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Plasma Physics Laboratory MAY 1978 PPPL-1445 UC-20f <-'/C-7- -/ TOKAMAK PLASMA DIAGNOSIS BY SURFACE PHYSICS TECHNIQUES BY S. A. COHEN PLASMA PHYSICS LABORATORY WISER PRINCETON UNIVERSITY PRINCETON, NEW JERSEY- This work was supported by the U. S. Department of Energy v v;- Contract No. EY-76-C-02-3073. Reproduction, translation, „ : >v| publication, use and disposal, in whole or in part, by or S» for the United States Govemme:"i: is -ipi^-h-•<• »,-• :;,•*'$$* NOTICE This report was prepared as an account of work sponsored by the United States Gov­ ernment. Neither the United States nor the United States Energy Research and Development: Administration, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express cr implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Printed in the United States of America. Available from National Technical Information Service U. S. Department of Commerce 5285 Port Royal Road Springfield, Virginia 22151 Price: Printed Copy $ * ; Microfiche $3.00 NTIS *Pages Selling Price 1-50 $ 4.00 51-150 5.45 151-325 7.60 326-500 10.60 501-1000 13.60 ]' i"i •:'.! -n t.ed a I : he Th rd International Con i"e ronce . m I'L Sut'' •><:•e lnti ir.tcV ion ; in Controlled I-'union Devices, I'll I 1,-ibor.j lory, ' J K '', •7 Apr i I 1978- ABSTRACT The utilization of elementally-sensitive surface techniques as plasma diagnostics is discussed with emphasis on measuring impurity fluxes, charge states, and energy distributions in the plasma edge. A model of plasma flow to the probe is presented and applied to the interpretation of data. Limits on time and energy resolution, and sensitivity are given. The overlap of these techniques with conventional plasma diagnostics is described. -2- I. Introduction Surface physics techniques are being routinely applied in tokamaks to characterize wall conditions. In this mode of opeiation, samples are inserted to the wall position ina tokamak, exposed to high power pulses or discharge cleaning, and then retracted into an analysis chamber. To date, AES [1],[2] ,[3] , SIMS [11 . RIBS [1],[4],[5], ESCA [4], SXAPS [6], thermal desorption [7], and nuclear reactions [4] have been used in such studies. The main goal of these efforts is to correlate wall conditions with plasma behavior. By this approach it should be possible to learn what state of the walls is associated with minimum plasma contamination caused by global sputtering and desorption. The question naturally arises, can the same surface physics techniques also be utilized to diagnose important plasma properties? To perform this function a sample must be inserted into the plasma to act as a collector for ions and neutrals which impact on it. Several groups [8] , [9 ] , [10] ,[11] are currently using probes in this manner. Surface analysis techniques applied to the sample would identify and quantify the deposited elements. A model for probe behavior is then needed to relate the data to plasma parameters. In this paper attention is restricted to the use of elementally-sensitive surface techniques in the determination of fluxes, energies and charge states of impurities present in tokamak plasmas. In section II are presented discussions of sample configurations, heat and plasma flow to the probe. -3- and the effect of varying probe potentials. Use of the probe as an ion thermometer and charge state discriminator is discussed in section III. The overlap of this approach with conventional plasma diagnostics is described in section IV. II. The Probe A. General Considerations If a cube-shaped probe is floated in an infinite, homogeneous, magnetized, collisionless plasma with the magnetic field, B , normal to two faces, then the two faces experience a greater ion flux than the other four. This is due to the fact that motion along the magnetic field, V„ , is free streaming, while motion perpendicular to B , Va , is diffusive, i.e., for the ions 1/2 iV„ =2ir„/ni ~ (2tT./ra.) , (1) ivi =irx/ni ~ DJ. ?i Vi ' (2) where