Microbeam analysis of deuterium in carbon probes exposed to plasma in the DITE tokamak

Nuclear Instruments and Methods 191 (1981) 383-390 North-Holland Publishing Company

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MICROBEAM ANALYSIS OF DEUTERIUM IN CARBON PROBES EXPOSED TO PLASMA IN THE DITE TOKAMAK C.J SOFIELD, G.M McCRACKEN *, L.B. BRIDWELL **, J SHEA ***, E S HOTSTON * and S K. ERENTS * UKAEA, Harwell, Dtdcot, Oxon, 0 X l l ORA, UK

Carbon discs have been used as collectors for plasma and impurity ions in the boundary layer of the DITE tokamak They were exposed either while stationary behind a circular colhmatmg aperture or else while being rotated behind an aperture during the plasma discharge The ions deposited on the carbon surface were analysed using d(aHe, p)4He nuclear reaction analyses for the deutermm and at the same time by measurement of characteristic X-rays from plasma impurity ions In the first mode of operation from measurement of the spatial distribution of the collected species on the stanonary targets the radius of gyration of the ions in the magnetic field can be obtained From this radius the ion energy and/or charge state can be deduced The second mode of operation provides time resolved data along the track laid down m the direction of rotation and spatial distrlbuUon at right angles to that direction The reqmred spatial resolution to obtain the distributions are provided by use of the Harwell mlcrobeam system to produce and focus the 3He ions

1. Introduction Plasma impinging upon and interacting with the hmlters and walls of tokamaks produces serious consequences for the operation of these plasma confinement devices For example escaped plasma particles may damage the walls and give rise to the injection of metal impurities into the main body of the hot plasma As the presence of such metal Impurities gives rise to a major source of energy-loss, an understanding of the ways in which lmpurmes are produced is of concern for the understanding of the operation of current tokamak devices. A review of thas important area of plasma surface interactions in Tokamaks has recently been given by McCracken and Stott [1]. The flux and energy of the plasma particles and impurities in the regions near and behind the hrmters are essential parameters in making any estimates of the consequences of plasma wall interactions. Recent developments m the use of carbon probes [ 2 - 7 ] to trap incident plasma and Impurity ions

* UKAEA Culham Laboratory, Abmgdon, OX14 3DB, UK (Euratom/UKAEA Fusion Association) ** Permanent address Murray State University, Murray, Kentucky, USA *** Now at CEGB Berkeley Nuclear Laboratories, Berkeley, Glos GL13 9PB, UK 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland

offer the opportunity to deterrmne the flux and energies or charge state of such ions A carbon probe consisting of a disc of vitreous carbon, mounted on an axle and placed behind a statable collimator, can be used to obtain time resolved data by rotating the disc through, say, 120 ° during the 2 0 0 - 3 0 0 ms of a plasma discharge. Ttus procedure produces a concentric track of plasma ions and metal ions deposited on the carbon disc. The trapped Ions are very near the surface of the carbon as their energies are typically of the order of 2 0 - 2 0 0 eV [1]. The density of plasma ions laid down on the disc is 1016/cm 2 during a smgle discharge Another aspect of the plasma may be studied by exposing a carbon disc to the plasma through various small circular apertures. The distnbution of trapped ions behind an aperture is influenced by the relative diameter of the aperture compared to the gyration (Larmor) radius of the ions about magnetic field lines Such distributions can be modelled m terms of the temperature and charge state of the ions so that comparison with observed distributions provides a means of obtaining the model parameters, as has been recently shown by Staudenmaier et al [8,9]. The trapping of hydrogen or deuterium also shows the interesting property of saturation at densities (atom/cm 2) that depend on the energy of the incident ion [6,7]. Measuring the density of trapped H or D as a function of the number of plasma shots to VIII NUCLEAR REACTION ANALYSIS

