22]. A new (trace) tritium experimental (TTE) campaign is being planned at JET for late 2003, and operations up to 2006 with JET-EP are proposed. ITER will probably be the first fusion device to produce energy at the level of a small power plant. It will provide the next major step for the advancement of fusion science and technology, and is a key element in the strategy to reach the planned next step, a pilot power plant (DEMO) [19]. Initially, ITER will be able to muster 73 MW of auxiliary heating divided on 33 MW of NB heating and 40 MW of electron and ion cyclotron resonance heating. With the goal of producing PF > 500 MW, a QF-value of 10 or more is within reach.

Introduction to fusion plasma research 11 2 Plasma diagnostics

Plasma diagnostics are used to deduce information about the state of the plasma from observations of physical processes and their effects. The information is used to verify performance of the experiment and for control of the plasma volume regarding its topology and boundary. It is important to be able to describe the plasma, which is done by comparing theoretical predictions with measurements. This is done in terms of a number of plasma parameters. As shown earlier, density and temperature are important parameters as well as fusion power. Numerous diagnostic systems are in use at today’s fusion facilities to perform the required measurements. Some of the diagnostics at JET are presented in Fig. 2-1. Many of these are based on measurements of electromagnetic radiation and particles emitted from the plasma. Examples are measurements of the electron temperature, Te, from electron cyclotron emission and evaluation of the plasma ion velocity distribution from fast neutrals escaping the plasma detected with a neutral particle analyser (NPA) [23]. The diagnostics can be divided into two classes, namely, active and passive systems. The active systems depend on probes, in the form of laser light, or injection of diagnostic beams, as is the case for charge exchange recombination spectroscopy (CXRS) [24]. Among the passive systems we find NPA, x-ray crystal spectroscopy (XCS) [25] and neutron diagnostics [1]. The instrumentation for neutron diagnostics at JET consists of three main classes, namely, those for total yield, emissivity distribution (profile), and neutron emission spectroscopy (NES) measurements. The absolute yield is measured with fission chambers located at three different positions outside the torus vessel (marked KN1 in Fig. 2-1). The neutron profile data are provided by the neutron camera consisting of 19 collimated detectors positioned in horizontal (marked KN3H in Fig. 2-1) and vertical arrays [1]. Several neutron spectrometers are employed at JET, but only three are dedicated DT diagnostics ([26], [27] and [28]), among which the magnetic proton recoil (MPR) neutron spectrometer has been the most successful. The high quality data obtained from the MPR spectrometer are used for the studies presented in this thesis.

12 Plasma diagnostics Fig. 2-1. Diagnostics employed at JET including neutron spectrometers (marked KM) and neutron camera and yield monitors (marked KN). The MPR spectrometer (at JET known as KM9) is seen in octant 4, to the right in the figure, behind the limb numbered 3/4. Figure taken from [29].

Plasma diagnostics 13 3 Neutron experiments

3.1 The discovery of the neutron Experimental evidence of the neutron was shown in the early 1930s when Bothe and Becker bombarded beryllium with Į-particles [30]. The resulting radiation was assumed to be high-energy γ’s, but when Marie Curie and Frédéric Joliot found that 5.3 MeV protons were knocked out from a paraffin target as a result of this new type of radiation, the γ-theory was questioned. The γ-particles had to have a much higher energy to account for this. Chadwick explained this in 1932 [31] by assuming that this particle was neutral. Moreover, it had about the same a mass as the proton, which was the key to explaining the energies of the two particles involved in this recoil experiment. Already in 1920 Rutherford had mentioned the word ‘neutron’ for this particle as found in Ref. [32]:

Such a particle, to which the name neutron has been given by Prof. Rutherford, would have novel and important properties. It would, for instance, greatly simplify our ideas as to how the nuclei of the heavy elements are built up.

3.2 Neutron sources and detection The neutron together with the proton builds up the nucleus of atoms that can have several isotopes. The (free) neutron has a finite lifetime (about 15 min) and therefore lives only bound in nuclei. To use nuclei as a neutron source one has to overcome the nuclear force that bind neutrons by knocking them out from the nucleus as was done in the experiments described above. There the 9Be(Į, n)12C reaction was achieved, i.e., when an incoming Į- particle colliding with the neutron-rich 9Be isotope results in a neutron and the stable 12C isotope. Another way to produce neutrons is to bombard a neutron-rich nucleus with γ-radiation, e.g., 9Be(γ, n)8Be. Neutron production is also obtained in nuclear fission reactions, as for example spontaneously from 252Cf or 240Pu and from fission reactors. Also fusion reactions generate

14 Neutron experiments neutrons, such as the dt-reaction (see Table 1-1, reaction #2a). The neutrons carry 4/5 of the fusion energy of the dt-fusion reactions and they can also be used to extract information about the reactants if properly measured. Neutrons in the MeV range are detected by observing secondary particles produced in nuclear reactions, like proton recoils, (n,p), or neutrons instigating fission reactions in fission chambers. The neutron energy can be determined by the time it takes between two neutron interactions over a flight path of a certain distance. This time-of-flight (TOF) technique was mainly used for low energy neutrons with velocities of 103-104 m/s (energies up to 1 eV). However, with timing electronics in the ns-range and longer flight paths also high-energy neutrons (of 1 MeV or more) can be measured with this technique today. The proton recoil reaction that Curie and Joliot made use of during the days of the discovery, (n,p) scattering, has been used in many experiments since then, especially, for neutrons of higher energy. The recoiling proton energy is [33]

= 2 θ EEpncos (7)

which for ș§ 0 gives Ep § En. The proton energy can be measured in different ways including magnet systems, were the proton momentum (non- relativistically) is determined from the radius, r, of the proton path and velocity, v, according to

mv2 = qvB (8) r where B is the magnetic field perpendicular to the proton path. For a proton of 14 MeV, the radius of curvature in a magnetic field of 1 T, would be about 0.5 m. The MPR neutron spectrometer utilises this method, and will be discussed more in detail in Chapter 5.

3.3 Fusion neutrons The thermonuclear fusion process involves neutrons in almost all reactions, and therefore the neutrons play a key role. The most important in a fusion reactor is to carry the energy out from the burning fuel to the outside of the plasma vessel. Here, the neutrons are also important for the breeding of tritium fuel as mentioned earlier. Finally, the neutron emission is an important carrier of information about the plasma state, especially, what concerns the fusion process itself. This is the basis for neutron diagnostics, both for machine control and plasma physics studies. One should also note

Neutron experiments 15 that the neutron emission is the cause of radiation problems. This makes the torus building inaccessible during and after operation and the induced radioactivity causes material damage. Placing sensitive instrumentation in such an environment is challenging and remote operation as well as radiation shielding is essential.

16 Neutron experiments 4 Information from the neutron emission

The kinetic energy that the neutron carries, En, from the dt fusion reaction can be expressed as [34]

mmm2 EmV=+1 2 αα() QKV ++cosθ n () QK + (9) nn2 CM++ CM mmααnn mm

where mn and mĮ are the masses of the products. The velocity of the centre- of-mass, VCM, is calculated from the masses and velocities of the reactants according to

mmvv+ V = dd tt (10) CM + mmdt

The Q-value of the dt-reaction is 17.589 MeV, in the zero kinetic energy limit as calculated from the difference in mass between the reactants and the products

QM=−() M c2 = before after (11) =ªº()()()mmm +−++−− m222 mm m +−− mm mc22 =−−() mmmc ¬¼pnd p nt p nαα n dt using masses from Refs [35] and [36]. The relative kinetic energy of the reacting particles, K, is

1 mm K = dtv2 (12) + rel 2 mmdt

with deuteron and triton masses, md and mt,, respectively, and their relative velocity, vrel. The angle ș in the last term of Eq. (9) is defined as the angle between VCM and the neutron velocity vector, vn, in the laboratory system. This means that if the distribution of the reactants is isotropic, as is the case

