Plasma Processes in a Low-Pressure Spark Discharge

Measurements on a 60-kV 1-μF vacuum spark discharge indicate the following sequence of events. During the early stages of the discharge, the current is carried mostly by electrons which are accelerated to the full tube voltage, as seen from intense bremsstrahlung x-ray production at the anode. The fast electron current and accompanying x-ray emission then diminish, but the total current continues to increase. During the time of reduced x-ray emission, the plasma emits a strong microwave pulse which peaks near the frequencies characterizing the electron drift instabilities. The dense beam of fast electrons then reappears for about 50 nsec and the whole cycle repeats, perhaps twice more. Although the fast electron stream repeatedly appears and disappears, the total current (fast stream plus drift current) usually rises monotonically. When a current of about 4×${10}^4$ A is reached (in ∼5×${10}^{-7\mathrm{}}$ sec), dense material vaporized from the anode reaches the cathode and the tube impedance collapses. These events are explained in terms of a model which treats the discharge as alternating between two current modes, one dominated by a fast electron beam, the other by electron drift instabilities.