EDGE ION FLUXES AND RECYCLING STUDIED IN TDEV BY USE OF BIASABLE BE-COLLECTOR, C-COLLECTOR AND SI-COLLECTOR PROBES AND LASER-DESORPTION

A pulsed laser desorption diagnostic was used to analyse, between two tokamak shots, the retention of hydrogen in a collector probe exposed in the scrape-off layer (SOL) of the TdeV tokamak. The probe can be rotated during a shot to obtain temporally resolved exposure, or biased electrically relative to the vacuum chamber. Hydrogen recycling was studied by measuring the ion saturation current drawn from the plasma by the probe and retention of H in the Be and C collectors. It is found that, at low incident energy, the H recycling coefficient is close to unity. The codeposition of carbon plays an important role in the retention of H. The main release mechanism for the collected hydrogen is thermal desorption. The effects of the conditions of the edge plasma, such as those produced by divertor plate biasing and boronization, were also studied. The former has a weak influence on the retention of H. A slight increase in the retention with positive biasing is believed to be due to the increase in the codeposition with the carbon flux. However, boronization shows a strong influence on the retention: the hydrogen retained in the probe is found to increase by, 80%, compared with a tokamak shot with the same plasma density and current before boronization.