Information from the neutron emission 17 for reactants with Maxwellian distributions, this last term vanishes and the mean energy is given by [34]

mm EmV=+1122αα() QKEmV +=++ K (13) nn22CM ++0 nCM mmααnn mm where the numerical value of E0 is calculated to 14.028 MeV from Eq. (11) together with

m EQ= α (14) 0 + mmα n with the masses given in [35] and [36]. For isotropically distributed, zero-temperature plasmas, approaches zero, which yields = E0. When the plasma ion temperature, Ti, increases, the additional terms in Eq. (13) give an energy shift,

m ∆=EmV1 2 + α K (15) Sn2 CM + mmα n of the same magnitude as Ti, e.g., ǻES § 20 keV at Ti = 5 keV, and ǻES § 36 keV at Ti = 10 keV [37]. A collective motion of the plasma (such as rotation) also adds an energy shift through the first term of Eq. (15). This additional shift can easily be measured by NES. Hence, depending on viewing angle, a rotation of the plasma can be measured. Apart from the mean neutron energy, the spread in energy is also of interest. This can be calculated from the reactants’ velocity distribution functions. For isotropic reactants in a thermal plasma, the ions have Maxwellian velocity distributions with the same temperature, Td = Tt= Ti,

33 §·§mm22 ·§·§ mm · f =−=−ddexpvf22 , t exp t v (16) dt¨¸¨ππdt ¸¨¸¨ ππ ¸ ©¹©22TTii ¹©¹© 22 TT ii ¹

Under the assumption that Ti << Q, [34], which is the case in the operational regime of a tokamak plasma, the analytical solution for the resulting neutron energy spectrum from these thermal ions is

§·− 2 1 ()EEnn fE()= exp¨¸ (17) nn s 2π ¨¸2s2 ©¹

18 Information from the neutron emission This distribution is a Gaussian, with the standard deviation, s, related to the spectral full width at half maximum (FWHM) according to

4mET FWHM=⋅=2ln2 s 2ln2nni ≈ 177 T (18) + i mmα n

From this, the temperature can be measured directly from the FWHM in a thermal plasma, which constitutes the basis for NES as a Ti diagnostic. The width of a neutron spectrum from dt-reactions would be FWHM § 0.9 MeV for a plasma of T = 25 keV. For dd-reactions (reaction #1a in Table 1-1), the neutron spectrum from a plasma of T = 25 keV would have a width (FWHM) of 0.4 MeV according to

4mEdd T =⋅=nn i ≈ (19) FWHMdd2ln2 s dd 2ln2 82.5 T i 2md

dd where the mean neutron energy, , is about 2.449 MeV for dd- reactions. Examples of neutron spectra are shown in Fig. 4-1. The neutron spectrum resulting from a thermal plasma of T = 5 keV is shown together with a case where Tt = 30 keV and Td =5 keV. If the ion populations are Maxwellian with Td Tt, the spectrum changes shape and has to be calculated numerically. For the general case of non-Maxwellian ion distributions, computer codes such as ‘Cauldron’ [38] and ‘FPS’ [39] have been developed utilising the Monte Carlo technique to obtain a numerical calculation of the neutron emission spectrum. A simplified case to study, only for illustration, is when mono-energetic ions are reacting with each other. Let us assume that deuterons of Ed = 150 keV interact with tritons of Et = 5 keV, to mimic what happens when neutral beam ions (initial energy, Eb, or velocity, vb) interact in a thermal plasma with ions of thermal velocities of average value, vth, i.e., the situation

=>>=22ETbi (20) vvbth mmbi

max The maximum neutron energy from this reaction is En = 14.96 MeV. This can be compared with the case where the NB ions would be tritons of the max same energy, Et = 150 keV, which gives En = 15.13 MeV. The difference is caused by the masses of the reactants. The latter example is illustrated in Fig. 4-1 (solid line) as a square distribution, showing that the neutron energy distribution starts from Emin = 13.1 MeV and extends to Emax = 15.1 MeV. If

Information from the neutron emission 19 the deuteron energies would be Maxwellian distributed with T = 5 keV, the slope of the square distribution is smoothed. These cases are important as illustrations of features that appear in spectra of the neutron emission from NB heated plasmas (see paper I). The neutrons produced can also be generated from second or higher order reactions. The triton burn-up neutron (TBN) emission, is such an effect. The tritons of about 1.01 MeV produced in the dd-reactions (#1b in Table 1-1) generate a secondary reaction with a deuteron, which results in a neutron emission with a distinct shape, different from the DTN emission. A third order effect is the Į-knock-on neutron (AKN) production from the 3.56-MeV Į-particles generated from dt-reactions. These particles transfer energy to deuterons or tritons in a second stage followed by a third stage where dt-reactions involving the internally heated fuel ions produce the AKN emission. The TBN and AKN effects are several orders of magnitude lower in probability than the DTN emission in DT-plasmas.

E =150 keV, T=0 keV t 0.16 E =150 keV, T=5 keV t T =30 keV,T =5 keV t d 0.12 T=5 keV T=30 keV 0.08 Intensity (a.u.)

0.04

0 12.5 13 13.5 14 14.5 15 15.5 E [MeV] n

Fig. 4-1. Neutron energy spectral components from dt-reactions. The solid line indicates a neutron spectrum from reactions between mono-energetic (150 keV) isotropically distributed tritons on deuterons in a cold plasma, while the long- short-dashed line corresponds to a background plasma of T = 5 keV. A spectrum from a plasma with different deuteron and triton temperatures is also shown (dashed with diamonds). Two spectra represent Ohmic plasmas of T = 5 keV (short-dashed with circle) and 30 keV (dash-dotted with square), respectively.

20 Information from the neutron emission 5 The MPR neutron spectrometer

The MPR spectrometer, suggested by Källne and Enge [40], was installed at JET in 1996 as a DT diagnostic for the DTE1 campaign. It was built, calibrated and tested in Uppsala and shipped over to JET, as a turnkey ready device.

5.1 MPR principles The proton recoil set-up used in this work is based on the same technique as was used in the early experiments when the neutron was discovered. In the present case, a collimated neutron flux from the plasma is converted into a proton flux through (n,p) elastic scattering. The protons are subsequently momentum analysed and thus spatially dispersed on a focal plane where they are registered by a detector array hodoscope. This is the principle of the MPR neutron spectrometer. The basic design is shown in Fig. 5-1 and consists of four parts, namely, a neutron collimator in front of a proton rich target, in this case made of polythene, (CH2)n, a magnet system and, finally, the hodoscope with 37 plastic scintillators. The collimator defines a narrow sight line cone where neutrons from the plasma pass through. In the target foil, a fraction of the neutrons scatter elastically on protons. These recoiling protons pass through an aperture to accept only those of (nearly) the same energy as the incoming plasma neutrons according to Eq. (7). The selected protons are momentum analysed in the magnet system consisting of one focusing magnet and one magnet of clamshell shape. The magnetic field of about 1 T curves the proton trajectory about 135 degrees towards the focal plane of the spectrometer (see Fig. 5-1). The plastic scintillators cover a distance of 518 mm, in steps of 8 mm (20 mm on the low and high energy ends) corresponding to an energy bite of about Ep = E0 ± 20 %. The protons impinging on the scintillators are counted and build up a proton position histogram, H(X), which, together with a well characterised spectrometer response function gives the desired information on the amplitude and energy distribution of the incoming neutron flux. The response function was calculated using a Monte Carlo code simulating neutrons impinging on the target foil, and generating protons that were

The MPR neutron spectrometer 21 tracked to the detector [41], [42]. An assumed neutron emission flux, Fn(E), was folded with the response to give a calculated proton histogram, Hcalc(X). This makes it possible, in an iterative manner, to fit a neutron spectrum to the proton data recorded by minimising the difference between H and Hcalc.