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C J Sofield et al / Mlcrobeam analysts o] deuterium

which the carbon disc is exposed provides information on the average incident ion energy The details of the physics underlying the use of such carbon probes in trapping and samphng plasma and impurity ions as well as the experamental details of their construction and use is in its own right a topic of considerable interest [ 2 - 7 ] However, here we wish to address the analytical problems involved in the extraction of the information stored on the carbon discs The samples we have exarmned were all exposed to plasma In the DITE tokamak at the Culham Laboratory The low energy plasma ions, hydrogen or deuterium, are trapped very near to the carbon surface so that exposure of the discs outside the Tokamak can cause serious contamination problems In the case of H discharges, problems Much are virtually absent for D discharges Thus, for the present, we confine our attention to discharges in deuterium 2 tzxpertrnental m e t h o d The requirements of the analysis problem posed by the DITE tokamak carbon probe samples are very stringent Firstly we require sufficiently lugh sensitivity to detect 1013 to 1016 D atoms/cm 2 with a spatial resolution across the sample of about 0 1 mm Secondly in the case of the trapped metal impurities we need to detect 1012 atoms/cm 2 or more, again with a spatial resolution of about 0 1 mm Furthermore In some of the work it is essential that the concentratton of trapped atoms (number/cm 2) be measured absolutely There are several techniques for the analysis of deuterium each with its own particular merits, for example secondary ion mass spectrometry [71, thermal desorptlon or laser ablation followed by mass spectrometry [5], and nuclear reaction techniques [11] Of these the nuclear reaction analysis using for example the d(3He, p)4He reaction offers the most reliable absolute quantitative analysis Indeed for tbas reason and for the added advantage of depth profiling, this reaction has been used already to study tokamak wall samples [12,13] If a microprobe lens system and associated equipment are available the spatial resolution requirements of this analysis problem are readily met without loss of sensitivity At Harwell we have available to us a microbeam system [14] on the 3 MV 1BIS Van de Graaff accelerator Thus we have the capability of a powerful combination of d(3He, p)He nuclear reaction ana-

lysIs and mlcrobeam techniques with wluch to solve the deutermm analysis problem The analysis of the metal impurities is a somewhat different problem, e g the sensitivity requirements are more stringent than in the D case Sensitive absolute measurements of the metal impurities on these carbon discs has been achieved in the past [15] with Rutherford backscatterlng using 4 MeV 14N Ions However the requirement for high spatial resolution could not be easily met on the 6 MV accelerator used to produce the 4 MeV 14N ions For tins reason a compromise solution to the problem has been adopted using the microbeam lens system A consequence of the 3He bombardment of the carbon discs during the deuterium analysis is that characteristic X-rays are also produced from the trapped metal impurities Although the sensitivity of this beam induced X-ray analysis is not sufficient to see heavy metals of nuclear charge as hagh as Pb, It IS adequate to detect the major metal impurities of interest in the DITE tokamak, namely T1, Fe and Cr Thas approach has the distinct time and cost advantage that it is done at the same time as the deuterium analysis 2 1 D e u t e r m m analysts A schematic diagram of the experimental arrangement is shown in fig 1 The vitreous carbon discs are mounted concentrically on a circular metal specimen holder wbach also has a quartz beam viewer and various standard samples mounted on It The specimen holder with its carbon disc is mounted on the shaft of a stepping motor, which can rotate the disc through successive steps of 0 1 ° This rotation drive motor IS mounted on three successive slides provlding motion in the following directions, horllontal (x) and vertical (.V) perpendicular to the beam, and horizontal parallel to the beam (z) The slides in the x and v directions are driven by stepping motors in steps of ~ 0 3 ~tn This drive assembly IS mounted in the vacuum chamber on the mlcrobeam line of the 3 MV IBIS accelerator The drives to the specimen mounting disc allow us to follow concentric tracks by rotating it, and to scan in the x or y directions across any distributions of interest The mlcrobeam lenses are used to focus a beam of 980 keV 3He ions onto the quartz viewer where its size is set to be typically 20/lna × 20 I~m for our purpose The 3 He energy, 980 keV, is chosen to gave a reasonable compromise In obtaining the largest d(3He, p)4He reaction cross section while also

