Behringer, K.

Carolan, P.

Duval, B.

Field, A.

Fielding, S.

Fußmann, G.

Hawkes, N.

Janeschitz, G.

Peacock, N.

Soeldner, F.

English

MPG.PuRe

- 267 - A P Fr 01 5 CHARGE EXCHANGE RECOMBINATION SPECTROSCOPY OF ASOEX P G Carohn. B P oLival'*. A R Fleld+. S J held,ng, Ne AaWkes and H J Peacock Cul han Laboratory . Ab; ngdon aX14 308. UK (Euratom/UKAEA Fusion Associ at ion) G Fussman. G Janeschitz and F Soldner Max Planck Institut fur Plasmaphysi k Euratom Association 0-8046 Garchi ng. Germany K H Behringer JET Joint Undertaking. Abingdon, Oxon, OX14. 3EA UK *Attached Clarendon Laboratory. University of Oxford. +Attached Roya l Holloway College. University of London. INTRODUCTION In this paper we report on experiments and theoretical predict ions rel at; n9 to the spect roscopi C observat i cns of charge exchange (eIX 1 processes occurring between highl y ionised impuri t ies in the core of tok amaK dev ices and high power beams of energetic neutral atoms. Our goals were to calibra te a Vac uum Ul trav i olet (VUV) spectrometer and to show that fibre opti cally trans -mitted visible C/X radiation could 'be used to diagnose remotely tokamaks up to and inc l uding the crucial active phase. The diagnostic potential of these C/X lines has been emphasised in recent years. and has been recently re viewed (Fonck, 1984 and Isler , .1985) • . Our observations · cover emission f rom hydro-gen ic ions in both the VUV and the visible, and we have developed a computer code to model the experimental data. This is based on calculated cross sect ions for cha rge exchange into the [ n,t ] resol ved qua ntum levels and the subsequent cascade radiative decay. VISIBLE SPECTROSCOPY A 1m Czerny-Turner vi sib l e spectrometer vi ewed the plasma vi a a quartz window as indi cated in Figure 1. This line of sight observes the i nteraction of one injector beam with hal f the pl asma cross section - a region where the beam attenuation is small «20t ). For survey work it was operat ed with a PARC Optical Multichannel Analyser (OMA). For faster time mea~urements a spectrometer fitted with a photomul tiplier detector sampled at 100 Ils in terva l s was used . The spectrometer was equi pped wi th grat; ngs of 1200 and 2160 lines/nm , gi vi ng di s pers ions of O.2A/OMA-l)i xe l and 0.05 (second order) A/O~ phel respect; vely. Inner phstl' SQectroet !r llne-of-slght / .I CUt er / ,. Fig.l Arran gement of. spectrometer line of ~ight intersecting i njector beam. C/X ~xcite<l transitions were observed froll Il j drogen-like Helium, Carbon, Oxygen , Fluorine, Sulphur and Chlorine . Several 1 ines that bedr the C/X signa ture of very rapid fal1 off of inte ns ity with the end of injection are as yet unidentified . Figure 2 shows rad; ation from C, 0, and S befol' e and during inject ion. Of all the cn lines only the C line was present before inj ection , but was much stronger dur ing injection. Least squ ares fi t ting a Gaussian to t he C line gave ion temperatures of 250 eV berol'e injection but during inject i on we measured ion temperatures of 750 eV from C and ..... 1.5 keV from 0 and S, the reason for th i s di fference 1 s not understood. Observat i ons of the Ooppler sh; ft of the C/X lines yie lded bul k rotational vel ocit ies of 1-2 x 10+5 m/s in the cO-injected ( beam-driven) di rection. In the case of the C I i ne, its appear ance before injection provide d a co nvenient wavelength r e ference. ~ :!! 13 ,. c " > ~ Vi % i!' " 10 0 1.915 NEUTRAL BEAM VOLTAGE 1.920 1(5) lmsfdiv - 268 -., " % ,!;939 10 ~ " tScon28 1 5 t:1L.s 11 ~ 5 I' er> ~ L I ~ " 0 ~) l ). 10fSC0f1 26 ! Scan 2S " I ;' I ~ 't '\ "J ) l , 1\ 2CO 400 " 600 CHANNEL NUM3!:R I 'I /_L.-~ 800 Fi g.2 Vi sib l e spectrum 3400-3500:', befor e injection (at 1.]5), duri ng injection (at 1.4s) and the re sult of subtracting the spectrum at \ . 3s from that at 1 . 4s (to remo ve il ll but the char~e exchil ng e feilture s) . The t ime his t or i es of the visible I.In=1 trans iti ons of 0 and C were exam; ned wi th the photomultiplier . Figure 3 shows the time hi story of the OVIII n=10-9 I f ne at the end of injection. The double step i n the f; gure is due t o the t wo Fig.] OVIl1 n=1 0 -9 C/X line intensity fall off ilt the end of injection. (Neutral Beam Voltil~e re fers to the sum of the vo I tilges across each of two sources i/l the beam l i ne, and is equivillent to totill bean current). 1.92: - 269 -sources in the neutral beam switching off at slightly different t imes. The line intansity falls off rapidly with the switch off of the two sources. This drop i n intensity i s far faster than any changes in the ionisation balance. wh i ch would be expected to occur on a timescale of about 1 ms. The step change in i ntensi ty is a measure of the CIX contr; but ; on to the line intensity. since the C/X excitation process tUrns off on a timescale much shorter than 100 ~s . For the accurate calculation of C/X li ne intensities needed for cali-bration of the VUV instrument, it was necessary to measure the fraction of beam power at each energy. Th i s was done (following Fielding . 