Insulator degradation due to radiation induced ion and dark currents in vacuum

Full text of publication follows: Severe surface electrical and optical degradation has recently been observed when oxide materials are implanted to low doses (1019 ions/m2) with keV H and He ions at 50 to 450 C. Similarly ITER ceramic insulators and windows may also degrade, as they will be bombarded by energetic H isotope and He ions due to ionization of the residual gas by gamma radiation and acceleration by local electric fields. To assess this potential problem one must determine the magnitude of radiation generated ion currents at low residual gas pressures. Experiments were performed in a chamber mounted in the beam line of a Van de Graaff accelerator. H, D, and He gases at a controlled pressure were irradiated through an aluminium window with 1.8 MeV electrons. Within the chamber a guarded volume between copper electrodes with a voltage applied, permitted the electric current flowing through the ionized gas to be measured. In this way radiation induced ion currents have been measured at gas pressures between 105 and 10-2 Pa, dose rates of 20 Gy/s, and applied voltages up to 1500 V (105 V/m). As anticipated a decreasing ion current was observed on lowering the pressure. However the measured current reached an unexpected constant saturation level for pressures below 10 Pa. The electrically insulated irradiation chamber permitted polarization of the chamber relative to the guarded volume. This allowed one to demonstrate that the saturation was due to a radiation induced dark current of electrons rather than ions. With the modified system it was possible to suppress most of this dark current and measure ion currents down to 10-2 Pa. At this pressure the radiation induced ion currents were about 10-6 A/m3/Gy/s, i.e. potentially high ion currents for large systems. Dark currents occur in large high voltage systems under vacuum, and can produce Bremsstrahlung (X-ray) radiation. In some cases they have been observed to degrade insulation performance in NBI test systems. To date radiation enhanced dark currents have not been considered as a potential problem. However the results obtained indicate that currents of the order of 10-7 A/m2/Gy/s may be generated from metal surfaces. (authors)