Electrostatic oscillations of the ion and discharge currents in the plasma of a Hall accelerator

In the plasma of a self-sustained abnormal glow discharge of three working gases (neon, argon, and krypton) generated in a Hall thruster with an anode layer, simultaneous recording of ion current and discharge current oscillations in the frequency range from 20 kHz to 40 MHz was performed for the first time. Oscillations with the largest amplitudes are observed as clusters – “bundles” of frequencies with a half-width Δi ∼ 1 MHz for ion current oscillations at frequencies 4 ≤ f ≤ 30 MHz and a half-width Δdi ≤ 100 kHz for discharge current oscillations at frequencies 20 kHz ≤ f ≤ 7.5 MHz. Ion clusters are absent in the frequency ranges of ∼500 kHz–4 MHz and 10–17 MHz in the plasma of all gases. Discharge current oscillation clusters also exhibit two “gaps”—in the ranges of ∼250–500 kHz and 900 kHz–2.5 MHz. The highest-amplitude discharge current oscillations at 3.75 ≤ f ≤ 7.5 MHz are observed only in neon—the gas with the minimum mass. There is almost no overlap in the ranges of ion current and discharge current oscillations—the currents respond to perturbations driven by different coexisting plasma instabilities. It was found that transitions to threshold regimes are necessary for the generation of oscillation clusters. These transitions occur when one of the following parameters changes: magnetic field induction, discharge voltage, or working pressure. In these regimes, an increase in the levels of oscillation and amplitude-frequency characteristics by more than an order of magnitude is observed with a change in magnetic field, working pressure, or discharge voltage of only 10%–15%. The recorded threshold values of magnetic field, working pressure, and discharge voltage differ for the various gases. The generation of ion oscillation clusters is marked by an increase in oscillation amplitude from an already existing stationary level of oscillations, followed by decay back to the same stationary level. Clusters, therefore, are not the result of excitation of oscillations when conditions arise for the buildup of some instability from the noise level; instabilities are obviously excited already at the stage of discharge ignition. This new result raises the need for a theoretical search for the conditions of threshold changes in the level of existing oscillations, which are determined by various plasma instabilities. The rise and decay times of oscillation amplitudes in clusters are comparable for all three working gases. Subsequent clusters form in a time that is shortest for krypton—the gas with the lowest ionization potential. An increase in the level of discharge current oscillations at frequencies of 150–200 kHz results in an approximately 30% growth in ion density. The article discusses possible connections between the experimental results and the operation of known plasma instabilities in the Hall thruster.