Reduced plume oscillation amplitude in hollow cathode discharge due to enhanced electron emission from keeper electrode

Abstract

Inspired by the fact that ionization oscillations in hollow cathode plume are triggered by electron loss on keeper electrode, this paper replaced the material of keeper orifice plate with emissive ones in order to re-compensate the electron loss and checked the influence on plume oscillations. Results show that plume oscillations were indeed sensitive to exterior electron emission. When keeper orifice plate was LaB6 and short-circuited with cathode negative electrode instead of electrically floating, the oscillation amplitude and ion energy dropped by 60% and 50%, respectively, and more under lower flow rates. However, the work function being too low would contrarily increase oscillation amplitude and ion energy. The results recommended the ratio between compensation and loss current to be unity, in which case the main emission mechanism was secondary electron emission due to ion impingement. These results would help increase thruster efficiency and extend lifetime from the perspective of thruster-cathode coupling.

Presence and statistics of a discontinuous potential distribution in the plume of an electric propulsion hollow cathode

“…From tantalum (Ta) to LaB 6 to BaO, the work function became lower and emission from the keeper orifice plate became higher. As can be seen, plume oscillations were not always suppressed with more keeper emission [13]. It was estimated that the lowest amplitude (∼40% lower) came when thermal emission and secondary electron emission together were almost equal to the electron beam loss on the keeper boundaries.…”

“…The trigger point happened to coincide with the point where electron relative amplitude was equal to ion relative amplitude, indicating that the triggering process also involves electron-ion non-equilibrium. In fact, the ion energy distribution function (IEDF) in the cathode plume did indeed shift to higher/lower energies with the keeper material changes [13], and therefore with the upstream/downstream movement of the step region. Currently the physics in the plume is more about electronion non-equilibrium, which is generally expected in lowtemperature plasmas.…”

“…The step region is so resistive (∼40% of total resistivity) that it could supply power for the ion acoustic turbulence (∼100 W for the step region and ∼16 W for energetic ion production in our case). Coincidentally, its location also affected plume ion energy greatly [13]. And all these things happen because of electron-electron non-equilibrium.…”

“…Meanwhile, Levko et al [9] and Qin [10,11] proposed the ion current saturation model and virtual anode oscillation model based on space charge limitations, which coincide with experiments at µA ∼ mA current levels. After that, the authors of this paper discovered that the ionization oscillations in the plume are triggered by electron loss on the keeper electrode [12], and reduced the oscillation amplitude by 40% by changing the keeper materials from refractory metals to thermionic materials [13]. As can be seen, plume physics is currently focused on plume oscillations, and the assumption is that (a) the anomalous resistivity is mainly due to plasma oscillations, (b) the main potential drop is located at the region with the highest oscillation amplitude and (c) the potential drop value is correlated with oscillation amplitude.…”

Role of thermionic emission in the formation of negative electric potential and oscillations in low pressure discharges

“…The oscillations were coherent with the potential fluctuations of the keeper. In their subsequent work [11] the authors replaced the material of the keeper orifice plate with the emissive material to compensate for the electron loss. The results obtained showed that the amplitude of oscillations dropped by 60%.…”