Neutron Spectroscopy Studies of Heating Effects in Fusion Plasmas

Transcription

1 Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 861 Neutron Spectroscopy Studies of Heating Effects in Fusion Plasmas BY HANS HENRIKSSON ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2003

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3 A Véronique Till mina föräldrar Ingrid och Alf

4 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 II III IV 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 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) 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. 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: Plasma physics and controlled fusion: iv

5 Contents Contents... v Preface...vii Glossary Introduction to fusion plasma research Fusion power for mankind Fusion reactor concepts A magnetic bottle The tokamak configuration Plasma heating externally and internally The JET and ITER reactors Plasma diagnostics Neutron experiments The discovery of the neutron Neutron sources and detection Fusion neutrons Information from the neutron emission The MPR neutron spectrometer MPR principles Contents v

6 5.2 The MPR data acquisition system Background suppression and correction MPR development work and operation Absolute energy calibration Operation at JET New developments Analysis of fusion experiments Basic and auxiliary heating Experimental aspects of neutron measurements Analysis model for neutron spectra Results and discussion Conclusion and outlook Synopsis of attached papers Acknowledgements References vi Contents

7 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 ( 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

8 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 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

9 Glossary ADC AIM AKN BES CXRS DAQ DDN DEMO DTE1 DTN ECE ET FPS HE ICF ICRH ITER JET / JET-EP LHCD LMJ MCF MPR / MPRu NB NES NIF NPA RF Analog to digital converter Advanced Instrumentation and Measurements Alpha knock-on neutron Beam emission spectroscopy Charge exchange recombination spectroscopy Data acquisition Neutrons from dd-reactions Demonstration electricity generating fusion power plant Deuterium-tritium experimental campaign at JET Neutrons from dt-reactions Electron cyclotron emission Epithermal Computer code calculating fusion product spectra High energy Inertial confinement fusion Ion cyclotron resonance heating International thermonuclear experimental reactor Joint European Torus / JET enhanced performance Lower hybrid current drive Laser Mégajoule (laser facility for ICF in France) Magnetic confinement fusion Magnetic proton recoil / MPR upgrade Neutral beam (injected for heating of the fusion plasma) Neutron emission spectroscopy National ignition facility (for ICF in the USA) Neutral particle analyser Radio frequency (frequency range for auxiliary heating on ions by means of ICRH) Glossary 1

10 RTN SNAP ST TBN TOFOR TRANSP TTE W7-X XCS ZETA µcf Residual tritium neutrons Spectroscopic neutron analysis program Supra-thermal Triton burn-up neutron Time of flight optimised for rate Transport analysis code for fusion plasmas Trace tritium experiments at JET WENDELSTEIN 7-X (proposed stellarator in Germany) X-ray crystal spectroscopy Zero Energy Toroidal Assembly Muon catalysed fusion 2 Glossary

11 1 Introduction to fusion plasma research ITER, "the way" in Latin (ITER home page: 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, 2 H and 3 H, respectively, releasing energy carried by a neutron and an -particle ( 4 He). The dd-reactions (#1a and #1b) can also contribute to fusion, but these have about a hundred times lower reaction cross section,. 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 t 1/2 = 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

12 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 ratio Reaction Q 1) [MeV] 1a 0.5 d + d 3 He + n b 0.5 d + d t + p c 10-5 d + d 4 He + γ a 1.0 d + t 4 He + n b d + t 5 He + γ t + t 4 He + 2n d + p 3 He + γ t + p 3 He + n a 1.0 d + 6b d + 7a 0.59 t + 7b 0.41 t He + 9 n + 10 n + 3 He 4 He + p He 5 Li + γ He 4 He + p + n He 4 He + d He 4 He + 2p+γ Li 4 He + t Li 4 He + t + n ) 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, increases with energy up to a peak value of 5 barns at about 100 kev. The 4 Introduction to fusion plasma research

13 rate of the fusion reactions is given by the product of and the relative velocity between the reactants, v. Integrating over the distribution functions of the reactants one obtains the reactivity, < v>, 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 10 8 K, or about 10 kev if it is converted into energy units, ev (1 ev = J), by the Boltzmann constant, k = 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 The fusion reactivity, < v>, for reactions between light nuclei as function of plasma temperature. Adapted from [3]. Introduction to fusion plasma research 5

14 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 (t ½ = 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 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

