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 approximately 20 min. is described. This has been used to confirm that the conditioning process is permanent and that there exists an upper field limit of approximately 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 approximately 0.4 eV to higher energies and a decrease of approximately 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.