High energy neutral particle analyzer

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Fusion Engineering and Design 34-35 (1997) 107-113 ELSEVIER

Fusion Engineer!ng and Design

High energy neutral particle analyzer A.I. Kislyakov a, A.V. Khudoleev a, S.S. Kozlovskij b, M.P. Petrov ~ a A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia b St. Petersburg Technical University, St. Petersburg 194064, Russia

Abstract

The paper describes a high energy neutral particle analyzer (HENPA) developed in the Ioffe Institute for the diagnostics of alpha particles in the megaelectronvolt energy range, as well as other charged fusion products and ion cyclotron resonance frequency-driven minority ions in a plasma. Megaelectronvolt ions can be neutralized in the plasma by charge exchange reactions with low energy atoms or impurity ions, and by recombination. The density of target particles for neutralization can be increased in the plasma by neutral beam or pellet injection. The operation of the HENPA is based on stripping megaelectronvolt atoms by means of a carbon foil, and subsequent E II B analysis of secondary ions. Eight (or 2 x 8, depending on the version) detectors that consist of (CsI + T1) scintillators 2-25 gm thick optically coupled to photomultiplier tubes are used to detect secondary ions. The detection efficiency of the detectors for megaelectronvolt ions is close to 100%; it drops to 10-4-10 8 for D - T neutrons, and to much lower values for D - D neutrons and gamma quanta in the energy range above 1 MeV. The HENPA is calibrated using megaelectronvolt H°-He ° beams from the cyclotron, starting from 0.4 MeV for He ° and 0.1 MeV for H. The energy ratio of simultaneously detected particles is Ema×/E~n ~ 4. The energy widths of the channels are 6%- 15%. The HENPA can operate in a stray magnetic field up to 0.07 T. Some examples of results obtained by the HENPA are presented. © 1997 Elsevier Science S.A.

1. Introduction

The physics of fusion plasma heating by alpha particles is one of the key issues of the largest plasma experiments, i.e. Joint European Torus (JET), JT-60 and T F T R . Some diagnostic techniques have been proposed to study the behaviour of fast alpha particles with an initial energy of 3.6 MeV born in the fusion D - T reaction [1]. A m o n g these techniques, charge exchange diagnostics that have clear physical bases and straightforward interpretation of measurement results are the most promising. They are

based on the neutralization of alpha particles in the plasma, and subsequent measurement of the energy distribution and intensity of escaping helium atoms with high energy neutral particle analyzers (HENPAs). This technique can provide radially resolved information on the distribution function of slowing down alpha particles confined in the plasma. In addition, the technique can be used for the diagnostics of other charged fusion products in the plasma, i.e. high energy protons and tritons, as well as ion cyclotron resonance heating-driven minority ions in the megaelectron energy range.

0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00668-0

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A.I. Kislyakov et al./Fusion Engineering and Design 34 35 (1997) 107 113

2. Principles of charge exchange diagnostics of alpha particles and megaelectronvolt ions Fast alpha particles (4He + 2) can be neutralized in a plasma by double charge exchange reactions with helium atoms (He °) or He-like impurity ions (MZ-2), i.e. 4He + 2 + He 0 __,4HeO + He + 2

(1)

4He + 2 _[_M z 2 ~ 4HeO + M z

(2)

He atoms can be introduced into the plasma by heating or diagnostic beams. The cross-section of reaction (1) steeply decreases with increasing energy (Fig. 1 [2-4]) and it was originally proposed [5] to use high energy He or Li beams (up to 1 MeV a.m.u.-1). In the case of a small relative velocity of the beam and alpha particles (Vre I < 2.5 x 108 cm s - I = 0.2Vvo, where vv0 is the alpha particle birth velocity), double charge exchange was expected to produce a flux of neutralized alpha particles sufficient for diagnostics, even with a low current probing beam of the order of 10 mA. Further computer simulations have shown [6] that charge exchange in He beams produced by heating injectors with energies of 50-150 keV enables measurements of the alpha particle distribution function f~(E) over a wide energy range. Charge exchange with He-like light impurity ions is less sensitive to the alpha particle energy (Fig. 1). The composition of He-like impurity ions in the plasma reflects the composition of the wall coating materials. For example, for a JET plasma, these ions are mainly C +4 and Be + 2. Their expected densities in the reactor are strongly dependent on the transport model used. Some estimations predict quite high-rate passive fluxes of neutralized alpha particles [7]. To enable diagnostics, the density of He-like ions can be increased by the injection of pellets. In this case, the probability of neutralization is determined by the equilibrium between charge exchange and ionization [9], and does n o t depend on the absolute density of impurities introduced (Fig. 1). Lithium and boron pellets with velocities of 300-700 m s-1 have been used in the T F T R experiments [8,9].

