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

LGEP 2014 ID = 1590International audienceField 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 H 2 , He, N 2 , 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 N 2 having larger effects at lower pressures than He and H 2 . 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

Development of a compact bushing for NBI

International audienceResearch into a novel type of compact bushing is being conducted through the HVIV (High Voltage holding In Vacuum) partnership between CEA-Cadarache 1 , GeePs-Centralesupélec 4 , LPGP 3 and LCAR 2. The bushing aims to concentrate the high electric field inside its interior, rather than in the vacuum tank. Hence the field emission current is also concentrated inside the bushing and it can be attempted to suppress this so-called dark current by conditioning the internal surfaces and by adding gas. LCAR have performed theoretical quantum mechanical studies of electron field emission and the role of adsorbates in changing the work function. LPGP studied the ionization of gas due to field emission current and the behavior of micro particles exposed to emissive electron current in the vacuum gap under high electric fields. Experiments at Geeps have clarified the role of surface conditioning in reducing the dark current. Geeps also found that adding low pressure nitrogen gas to the vacuum is much more effective than helium in reducing the field emission. An interesting observation is the growth of carbon structures after exposure of an electrode to the electric field. Finally, IRFM have performed experiments on a single stage test bushing that features a 36 cm high porcelain insulator and two cylindrical electrode surfaces in vacuum or low-pressure gas. Using 0.1 Pa N 2 gas, the voltage holding exceeded 185 kV over a 40 mm "vacuum" gap without dark current. Above this voltage, exterior breakdowns occurred over the insulator, which was in air. The project will finish with the fabrication of a 2-stage compact bushing, capable to withstand 400 kV