Observation of Arc Spots Initiated on Nanostructured Tungsten

When tungsten is exposed to helium plasmas, it was found that nanostructures are grown on the surface. Arcing can be easily initiated in response to a transient heat load with a pulsed laser, which demonstrates transient heat loads in fusion devices. In this paper, we show the behavior of arcing initiated on the nanostructured tungsten. From the measurement of field-emission current, it is thought that the increase in the field emission by helium irradiation may be a key process for the initiation. The behavior of the free running arc spots are observed with fast framing camera. Interesting behavior of initiated arc spots including retrograde motion, bifurcation, and random motion are shown. The spectroscopic study of the emission from the arc spot revealed that the emission contained a continuum emission in visible range in addition to many line emissions.