385

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obtaining a stable well focused mxcrobeam from the 3 MV accelerator Early tests on carbon probe samples indicated that beam induced loss o f trapped D ions occurred when the beam spot was as small as 20 /~m × 2 0 /lm and the beam current was ~ 5 0 nA Because currents as large as this were required, the beam was swept by electrostatic scanners over a 0 1 m m by 0 1 mm area at 500 Hz In the vertical and 50 Hz in the horizontal direction This procedure reduced the level of beam induced D loss sufficiently to give reproducible results Having no requirement for depth profiling of the deuterium in the carbon discs as it is trapped very close to the surface ( ~ 2 0 0 A), we have taken advantage of the low background offered by detecting the ~ 1 5 MeV protons produced by the d(3He, p)4He reaction The protons were detected with an annular surface barrier detector placed on the beam axas at the entrance of the vacuum chamber and about 8 cm from the target, thus giving a sohd angle of about 0 56 sr The active area of the detector was covered with a 50 /am Ta plate which stopped all the backscattered 3He ions while slowing the protons enough to stop them in the 60 /am depletion layer o f the detector The signals from the annular detector were amplified and passed through a lower level discriminator

and pulse shaper before being counted in a scaler The broad spectrum of the slowed down protons, and a stable pulser injected at the pre-amp were examined periodically with a multlchannel analyser to ensure gain stablhty and thus constant detection efficiency Ttus arrangement provided a virtually background free detection system for the high energy protons which thus gwes the optimum sensitivity for deuterium detection The relation between the measured proton yield (V), and the deutermm concentration (/9 atoms/cm 2) is gwen by the thin target yield expression y = NDed~Z where o is the differential reaction cross section which IS known to ~+5% at 980 keV 3 He energy [16,17], d~2 is the sohd angle at the detector, a n d N is the number of incident 3He ions The number of 3 He ions incident upon the target IS obtained by measurement o f the beam current using the whole vacuum chamber as a Faraday cup, a procedure known to give reproducible results for tins apparatus [141 Absolute beam current measurements and measurements of the detector sohd angle are difficult to make with high precision wath this target chamber arrangement Thus it was decided to calibrate the detection system by use o f standard deuterium target The standard was produced by implanting 10 Is, 40 keV D ÷ VIII NUCLEAR REACTION ANALYSIS

C J Sofield et al / Mwrobeam analysts of deutertum

386

tor A Be window was placed in front of the SI(L1) detector to stop backscattered 3He ions entering The 15 MeV protons that enter the detector show no signs of interfering with the X-ray signals and their dose rate (~ 104/cm 2/day) is sufficiently low to avoid radiation damage problems m the detector. The signals from the detector were fed to a Link System 2010 s:gnal processor, and pulse height analysed with a Nuclear Enterprise Mecam II data acqmsltlon system using a PDP 11/34 computer Spectra were collected and stored on floppy disc for subsequent detailed analysis The energy scale of the X-ray spectra was calibrated using a SSFe source as an mltlal guide This cahbratlon served to easily identify the prominent peaks in the metal impurity spectrum winch were then used to fine tune the cahbration The heaxaer metal impurities TI, Cr, Fe that were detected were just those expected from previous observations of heavy ion Rutherford backscattermg [15] However, heavaer metals seen by RBS were not detected in our X-ray spectra due to insufficient energy for generating their K X-rays with 980 keV 3 He Lighter elements such as $1 were readily identified using the X-ray

ions into a vitreous carbon disc. It turned out that the yield of protons measured for ttus standard and that obtamed from the beam current measurement by integrating the yield equation over the depth distribution of the deutermm agreed to watinn 5% The deutermm chstrxbutlon was calculated using the program EDEP [18]. The standard sample (10 ts atoms/cm 2) gave a proton yield of "~100 counts for a beam charge of ~1 0 IJC winch took about 2 mln to dehver to the target Clearly tins sensitlwty is adequate to detect 1013 D/cm 2 using ~1 h irradiation time However, perhaps more importantly, tins yield lS sufficmnt to allow the large number of measurements required to cover several 120 ° tracks on an exposed carbon disc to be made In a period of about 3h

2 2 X-ray analysts Characteristic X-rays generated by 980 keV 3He ions interacting w:th various metal lmpuratles are detected by a Ingh resolution SI(L1) detector placed at 45 ° to the target and m our case some 8 cm away so as not to shadow the annular surface barrier detec-

