Studies of unipolar arcing in the mini-torus

Unipolar arcs have been investigated in a small tokamak, the Mini-Torus, on probes made of graphite, aluminium, stainless steel, titanium, and molybdenum, exposed to the plasma by time-resolved in situ observations with an endoscope system and a high speed framing camera in conjunction with photoelectric spectral recordings from an optical monochromator. Various types of arcing tracks are observed. Visual and SEM examination of the details of arc craters reveal several interesting features of the structure and motion of unipolar arcings. Special attention has been paid to the “stationary arcs” with fixed arc spots (giant craters), some of which show a rotational structure.