Single-charged hydrogen, deuterium or tritium megaelectronvolt ions can be neutralized by recombination or by single charge exchange on hydrogen, deuterium or helium beams, as well as by charge exchange with H-like impurity ions, i.e. (H, D, T) ÷ + (He, D) ° -* (H, D, T) ° + (He, D) ÷ (3) (H, D, T) + + M z - 1 ~ (U, D, T) ° + g z

(4)

Reactions (1)-(4) produce a flux of He or H, D or T atoms. In the case of charge exchange with beam atoms or plasma impurity ions, the energy spectrum of the flux (dFo/dE) correlates with the energy distribution of alpha particles or ions in the plasma (dG,i/dE) as dF0/dE = (47r) - lntao~(vrel) dn~,,i/dE

(5)

where n t is the density of target atoms or impurity ions for charge exchange, Vrel is the relative target-alpha particle velocity, O-cx(V~d ) is the charge exchange cross-section as a function of Vre], and F o is the flux produced by a plasma layer 1 cm thick. In the case of neutralization in a pellet ablation cloud, the energy distribution of the flux emitted by 1 cm 2 of the cloud is given by dFo/dE = (4~r)-%~,iFo(E) dG,i/dE 10-17

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Fig. 1. Cross-sections for charge exchange of alpha particles with He atoms (experiments: © [2], • [3], • [4]; , theory [4]), and with He-like impurity ions (theory [7]: , Be+2; C + 4) and the probability of neutralization in the Li pellet cloud (. -., theory [8])as functions of the alpha particle energy.

A.L Kislyakov et al. / Fusion Engineering and Design 34-35 (1997) 107-113

The energy distribution of the atomic flux is detected outside the plasma by multichannel neutral particle analyzers (NPA) that have known detection efficiencies for atoms (r/(E)) and dispersion parameters of the analysing system, and dn~,i/dE can be derived from measurements. Parameters of NPAs should be obtained by direct calibration. For local measurements, the source of neutrals should be well localized in the plasma. The best localization takes place when the diagnostic or heating beam crosses the line of sight of the NPA, or in the experiments with pellet injection. The plasma transparency for megaelectronvolt particles, which is quite high (0.3-0.9 for typical plasma conditions), should be taken in account.

3.GEMMA-type NPAs GEMMA-type HENPAs are based on the ionization of atoms by stripping in a thin carbon foil, and on the analysis of secondary ions in magnetic and electric fields (EI] B type), These analyzers are designed for operation in the presence of strong neutron and gamma radiation, with possible penetration of tritium into the instrument. The instrument layout and the design of the particle detectors have been affected by the presence of a background. Fig. 2 shows the instrument layout. The entrance slits (2) of the analyzer collimate the incoming atomic beam, and enable independent variation of both the height and the width of the beam from 1 m m x l mm to 5 m m x l 0 mm. A carbon stripping foil (4) (20-40 nm thick) is located between the poles of the electromagnet (3) in a uniform magnetic field to avoid defocusing by the stray magnetic field at the entrance of the magnet. The foil is supported by a mesh that has a transparency of 95%. The energy losses of helium particles in the foil do not exceed 25 keV in the energy range above 0.5 MeV, and are much less for hydrogen particles. After stripping in the foil, the resulting secondary ions are deflected in the magnetic field of the electromagnet (3) by 90 ° . This angle was

109

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Fig. 2. Layoutof the GEMMA-2 HENPA. chosen to place particle detectors (7) far away from the axis of the neutral beam collimator that looks into the plasma. Having passed the magnet, the ions with different momenta move along parallel trajectories, being dispersed by momentum in the direction perpendicular to the trajectories. For mass separation, the ions are deflected from the middle plane of the analyzer by the electric field produced by an electrostatic deflector (6), and are detected by eight detectors (7) each with an input window of dimensions 10 mm x 22 mm. The detectors are shifted by a distance h = 5 cm from the middle plane of the NPA. One more detector (8) of the same type but with a larger input window, installed on the axis of the NPA entrance collimator behind the magnet, is used for absolute calibration of the NPA. The instrument is equipped with a Hall probe (5) for measuring the magnetic field of the electromagnet, and with a vacuum valve (1). A radioactive alpha particle source can be installed on the collimator axis in front of the entrance slits (2) for express checking of the NPA and data acquisition system. The vacuum chambers of the instrument are made of soft iron with a thickness not less than 10 mm. This permits normal operation of the NPA in stray magnetic fields of up to 0.07 T.

