Surface electrical degradation due to ion bombardment of ITER insulators

Abstract

Insulators will be used in ITER diagnostic systems where they will play an important role for optical components, as well as in neutral beam injector system. In addition to neutron and gamma radiation, these materials will be subjected to bombardment by low energy ions and neutral particles. Previous works carried out in silica have shown a dramatic surface electrical and optical degradation with a marked temperature dependence when the material is subjected to proton or alpha particle bombardment, due to loss of oxygen from the surface caused by preferential sputtering by energetic ions. Therefore, it was necessary to extend the study to the main candidate ceramic insulators for ITER (alumina, BeO and AlN), in order to evaluate possible issues for both diagnostic and heating and current driven systems. A clear optical and electrical degradation have been found in all the materials studied after implantation, in general this appears to be an issue common to all the insulator materials studied. This degradation is related with oxygen loss from the implanted zone for alumina and BeO, as observed previously for silica, and nitrogen loss for AlN. For silica implanted at higher energies data show surface degradation in all cases again due to loss of oxygen with a markedly different behaviour for each implanted ion.