Potential fluctuations and energetic ion production in hollow cathode discharges

Abstract

Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in the discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent “spot mode” to the noisy “plume mode” characteristic of these discharges is found to be a gradual transition of increasing fluctuation amplitudes.

Impact of plasma noise on a direct thrust measurement system

“…It should be noted that HCs operate in distinct modes where the extreme cases are referred to as the spot mode (a low noise discharge) and plume mode which generates high levels of electrical noise relative to the spot mode. 18 The results presented correspond to the HCT operating in the spot mode.…”

“…the thrust generated by the device. 18 Although a comprehensive theory for thrust generation mechanisms has yet to be firmly established, the primary focus of the work presented is to measure the thrust produced by a HCT by means of direct thrust measurement and assess the interaction of the plasma noise generated by this device on the LVDT and laser sensor systems. It should be noted that HCs operate in distinct modes where the extreme cases are referred to as the spot mode (a low noise discharge) and plume mode which generates high levels of electrical noise relative to the spot mode.…”

Thrust production mechanisms in Hollow cathode microthrusters

“…However, as suggested, plasma density significantly decreases downstream of the orifice (due to decreasingα ) and as such, so does the anodes capacity to passively collect current from the plasma. Measurements by Goebel 40 , shown in Figure 7, confirmed by the plasma models of Mikellides 39 have shown that plasma density can decrease by nearly two orders of magnitude only 8mm downstream of the orifice in the NSTAR discharge cathode. The NSTAR cathode in this case is a very useful comparison since the cathode orifice is of identical diameter to the T6 cathode used in our both our experiments and previous HCT experiments by Gessini on which our model is based.…”

Controlling the Plasma Flow in the Miniaturized Cylindrical Hall Thruster

“…It seems quite plausible that the coupling between the cathode plasma and the main CHT discharge plasma could affect the stability of some plasma modes. The cathode discharge is known to be the source of noise [20], which can propagate to the thruster plasma [21]. Besides, the cathode plasma sets a boundary condition for the thruster discharge, and, thus, can directly affect the global discharge behavior [22].…”