Experimental study of the reduction of field emission by gas injection in vacuum for accelerator applications

Field emission current from surfaces under vacuum and at high field strengths can be reduced by the injection of gas into the evacuated volume. In this paper, the effects of H₂, He, N₂, and Ar on this ``dark'' current emitted from a tungsten carbide point cathode for 2 cm gap distance is studied. Exposure to any of these gases at pressures on the order of 10^-3--10^-2 Pa was found to reduce the emission current by up to 90% with a time constant on the order of ~1 minute as compared to the current at 10^-6 Pa. The effect was strongly dependent on the gas nature, with Ar and N₂ having larger effects at lower pressures than He and H₂. The reduction was reversible, with the current increasing to near its original value with a time constant on the order of ~1--10 minutes after pumping down. The effect of the gas remained in the absence of electric field, whatever the gas pressure. Mechanisms for these and related phenomena are discussed.