Studies of radiation blistering effects on voltage holding

The surfaces of niobium and tungsten wires were blistered by 300-keV helium-ion irradiation and then tested for voltage holding. A cylindrical projection-tube technique was employed so that regions of strong electron emission could be observed and later examined with a scanning electron microscope (SEM). Blistering was found to cause significant increases in pre-breakdown currents. However, these currents tend to saturate over a region corresponding to around 200-400 kV/cm surface field such that the ultimate voltage breakdown limit is not seriously reduced. Emission image observations and SEM photographs suggest that, in many cases, parts of the blistered surface are gradually erected by the strong surface fields, but this may not occur until after several arc breakdowns. SEM photographs also indicate that vapor from the anode may play an important part in the breakdown mechanism. Implications of these results to the design of devices important to fusion development, such as direct collectors and ion sources, are briefly discussed. The importance of future in situ irradiation-voltage experiments is also stressed. (U.S.)