Silicon fluxes in the scrape-off layer plasma during silicon-assisted operation of TEXTOR

Abstract

Surface collector probes were applied at TEXTOR for the investigation of silicon fluxes in the scrape-off layer during the first silicon-assisted (silane puffing, siliconization) operation of a tokamak. Probe exposures were made in order to measure the evolution of Si fluxes and the influence of silicon on the behaviour of other impurity fluxes like boron, oxygen and metals. Studies were performed under different conditions: heating mode, plasma density and gas filling. Comparative exposures were made before introduction of Si into the machine as well as immediately and long time after the siliconization. The exposed graphite samples were examined by surface analysis techniques, including Auger electron and Rutherford backscattering spectroscopies, nuclear reaction analysis and ultra-high resolution microscopies. The most important findings are concerned with: (i) the relation between silicon to carbon and silicon to oxygen in the deposits; (ii) the change in radial profiles of Si, B and D fluxes during consecutive stages of the silicon-assisted operation, and the retention of deuterium in the Si containing codeposited layers. The influence of plasma density on the fluxes is considered and gettering of oxygen by silicon is also addressed. Comparison is also made to the results of VUV spectroscopy signals of silicon and oxygen impurities in the plasma.