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C J Softeld et al / Mtcrobeam analysts o f deuterium

spectra and helped to confirm observations of these elements made with the RBS analysis A typical spectrum is shown in fig. 2. This spectrum t o o k about 10 min to obtain with 40 n A of aHe, a somewhat longer time than required to carry out a deuterium measurement with the same beam current However there is sufficient signal from the trapped T1 to obtain significant data in the 1 to 2 nun taken to obtain a deuterium analysis. The T1 analysis is made much easier by the virtual absence o f background at the T1 X-ray energy The low backgrounds observed are partly due to the use o f 980 keV 3 He ions and partly due to using a high purity low Z (carbon) substrate The calculation of absolute values o f metal Impurity content from these X-ray spectra may be straightforward as the metals are very near the surface of the carbon discs and thus X-ray absorption effects are negligible However rehable X-ray excitation crosssections would be required We have chosen to calibrate our X-ray data by comparison with RBS Even the relative concentrations o f the metal impurities as a function of position are however proving to be very valuable data The dual mode o f operation of the mlcrobeam to obtain deuterium and metal impurity analysis has not yet been fully optlnused For example, the geometrical arrangement o f both detectors relative to each other and the target, especially the SI(L1) detector to target dtstance, has not yet been chosen to best advantage 2 3 Data acqutsttton

The information content of the carbon discs exposed in the DITE Tokamak is very large One requires o f the order o f 300 deuterium and heavy metal analyses to be performed on each disc Using a beam current restricted by target damage conslderations to ~ 5 0 n A and accepting 10% counting statistics with l 0 is D atoms/cm 2 it takes at best ~ 1 0 h to analyse one disc Clearly automation o f the equipment for moving the disc between measurements, recording the data and carrying out preliminary data analysis is required to obtain the shortest measurement time When deuterium was the only element of prime interest we used an arrangement of hard wired timers, scalers, and a stepping motor drive unit to actueve automatic recording o f the proton yield for a fixed beam charge, followed by repositlomng of the target An automatic repetition of ttus cycle for a desired number of positions com-

pleted the analysis However, ttus system is inadequate for simultaneous recording and analysis of the X-ray spectrum. A fully computerlsed system is thus being implemented to drive the stepping motor, collect the X-ray and proton spectra and perform integrations over desired regions o f the spectra (e.g. the Ti X-ray peak)

3 Results Let us recall that there are two modes of operatlon of the carbon disc probes, one to obtain time resolved data and one to obtain spatial distribution data We shaU examine examples of each h n d o f data in turn. As an indication of the value o f such analyses for plasma diagnostic purposes we shall give outlines o f the Interpretation of the D ÷ ion distribution data to obtain the D ÷ ion temperature, and the analysis of the T1 metal impurity distribution data to estimate the T1 charge state A more detailed description is to be pubhshed later. 3 1 Ttrne resolved data

Data obtained by exposure of a carbon probe m the DITE tokamak are shown in fig 3. The carbon probe was exposed through a 2 mm aperture at a minor radius o f 235 mm behind the separatrlx Six 60 k A diverted discharges were collected on the probe The tugh flux at the start o f the discharge IS correlated with the high impurity flux observed at the same time and with MHD activity and arcmg [19]

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The spatial wadth corresponding to this lnitml peak is set by the width of the aperture in front of the disc and by some spread due to the finite karmor radii of incoming D ÷ ions Thus this feature may be the result of a very fast event It is common to most time resolved data so far examined at the start of a discharge The increase in flux as a function of time during the rest of the discharge is proportional to the increase In the mean electron density This second broad peak is the result of feeding the Tokamak discharge with D2 by gas "puffing" during the discharge Data obtained for 9 exposures to 150 kA discharges during which 1 MW of H ° neutral injection took place are shown in fig 4 This disc was also exposed to several discharges while stationary at the position corresponding to t = - 2 0 ms The probe aperture was set at a minor radius of 255 mm The rise in density during the period of neutral injection may partly arise from the injection but may also be Influenced by gas puffing as was the case for the data of fig 3 3 2 Spatml distributions

Fig 5 shows data for the distribution of deuterium on a carbon disc probe exposed to the plasma behind a 1 mm hole in another carbon disc acting as a colh-

