Abstract
When a helium beam is extracted, a significant decrease in arc current is observed in the filament arc ion source of the Neutral Beam Injector. Microstructural changes in the tungsten filament surface of the ion source are observed by electron microscopy to investigate the cause of the reduction of arc current. Helium bubbles with 100–800nm diameters appear immediately beneath the surface within 1μm, and the bubbles develop into surface hole structures with surface erosion by sputtering. The decrease in arc current with helium beam extraction can be explained by a reduced effective surface area, due to the increase in hole porosity. It has been shown that the reduction of the arc current can be recovered by removing the helium-irradiation-affected layer by sputtering with argon discharge.
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