The discharge modes and the nonlinear potential relaxation instability of a thermionic argon discharge operated at pressures of 0.02-0.2 Pa are studied experimentally and compared to particle-in-cell (pic) simulations. The plasma produced in a steel vessel with a filamentary cathode is confined by a magnetic box arrangement of permanent magnets. Depending on the discharge voltage, the neutral gas pressure, and the filament heating current, the discharge shows two stationary states with a pronounced hysteresis in the current-voltage characteristic. For a certain range of discharge parameters, sawtooth oscillations can be observed in the discharge current. One-dimensional (1D) particle-in-cell simulations reveal details about the mechanism of the oscillations which are confirmed by experiment. The plasma potential which reflects the microscopic mechanisms is measured by means of emissive probes with high spatial and temporal resolution. The coherence of the self-oscillating system is improved by mode locking it with an external driver. The results of the simulation-specifically formation of an electron hole and its transformation into a double layer-are confirmed experimentally.
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Klostermann et al. (1994) studied this question.
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