Radiation induced ion currents in vacuum due to residual air, He, and H, and their expected effect on insulating surfaces

Ceramic insulators and windows in ITER will be subjected to bombardment by energetic hydrogen isotopes and helium as a consequence of ionization of the residual gas by gamma radiation and acceleration of the ions by local electric fields. Most of the energy carried by these particles will be deposited at or very near the surface giving rise to possible electrical and optical degradation. In this work, radiation induced electrical conductivity for air, helium, and hydrogen for pressures from 1000 to 10 −3 mbar, at 3 Gy/s, and applied voltages up to 1500 V (10 5 V/m) has been measured. The ion currents at the low pressure ((0.2–2) × 10 −6 A/(Gy s −1 m 3 )) are much higher than those expected from both theoretical extrapolations and experimental measurements near to atmospheric pressure (10 −8 A/(Gy s −1 m 3 )). If such high ion currents bombard the insulator surfaces, severe degradation will occur.