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A.L Kislyakov et aL / Fusion Engineering and Design 34-35 (1997) 107-113

The maximum energies which can be detected by the described analyzer is 4 MeV for alpha particles and 2 MeV for protons. The energy range is restricted by the voltage which can be applied to the deflector (6). To extend the maximum detectable energy for protons up to 4 MeV, the second detector array was installed in the last version of the NPA (GEMMA-3). It is shifted 2 cm from the middle plane of the NPA in the direction opposite to the shifting of the first detector.

4. Secondary ion detectors The detectors (7) of secondary ions were specially developed for the H E N P A to have a high efficiency for megaelectronvolt alpha particles and hydrogen ions, a low sensitivity to background and a high radiation stability. In fact, each detector (Fig. 3) is a spectrometrical scintillation counter, which in this case consists of a very thin scintillator layer (1)evaporated on to a glass window (2); a glass filter (3); and a photomultiplier tube (4). CsI(T1) is used as a scintillator. The thickness of the scintillator layer is defined by the ion range in the sensitive layer and varies from 2

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Fig. 3. Schematic diagram of the HENPA detector. In the HENPA, ions strike the scintillator at an angle of 20° to its surface.

to 25 gm, depending on the energy and type of particles to be accepted by the channel. Before evaporating the scintillator, a layer (5) 100 pin thick of CsI without T1 was deposited on the surface of the glass window in some version of the detectors. This layer absorbs high energy protons and alpha particles which are produced in the glass by (n, p) and (n, c~) reactions with thresholds above 2 - 3 MeV. The detectors with an absorption layer are expedient in experiments with D - T plasmas. The scintillator is covered with an A1 layer (6) 0.05 lam thick for light reflection. A glass filter (3) serves to scatter light from the scintillator to minimize the zone effect caused by the inhomogeneous structure of the photocathode. The phototube is surrounded by three layers (7) of magnetic shielding to avoid the effect of stray magnetic fields. To reduce further the neutron sensitivity, the inner surfaces of the NPA detector chamber were covered with a Ta foil that had a small cross-section for (n, p) and (n, e) reactions. The detection efficiency of the detectors for alpha particles and hydrogen ions in the megaelectronvolt energy range is very close to 100%. The ions produce a Gaussian amplitude spectrum at the output of the photomultiplier tube with a mean amplitude proportional to the particle energy. The energy resolution of the detectors (AEd/E, where AEd is the full width at half-maximum (FWHM) of the spectrum) depends on the particle energy. For 1 MeV alpha particles, the resolution is equal to 0.11, improving to 0.07 for an energy of 4 MeV and falling to 0.25 for 0.150 MeV. The sensitivity of the detectors to neutrons and gamma quanta was measured using a pu238-F Be neutron source, and later in D - D plasma experiments with JET and JT-60 [10]. The P u - B e neutron source produces fast neutrons in the energy range up to 10 MeV and 4.4 MeV gamma-rays. The neutron efficiency, defined as the number of counts per neutron above a certain energy threshold, is presented in Fig. 4. The energy scale is calibrated against the energy of the alpha particles, so 1 MeV of the abscissa corresponds to the amplitude of the output detector signal from 1 MeV alpha particles. For protons, the scale should be divided by about 2, because protons

A.L Kislyakov et al./Fusion Engineering and Design 34-35 (1997) 107 113

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Fig. 4. Absolute neutron efficiency of the HENPA detector: - - - , Pu + Be radiative source; 0 , JT-60 without absorption layer of CsI; e, JET with absorption layer and Ta foil surrounding.

have a twice higher photo-particle yield. Below 0.25 MeV, background signals are produced by Compton electrons generated by gamma quanta both in the scintillator and in the phototube. Above 0.25 MeV, signals mainly result from heavy particles created by (n, p) and (n, c~) reactions in the scintillator and surrounding materials. It will be seen that the detection efficiency for neutrons is in the range 10-4-10 -8 , depending on their energy. In experiments with D - T plasmas, some background signals can be initiated by tritium coming from the reactor. The amplitude of the pulses produced by /?-decay electrons with a mean energy of 10 keV is two orders of magnitude smaller than the amplitude of pulses produced by 1 MeV alpha particles; hence, they can be eliminated by amplitude analysis.

