Grigull, P.

Hildebrandt, D.

McCormick, K.

Roth, J.

Schneider, W.

W7-AS Team

English

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Erosion Measurements on the Inner Wall of the Stellarator W7-AS

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Erosion Measurements on the Inner Wall of the Stellarator W7-ASD. Hildebrandt1, P. Grigull2, K. McCormick2, J. Roth2, W. Schneider1 and the W7-AS Team21Max-Planck-Institut für Plasmaphysik, Euratom Association, D 10117 Berlin, Germany2Max-Planck-Institut für Plasmaphysik, Euratom Association, D 85748 Garching, Germany1. IntroductionThe new stellarator W7-X presently under construction will be equipped with many opticallenses and mirrors for diagnostic purposes. Due to the interaction with the plasma anddepending on the specific location with respect to the magnetic configuration erosion anddeposition phenomena will occur at the surface of these components. Even at larger distancesfrom the plasma these in-vessel components will be subjected to the same kind of radiationsas the first wall if they are positioned in view of the plasma. It is the aim of this experiment tomeasure surface modifications of wall elements caused by the plasma exposure and inparticular impurity deposition and erosion by charge exchange neutrals on the stellarator W7-AS.Similar to the magnetic configuration of W7-X the stellarator W7-AS exhibits magneticislands at the plasma boundary which are the basis of the concept of the presently investigatedboundary-island divertor [1]. In preparation for this phase, preliminary studies of the relevantplasma configurations and edge parameters have been carried out in W7-AS with inboardlimiters. During that time a number of long term samples were mounted at the inner wall ofW7-AS protected from direct impact of plasma ions by the inboard limiters. Earlierinvestigations with collector samples at the outer wall and with top and bottom limiters haveshown that deposition of plasma impurities and wall erosion by ion sputtering in dischargeswith magnetic islands is concentrated in regions intersected by plasma fans emerging from theX-point of the helical edge [2,3] .2. Experimental DetailsW7-AS consists of five modules, each of which had an upper and lower inboard limiter,toroidally separated [4]. The long term samples (LTS) were mounted in the shadow of theselimiters in the equatorial plane at different toroidal locations of the inner wall in module 3.The sample surface was 2 cm in front of the wall and about 7 cm behind the limiter edge. Aspecial sample holder with a ferromagnetic shutter also used in the tokamak ASDEX-Upgrade[5] allowed an exposure of the sample surface only when a magnetic field is applied in orderto avoid surface contamination during boronization or glow discharge cleaning procedures.The samples were made of polished fine-grain graphite and silicon. They were covered withan amorphous hydrogenated 13C-layer of 360 nm thickness giving the possibility ofsimultaneous measurements of 12C-deposition and 13C-erosion. After 9 months of operation(from November 1998 until August 1999) with about 4000 discharges (total plasma exposuretime of ~ 2000 s) and 5 wall boronization procedures the surfaces of the samples and theshutters were investigated using Secondary Ion Mass Spectrometry, Rutherford backscatteringand Elastic Recoil Detection (ERD). The dominant plasma impurity observed by spectroscopywas carbon mainly released from the inboard limiters.3. ResultsFigure 1 shows the depth distribution of the carbon isotopes 12C and 13C and deuteriummeasured on a graphite collector sample by Secondary Ion Mass Spectrometry just before theplasma exposure. The depth profiles were obtained by ion sputtering with 10 keV O2-ions. Toavoid crater edge effects in depth profiling only ions from the 16% central part of the erodedarea have been analysed. The depth scale was calibrated by measuring the final crater depthby laser profilometry. As shown in fig.1 the original 13C-layer had a thickness of 360 nm. Itcontains the carbon isotope 12C with a concentration of 10 % due to the gas compositionduring layer manufacturing. The signal of mass 13 in the graphite substrate originates from12CH-molecules. Rutherford Backscattering and ERD yielded for this virgin 13C-surface layertotal amounts of 1.64*1022  13Catoms /m2, 1.6*1021 12C-atoms /m2 and  0.7*1022 H-atoms /m2,respectively. This indicates an atomic density of the 13C-layer of 0.5*1023 C-atoms /m2. Theobserved deuterium signal in fig.1 