Simulation experiments on unipolar arcs

A plasma source is described which produces a plasma beam with a density n e =10 13 cm -3 and a temperature of 5-25 eV, 50 cm outside the actual discharge. In this plasma unipolar arcs are ignited without applying an external voltage. It is shown that by plasma cleaning of the surface this type of arcing can be completely suppressed for the above stated parameters. A microcontamination deliberately attached to a wall probe of stainless steel enables one to predetermine the ignition point of a unipolar arc, and gives the possibility of measuring the arc current. The erosion phenomena of single arc events are compared with those from bipolar arcs and wall probes in tokamaks.