15 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

16 Fig 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

17 1.5 Plasma heating externally and internally When the plasma is hot enough, internal heating from -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, P Ω. This heating scheme is inefficient at higher temperatures because of decreasing resistivity and other methods are needed. Auxiliary heating, P AUX, 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 The produced power from the dt-reaction (per unit volume) is [2] p = n n σ v Q (1) F d t where n d and n t are the densities of the fuel ions and Q is the energy released per reaction (see Table 1-1, #2a). The produced -particles in the fusion reaction between deuterium and tritium carry some of the energy released 2 n mn pα = ndnt σv Eα = σv Q 4 m + m α n (2) which can be used to heat the plasma internally. Here n d = n t, each equal to half the ion (or electron) density (n i = n e = n). This power may be enough for a self-sustained fusion burn if the power losses, p loss, are balanced. The energy stored in the plasma, W, together with the energy confinement time, E, gives [2] p loss W 3 ( ne + nd + nt) T 3nT = (3) τ 2 τ τ E E E (where T is the plasma temperature). Radiation losses from bremsstrahlung, p = cz n n T (4) 2 br eff e Z would at most account for p br MW/m 3 << p for a reference plasma of n e = n Z = n = m -3, T 25 kev, and an effective charge, Z eff, (impurity level) of Z eff =1.5 (Z eff = 1 for a pure DT-plasma); however, for lower temperatures, T < 5 kev, p br dominates over p, but the absolute power level is very low as compared to p loss [2]. The needed total external power per unit volume, varies according to the power balance Introduction to fusion plasma research 9

18 p = p p TOT loss α 2 3nT n pω+ paux = σ v E τ 4 E α (5) The different power contributions are shown as function of temperature in Fig. 1-3 for a reference plasma of n = m -3 and E = 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 The ratio between P F and the external power required Q F P P F = (6) TOT indicates a figure of merit for the experiment (Fig. 1-3). Break-even is reached for Q F = 1 and ignition is reached for Q F. 10 Power (per unit volume) [MW/m 3 ] Q F 0.2 P α 0.15 P loss P AUX T [kev] QF -value Fig Internal (p, circles) and auxiliary (p AUX, diamonds) input power together with output losses (p loss, triangles) as function of temperature (assuming T i = T e = T) for a DT fusion plasma with the reference values, n = m -3, E = 3.5 s, and reactivities taken from Ref. [20]. The Q F -value (crosses) is shown on the right axis. The lines are only guides for the eye. 10 Introduction to fusion plasma research

19 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 JET is a tokamak with a major radius of 3 m and an overall plasma volume of almost 90 m 3. This should be compared to the proposed ITER, with a major radius of 6.2 m, and a plasma volume of about 850 m 3 [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, P NB, 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 P F = 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 Q F = 1, and it was almost reached during DTE1 with Q F = 0.65 during fusion power increase, which gives a calculated Q F 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 P F > 500 MW, a Q F -value of 10 or more is within reach. Introduction to fusion plasma research 11

20 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 Many of these are based on measurements of electromagnetic radiation and particles emitted from the plasma. Examples are measurements of the electron temperature, T e, 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

21 Fig 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

22 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 -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 9 Be(, n) 12 C reaction was achieved, i.e., when an incoming particle colliding with the neutron-rich 9 Be isotope results in a neutron and the stable 12 C isotope. Another way to produce neutrons is to bombard a neutron-rich nucleus with γ-radiation, e.g., 9 Be(γ, n) 8 Be. Neutron production is also obtained in nuclear fission reactions, as for example spontaneously from 252 Cf or 240 Pu and from fission reactors. Also fusion reactions generate 14 Neutron experiments

23 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 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 Ep Encos θ = (7) which for 0 gives E p E n. The proton energy can be measured in different ways including magnet systems, were the proton momentum (nonrelativistically) is determined from the radius, r, of the proton path and velocity, v, according to 2 mv r = qvb (8) 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 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

24 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

25 4 Information from the neutron emission The kinetic energy that the neutron carries, E n, from the dt fusion reaction can be expressed as [34] m 2m m = α n ( ) cosθ ( ) 1 2 α En 2 mv n Q K V Q K CM CM mα + mn mα + mn (9) where m n and m are the masses of the products. The velocity of the centreof-mass, V CM, is calculated from the masses and velocities of the reactants according to m v + mv d d t t V CM = (10) md + mt The Q-value of the dt-reaction is MeV, in the zero kinetic energy limit as calculated from the difference in mass between the reactants and the products 2 Q= ( Mbefore Mafter ) c = ( p n d) ( p 2 n t) ( 2 p 2 n ) α n ( α d t) (11) 2 2 = m + m m + m + m m m + m m m c = m m m c using masses from Refs [35] and [36]. The relative kinetic energy of the reacting particles, K, is K = 1 2 mm d t v m + m d t 2 rel (12) with deuteron and triton masses, m d and m t,, respectively, and their relative velocity, v rel. The angle in the last term of Eq. (9) is defined as the angle between V CM and the neutron velocity vector, v n, 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

