Evaluation of the scavenging effect by low temperature laboratory plasmas driven with radiofrequency

Abstract

Proposed materials for ITER are beryllium for first wall and tungsten tiles (plus carbon fibre composites) for the divertor. The use of carbon materials at the divertor plates of ITER can be problematic due to the formation of tritium-containing co-deposits coming from the products of chemical erosion and physical sputtering. Scavenging effect in the hidden regions of the divertor has been proposed as a possible technique to eliminate the formation of amorphous carbon films. Nitrogen gas has been proposed as a good scavenger. Although a complete suppression of the formation of amorphous carbon films by nitrogen was achieved in laboratory experiments (in a low temperature plasma), only a moderate reduction (<30–40%) was seen at JET in type-III ELM discharges (Tabares et al 2005 J. Nucl. Mater. 337–339 867–71). This justifies further analyses for an understanding of physicochemical processes for the deposition/inhibition of carbon films. In this work, experiments in RF plasma discharges at low pressures of CH4/N2 admixtures were carried out. The formation of amorphous carbon films was investigated under different physical conditions, with high and low energetic ions on powered and floating electrodes, respectively. Optical emission spectroscopy of ion and neutral species was employed as the main diagnostic.