Improving the performance of stainless-steel DC high voltage photoelectron gun cathode electrodes via gas conditioning with helium or krypton

Gas conditioning was shown to eliminate field emission from cathode electrodes used inside DC high voltage photoelectron guns, thus providing a reliable means to operate photoguns at higher voltages and field strengths. Measurements and simulation results indicate that gas conditioning eliminates field emission from cathode electrodes via two mechanisms: sputtering and implantation, with the benefits of implantation reversed by heating the electrode. We have studied five stainless steel electrodes (304L and 316LN) that were polished to approximately 20 nm surface roughness using diamond grit, and evaluated inside a high voltage apparatus to determine the onset of field emission as a function of voltage and field strength. The field emission characteristics of each electrode varied significantly upon the initial application of voltage but improved to nearly the same level after gas conditioning using either helium or krypton, exhibiting less than 10 pA field emission at −225 kV bias voltage with a 50 mm cathode/anode gap, corresponding to a field strength of ~13 MV/m. Field emission could be reduced with either gas, but there were conditions related to gas choice, voltage and field strength that were more favorable than others.