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mating plate The front face of the colhmating carbon disc traps the incident flux while the second disc traps flux transmitted through the hole The back of the collimating disc traps the deuterium backscattered off the second disc The probe consisting of these two discs had holes of 1 ram, 2 mm and 4 mm in the first disc and was placed at a minor radius of 265 nmi during exposure m the tokamak Monte Carlo modelhng of trajectories of ions gyrating around magnetic field hnes using the experimental geometry has been t a m e d out A Maxwelhan distribution of energies and random direction of pamcles was assumed The distributions expected on the collector were calculated for a variety of 1on temperatures and charge states These show how the ratio of transmitted to incident mn flux depends on the 1on l_armor radius and on the diameter of the colhmating aperture In the case of deuterium ions the charge state can only be unity and thus the energy can be obtained from the data of fig 5 In this case the D + energy was found to be (60 -+ 12) eV The value of the ion temperature chosen to fit the data is based on the transmission, 1 e the flux measured at the centre of the hole compared with that on the front disc with the colhmating hole The shapes of the experimental and theoretical distribution fit satisfactorily only in the centre, the experimental wings being much greater than the calculated values The most

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Radla( Dlstar~ce (rnrn) I lg 5 Comparison ol experimental distribution of deuterium deposited on a collector below a 2 mm bole with a Monte Carlo calculation for 60 eV D+ ions The calculation was nornIahsed so that the 100% level corresponded to the concentrauon on the surtace with the dehning bole The exposure was at a tokamak minor radius of 265 mm tor 1 discharge x~uh a plasma current ot 130 kA o experimental poinls on disc behind aperture A, experimental points on lront ol disc containing aperture ×, experimental points on back ol disc containing aperture

o b v m u s e x p l a n a t m n is t h a t t h e r e is b a c k s c a t t e r l n g f r o m t h e c o l l e c t o r plate F o r t h e average i o n e n e r g y a n d angle o f i n c i d e n c e a r e f l e c t i o n c o e f f i c i e n t o f a b o u t 0 4 is e x p e c t e d f r o m t h e data o f E c k s t e l n a n d V e r b e e k [20] M e a s u r e m e n t s o f t h e d e u t e r i u m in t h e b a c k o f the c o l h m a t l n g plate, 1 e t h e surface facing t h e c o l l e c t o r plate, s h o w e d t h a t t h e r e was a p p a r e n t l y as m u c h d e u t e r m m in t h a t as m t h e coll e c t o r plate itself F u t u r e c a l c u l a t i o n s wall take i n t o a c c o u n t t h e b a c k s c a t t e r m g effects V a l u e s o f t h e i o n t e m p e r a t u r e o b t a i n e d f r o m d i f f e r e n t holes varied f r o m 6 0 to 100 eV, t h e larger holes giving tugher t e m p e r a t u r e s This again IS c o n s i s t e n t w i t h b a c k scattering The d i s t r i b u t i o n o f TI m e t a l collected b e l u n d t h e 4 m m a p e r t u r e was o b t a i n e d f r o m t h e same disc a n d is s h o w n In fig 6 The t i t a n i u m b a c k s c a t t e r i n g was

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t lg 6, Comparison of experimental distribution of titanium deposited on a collector below a 4 mm hole with a Monte Carlo calculation for 80 eV T1 ions in charge state 4 The calculation was normahsed so that the 100% level corresponded to the measured concentration on the surface with the defining hole The exposure was at a tokamak nunor radius of 265 mm for 10 discharges with a plasma current ol 130 kA o, experimental points on disc behind aperture X, experimental points on disc containing aperture

n o t d e t e c t a b l e a n d e s t i m a t e d t o b e < 1 % o f t h e mcl' d e n t flux If we assume t h a t t h e d e u t e r m m p l a s m a a n d t h e i m p u r i t y ions are in t h e r m a l e q u i l i b r i u m t h e n we can take the t i t a n i u m 1on e n e r g y to be ~ 8 0 eV (the mean of the D + temperatures obtained) T a k i n g tbas t e m p e r a t u r e we can fit t h e t h e o r e t i c a l curves t o t h e e x p e r i m e n t a l data, b a s e d o n t h e transmission as b e f o r e , a n d h e n c e o b t a i n t h e i o n charge state A charge o f 4 was d e d u c e d It is seen t h a t in this case t h e r e is m u c h b e t t e r a g r e e m e n t t h a n for t h e deuterium, consistent with the observation of no b a c k s c a t t e r i n g It also agrees w i t h d a t a f r o m T F R

