THE SPATIAL AND TEMPORAL BEHAVIOR OF ELECTRON-EMISSION SITES DURING HELIUM CONDITIONING

An optical imaging technique for visually displaying the spatial and temporal behavior of electron emission sites during helium conditioning over a time scale of 20 min. is described. This has been used to confirm that the conditioning process is permanent and that there exits an upper field limit of 40 MVm**-**1 beyond which no conditioning occurs. Electron spectroscopy measurements have been made at emission sites before and after a period of conditioning and reveal a spectral shift of 0. 4 eV to higher energies and a decrease of 0. 6 eV in the spectral FWHM (full width at half maximum) after conditioning: these findings suggest that an electronic process could be involved in the conditioning mechanism. The optical imaging technique has also been used to confirm that there is a spatial correlation between emission sites and breakdown events.