Fig. 5-1. Schematic drawing of the MPR spectrometer set-up and its measurement principle. The top part shows the spectrometer’s main components, such as the scintillator hodoscope and the magnet poles, surrounded by the extensive radiation shielding (concrete), with an opening only for the neutron collimator in front of the target. The lower part illustrates how the neutron flux is converted into a proton position histogram (see text for details).

The MPR is optimised for high count-rate capability at an energy resolution, ǻE/E, of typically better than 5 %; the energy resolution is here

22 The MPR neutron spectrometer defined as FWHM of the proton distribution at the focal plane for mono- energetic neutrons impinging on the target. The efficiency, İ, defined as the ratio between the proton rate and the incoming neutron flux (expressed in 2 units of cm ), gives the maximum count rate, Cn, that can be obtained. The efficiency can be varied by changing the target thickness and the solid angle of the proton aperture; with thicker target, İ increases at the expense of coarser energy resolution. Several targets are available, so that the settings can be changed depending on what is required for the specific experiment. The most commonly used settings are listed in Table 5-1 from Refs [42], [43] and [44]. The energy resolution and efficiency vary slightly over the hodoscope, why it is given for the central energy in Table 5-1. Radiation shielding is an important part of the MPR system. The vacuum chamber and magnet yoke are surrounded by concrete walls that weigh 65 tonnes; the total weight of the MPR system is about 90 tonnes. This extensive shielding is necessary in the harsh environment that surrounds the MPR positioned outside octant 4 of the JET vacuum vessel in the torus hall with a distance of about 4.3 m between the torus vessel port and the MPR target foil. The overall size of the MPR spectrometer is 1.9 × 2.6 × 0.8 m3 (l × h × w) with circumference dimensions of about 4.3 × 3.9 × 2.3 m3, including shielding.

Table 5-1 Main instrumental settings of the MPR used for the acquisition of data from deuterium-tritium, triton burn-up, and deuterium-deuterium neutron emission (DTN, TBN, and DDN, respectively).

Setting Reaction Target Efficiency Solid ǻE/E Magnetic # thickness angle field [mg/cm2] [10-5 cm2] [msr] [%] [T]

1a DTN 8.01 5.3 40.0 2.5 1.03 1b DTN 8.01 5.3 40.0 2.5 1.04 2a DTN/TBN 17.7 14 52.5 4.0 1.03 2b DTN 17.7 14 52.5 4.0 1.02 3 DTN 50.1 40 52.5 10 1.03 4 DDN 1.58 2.8 52.5 8.0 0.43 5 DDN 0.938 1.8 52.5 4.5 0.43

The view of the plasma is slightly up-shifted and quasi-tangential (as seen in Fig. 5-2) with an average angle of 47º relative to the magnetic axis [45]. The MPR spectrometer is sitting next to one of the NB injector boxes in Octant 4, sharing the same torus vessel port, with a different sight line as compared to the NB injector (Fig. 5-2).

The MPR neutron spectrometer 23 Fig. 5-2. The MPR position near the NB injector (NBI) shown with respect to the JET torus vessel (in an equatorial cut). Also indicated is the NB injection lines and the MPR line of sight, passing the plasma centre twice in a quasi- tangential view.

5.2 The MPR data acquisition system The data acquisition and monitoring (DAQ) system of the MPR collects data during the JET pulsing sequence in a time window covering the flat top plasma current pulse of 5-30 s (typical values during DTE1); afterwards data are stored in a dedicated computer. The DAQ also monitors the detector system concerning magnetic field and temperatures at different positions, such as the cooling water to the magnet system, inside the vacuum chamber, etc. The pressure inside the vacuum chamber is also monitored regularly with a vacuum gauge. Another important part of the DAQ system is the possibility to test the spectrometer with artificial signals generated by a pulse generator or by a light emitting diode (LED) system [46]. The plastic scintillators constituting the hodoscope are optically connected to photomultiplier tubes (PMTs). Protons losing energy in the plastic scintillation material causes light to be emitted that is converted into electric charge in the PMTs. The signal is fed into a system of electronic modules providing, mainly, discrimination and counting. The voltage amplitudes of the proton-induced PMT signals have to be over a certain

24 The MPR neutron spectrometer threshold to be recorded; if not, the signals are discarded, since low- amplitude events are mostly associated with noise or radiation background. The data collection is practically dead-time free since it employs fast counting electronics with latching scalers in conjunction with the entirely passive spectrometer system. A supplemental part of the DAQ system, utilising charge integrating analog-to-digital conversion (ADC) modules, requires more time to store the information, hence generating dead-time, which can be monitored due to parallel data acquisition using both scalers and ADCs for each hodoscope channel. This dead-time can, therefore, be handled off-line in a correction phase which also deals with the remaining radiation background. It should be noted that dead-time only affects the ADCs, which are needed for background correction of the data from the low- and high-energy wings of the hodoscope. Because of one common trigger for all 16 channels in one ADC unit, the central channels of the hodoscope, ch with the highest individual count rate, Cn , should be distributed between different ADC units. This also means that in order to avoid loss of events the ch low and high energy hodoscope channels with low Cn , should be separated ch from channels with high Cn in the same ADC unit. An example of ADC data is shown in Fig. 5-3 where typical threshold levels are indicated.

Data, #43013 1000 Total fit Proton signal L Background 100 Data, #43011

G S 10

Counts / 12 bins 12 / Counts 1

0.1 0 100 200 300 400 500 600 Pulse height (ADC bins)

Fig. 5-3. Pulse height histogram for hodoscope channel #21 of JET discharge #43013 (filled diamonds) recorded with low (L) threshold setting as compared with discharge #43011 (circles) with the signal (S) threshold level. Indicated is also the position of the gain (G) discriminator level and a spectral fit (solid line) used for separation between signal (dashed line) and background (dotted line). (See text for details.)

The MPR neutron spectrometer 25 High count rates can have an effect on the gain of the PMTs, which manifests itself in a shift of the peak position of the pulse height distribution for the signal events, i.e., those caused by recoil protons. To monitor this effect, a high discriminator level (G) was used. The G setting is higher than the signal threshold (S) used normally. The G-level was aligned to coincide with the proton peak position in the ADC pulse height histogram (see Fig. 5-3). The event rate recorded with this setting is sensitive to gain shifts, which is not the case for events recorded with the S-setting. The ratio between the number of counts for the G and S thresholds, NG and NS, in scaler data was used as a gain change monitor. The results from this monitor showed that for the highest obtained count rate reached at JET in an ch individual hodoscope channel (Cn = 50 kHz for a total Cn = 0.61 MHz), the gain shift was about 3 % [46]. At present this has a negligible impact on the quality of the measurements.

5.3 Background suppression and correction Even if the pulse from an event is large enough to pass the discrimination threshold for a signal event, it can still be caused by background radiation. This contribution can be removed by detailed study of the collected signals in the charge integrating ADCs. The ADC data builds up a pulse height histogram, of which one, for hodoscope channel #21, is shown in Fig. 5-3. Here, a spectral fit is presented used for separation of proton events and background in data from JET discharge #43013. A low discriminator threshold level (L, significantly lower than the S and G levels) was used here to illustrate the difference in shape between the exponential fall-off of the background and the almost Gaussian-shaped full energy proton signal∗. With a higher threshold setting, most of the background is already rejected as seen for the same hodoscope channel of data from JET #43011, also shown in Fig. 5-3, where the S threshold setting was used. The signal-to-background (S/B) ratio is about 2·103 for the central channels when a discriminator threshold positioned at about 65 % of the proton peak is used [28], without performing any background corrections to the data. With the S-threshold level just below the proton peak in the ADC histogram, the background ratio decreases further. By the use of a spectral fitting routine to ADC data (see Fig. 5-3) the S/B ratio increased to 5·104. This has been used for the search of the weak AKN component that was predicted to be below 10-4 of the signal peak amplitude for the studied DT- plasmas [47]. The successful results on this are presented in Ref. [48].

∗ Experience has shown that an asymmetric Gaussian, with different widths for the two sides of the peak position gives a good fit to the signal peak in the ADC histogram.