D^ is the diffusion coefficient perpendicular to B and T. , m. and n. are the ion temperature, mass and density. In the edge plasma of tokamaks V„ = 1 x 10 cm/s and Vx - 5 x 10 cm/s [12,13] for 10 eV oxygen ions. Assuming, for the moment, that all ions which impact on the sample surface stick to it, and that there is no erosion or diffusion, then the probe collects 20 0 times more ions/per unit area by parallel flux .r„ , than by perpendicular flux , .T± . Of course, the ion -4- flux to a probe can vary without there beiri</ a change in tie- plasma density. Throe obvious ways for Hi i s to occur are: va r i I- i or in in t fie put en t i a I of the prole- re I a I i ve in I iM • p 1 .is.vu ; Viiri.it \> ol I tie ior. HI <• ler-t ron t cirper.it me:,; .md va r i a I i on o I t he n, 1 I i s i on a I i I y o ! I he p I .ism.i cans i ii'| tin- I r ee si. re.mi i Hi | II.'J I i on fia r a I I e I to V, t i, ),< -come v i scon::. Tin •: ;• • v/ i I 1 he d i SCHSSI -d la t e i in iw,r <• <!. a ,, j \ To determine the perpendicular and par..-lie) energies, !•'. L and K,, , of the impact i ri'i inns. , use cm be made of the fact that I he i i • |y i . i ••• i ad i i are in. ic i oscop i i • . r'ons i de i tie- cube I .ice which is pe rpi-nd i en I .< r to I; , and a shield with an aperture smaller than I he ion cy ro--rad j us , ''. , in I mu t ol 1.1) i .': 1 ace . With the sh i •• I d : :u I I i c i en I I •/ I a i I ' from the cube hire, the ions that p.iss tbromjb the apertuie (see I'ii). I) can I and on the cube f .,ce .n: fat as V '' . I r om the ijeoinelric sharow of the ,i|,i-rl.ure on the cube tire. In ri.nl i,i:;l (!•' i <) . I ) , with a shield in I r on t ol a I'.-pa r a I I <• I face, most ions would not impact on that face .it .ill because ol the small V„ required. '1'hose ions with V„ '> x I 0 'em/s repri'Sent rjii ) y (mV„ '/li 2 k'l'I ' of the Iota) n. which, in this case j ;; about Z z 1(1 n . 1 In the above; corif i<|iir.il ion, t i me resoiut ion it the ion f lux may be obtained by moving the probe lace parallel to the shield. Jn this manner ions arriving at the probe at different times will land at different location:-;. Probes of this type have used spinninri "mov ie" f i liti (9| and rotating djscsll] or cyl inders [111 . Another approach has been to keep the probe stationary and open and close a shutter 11-41 _ In the case whore tin- probe is moved behind a hole, I lie I i me rosolnt ion, ,'. I , cm be a:; :;horl .1:; W/!'. , where W is Hie diameter of (ho hole .incl :; Ihi' speed at which I he probe is moved. This value ol /.1 may lie obtained only il I he probe is less than .2 '•' i I rum I lie shield. As shown in I-'i<|. 2 , when the distance x , between the probe and shield is increased to 1 . r> '•' i , I he time resolution degrades to (Wt^.'> » 1 ) :: and then asymploles lo (W I 2 . I '•' i ) s as >: • ••• . The n ih-rioii m.t-il i :. I li.i t "I',- o< t he deposited i on:; be wi t h i 11 s '.'<!. \ /'.>. ol I he .11 e.i be i nu. all, 1 I y y.cd . The 1 let a i I:; ol I 11 < • . 11 • 1,1 J :; j t j 1, j 1 j 11 1 j 1 i I . •:; I e.jd i r 1 < j t o I he above .11 e shown in Sec . III. Al this juncture il is .iq.iin empli.i:. i zed th.il any poteiil ial dillcience between I lie plasma and I h< • probe will ohanqe I he I I 11 >: . IJ i< I enel i|y spec I I inn ol I he i on:; which r e.lcll I he pi obe . Thus loi I In- probe lo be used ,1:; a Mux meter- 01 ion I he 1 moiiiel c t , seem i in 1 I y i I shriii 111 be in. 1 i 111 ,1 i ned a I the space pot en I i ,1 I . In I . 101 , this is no I I he cise, ,is will be shown in section MC. A 110 I her el I col o I Opel ,1 I i |H| I he | > I (ibe .it space po I ell I i ,1 I is. that t lie power I lux to I tie probe, <| , duo lo the nearly s.i I ill a I ed I'IIM'I idii current t I ow i nq to i I , slum I d be alum I I I) I i mes i|ii'.iln t h.in i I I lie pi obe were ma i 111 a i lied ,11 the I loat i ni| inilciil i.il , i.e., 11 n V Z'l' /'?.
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