Investigation of the ETA-BETA II plasma edge by surface analysis of collector probes

To investigate the ion flux escaping from the plasma and the impurity flux released by the wall, collector probes made of graphite, silicon and titanium have been exposed to the deuterium plasma confined in the toroidal device ETA BETA II. The damages on the collector surfaces have been surveyed by a scanning electron microscopy (SEM) apparatus. The deuterium and impurity retention have been measured by elastic recoil detection (ERD) and Rutherford backscattering spectroscopy (RBS) techniques respectively. The implantation build-up has been investigated as a function of the exposure time. The deuterium dose in graphite saturates after a few discharges, whereas the metal impurities exhibit a linear increase in time. The deuterium flux and its radial dependence, inferred from the implanted concentrations, have been compared with those measured by Langmuir probes. Metal impurities have been identified and their relative abundances have been compared with the material wall composition. The impurity flux is found consistent with the global content in the plasma derived by spectroscopic measurements. The deuterium dose measured in different samples has been related to the backscattering coefficient of the materials. Finally, to investigate the damage on sample probes facing the plasma particle flow, erosion probes made of vitreous graphite with silver implanted at a fixed depth have been exposed to the plasma and the thickness change after exposure recovered.