26 The MPR neutron spectrometer 6 MPR development work and operation

Since the installation of the MPR at JET in 1996, it has not been necessary to open the concrete shielding and the vacuum chamber. The reliability is high even if some changes were needed over the years. Because of maintenance work on the JET vacuum vessel, the entire MPR instrument had to be lifted out from its position in 1999 and in 2002. These changes with corresponding disconnection and re-cabling has caused no interference in the operation. The magnet power supply has been replaced after a water leak in the cooling system, and two roughing pumps on the outside of the vacuum chamber have been substituted after failures. However, no interior parts have failed, confirming that the MPR has been a reliable and robust instrument. Modifications to the DAQ electronics have been performed over the years, to improve the data readout since the installation, and to permit easier maintenance and control of the system. One such modification is the implementation of a high voltage (HV) controller module for the PMTs, as well as automatic logging of temperature and magnetic field for each recorded JET discharge. One way to improve the readout is to make use of more ADC channels. Each of the 37 hodoscope channels goes to one scaler and one ADC channel. For some of the channels, a second scaler-ADC pair is attached, with another discriminator setting. By permitting more channels to have two settings, the background can be suppressed more efficiently. Another positive effect is that the dead time in the ADCs can be reduced. To this end, an extra ADC module (with 16 channels) was added to the electronics set-up in October 1999.

6.1 Absolute energy calibration The MPR spectrometer is ‘ab initio’ energy calibrated. This is based on detailed measurements of magnetic excitation curves and field maps, as well as the physical survey of the MPR elements. These data were included in the response function calculations and the calibration was confirmed by measurements of Į-particles from a 241Am-source in the target position of the MPR.

MPR development work and operation 27 Since Ohmic discharges are assumed to have only a small or no toroidal rotation (as opposed to NB heated plasmas, for example), they were used for a verification of the ‘ab initio’ energy calibration with neutron data. MPR data in the form of proton position histograms, collected during Ohmic JET DT discharges are shown in Fig. 6-1, for three different instrumental settings (see Table 5-1), together with calculated neutron spectra; best fits to data are also shown. The energy distribution of the neutron emission is Gaussian- shaped due to thermal dt-neutrons. The histograms exhibit a tail at low X- values associated with energy down-degraded neutrons caused by scattering of full-energy neutrons in various structures of the MPR and JET machine. This effect is included in the data analysis model as a phenomenological spectral component.

Setting 1a 103 Setting 2a Setting 3 102

101 Counts / bin / Counts

100

0 100 200 300 400 500 X [mm]

Fig. 6-1. MPR proton position histograms from JET discharges heated solely with Ohmic heating. Data (symbols) for three different instrumental settings (see Table 5-1) are presented together with best fits to data (lines). (Note that no background correction is applied to the 1a and 2a settings, hence, the presence of a tail for high-X values).

The parameters used to calculate the fits presented in Fig. 6-1 are shown in Table 6-1, where the peak shifts and widths are given, together with the corresponding reduced Ȥ2 and the number of counts in the spectrum. The weighted average energy shift for all three MPR settings is calculated to ∆E = 0.75 ± 0.47 keV. It should be noted that an uncertainty in the

28 MPR development work and operation determination of the actual position of the hodoscope of ± 2 mm results in an absolute uncertainty of the energy shift of about ± 22 keV [28].

Table 6-1 Neutron emission energy shifts (ǻE) and temperatures (T) derived from best spectral fits of collected MPR data from several Ohmic JET discharges, with different instrumental settings (see Table 5-1 for details). The number of counts in the hodoscope channels 3-28 is shown together with the reduced Ȥ2 of the fits.

MPR setting # ǻE [keV] T [keV] Counts Ȥ2

1a -2.0±3.2 1.8±0.2 3726 1.88

2a 3.6±3.0 1.6±0.2 7551 1.38

3 -4.8±10.0 2.4±0.6 2667 0.871

6.2 Operation at JET In 1997, a long experimental campaign with DT-plasmas (DTE1) was performed at JET. Over 900 discharges were carried out and now constitute the basis of an extensive NES DT database. The MPR is optimised for 14- MeV neutrons in DT measurements but has routinely collected data also during D-plasma operation at JET. In total data obtained from over 6600 JET discharges have been stored so far (spring 2003). Since 1998, JET has only run D-plasma experiments, and MPR data have been collected for three main reasons. First, measurements were made to test the MPR as a 2.45-MeV neutron spectrometer [44]. Second, the TBN component could be studied by the MPR, which has been carried out both theoretically and experimentally [43]. Third, the MPR was used to measure the levels of residual tritium in the torus after the DTE1 [49]. The first objective, to measure dd-neutrons (DDN) from reaction #1a in Table 1-1, is possible due to the availability of a range of different conversion target foils as well as a magnet power supply that can alter the magnetic field to any preferred setting (see Table 5-1). It was concluded though, that this task is difficult with the present instrument, due to a high background level in the relevant energy region from ȕ-radiation produced in structures surrounding the detector; this radiation has a very similar energy deposition to 2.45-MeV neutrons. The S/B ratio was estimated to 0.1 [44]. Measurements of the tritium in the torus vessel are performed by comparing spectra from the two sources of dt-neutrons, namely, neutrons

MPR development work and operation 29 originating from residual tritium (RTN) in the vessel walls interacting with bulk deuterons, and neutrons from TBN emission. The strength and shape of the TBN component can easily be distinguished from the RTN contribution, which is narrow due to the cold triton population. The ratio between TBN and RTN can therefore be measured and gives a relative estimate of the tritium concentration. This ratio varied from about RTN / TBN = 44, two months after DTE1, to 0.3 three years later. An absolute estimate of the tritium density can also be calculated from the deuterium density and

RTN n2 σσvv n = d dd th dt TBN (21) t σ TBN2 dt v th where the reactivities reflect the different populations in the plasma. From calculated reactivities and data on the deuterium density, the relative tritium -5 density was estimated to be below 3·10 (with respect to nd) three years after DTE1 as compared to about 0.2 % in December 1997 [49].

6.3 New developments As discussed previously, the measurement of the 2.45-MeV neutron emission is difficult with the present MPR spectrometer. This is true for pure D-plasmas, and to study this emission in DT-plasmas is an even more challenging undertaking due to the lower reactivity of dd-reactions of about 2 orders of magnitude as compared to dt-reactions. To accomplish this with the MPR, an upgrade (MPRu) is under development as part of a general programme to upgrade various JET systems under the JET-EP activity. An increase of the sensitivity is envisaged with the MPRu by reducing the background with improved passive shielding and by digital pulse shape analysis. The latter is made possible with a new hodoscope consisting of phoswich detectors permitting separation between signal protons and background due to their different response [50]. The MPRu will hence be capable of measuring 2.45-MeV neutrons in D-plasmas as well as 14-MeV neutrons and possibly also the 2.45-MeV neutrons in DT-plasmas. Another line of development for JET-EP is a time of flight spectrometer optimised for rate (TOFOR) [51]. This spectrometer is dedicated for 2.45- MeV neutrons in D-plasmas, with an expected Cn potential of over 300 kHz, which is comparable with what the MPR has achieved so far. Data obtained by TOFOR should permit analysis of D-plasmas on the same detailed level as what the MPR has provided in DT-plasmas. Both TOFOR and MPRu are to be installed at JET in 2004, during the shutdown period before the JET-EP phase.

30 MPR development work and operation 7 Analysis of fusion experiments

JET has mainly operated with deuterium plasmas, but has the capability of handling tritium, which was shown during DTE1* of 1997. MPR data are being collected for both D- and DT-plasmas, mainly with results concerning the dt-neutron emission. The JET fusion plasmas are normally subjected to basic and auxiliary heating, which is described below in more detail.

