Glow discharge initiation with electron gun assist

Helium glow discharge conditioning is used before every discharge in the DIII-D tokamak to desorb hydrogen and low Z impurities from the graphite and Inconel plasma facing surfaces. However, high gas pressure is required to initiate each glow discharge session and this requires frequent cycling of valves to protect pressure sensitive devices. To alleviate this mechanical fatigue an electron gun assisted glow system is being installed on the DIII-D vessel to lower the initiation pressure. Through the injection of electrons the initiation pressure of the helium glow discharge has been lowered by a factor of 70, bringing the initiation pressure within a factor of 2 of the minimum sustaining pressure of the glow discharge. This might also make possible pulsed glow conditioning, which would allow a lower average pressure during glow conditioning, reducing the heat load on proposed cryogenic pumping panels. Experimental results of the electron assist on He glow initiation and a scaling model for the electron gun assisted glow are presented.<>