5. H E N P A calibration

Each HENPA of the GEMMA type is calibrated using He + and H + beams produced in the energy range 0.3-4 MeV by a cyclotron. The calibration unit that produces beams of He or H atoms consists of a collimation system, a magnetic

111

separator, a neutralizing cell filled with gas, and a cleaning magnet which removes non-neutralized ions from the beam and sends them to an ion detector to monitor the beam intensity. The intensity of the He or H beam entering the NPA is measured by a detector (8) (Fig. 2). When the NPA magnet (3) is switched off, the counting rate R , measured by the detector (8) is proportional to the full intensity Io of the incoming beam, i.e. R , =IoT, with T being the geometric transparency of the mesh that supports the foil and I o being measured in particles per second. The efficiency of the detector (8) is very close to unity both for neutrals (He or H) and for ions (He +, He + 2 or H +) produced by stripping in the foil (4). When the NPA magnet (3) and electrostatic deflector (6) are switched on, secondary ions produced by stripping can be detected by one of the detectors (7), and the NPA detection efficiency is defined as rl(E) = R,/Io = Ri T / R ,

where R i is the counting rate in the ith channel optimally tuned to the beam energy. It was found that the detection efficiency ~/(E) of the HENPA is insensitive to the horizontal size of the incoming beam, instead being affected by its vertical size. As a result, r/(E) was measured for two sets of input slits, i.e. 1 mm x 10 mm and 5 mm x 10 mm (height x width), for He and H atoms. The energy dependence of ~/(E) averaged over measurements for analyzers which have been manufactured up to now is shown in Fig. 5. Owing to scattering in the stripping foil and the defocusing effect of the edge magnetic field, the measured efficiencies are smaller than would be expected from the equilibrium fractions in the carbon foil and from the geometric transparency of the supporting mesh. Parameters of an N P A dispersion system measured with the same calibration equipment are presented in Table 1. The inductance of the magnetic field in the gap of the magnet (3) needed for deflection of 4 MeV particles in the eighth NPA channel is equal to 1.1 T and the voltage applied to each plate of the electrostatic deflector is _ 20 kV.

112

A.L Kislyakov et al./Fusion Engineering and Design 34-35 (1997) 107 113

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6. Results of plasma experiments HENPAs of the GEMMA type have been used in several plasma experiments [9-12]. Experiments on JET and JT-60 mainly deal with measurements of hydrogen fluxes produced by ion cyclotron resonance frequency (ICRF) minority heated discharges. On JET [11], the analyzer was installed on th e top of the machine and its vertical line of sight crossed the heating beams. When one of the heating injector modules was switched to Table 1 Parameters of G E M M A - 3 H E N P A dispersion system with input slits 1 mm x 10 mm (height x width) ' E . / E a A G / G (%)

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Fig. 5. Detection efficiency ~/(E) of He (A) and H (e) atoms by the H E N P A as a function of the energy. Slits are 1 mm x 10 mm.

Channel

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Hydrogen detector array

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14.1 12.1 12.3 8.5 9.3 7.6 6.2 6.0

Fig. 6. Energy spectra of H and He fluxes measured by the G E M M A - 2 analyzer during the ICRF minority heating phase in JET. Shot 27368, D plasma; - - , t = 6.4 s; - - - , t = 6.6 s; ' • ", shot 27385, 4He plasma.