7.1 Basic and auxiliary heating Ohmic heating, generated by the induced plasma current is providing the initial heating in a tokamak. The Pȍ heating is inefficient at high temperatures as the resistivity of the plasma decreases proportionally to T -3/2. A plasma temperature of at most 2 keV can be reached for JET conditions in this way. For further heating, two main schemes of auxiliary heating are applied, namely, NB and RF heating. The NB and RF power is thus injected into a base plasma formed by Pȍ, leading to drastic changes in the plasma conditions. The NB heating is applied using two injector boxes on JET, one in octant 4 and one in octant 8 (cf. Fig. 5-2). One injector box consists of eight individual beam lines that each deliver about 1.3 MW at full power at an energy up to Eb = 160 keV. The neutrals are stripped in the plasma leading to the deposition of ions with energies of Eb, Eb / 2 and Eb / 3 depending on being injected as D, D2 or D3 (or T, T2 or T3 for tritium). The proportion of the neutrals can vary about typical ratios of 70:15:15 for the three components. The neutral beam injectors can also give rise to an Eb/18 component (due to D2O). When neutral beams are injected into the plasma, an anisotropic ion source is introduced. The injected neutrals become ionised through collisions with the bulk plasma and are slowed down to thermal velocities. Once ionised the particles follow orbits in the magnetic field determined by their energy, point of deposition and injected ‘pitch’ angle.

* JET is the only fusion test reactor with this capacity since the TFTR (Tokamak Fusion Test Reactor) at the Princeton Plasma Physics Laboratory, USA, was stopped in 1997.

Analysis of fusion experiments 31 The pitch angle, ș, is given by

V sinθ = ⊥ (22) V and is the angle between the direction of the injected ions, V, and the velocity perpendicular to the magnetic field, Vŏ. The ions enter passing orbits, or become trapped (for ș § 90º). In the trapped orbits, the ion velocity vector oscillates between being perpendicular and parallel relative to the magnetic field according to constant magnetic moment in a magnetic mirror [52]. This is illustrated in Fig. 7-1 where orbits of passing and trapped particles (banana shaped orbits) are shown in a poloidal plane with respect to the torus.

Fig. 7-1. Passing (left) and trapped (right) particle orbits as seen in a poloidal plane of the torus [2].

The initial energy of the injected atoms is transferred to the plasma by Coulomb collisions with electrons and ions [2]. At high Eb, the electron heating is dominant, while ion heating takes over as NB ions slow down further. The angular momentum transfer is small for the electron collisions due to the difference in mass, which is not the case for ion collisions where, especially, pitch-angle scattering must be considered. In the case of low- mass beam ions, with respect to the background plasma, the dominant effect will be a deflection of the velocity vector from its initial direction.

32 Analysis of fusion experiments The total direct NB power transfer to electrons and ions can be expressed as [2]

ne4 §·2 mmmEln ()Λ m2 ln ()Λ =+=¨¸ebbe + b i (23) PPPei 22 3 πε π 2 2320 mEmbi¨¸ 22 ©¹Te where me, mi and mb are the masses of the electrons, ions and beam ions, respectively, n the plasma density, E the ion energy (initially equal to Eb), e the elementary charge and İ0 the permittivity. The coulomb logarithm for electron-ion collisions, ln(Λe), can be approximated by ln(Λe) § 17 under typical tokamak conditions (n § 1019 m-3, and T § 5 keV) and the Coulomb logarithm for ion-ion collisions is approximately ln(Λi) § 1.1 ln(Λe) [2]. At a certain energy during the slowing down, fast ions undergo scattering with electrons and ions with equal probability, which is referred to as the critical energy given by

2 §·3 π mmln ()Λ 3 ET= ibi (24) crit¨¸()Λ e ©¹4lnmmeei

This means that Ecrit § 26·Te and Ecrit § 17·Te for fast tritons and deuterons, respectively, assuming a DT-plasma of nd = nt. The total heating in Eq. (23) can therefore, together with Eq. (24), be expressed as

3 2lnmmne4 ()Λ §·E 2 =+eb i §·crit (25) PE3 ¨¸1 ¨¸ ()πε2 22 ¨¸©¹E 32Tmeb0 ©¹ which can be rewritten [2], if we insert the Spitzer electron slowing-down time, Ĳse, [53]

3 2 §·E 2 PE=+¨¸1 §·crit (26) τ ¨¸¨¸E se ©¹©¹

The total slowing-down time for ions on electrons and ions down to thermal energies has been calculated with the SNAP code [41], [39] and is plotted in Fig. 7-2 versus Te for tritons from NB injection at Eb = Et = 150 keV into a plasma with density ratios of nt / nd = 1 and nt / nd = 10; comparison is made with NB deuterons injected with Eb = Ed = 75 and Ed = 150 keV for a plasma with nt / nd = 1. The electron 19 -3 density is assumed to be ne = 10 m for all four cases. This shows that the slowing down time in a hot plasma (T > 10 keV) is of the order of 1 s for

Analysis of fusion experiments 33 Eb = 150 keV, while it is a factor of 2 lower for Eb = 75 keV. Note, at the onset of NB heating, the plasma is normally colder, resulting in shorter slowing down times initially. The electron density is approximately inversely proportional to the slowing down time resulting in shorter times for denser plasmas.

E =150 keV, n /n = 1 t t d E =150 keV, n /n = 10 1.5 t t d E =150 keV d E =75 keV d 1.0 [s] s τ

0.5

0.0 05101520 T [keV] e

Fig. 7-2. Spitzer slowing down time of tritons injected at Eb = 150 keV versus Te, for two different tritium concentrations (solid and dash-dotted lines) in the plasma. Also shown is the slowing down time for deuteron beam injection at Eb = Ed = 75 (dotted line) and 150 keV (dashed line with circles) in a similar plasma (nt / nd = 1). Data are calculated with the SNAP code [41], [39].

A steady state distribution function of the NB ion population slowing down in a plasma can be obtained from calculating the source rate, S, of the beam

dE SfEPfE=−() = ⋅ () (27) dt

Rewriting Eq. (27) using Eq. (25) above gives

34 Analysis of fusion experiments Fast slowing-down populations of tritons with a birth energy of Eb = 150 keV in a plasma of Te = 0, 2 and 5 keV are shown in Fig. 7-3. A half-box (HB) distribution is a good approximation to the slowing down distribution over the region Eb to Eb / 2 for realistic temperatures T > 2 keV (see Fig. 7-3). This simplified energy distribution has been used in the analysis model of NB heated plasmas to calculate the neutron emission spectral components of the high-energy ions from NB action. The remaining (low energy) part of the slowing down population is included in the analysis as an isotropic epithermal component.

E =150 keV, HB T=2keV b T=5 keV T=0keV 1.0

0.8 A.u. 0.6

0.4

0.2

0.0 0 50 100 150 E [keV] t

Fig. 7-3. Energy distributions of Eb = 150 keV tritons slowing down in bulk plasmas of T = 0, 2, and 5 keV (dashed lines). The three distribution functions are compared with a flat energy distribution (box distribution, solid line) between Eb/2 and Eb.

ICRH is applied by RF wave power injected by four antennas located on the inside of the torus vessel. The frequency range at JET is 23-56 MHz [2]. The system has an installed generator capacity of 32 MW with a record value of PRF = 22.3 MW coupled to the plasma [54]. The ICRH system allows heating of ions in the plasma at their fundamental, second or higher harmonic of the resonance frequency (Ȧc). The most favourable scenarios for DT operation are the second harmonic tritium heating (Ȧ = 2ȦcT) and minority deuterium heating (with deuterium concentrations, cd < 30 %) at the fundamental ion cyclotron frequency

Analysis of fusion experiments 35 (Ȧ = ȦcD) [55]. A special case has also been studied with fundamental minority heating of hydrogen (ch < 5%), which is also the second harmonic frequency of deuterons (ȦcH = 2ȦcD). This scheme was carried out in a number of NB heated high performance discharges at JET, for instance, #41759 [56]. This has been the subject for NES studies as presented in paper II. The RF power accelerates ions in the direction perpendicular to the magnetic field generating an anisotropic velocity distribution. The impact of the RF wave heating on the plasma can be described with the parameter

P τ ζ = se (29) 2nTme

Here,

is the average RF power density absorbed by minority ions, and nm is the density of the minority ions. This impact parameter is a direct measure of the effectiveness of minority RF heating and can be estimated from a measurement of T⊥ representing the (perpendicular) high energy tail temperature of a bi-Maxwellian distribution, where

=+()ζ TT⊥ 1 e (30)

Typical values of T⊥ are above 100 keV, resulting in ȗ > 20 for Te>5 keV [57].

