Dielectric loss during irradiation of a candidate ITER NBI RF ion-source insulator

Abstract

Alternative NBI RF sources will require insulation for RF coils operating at about 1MHz, 60kV and moderate temperatures, in a radiation field. This is a situation which may cause electrical degradation. This is a situation known to cause electrical degradation. To test degradation and breakdown issues, a selected high purity alumina grade (Deranox 999) has been subjected to 1.8MeV electron irradiation at the expected temperatures, with a 1MHz RF high voltage applied. Dielectric loss has been studied to check degradation at the working frequency at 120°C and 240°C chosen to cover the expected temperature range. On irradiation at 120°C dielectric loss increased due to RIC and then remained constant up to 110MGy, and when irradiation ceased recovered the initial value with no permanent degradation. However at 240°C degradation was observed during irradiation, with dielectric loss increasing from 1.6×10−3 to about 3.1×10−3. Following irradiation the initial value showed permanent degradation, indicating that insulation temperature must be kept close to 100°C during operation.