has to be considered as background.Figure 2 shows as an example the depth distribution of 12C, 13C and D on a graphite sampleafter the plasma exposure in W7-AS. It indicates a reduced thickness of the 13C-layer of 240nm which corresponds well to 1.05*1022 13C-atoms / m2 found by Rutherford Backscattering.At the surface the implanted deuterium is found with an exponential decay up to a depth ofabout 160 nm in fig.2. It has to be noted that the measured depth profiles of 13C and D werefound not to depend on the used substrate material (polished graphite and silicon) indicating anegligible influence of the surface roughness of the used graphite material on the measuredprofiles. The total amount of the remained hydrogen and the trapped deuterium wasdetermined by Nuclear Reaction Analysis to be 0.,3*1022 H-atoms /m2 and 0.05*1022 D-atoms/m2, respectively. Significant deposition of plasma impurities like 12C, boron or metals werenot detected on the 13C-layer. In contrast the movable shields of the sample holders arecovered with a thick insulating layer consisting mainly of boron. This contamination is theresult of wall boronization procedures.In fig.3 the measured depth distribution of the implanted deuterium of fig.2 is compared withimplantation profiles calculated by the TRIM programm [6]. The calculations have been donefor implantation of deuterium in amorphous carbon for a Maxwellian energy distribution andan isotropic angular distribution of the incident particles. Parameter is the particletemperature. The comparison is based on an atomic density of 5*1028 Catoms/m3 (upperscale). As can be seen the simulated results show good agreement for a particle temperature ofabout 50 eV which seems to be a plausible value for the temperature of the edge plasmawhich is the main source of the charge exchange neutrals [7].In fig.4 the thickness of the 13C-layer on the carbon samples before and after the plasmaexposure is presented in dependence on the toroidal location with respect to the inboardlimiters. The plasma exposure during high power discharges caused erosion of the 13C-layerranging from 120 and 240 nm. This corresponds to a mean erosion rate of 13C between 0.06and 0.12 nm / s or 3 – 6 * 1018 C-atoms /m2 s. The lower value is similar to that found for theouter wall of ASDEX-Upgrade [5]. The higher erosion for the sample located between thelimiters in fig.4 can be explained by the fact that the limiters are a strong neutral gas sourcecausing higher CX-fluxes locally [7].4. ConclusionLarge parts of the inner vessel wall of W7-AS are erosion dominated in high powerdischarges. No significant material deposition is expected on lenses and mirrors provided thatshutters are used during wall conditioning procedures as boronization. Erosion and relatedsurface roughening due to the impact of CX-neutrals remain as serious problem.References[1] P. Grigull et al., this conference[4] K. McKormick et al., Plasma Phys. Control. Fus. 41 (1999)  B285[2] D.Hildebrandt et al., J. Nuclear Mater. 220-223 ( 1995) 703[3] F.Wagner et al., 15th International Conference on Plasma Physics and Controlled NuclearFusion Research, Sept. 1994, Seville (Spain) IAEA –CN-60/A-4-II-1[5] A. Tabasso et al., J. Nuclear Mater. 290-293 (2001) 326-330[6] W. Eckstein, IPP-report 9/33 (1980)[7] H. Verbeek, Nuclear Fusion 38 (1998) 17891 001 011 021 031 041 050 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0DC12C13ion intensity / a.u. depth* / nmFig.1: Depth profiles of different isotopestaken at the surface before plasma exposure1 001 011 021 031 041 050 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0DC12C13ion intensity / a.u. depth* / nmFig.2: Depth profiles of different isotopestaken at the surface after plasma exposureindicating the implanted deuterium, theerosion of the 13C-layer and negligible 12Cdeposition1 001 011 021 031 040 5 0 100 150 200deuterium signal / a.u.depth* / nm10 eV20 eV50 eV100 eV200 eV5 01 02 0 C-atoms / m21000 2 5 7 5Fig.3: Comparison of measured depth profileof deuterium and calculated implantationprofiles using the TRIM-code [6]01 0 02 0 03 0 04 0 0- 4 0 0 - 2 0 0 0 2 0 0 4 0 0 6 0 0 8 0 0before exposure after  exposurethickness of C13  -layer / nmtoroidal position / mml i m i t e rl i m i t e rFig.4: Toroidal dependence of the measured13C erosion in respect to the limiter position

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Erosion Measurements on the Inner Wall of the Stellarator W7-AS

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