He, its beam could be used for diagnostic purposes. During the ICRF heating phase, both passive (before the beam injection) and active (during the beam injection) high energy hydrogen fluxes have been detected, and their energy spectra have been measured (see Fig. 6). This was the first observation of a megaelectronvolt energy neutral hydrogen flux from fusion plasmas, confirming the importance of charge exchange with H-like impurity ions for the production of a megaelectronvolt hydrogen flux (reaction (4)). The increase in the flux during beam injection was explained by an increase in the density of H-like impurity ions that resulted from changes in the ionization balance that occurred when the neutral beam was injected. Measurements of the passive and active megaelectronvolt energy hydrogen fluxes provide a good diagnostic tool for investigating ICRF plasma heating. A flux of 3He atoms of megaelectronvolt energy was detected from a 3He minority ICRF-heated 4He plasma only during 4He° diagnostic beam injection, and it was much less intense than the hydrogen flux (see Fig. 6). It is produced by charge exchange between 3He + 2 plasma ions and probing beam atoms (4He°) (reaction (1)), and confirms the possibility of using active charge exchange for fusion alpha particle diagnostics.

A.L Kislyakov et al./Fusion Engineering and Design 34-35 (1997) 107 113

crashes transport alpha particles outwards, which may lead to enhanced alpha particle losses.

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Fig. 7. 1.21 MeV alpha particle density profiles measured in TFTR by pellet charge exchange diagnostics before (e) and after ( i ) a sawtooth crash. The full line is given by theory for a sawtooth-free case.

In recent D - T experiments on TFTR [8,9,12], a GEMMA-2 analyzer was used to detect alpha particles neutralized in the pellet ablation cloud. Lithium and boron pellets were injected into the plasma almost along the line of sight of the H E N P A in the direction of the major radius. Signals obtained by the H E N P A are very short (of the order of 0.2-0.3 ms), because of the short life of the pellet cloud, and detectors operate in the analog mode. The pellet ablation cloud is quite narrow in the direction along the observation line, and it moves inside the plasma together with the pellet. As a result, time-resolved measurements of the He flux provide radially resolved information on the alpha particle distribution function. Fig. 7 shows two radial density profiles of 1.21 MeV alpha particles measured in two sequential shorts before and after a sawtooth crash, together with a computer simulation of the sawtooth-free case. The simulation that takes into account the stochastic diffusion of alpha particles in the ripples of the magnetic field is based on a numerical solution of the drift-averaged Fokker-Planck equation. It can be clearly seen that sawtooth

An HENPA for neutral particle diagnostics of slowing down alpha particles and ICRF-driven minority particles has been developed. It can operate in the environment conditions of the most modern tokamak and can be used as a prototype for appropriate instrumentation for ITER. The experimental results obtained for the largest tokamaks confirm the productivity of neutral particle diagnostics in the megaelectronvolts energy range.

References [1] D.E. Post et al., Basic and Advanced Diagnostic Techniques for Fusion Plasmas, Proc. Course and Workshop of Int. School of Plasma Physics 'Piero Caldirola', Varenna, 1986, vol. III, p. 721. [2] C.F. Barnett (ed.), Atomic Data for Fusion, vol. 1, Oak Ridge, TN, 1990. [3] N.V. de Castro Faria et al., Phys. Rev. A 31 (1988) 280. [4] V.V. Afrosimov et al., Zh. Eksp. Teor. Fiz. 104 (1993) 3227; JETP 77 (1993) 554. [5] D.E. Post et al., J. Fusion Energy 1 (I981) 129. [6] M.P. Petrov et al., Rep. A.F. Ioffe Physical Technical Institute, Leningrad; 1990. A.B. Izvozchikov et al., JET Rep. JET-R(91) 12, 1991. [7] A.A. Korotkov and A.M. Ermolaev, Proc. 22nd EPS Conf. on Controlled Fusion and Plasma Physics, Bournemouth, 1995, Vol. 19c, part III, p. 389. [8] S.S. Medley et al., Proc. 22nd EPS Conf. on Controlled Fusion and Plasma Physics, Bournemouth, 1995, Vol. 19c, part I, p. 409. [9] R.K. Fisher et al., Phys. Rev. Lett. 75 (1995) 846. [10] V.I. Afanassiev et al., Proc. 22nd EPS Conf. on Controlled Fusion and Plasma Physics, Bournemouth, 1995, Vol. 19c, part II, p. 57. [11] M.P. Petrov et al., Proc. 19th EPS Conf. on Contr. Fus. and Plasma Phys., Innsbruck, 1992, vol. 16C, part II, p. 1034. [12] M.P. Petrov et al., Nucl. Fusion 35 (1995) 1437.