7.2 Experimental aspects of neutron measurements An important figure of merit for neutron emission studies of plasmas subjected to auxiliary heating is Cn since this determines the quality of the data acquired for a discharge divided in time bins, ǻt. The extraction of plasma information from a measured neutron spectrum requires a certain number of counts N = ǻt·Cn as this determines the statistical uncertainty in the measurement and hence the accuracy in the information derived. For instance, the statistical uncertainty of the measured width of a Gaussian distribution is proportional to the square root of the number of counts [58], [59], [60]. The spectrum of the neutron emission from a thermal plasma is nearly Gaussian as is often also the response function of the spectrometer. With these assumptions, and the widths of the two contributions, being WT and WR, respectively, one can determine the uncertainty in the extracted value for WT to be

This gives that (if the quadratic term of the widths can be neglected)

22+ ∆∆WW()WWRT 1 TR→→ (32) 2 for 0 WWNWTT2 R

Assuming WR << WT we find that about 200 counts would give an uncertainty of 5 % in WT corresponding to 10 % in T. Similarly, if WR § WT the number would increase to N § 800. As a result of the flexible solution with different target conversion foils and collimator lengths in the MPR, the instrumental resolution can be improved at the expense of a lower Cn. For the MPR spectrometer, one setting gives an instrumental resolution of about ǻWR/WR = 2.5 % according to Table 5-1, which would result in N > 1000 counts for the same uncertainty as before. With a 4 % instrumental resolution N > 2200 counts are needed in the spectrum. For several DT-plasma discharges during DTE1, over 1018 neutrons/s were produced. Of these, about 1010 neutrons reached the MPR conversion target [42]. According to Table 5-1, the efficiency differs for different energy resolution settings. For the DT setting with 4 % energy resolution (setting #2), about 1.4·106 protons were collected, while only 5.3·105 protons would have been collected for the 2.5 % setting (setting #1). For a time resolution of 10 ms, this means that 14000 and 5300 counts are recorded per spectrum, respectively, for the two settings. This is enough for satisfactory spectral analysis of thermal plasmas as discussed above. For plasmas with several underlying non-thermal ion reactions, the calculation becomes more complex and a larger number of counts is needed in the spectrum. Therefore, longer collection times (than 10 ms) are used for a detailed spectral analysis. A simple case has been studied for this purpose, with a calculated spectrum consisting of two Gaussian distributions of different temperature (T1 = 10 and T2 = 22 keV, respectively). The spectrum was simulated based on Poisson statistics, followed by a fit to reproduce T1 in the simulation. For spectra with N > 5000, the standard deviation was below 0.2 [61].

7.3 Analysis model for neutron spectra The thermal (TH) component in the neutron spectrum originates from isotropic fuel ions in thermal equilibrium. This component is of Gaussian

Analysis of fusion experiments 37 shape and specified by its amplitude, energy shift (peak position) and width, where the amplitude is a measure of the spectral strength, hence, the amount of thermal fusion reactions. The energy shift is a reflection of a global plasma toroidal rotation, apart from neutron kinetic effects. The width is directly correlated to the bulk ion temperature as shown earlier. A single TH component is sufficient to describe the neutron emission spectrum from Ohmic plasmas, which are assumed to be in thermal equilibrium with a plasma temperature of T = Te = Ti. For auxiliary heated plasmas one Gaussian component is not enough to describe the energy distribution of the neutron emission but additional components must be added to obtain a good spectral fit. These components have been calculated with computer codes, for instance, using Monte Carlo methods to simulate reactions between NB or RF ions and thermal ions in the bulk plasma [38], [39]. There are two types of components, namely, those with isotropic and anisotropic velocity distributions. In the case of NB heating, the fast ions of high energy (HE) along the direction of the injection are strongly anisotropic. This anisotropic ion population can be sub-divided into ions on passing (HEP) and trapped (HET) orbits. These ion populations give rise to specific neutron spectral components when reacting with the bulk plasma and are denoted likewise. The HEP component originates either from neutron emission of ions in the co-direction (HEP-co) of injection or counter (HEP-cr) relative to the plasma current. The HEP-cr component is always weaker than HEP-co. The calculated components are shown in Fig. 7-4 for a plasma that was heated by the injection of tritons of Eb = 150 keV. The background plasma is 10 keV, as indicated by the TH component in Fig. 7-4a. The HET component (shown in Fig. 7-4b) is centred around E0 of the neutron spectrum, with a double-humped shape. This shape is due to the narrow spread in pitch angle of reacting ions. The HEP components also show this double-humped shape with an overall shift in energy due to the preferred direction of the injected ions; there is an up-shift in energy for HEP-co and a corresponding down-shift for HEP-cr. After slowing down to below the critical energy, the ions become isotropic while they are still supra-thermal (ST) as compared to the bulk plasma. This epithermal (ET) part of the ion velocity distribution gives rise to a neutron emission that is modelled with a Gaussian component. It is distinguishable from the TH component by its larger width, here denoted as a fictitious temperature, TET (with TET > TTH). The effect on the neutron emission spectrum due to RF heating can be modelled in a similar way to that of NB injection. The components are divided into isotropic (TH and ET) and anisotropic HE ones, where the latter are calculated based on the assumption that the ions are heated perpendicularly to the magnetic axis, giving a similar shape as HET but with more pronounced tails of the energy distribution (as shown in Fig. 7-4b).

38 Analysis of fusion experiments This HE component is parameterised so that a tail temperature, Ttail, can be derived [57].

0.1 HE , HB Pco (a) HE (FPS) 0.08 Pco TH (10 keV) o 0.06 60 , T=0 keV 4π, T=0 keV 0.04 Intensity (a.u.)

0.02

0 HE T (b) 0.05 HE (FPS) T RF (T =150 keV) 0.04 tail

0.03

Intensity (a.u.) 0.02

0.01

0 12.5 13 13.5 14 14.5 15 15.5 16 E [MeV] n

Fig. 7-4. Spectral components used to model plasmas subjected to auxiliary heating. Panel (a) shows components from high energy passing (HEP) particles calculated with the half-box (HB) distribution (solid line) and with a full slowing down distribution (dashed line with full squares) compared with a thermal (TH) component (dash-dotted line) of a plasma of 10 keV. Also shown are two components with mono-energetic tritons on a cold plasma injected at 60 º (dotted line) and isotropically distributed (dashed with unfilled squares). Panel (b) presents components caused by trapped ions calculated with the HB (solid line) and full slowing down distribution (dashed line with squares). The long dashed line (with circles) indicates the component resulting from RF heating (here, Ttail = 150 keV). Components were calculated with the help of the FPS code [39] as well as other codes [38].