26 for reactants with Maxwellian distributions, this last term vanishes and the mean energy is given by [34] m mα E = m V + ( Q+ K ) = E + m V + K m + m m + m 1 2 α 1 2 n 2 n CM 0 2 n CM α n α n (13) where the numerical value of E 0 is calculated to MeV from Eq. (11) together with mα E0 = Q m + m α n (14) with the masses given in [35] and [36]. For isotropically distributed, zero-temperature plasmas, <K> approaches zero, which yields <E n > = E 0. When the plasma ion temperature, T i, increases, the additional terms in Eq. (13) give an energy shift, = + m 1 2 α ES 2 mv n K CM mα + mn (15) of the same magnitude as T i, e.g., E S 20 kev at T i = 5 kev, and E S 36 kev at T i = 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, T d = T t = T i, d d 2 t t 2 exp, exp d t t Ti Ti Ti Ti m m m m fd = v f = v 2π 2π 2π 2π (16) Under the assumption that T i << 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 f n ( E ) n = s 1 exp 2π ( E ) 2 n En 2s 2 (17) 18 Information from the neutron emission

27 This distribution is a Gaussian, with the standard deviation, s, related to the spectral full width at half maximum (FWHM) according to 4mn En Ti FWHM = 2 ln2 s = 2 ln2 177 Ti m + m α n (18) From this, the temperature can be measured directly from the FWHM in a thermal plasma, which constitutes the basis for NES as a T i 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 dd 4mn En Ti FWHM dd = 2 ln2 sdd = 2 ln T (19) i 2m where the mean neutron energy, <E n dd >, is about MeV for ddreactions. Examples of neutron spectra are shown in Fig The neutron spectrum resulting from a thermal plasma of T = 5 kev is shown together with a case where T t = 30 kev and T d =5 kev. If the ion populations are Maxwellian with T d T t, 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 E d = 150 kev interact with tritons of E t = 5 kev, to mimic what happens when neutral beam ions (initial energy, E b, or velocity, v b ) interact in a thermal plasma with ions of thermal velocities of average value, v th, i.e., the situation b i d 2Eb 2Ti v = b vth m >> = (20) m The maximum neutron energy from this reaction is E n max = MeV. This can be compared with the case where the NB ions would be tritons of the same energy, E t = 150 kev, which gives E n max = 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 E min = 13.1 MeV and extends to E max = 15.1 MeV. If Information from the neutron emission 19

28 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 -knock-on neutron (AKN) production from the 3.56-MeV -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. Intensity (a.u.) E =150 kev, T=0 kev t E =150 kev, T=5 kev t T =30 kev,t =5 kev t d T=5 kev T=30 kev E [MeV] n Fig 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 longshort-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

29 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, (CH 2 ) 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 E p = E 0 ± 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

30 tracked to the detector [41], [42]. An assumed neutron emission flux, F n (E), was folded with the response to give a calculated proton histogram, H calc (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 H calc. Fig 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, E/E, of typically better than 5 %; the energy resolution is here 22 The MPR neutron spectrometer

31 defined as FWHM of the proton distribution at the focal plane for monoenergetic neutrons impinging on the target. The efficiency,, defined as the ratio between the proton rate and the incoming neutron flux (expressed in units of cm 2 ), gives the maximum count rate, C n, 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, 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 m 3 (l h w) with circumference dimensions of about m 3, 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 thickness [mg/cm 2 ] Efficiency [10-5 cm 2 ] Solid angle [msr] E/E [%] Magnetic field [T] 1a DTN b DTN a DTN/TBN b DTN DTN DDN DDN 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

32 Fig 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 quasitangential 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

33 threshold to be recorded; if not, the signals are discarded, since lowamplitude 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 lowand 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, with the highest individual count rate, C n ch, should be distributed between different ADC units. This also means that in order to avoid loss of events the low and high energy hodoscope channels with low C n ch, should be separated from channels with high C n ch in the same ADC unit. An example of ADC data is shown in Fig. 5-3 where typical threshold levels are indicated. Counts / 12 bins L S Data, #43013 Total fit Proton signal Background Data, #43011 G Pulse height (ADC bins) Fig 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

34 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, N G and N S, 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 individual hodoscope channel (C n ch = 50 khz for a total C n = 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 Here, a spectral fit is presented used for separation of proton events and background in data from JET discharge # 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 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 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 DTplasmas [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

35 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 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 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 -particles from a 241 Am-source in the target position of the MPR. MPR development work and operation 27