Influence of ambient gases on the dc saturated breakdown field of molybdenum, tungsten, and copper during intense breakdown conditioning

A comprehensive investigation of the effect of Ar, H-2, air, and CO on the saturated breakdown field (E-sat) of molybdenum, tungsten, and copper has been performed at intensive breakdown conditioning. A significant decrease in E-sat is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of similar to 10(-5) mbar, the decrease in E-sat is found to be up to 50% and similar to 30% compared to UHV conditions for molybdenum and tungsten, respectively. In addition, a similar to 30% decrease is found when molybdenum is conditioned with a similar to 10(-5) mbar CO pressure. Surface analysis measurements and breakdown conditioning in O-2 ambience imply that the origin of the decrease in E-sat is closely linked to oxide formation on the electrode surfaces. Ex situ heat treatment of molybdenum is shown to improve the breakdown characteristics drastically.