1981) by injecting the beams into the torus . when fi ll ed with neutral gas, and observing the Doppler shifted components of Ha light corresp,onding to the diffe rent beam neutral velocities . This yielded a power ratio of 40% :40t :20~ at energ ies E, E/2 and E/3 (where E is the primary beam energy component of 42keV) • VACUUM ULTRAVIOLET MEASUREMENTS A vacuum ultraviolet spectrometer (F,onck, 1982) covering the wavelength range lOO-1700A (wi th modest resoluti on -2A) viewed the plasma radially, at the same posit i on as the vi sible instrument in Figure 1. The spectrometer was equippE.d with a multichanne l detector consisting of a microchannelplate intensifier coupled to an OMA of the same type as used on the visible spectromet er above. The important C/X l ines were i dentified by scanning the entire spectrum , which could be achieved once every 16 ms. Figure 4 shows half a survey spect rum during beam injection. Thi s clearly illustrates the strong C/X lines, Idlich have negligible intensity before and after injection. Ulna " I If) • c '" '" "' H > 600 '" I " " c ~ The C/X line intensities could be 7 recorded once every 1-2ms by restricting c ~(\j the detector t o scan only spec; fic parts ru o;r 1 of the spectrum . However the fa ll off C'1 ~ ~ ~ time of the C/X line intensity at the end ..!r C'1 C of injection was still detector limited . [T1 11 .h;;;' ~~ Previ ous observat ion of these lines in .h ~ ~u........ OITE (Duval, 1985) and observat ions of ~ en en ;;;,~ C/X lines in the visib l e (discussed ~ : en u~ ~~:~ie~eil~a~liSo~~P~~~eg\vse ~~t~~~~;d~~~~ M::: ~ 0 than lO~s . Despite the relatively poor ~ ~;: temporal response of the detector. the Angstr om 100 o time behaviour of these lines is seen to be mar k. edly di fferent from that of the resonance li nes in the spect rum. Thi s diffe.rence is due to the fact that the resonance 1 ines apparentl y take ma ny i on 'i sation times to respond to the neutral beam switch- of f. whil e the C/X line i ntensities change within one scan t fme of the detector. F1g.4 Portion of vuv spectrum showing the strong CjX lines. - 270 -The relative sensitivity of t he spectrometer as a function of wavelength was derived us i ng the theoretically mode lled excitation cross sec t ions of the C/X transitions. (lsler, 1985 ). (To this end alson (1985, 1981) has calculated the C/X cross sec ti ons at 42 keY into [n.l] l evels up to n=12, and n=14 for Carbon and Oxygen respectively.) The results obtained were cons; sten t wi th a spectrometer sensi tivity cal ib rati on constructed pi ecemeal from line r ati os observed in JET discharges, and with that of t he prototype spectrometer, SPREO. The absolute sens i tivity of the spectrometer was establ ished at wavel engths above 110011. by reference to a cal ibrated deuteri um di scharg€ 1 amp. The spectrometer, now abso lutely calibrated, was used to make two measurements of impurity concentrations. Firstl y, the concent r ations of Carbon and Oxygen were calculated using a code for beam decay and impurity excitation r ates and the measured volume emissivities of the cn transitions, this gave typical impurity levels of 2%: A second meas urement of impurity concentrat i on was made using a 1-0 transport model (Oenne, 1985) and line emiss ion from lower ionisation states during the ohmic part of the discharge. The latter measurement gave a )'ather lower fi gure for the concentrations of t he impuriti es (about hal f). It is to be expected that the impurity content of the pl asma during i njection should be higher than during the ohmic phase. CONCLUSIONS Our experiments in the VUV have borne out cal cu l ati ons of C/X 1 ine ;ntensities for the 42 ~eV primary neutr al energy of the ASOEX injec tors. and enabled us to calibra te a VUV spec trome ter . The time histor ies of the visible signal s conf1rll them as being CIX in origin . Our efforts are now aimed at correl ating the observed intensities in the visible (n>8) with the VUV measur'ements (n<5). From the VUV data, the impurity concentrat ions have been measured, and the visibl e data has given ion temperatures and bulk plasma vel ociti e s. The results appear encourag ing regarding the use of visible i nstruments to derive important pl asma parameters which previousl y requi red the use of cl ose-coupled vacuum spectrometers. Our experiments are continuing in order to i ncrease confidence in the theo retical model s. ACKNOWLEDGEMENTS the authors wish to acknowledge the valuable assistance given by M Ke n hacker and the ASDEX and Neutral Injection groups in these exper imen t s. We are indebted to RC Is l er (ORNL) """0 did much of the orig i nal I«)rk on the codes, and to R E alson (Mi ssouri - Rol la) for computing the hi gh n C/X cross sections. The enthusiastic encouragement of W W Engelhardt (JET) is grateful ly acknowledged. REFERENCES ,6 oenn e et al , this Conference. B P Duval et al, Nucl Inst Methd. J uly Issue (1985), 5 J Fi el di ng Md 0 Sto rk. Nuc l ea r Fus ion 22 No.S 617 (1982 ). R J Fonck, A T Ramsey, RV Yell e , AppliedOpti cs '21 No . 12 2115 (1982 ) . R J Fonck . D S Darrow, K P Jae hn i g . Phys Rev. A ~0.6 (984). RC Isl er, Nucl Inst Methd July Issue (1985), Proi: ~ Conf. HIA (lgB4) Oxford. RC Is l er. R A.langley , Applied Optics 24 No.2 254 (985). RE 01son. Phys Rev. A 24 No . 4 1726 -1 73~(1981). RE 01 son, Private coouniTrlication (1985). 

Charge Exchange Recombination Spectroscopy of ASDEX

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Charge Exchange Recombination Spectroscopy of ASDEX

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