Berlin 2001 – scientific programme
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P: Plasmaphysik
P 12: Plasma Wall Interaction, Diagnostic and Low Temperature Discharges (Poster Session)
P 12.10: Poster
Friday, April 6, 2001, 12:30–15:00, AT2
Material Accumulation in Local Shadow Regions of TEXTOR- 94 — •Peter Wienhold1, Andreas Kirschner1, Dieter Hildebrandt2, Matej Mayer2, Marek Rubel3, and Wolfgang Schneider2 — 1Forschungszentrum Jülich, D-52425 Jülich, Germany — 2Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany — 3Royal Institute of Technology, S-10044, Stockholm, Sweden
The paper describes an experiment with a self shadowing target covered with a thin a-C:H film (200nm) and exposed in the SOL of TEXTOR-94. The target was inclined by 20 degrees with respect to the toroidal direction and partly protected with an aluminium plate having a sharp diagonal edge. This edge caused an 8mm wide shadow region. Ions out of the background plasma could not enter because of there small gyro radii (1mm). Thickness changes of the film were quantified in-situ by colorimetry. Outside the shadow the erosion and deposition of carbon is due to deuterium and carbon ion fluxes in the SOL. But, part of the eroded carbon is locally transported into the shadow region and accumulates with rates up to 7nm/s. Such rates can cause micro- meter thick deposits in a couple of minutes and, hence flakes and dust formation during long pulse operation. Simultaneous erosion due to neutrals was measured to be 0.1nm/s only and can’t hinder the growth of the deposit. Their energy is sufficient, however, to cause implantation into the near surface layer. Results and interpretation are confirmed by later ion beam analysis (NRA, ERDA, SIMS) and by simulation calculations with the ERO-TEXTOR and the EIRENE codes. The accumulation of tungsten was found in experiment carried out very recently.