Analysis of fusion experiments 39 8 Results and discussion

Results from NES measurements with the MPR have provided a wealth of plasma information illustrating the capabilities of this diagnostic method. This thesis describes results related to NB heated plasmas as well as certain aspects of RF heated plasmas and the combination of the two heating schemes. The details are presented in the papers, which are appended and part of this thesis. Here, examples of results are given with the aim to point out the main features of the information and the experience gained. In addition, illustrations will also be taken from data not presented in the enclosed papers to emphasize certain points. The NES data are the results of unique measurements of how the fusion reactivity is divided into reactions involving thermal ions only, and those involving supra-thermal contributions. The analysis of the neutron emission from NB heated plasmas requires several spectral components. A TH component is separated from those of ET and HE. The HE components are further subdivided into HEP and HET, where the latter often merges with the ET component in the analysis. A small contribution from HET could be seen apart from the HEP and TH components as seen in paper I at the onset of the NB heating with tritons of 150 keV. A special study in paper III showed that an inclusion of HEP and HET components could yield further information in some cases. The fraction between ion populations in co and counter passing orbits could be estimated as well as the trapped ion population. The analysis of NB heated plasmas with deuterium atoms was included in paper III, with the conclusion that it requires higher statistics than tritium beam heating. The analysis of plasmas heated with a combination of NB injection of D and T beams has been addressed in paper III and is also shown below for discharge #42647. The analysis model used is exploratory and experience has shown that the number of free parameters in the analysis model must be reduced for these discharges, for example by locking the bulk temperature. The study presented in paper II illustrated how to estimate the relative density of the fast ion population in RF heated plasmas. Applying this to the results in paper IV, one can see that the fast ion density is enhanced by a factor of 2 to 3 during combined RF and NB heating for the deuterium heated discharges, and a factor 4 to 5 for the corresponding effect in tritium heated plasmas. This is due to synergy effects from the simultaneous NB and RF heating.

40 Results and discussion One important aspect of the NES measurements is to provide information on the produced fusion power from thermonuclear reactions (PTH) and how much the fusion power is enhanced by the ST component in the fuel ion populations brought in or accelerated by the auxiliary power. A high value of the ratio PST/PF means that the fusion state of the plasma is dominated by the fast ion reactivity and very much perturbed, for example when injected auxiliary power heats the plasma. The ST fraction is difficult to estimate qualitatively, in part due to difficulties separating the TH part of the fuel ion population and the perturbed ST part. The time evolution of the total fusion power can be well tracked by the measured time dependence of the Cn recorded with the MPR. An example is shown in Fig. 8-1a of JET discharge #42647, where the Cn, and hence the fusion power, increases rapidly as an NB power pulse of 11 MW is applied. The NB power is here divided between 4 MW of tritium beam injection and 7 MW of deuterium injection.

5 10 C (a) n P 104 NB [MW] P [Hz]

n 3 NB(D)

C 10 8 P NB(T) 102 4

1.2 0 (b) 1 ST, MPR TRANSP 0.8 0.6 0.4 0.2 Spectral fraction 0 -0.2 12 13 14 15 16 Time [s]

Fig. 8-1. Count rate, Cn, (a, dots) and spectral fraction of supra-thermal components (b, diamonds) from the fit of MPR data obtained from JET discharge #42647, shown together with injected NB power (a, solid and dashed lines) and beam-thermal contribution from TRANSP calculations (b, solid line).

Results and discussion 41 About 0.3 s into the NB pulse (at time t = 12.3 s) the fusion power increases more slowly. There is a pause in the rise at t = 13.6 s followed by a steady exponential increase for a period of 1 s reaching the peak Cn of 120 kHz at t = 14.5 s with an exponential decrease at the NB power switch off at t = 15.3 s. The ST fusion power ratio was derived from MPR measurements (Fig. 8-1b) showing a value of 100 % for the initial 0.2 s of the NB pulse and then decreasing to a minimum of about 26 % at about t = 14.2 s, before the fusion power peaks. The ST fraction has a second maximum of about 68 % during the period when only the tritium beam heated the plasma. Noticeable is that a finite value of the ST fraction persists even after the switch off of the NB power indicating that fast ions remain during their slowing down. The similarity in shape between TRANSP calculations [62] of the ST component (normally referred to as beam-thermal and beam-beam reactions in the TRANSP calculations) and MPR data is striking. It should be noted, however, that the TTH is locked to Ti from CXRS measurements in this analysis. The ST fraction derived from MPR data is dominated by ET, except for t = 12.2 s, where the HEP component contributes 56 % of the total neutron emission. (Note that the ET and HEP components are not shown in Fig. 8-1.) The derived temperatures from NES measurements are available during the entire discharge and only limited to statistical uncertainties. This is of special interest for discharges where few diagnostics are accessible, for example CXRS measurements during RF heated plasmas. There, need for short periods of NB injection result in transient phases that are difficult to model. Results from several JET discharges with NB power blips are presented in Paper IV. It has been shown experimentally that the volume integrated MPR data can be approximated with temperature measurements off-axis as first discussed in Ref. [61]. Derived temperatures from MPR data have been compared with temperature profile data from CXRS measurements for different radial positions in the plasma as shown in paper III. The energy shift of the neutron emission, apart from a kinematical shift, as described in Chapter 4, gives an estimate of the toroidal plasma rotation, vtor. This is under the assumption that all ion populations rotate with the same velocity. At the onset of the NB-pulse it is clear that the beam ions have a different rotation than the bulk plasma. Taking into account the energy shifts for the different ion populations, the plasma rotation of the thermal bulk can be separated from the energy shift introduced by the beam injection as shown in Fig. 8-2 for the first 200 ms of NB injection in a JET discharge. A fit of one TH component gives an energy shift of 160±35 keV (Fig. 8-2a), which is only due to the intrinsic energy shift of the HEP component. This can be seen in a spectral fit of the same data to two HE components (HEP and HET) instead (Fig. 8-2b). This fit is slightly better than

42 Results and discussion the TH fit as measured by the reduced Ȥ2-value (1.2 and 0.9 for the TH and HE fits, respectively).

Data TH fit 10

1 Counts / bin Counts

0.1

Data

10 HE-fit HE P-co HE T 1 Counts / bin Counts

0.1

0 100 200 300 400 500 X [mm]

Fig. 8-2. Proton position histograms (diamonds) collected for 200 ms after the onset of NB heating of JET discharge #42780. Also shown are two fits to data (solid lines). The top panel illustrates the situation with a pure TH component, while the lower panel displays a fit with HE components (dashed lines) from the NB heating.

The derived toroidal rotation from NES data of JET discharge #42647 is shown in Fig. 8-3 for two different fits to data. Data divided in short time bins (50 ms) were here used in a one-component fit while data with longer time bins (0.2 s) made it possible to analyse the spectra with both TH and HEP components. The energy shift of the HEP component was not a free * parameter in this fit (indicated with v tor in Fig. 8-3). The two initial time * bins of vtor (t = 12.1 and 12.3 s) give a slower increase than what was obtained for vtor with the one-component analysis. This is in line with other

Results and discussion 43 JET data (i.e., CXRS and XCS measurements) for the same discharge, here shown for two radial positions (R = 3.0 and R = 3.3 m). The MPR data follow each other closely from t = 12.5 s, and agree with vtor(3.0) data except for the time period t = 14-14.6 s, where MPR data are smoother. This example illustrates the capability of NES measurements as a complement to other data.

800

600

400

[km/s] v* (0.2 s) tor tor

v v (50 ms) 200 tor v (3.0 m) JET tor v (3.3 m) JET tor 0 12 13 14 15 16 Time [s]

Fig. 8-3. Derived toroidal rotation, vtor, for JET discharge #42647 from a one- component analysis of 50 ms long time bins (solid line) and a two-component (TH+HEP) fit using 0.2 s long time bins (circles), where the energy shift of HEP is fixed in the fit. MPR data is compared with JET data from CXRS at two different radial positions R = 3.0 m (dotted line with diamonds) and R = 3.3 m (dashed line with squares). See text for details.

44 Results and discussion 9 Conclusion and outlook

The outcome of any serious research can only be to make two questions grow where only one grew before. Thorstein Veblen, “The place of Science in Modern Civilization and Other Essays.”

