ESTIMATING THE ELECTRON-TEMPERATURE FROM FLUCTUATIONS IN A VACUUM-ARC PLASMA - CATHODE SPOT OPERATION ON A CONTAMINATED SURFACE

High frequency fluctuations (up to 50 MHz) in the ion current, arc voltage, and plasma floating potential have been studied, together with the influence of the surface history on these fluctuations for arc currents ranging from 2 to 50 A. In the process of surface conditioning by repeated arcs, the amplitude of fluctuations increases. The fluctuations of a signal from the grounded probe situated at a distance of less-than-or-equal-to 0.1 mm from the cathode spot are accompanied by positive and negative bursts. The probe floating potential fluctuates between 0 and 10 V. The conclusion is drawn that these instabilities are caused by fluctuations in the plasma electron temperature T(e.) T(e) is estimated to be about 4.6 +/- 0.5 and 5.8 +/- 0.5 eV, for Cu and W arcs respectively. The influence of surface contaminations on arc behaviour is discussed. The model proposed is based on the breakdown of thin non-metallic films under the cathode spot plasma, and the appearance of emitting sites in the form of craters. The current is supported by field emission, enhanced by the influence of a non-metallic film.