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

Ceramic insulators and windows in ITER will be bombarded by H isotope and He ions due to ionization of the residual gas and acceleration by local electric fields. Most of the energy will be deposited at or near the surface giving rise to possible electrical and optical degradation. To quantify the problem gases at low pressure have been irradiated and the induced ion currents measured. The current decreases with decreasing pressure as expected, however it reaches an unexpected constant saturation level for pressures <10−1 mbar (10 Pa). Modification of the irradiation chamber indicated that the saturation was due to electrons, i.e., a radiation induced dark current from the metal surfaces. With the modified system it was possible to suppress this dark current and measure ion currents down to <10−4 mbar (10−2 Pa). In this low pressure region ion currents of about 10−6 A/m3/Gy/s may be generated. In addition dark currents of the order of 10−8 A/m2/Gy/s may be produced from metallic surfaces. The ion and electron currents are potentially high for large systems and must be considered as possible sources of insulator degradation.