[81 4 Conclusion T h e use o f p r o b e s c o n t a i n i n g c a r b o n discs t o t r a p plasma a n d i m p u r i t y ions requires a q u a n t i t a t i v e analysis o f t h e t r a p p e d species t o realize t h e p o t e n t i a l o f these p r o b e t e c h n i q u e s m p l a s m a diagnosis The r e q u i r e m e n t s o f thas analysis p r o b l e m are a c c u r a t e VIII NUCLEAR REACTION ANALYSIS

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absolute quantitative determination with high sensitivity and high spatial resolutmn The use o f the d( s He, p)4 He reaction at 980 k e V s He m m d e n t energy has provided adequate accuracy and sensmvlty for the absolute d e t e r m i n a t i o n o f the trapped d e u t e r i u m ion flux The SHe ions used in the deuterium analysis also generate c h a r a c t e n s n c X-ray spectra which provide an adequate analysis o f the metal Impurities such as Tl, Fe and Cr The c o m b i n a t i o n o f nuclear reaction analysis, a sophisticated m l c r o b e a m system, and carbon disc plasma samphng probes has provided a powerful new means o f deterrmnang the plasma parameters o f m n energy and charge state for runs in the region near and behind the fixed hmlters o f a T o k a m a k We wish to thank Dr J A Cookson for m a n y helpful discussions on the Harwell m l c r o b e a m system We are grateful to Mr J W Partridge and Mr J Vlnce for the exposure o f the samples in the DITE t o k a m a k , and to the IBIS accelerator crew for their support The help o f Mr F P u m m e r y with the a u t o m a t e d stepping m o t o r drive and scaler readout system is also valued

References [1] GM McCracken and PE Stott, Nucl l, us 19 (1979) 889 [2] S K Erents, G M McCracken and J Vmce, J Nucl Mat 76/77 (1978) 623

[3] G M McCracken, S A Cohen, H 1 Dylla, C W Magee, S T Plcraux, S M Rossnagel and W R Wampler, Controlled fusion and plasma physics (Proc 9~h Eur Conl , Oxford, 1979) p 89 [4] S A Cohen, HI: Dylla, WR WamplerandCW Magee, J Nucl Mat 93/94 (1980)109 [5] SK Erents, ES Hotston, GM McCracken, CJ Sofleld and J She,t, J Nucl Mat 93/94 (1980) 115 [6] G Staudenmamr, J Roth, R Behnsch, J Bohdansky, W Eckstem, P Statb, S Matteston and S K Lrents J Nucl Mat 84 (1979)149 [7] S A Cohen and G M McCracken, J Nud Mat 84 (1979) 157 [8] O Staudenmaler, P Stalb and W Poschenneder, J Nucl Mat 93/94 (1980)121 [9] SA Cohen, J Nud Mat 76/77 (1978) 68 [10] H Lmbl, lnt J Mass Spectrom Ion Phys 6 (1971) 401 [11] J k Zlegler et al, Nucl Instr and Meth 149 (1978) 19 [12] SA Cohen, H t D3AIa, SM Rossnaget, ST hcraux, J A Borders, CW Magee, J Nucl Mat 76/77 (1978) 459 [13] 1 Shea, CJ Sofleld, GM McCracken, JM kvans and LB Bndwell, J Nucl Mat 93/94 (1980)299 [14] J A Cookson, Nucl lnstr and MeSh 165 (1979) 477 [15] G Dearnaley, G M McCracken, J 1 Turner and 1 Vmce, Nucl Instr andMeth 149 (1978) 253 [16] CJ Altstet(er, R Behnsch, J Bottlger, l, Pohl and B M U Scherzer, Nucl lnstr andMeth 149 (1978) 59 [17] W Moiler, t Besenbacher, Nucl lnstr and Meth 168 (1980) 111 [181 M D Mathex~s, UKALRE Reporl R 9166 (1978) [19] D H J Goodall, J Nucl Mat 93/94 (1980) 154 [20] W Eckstem and H Verbeek, Report of the Max Planck lnstltut lur Plasmaphyslk 1PP9/32 (1979)