Characterization of insulators for electric insulation in vacuum

Flashover has been demonstrated to find its origin in the relaxation of the energy that is stored during insulator charging. Charging can occur under bombardment by electrons emitted from the cathode or under charge injection at electrode-insulator interfaces. Therefore, the measurement of the energetic characteristics of charged insulators is used for selecting materials and improving technologies (surface finishing, insulator-electrode brazing, etc.). Many space charge characterization techniques that have been developed over the last twenty years measure an average charge distribution. However, this measurement is not straight-forwardly correlated with the electric strength. A technique that uses the Rutherford's diffusion of low energy electrons deflected by trapped charges seems more promising. Measurement of the permittivity, /spl epsi//sub r/, allows calibration of the apparatus and determination of the electron dose that can be stored in the material before energy relaxation. The variation of the trapping efficiency as function of the temperature gives the trap energy distribution. The parameters that trigger the energy relaxation are determined by varying the temperature, the electric field or the mechanical stress.