Suprathermal electrons in a stationary magnetic arc discharge

Abstract

Stochastic short voltage spikes occur in the stationary arc discharge of the linear plasma generator PSI-2. Similar spikes are also found when detecting the infrared bremsstrahlung emission at various plasma positions. They are related to suprathermal electrons which have energies up to 150 eV, i.e. 1.5 times the average discharge voltage and 15 times kBTe, the thermal energy of the bulk electrons. These electrons are examined by different diagnostic methods, in particular a newly constructed segmented neutralizer plate was used as a diagnostic tool. The suprathermal particles are found to exist in a thin circular ring of the plasma column which is the region of field lines connected directly to the cathode. For low neutral gas pressure the suprathermal electrons can be treated as collisionless, but when increasing the neutral gas background, scattering with the molecules must be taken into account. In some cases the peak in the potential of the collecting neutralizer plate exceeds the peak of the accelerating voltage. This is explained as a transient event occurring when a bunch of electrons is approaching the collecting surface. The maximum current associated with these electrons is found to compensate the ion saturation current. Their peak density is thus estimated to be in the range of 10−3 of the thermal electrons; on temporal average the ratio ⟨nst⟩/ne is of order 10−8.To the best of our knowledge this is the first extensive study on suprathermal electrons in magnetized arcs. They provide an interesting physical phenomenon but are unlikely to affect the interpretation of electrical probes or optical diagnostic measurements.