R. Kawakami

National Institute for Fusion Science (NIFS-Repository)

References:1) ITER Physics Basis Editors: Nuc1. Furion 39, (1997).2) Huber, A. et al. : 1. Nuc1. Mater. 290-293, (2001) 276.3) Wang, W. et al. : 1. Nucl. Mater. 241-243, (1997) 1087.Fig. I. (a) Ionization eventmnnber ofC to for C atoms and CD4 molecules emittedfrom a C-side of a W-C twin test limiter exposed to the 1EXTOR-94 plamm (thickcurve) and the measured crr emission profile from the C-side (open circles).2J(b) Ionization event number ofC to for C atoms and CD4 molecules emitted froma C-mono test limiter exposed to the 1EXTOR-94 plamm (thick curve) and themeasured crr emission profile from the C-mono test limiter (open circles)?) In thesefigures, Yern is the chemical sputtering yield ofCD4 due to D+ bombardment ofC. Thethin curves correspond to the simulated results for reflection and physical sputtering.The others correspond to the simulated results for chemical sputtering.reflection + physical sputtering+ chemical sputteringDistance from the limiter (cm)(b) C-mono test limiterDistance from the limiter (cm)as shown in Fig. 1 (a). This figure shows that there occurs the weakcontribution ofthe chemical sputtering at a distance of less than 1.5cm from the C-side. Thus, the C impurity emission from the C-sideis found to be due to the same emission process as that for theW-side. This result is in disagreement with that for C impurityemitted from a C-mono test limiter exposed to the TEXTOR-94plasma. For the C-mono test limiter, ifYCD4 = 0.03 (which is higherthan that for the C-side), the simulation reproduces the experimentaldata, as shown in Fig. 1 (b). There is the significant contribution ofthe chemical sputtering at a distance of less than 0.5 cm from theC-mono test limiter. The decay length ofthe crr emission profile isquite similar to that for the chemical sputtering. Thus, the Cimpurity emission from the C-mono test limiter is caused mainly bychemical sputtering ofCD4 molecules from the C target.Eventually, a comparison between the C impurity emissionfrom the C-side and that from the C-mono test limiter shows that,for the C-side, the contnbution of the chemical sputtering issignificantly suppressed. The suppression ofthe chemical sputteringfrom the C-side is due to redeposition of W atoms, which arephysically sputtered from the W-side and locally back onto theC-side. In the W-C twin test limiter experiment, the redeposition ofthe W atoms on the C-side has been actually obseIVed. Forchemical sputtering ofCD4 due to D+ bombardment of C, an otherexperimental result shows that the chemical sputtering fromW-contaminated C material is suppressed compared with that frompure c.3) This experiment result also supports the explanation for thesuppression ofthe chemical sputtering from the C-side.104--;;;- (a) C-side ofW-C twin limiter+ .-:::U ;B.e+u""' 0):§ reflection + physical sputteringI: 1: + chemical sputtering0) QI):: 103§ U'p0'= co 2... I::0) .-..0 "0§ §Zo 3 0Kawakami, R. (Fac. Engin., Tokushima Univ.)§ 20. Simulation Study on Interactions ofWalls with Plasmas Including CarbonImpurityIn the future fusion plasma device, ITER, simultaneous use ofCarbon (C) and Tungsten (yV) materials has been planned asplasma-facing materials (pFMs) at the divertor plates.I) However,the simultaneous use causes a problem with mutual contaminationsof the PFMs, such as C-contaminated W material andW-contaminated C material. The contamination is due to depositionof impurity transported by the core plasma or by the SOL plasmafrom other material. How the contamination affects plasma-wallinteractions such as impurity emissions is an open question.Regarding this problem, an exposure experiment ofa W-C twintest limiter composed of W and C materials has been perfonned inTEXTOR-94?) In ref 2, a result that a crr emission profile from aC-side ofthe test limiter to the core plasma is similar to that from aW-side ofthe test limiter has been reported In this study, in order tounderstanding the experimental result, ionization distributions ofCto C2+ for C atoms and CD4 molecules emitted from C and Wmaterials exposed to D+ plasmas including C4+impurity have beencalculated by the EDDY code. A comparison of the calculatedresults with the experimental results has been described.The simulation code for plasma-wall interactions, EDDY, iscomposed of 4 models: (a) transport of bombarding ions in apre-sheath plasma, (b) emission of impurity from PFMs due to thebombarding ions, (c) the resultant mixing of the PFMs, and (d)transport ofthe emitted impurity in the plasma. Especially, in the (d)ionization and dissociation processes ofchemically sputteredCD4 due to electron collisions in the plasma are taken into accounttogether with the ionization of physically sputtered C. Based on afeature of the TEXTOR-94 plasma, the plasma temperature isassumed to be decreased from 72.4 eV to 12.6 eV with increasingminor radius r (r = 41 cm - 51.3 cm). The plasma density is alsoassumed to be decreased from 8.03 x1012 cm-3 to 0.79x1012 cm-3• Incontrast, the CD flux ratio defined by the ratio ofC impurity flux toD fuel flux is assumed to be increased from 2.2% to 6.7%.The simulation results obtained by the EDDY code are similarqualitatively to the measured crr emission profiles from the W-andC-sides. In particular, the similarity presents a difference between Cimpurity emitted from W-contaminated C material and that frompure C material. Ifthe chemical sputtering yield YCD4 is YCD4 = 0.006,C impurity emitted from the W-side is reproduced. This shows thatthere is little contribution of the chemical sputtering to the Cimpurity emission from the W-side. Thus, the C impurity emissionfrom the W-side is caused by reflection of C+ impurity in thebombarding ions and by physical sputtering of C deposited on theW-side. This result is the same as that for C impurity emitted from aW-mono test limiter exposed to the TEXTOR-94 plasma.IfYCD4 = 0.01, C impurity emitted from the C-side is reproduced388

Simulation Study on Interactions of Walls with Plasmas Including Carbon Impurity

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