Gong, X.

Hildebrandt, D.

Krieger, K.

Neu, R.

Rohde, V.

Roth, J.

Schneider, W.

English

MPG.PuRe

Tungsten Migration between Main Chamber and Divertorof ASDEX UpgradeW. Schneider , D. Hildebrandt , X. Gong, K. Krieger, R. Neu, V. Rohde, J. Rothand the ASDEX Upgrade Team Max-Planck-Institut für Plasmaphysik, EURATOM Association, Berlin, GermanyMax-Planck-Institut für Plasmaphysik, EURATOM Association, Garching , GermanyIntroductionUsing graphite or CFC as plasma-facing material, strong erosion and intense carbon deposi-tion occur and limit the performance of a fusion device. Especially, the formation of thickgas-containing carbon layers is a serious problem in fusion devices with long tritium pulseoperation. As alternative tungsten is being considered for use as a plasma facing componentin the main chamber. In the tokamak ASDEX Upgrade about 10 % of the graphite tiles at thebottom part of the inner heat shield were replaced by tungsten coated tiles (    nm) dur-ing the experimental campaign in 1999/2000 [1]. After this period two tungsten-coated tilesfrom the inner heat shield were removed and investigated by surface analysis techniquesin order to measure the total amountFigure 1: Typical plasma equilibria and arrangement ofthe samples exposed in ASDEX-Upgradeof eroded tungsten. In addition, agraphite tile from the entrance plateof the inner divertor (facing the heatshield) as well as a CFC-tile fromthe inner divertor were removed andinvestigated with respect to tung-sten deposition to get informationon the tungsten migration.ExperimentalThe temporal development of plas-ma equilibria of a typical dischargeof ASDEX-Upgrade and the arrange-ment of the samples are shown inFig. 1. The W coated tiles of the in-ner heat shield (samples 1, 2) wereexposed from November 1999 upto June 2000, whereas the graphite tile of the divertor entrance plate (sample 3) and the CFC-tile of the inner divertor (sample 4) were exposed from November 1997 up to June 2000. Be-tween November 1997 and August 1999 11 boronizations and 3 siliconizations were carriedout. In the experimental campaign from November 1999 until June 2000 2 siliconizationswere sufficient to guarantee an acceptable surface conditioning. In this last period with atotal discharge time of about 4000 seconds the erosion and migration of tungsten in ASDEX-Upgrade were studied. This experimental campaign represents 792 ohmic and NB-heateddischarges with heating powers up to 15 MW. The plasma densities near the last closed fluxsurface varied in the range between 	     and      m   [2] and the electron tempera-ture in the range between 20 and 200 eV depending on the discharge conditions. After theexposure the tiles were investigated by Auger Electron Spectrometry (AES), Secondary IonMass Spectroscopy (SIMS) and Rutherford Back-Scattering (RBS).ResultsAfter plasma exposure the inner heat shield tiles exhibit different zones with prevailing ero-sion or deposition. Fig.2 shows the areal densities of the tungsten coating in toroidal directionof sample 1 measured by AES. A sharp transition from the deposition to the erosion zone wasfound at x=38 mm. The impurity contamination in the region with prevailing deposition ofsample 1 measured by AES consists of carbon, oxygen and silicon as main components ofabout 85 % and iron, chromium, nickel and boron as minor components of about 15 %. Thedeposition layer was measured to be about 20 to 30 nm thick. Well-developed tungsten car-bide layers were formed in the deposition areas indicating a temporary enhancement of thesurface temperature up to values higher than 1000 K [3]. In the zone with prevailing ero-sion (x  38 mm) about 	     W-atoms m were removed in comparison to the zone withprevailing deposition.Figure 3 shows the results of sample 2 by means of RBS-measurements. Sample 2 wasclosely located in mirrored symmetry (see Figs. 2 and 3). The areal density of tungsten beforeplasma exposure was determined by RBS to be nearly constant across the sample surface05 102 11 102 21,5 102 22 102 22,5 102 23 102 20 2 0 4 0 6 0 8 0 1 0 0-2Btoroidal coordinate x  /  mmareal density  /  at mdeposition zone erosion zoneWFigure 2: Toroidal dependence of the tung-sten areal density of sample 1 measured byAES1  1 02 21 ,5  1 02 22  1 02 20 2 0 4 0 6 0 8 0 1 0 0areal density  /  at m-2Wtoroidal coordinate x   /  mm’erosion zone pre-exposurepost- exposure2 ,5  1 02 23  1 02 25  1 02 10◊◊ ◊ ◊ ◊■ ■■BFigure 3: Toroidal dependence of the tung-sten areal density of sample 2 measured byRBSof about       W-atoms m . This value agrees with the tungsten amount of the depositiondominated zone of sample 1 measured by AES. Sample 2 exhibits an eroded amount of thetungsten coating between to 6 and      W-atoms m .In addition, extremely non-uniform erosion of the inner heat shield tiles due to arcing hasbeen observed. These arcing processes remove the total tungsten coating within the trackwidths of about 50 to 100 " m (see Fig.4). Regarding the erosion zone of the investigated tilesof the inner heat shield, the eroded amount due to arcing can be approximated to # 1% of thetotal eroded amount in this area.C WFigure 4: SIMS-images of the lateral distribution of C and W in the erosion area of theinner heat shield with an arc track. The images represent a part of the sample surface of 500 500 " m.A fraction of the eroded tungsten was found in the deposition layer on the entrance plate ofthe inner divertor (sample 3). Increasing amounts from 8     up to 3    %W-atoms m were detected in the radial direction towards the plasma (Fig.5). The tungsten found on theentrance plate of the inner divertor indicates direct transport via the scrape-off layer plasma.Investigating the CFC-tile of the inner divertor (sample 4) by means of AES-depth profilemeasurements tungsten deposition was only found in the area of the separatrix position withamounts of 1 to 3    %W-atoms m (Fig. 6). A few centimetres away from the separatrixposition no tungsten could be detected. The sensitivity limit was about 2     atoms m .The occurrence of deposited tungsten near the strike zone of the separatrix at the inner targetsmay indicate that tungsten penetrated partly towards the separatrix region.We estimated the total amount of tungsten deposited on the inner CFC-divertor tiles to beabout 3 to 5     W-atoms and compared this value to the total amount of tungsten erodedfrom the surface of all 128 tungsten coated heat shield tiles using the values of Fig. 2 to beabout 3     W-atoms. In that way, one gets, that only 1 to 2 % of the eroded tungsten wasdeposited on the inner divertor tiles. In similar way, the part of deposited tungsten on allgraphite entrance plates of the inner divertor was evaluated to 5 to 10 %.Tungsten erosion has been simulated with B2/EIRENE modelling, based on sputtering byB1 02 01 02 11 02 21 02 30 2 0 4 0 6 0 8 0 1 0 0WS iareal density  /  at msample coordinate x  / mm-2Figure 5: Deposition of W and Si onto theentrance plate of the inner divertorB-21 02 01 02 11 02 21 02 30 2 0 4 0 6 0 8 0 1 0 0WBareal density  /  at msample coordinate x / mm0 204060Figure 6: Deposition of W and B onto aninner CFC-divertor tilecharge exchange neutrals (CX). These calculations yield a tungsten erosion at the inner heatshield tiles of about 1    %atoms m [4,5].The measured erosion values are higher than the calculated ones by more than one order ofmagnitude. It seems that the process of CX-sputtering is not sufficient to explain the erosionprocesses at the central column of ASDEX-Upgrade. The different erosion in the toroidaldirection in dependence on the angle between the surface and the magnetic field gives furtherindication that the dominant erosion process is sputtering by ion impact [5].With 10 % W-coverage of the area of the central column heat shield (1 m) no significant im-purity contamination of the central plasma has been found despite the relatively large erosionat the inner heat shield. The plasma performance was not affected [1], pointing to a very lowpenetration of tungsten into the confined plasma. This encourages the use of tungsten as afirst wall material.References[1] R. Neu et al., J. Nucl. Mater. 290-293 (2001) 206-210[2] H.S. Bosch et al., Plasma Phys. Contr. Fusion 41 (1999) A401[3] D. Hildebrandt et al., J. Nucl. Mater. 290-293 (2001) 89-93[4] H. Verbeek et al., Nuclear Fusion, 38 (1998) 1789-1803[5] A. Tabasso et al., J. Nucl. Mater. 290-293 (2001) 326-330

Tungsten Migration between Main Chamber and Divertor of ASDEX Upgrade

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Tungsten Migration between Main Chamber and Divertor of ASDEX Upgrade

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