The work with this thesis involved the development of analysis tools for the study of neutron emission from mainly NB heated high power DT plasmas. The study was carried out on data obtained with the MPR neutron spectrometer at JET. The analysis included a comparison with data from other diagnostics as well as simulations of the plasma response to auxiliary heating. Results from these studies have given unique information about the plasma conditions observed. Examples of results from neutral beam heated plasmas have been shown, as well as special cases with RF heating added to the plasma. The synergetic effect of using both NB and RF heating in the plasma has been presented from the NES point of view. The time resolution and, hence, statistics have been limited for the DTE1 experiments. This situation should improve in the planned enhanced performance (EP) programme for JET, due to higher auxiliary heating power expected from the installation of new ITER-like RF antennas and an upgraded NB system with higher injection energy of the neutrals. The experience from work on the analysis of DTE1 data will be useful for a new tritium experimental campaign being planned at JET this year (2003), where new high quality measurements with the MPR are envisaged. Absolute neutron yield determination can be performed with the MPR thanks to improved attenuation and sight line integration calculations. This thesis has contributed to the exploration of the NES diagnostic capability. NES is an important tool in high performance fusion experiments such as those carried out at JET during DTE1 and for the planned operation at ITER.

Conclusion and outlook 45 10 Synopsis of attached papers

The four papers that are included in this thesis are listed below. A summary of each paper is also given.

I Neutron emission study of DT plasmas heated with tritium neutral beams The analysis of neutral beam (NB) heated plasmas requires spectral components that describe the slowing down of injected particles deposited in the plasma. This paper presents a study of the neutron emission from an NB heated DT-plasma with the injection of 150 keV tritons. Time resolved data representing the energy distribution of the neutron emission were analysed in two ways, namely, in terms of the statistical moments of the distribution (i.e., intensity, position and width) and in terms of spectral components. The first approach is studied with short time bins (down to 10 ms), while the second method has more limited time resolution, but permits to study the underlying ion states during major perturbations of the plasma, as for example the onset of NB heating in comparison with a steady state condition.

II Neutron emission from JET DT plasmas with RF heating on minority hydrogen A special case of radio frequency (RF) heating was performed at JET in a tritium plasma with minority components of hydrogen and deuterium, i.e., (HD)T. The RF power was tuned to couple to hydrogen (which were admixed for this very purpose) and to a lesser degree to deuterons (fundamental and 2nd harmonic resonance of the RF, respectively). This resulted in an enhancement of the neutron yield recorded by detectors having no energy threshold as compared to the MPR spectrometer studying neutrons in the energy range: En = 11 to 17 MeV. This implied that the endothermic pt-reaction was producing low energy neutrons due to the presence of RF accelerated protons with energies above 1 MeV. The spectrum of the 14- MeV neutron emission was measured with the MPR spectrometer and was analysed in terms of two components due to thermal (TH) and high energy (HE) deuterons interacting with tritons from which were derived

46 Synopsis of attached papers temperatures (TTH = Ti and THE, where Ti is the ion temperature) and densities of the bulk and fast deuterons; Ti was also determined for the reference plasma of Ohmic heating, preceding the RF pulse. Combined with results from other sources, the proton and deuteron populations of this RF powered (HD)T plasma was characterised with respect to its TH and HE components as well as their relative contributions to the neutron yield rate; estimates for the corresponding particle energy densities were also made.

III Systematic spectral features in the neutron emission from NB heated JET DT plasmas This paper presents a systematic analysis of the neutron emission of DT fusion plasmas heated with both deuterium and tritium NB injection. The importance of different spectral components in the neutron spectrum is discussed together with assumptions necessary to carry out a systematic approach to this study. Results of derived plasma parameters such as ion temperature, plasma rotation and the fraction of produced fusion power from different fuel ion populations are presented and compared with other experimental and calculated JET data.

IV Synergetic RF and NB heating effects in JET DT plasmas studied with neutron emission spectroscopy A number of JET discharges with applied RF and NB heating have been studied. The analysis of the neutron emission is divided into four distinct time periods (where applicable), namely, Ohmic, RF, NB and combined RF and NB heating. The auxiliary heating was applied to either tritons or deuterons (same species for both NB and RF), so that the effect of supra- thermal ions on the bulk plasma could be compared for the different applied auxiliary power levels. Results from the analysis of the time periods are presented. The synergetic effect between RF and NB heating is reported and, according to obtained results, the effect is strongest for plasmas with minority heating.

Synopsis of attached papers 47 Acknowledgements

I love deadlines. I love the whooshing noise they make as they go by.

Douglas Adams (in the prologue by Nicolas Wroe of “The Salmon of doubt” by Douglas Adams).

Many people have supported me during my years as a PhD student. First of all I want to thank TTA Technotransfer that gave me a fellowship in 1998 for doing my Diploma work at JET on studies of thermonuclear fusion. An opportunity to continue with fusion research evolved through the Advanced Instrumentation and Measurements (AIM) graduate research education thanks to Jan Källne at INF. AIM financed my PhD studies via the Swedish Foundation for Strategic Research (SSF). Jan has been my supervisor and my deepest gratitude therefore goes to him, for being supportive and enthusiastic from the very moment I first stepped into his room in ‘Gula Villan’. Thank you for the introduction to research in general, to nuclear physics, JET and the possibilities you offered, and thank you for all the time you have spent with me. I have felt very privileged being a member of the fusion neutron group at INF. It has been a great pleasure to work with Göran Ericsson, who always has answers to a puzzled PhD student in the mysteries of MaW, data acquisition and background corrections. I should also like to thank Sean Conroy for proofreading material and for his great humour. He explained the broader perspectives of fusion research at JET together with Giuseppe Gorini whom I also thank for helping me out concerning the data analysis. Thanks also to Erik Traneus for the thorough introduction to JASA and mysterious updates of MaW once in a while. The time as a PhD student wouldn’t have been as fun without fellow colleagues and students. Thank you Marco Tardocchi for all the help with the data analysis and all time we spent together. It has also been great working with Anders Hjalmarsson, with fruitful discussions concerning all parts of the work in the group. Thank you Johan Thun for the support when testing hardware at JET, and the growing interest in folk music… Johan Frenje, also with indirect folk music interest, has been an inspiration to me

48 Acknowledgements and his thesis is a bible. I would also like to thank Luigi Ballabio for all analysis codes. Too bad you ended up writing your thesis outside ‘Little Italy’. It was conquered shortly after you left by Anders and me, with only Marco defending it. Thank you Joakim Klug for all enjoyable occasions and the support you have given me. Thanks Joakim and Somsak for the nice atmosphere we had together in Gula Villan and not to mention in Tecklenburg. My gratitude goes to Matthias (with his twin choir), Henrik (the torus- baker), Luca (our Italian Viking), Pär (the tree-climber), Cecilia (the owlophile), Stephan (once a one-foot-skater), Bel, Klas, Martin, Niklas, Diego, Udomrat, Philippe, Angelica, Fredrik and Simon for the nice atmosphere at the department. Thank you Elsa and David for the great dyng- party! I’m sure Göran and Sean agree with me on that point. Thank you, Joa, Peter, Staffan, Christofer, Osifo and Ingvar for all joyful moments over the years. I feel in debt to mention Nils Olsson, Jan ‘Bumpen’ Blomgren and Jan Wallenius for contributing to my work in various manners. For all practical and administrative doings at the workplace I am grateful we have Susanne Söderberg at INF. You have really helped me to sort things out concerning AIM, travel expenses, and so on. I will also take the opportunity to thank Sven Kullander, Erkki Brändas, Bo Thidé, Eva Forsman and Inger Ericsson involved in AIM. Thank you Gunnar Tibell for proofreading this summary and all helpful final suggestions. There are so many more people I would like to thank, so I hereby thank you all again, also those I might have forgotten... Jag skulle slutligen vilja tacka mina allra närmaste. Först dem som känt mig längst, mina föräldrar Ingrid and Alf, tack för all support och för att ni trott på mig, även om ni kanske inte alltid kunnat förstå er på detaljerna av vad jag egentligen hållit på med (precis som jag själv ibland…). Véronique, ma bien-aimée, tu as toujours été celle sur qui je pouvais compter. Tu m’as donné le courage de continuer ma thèse quand j’en avais besoin. Je suis heureux que tu sois à mes côtés.

Uppsala, May 2003

Hans Henriksson

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