Experimental scaling laws for the discharge oscillations and performance of Hall thrusters

Abstract

Low-frequency (5–30 kHz) discharge current oscillations characterize the operation of Hall thrusters and represent a valuable metric not only to monitor the thruster behavior, but also to optimize the thruster performance. Two types of low-frequency oscillation modes are commonly observed: a global “breathing” mode, associated with the propellant ionization, and a local mode, typically characterized by a lower oscillation amplitude and the appearance of azimuthal spokes. The main characteristics of discharge oscillations and the transition between the two oscillation modes vary greatly with the thruster geometry and the operating condition. In this work, we present the results of an experimental campaign carried out on a 20 kW-class thruster prototype, SITAEL’s HT20k, with an exchangeable discharge channel and a magnetic circuit. Three different channel sizes were tested over a wide range of operating conditions and magnetic fields. For each operating point, a high frequency measurement of the discharge current was performed, recording the main characteristics of the oscillations. The data collected were then processed to derive the influence coefficients of each thruster parameter on the discharge current characteristics, as well as their dispersion. Finally, this allowed us to formulate general, data-driven scaling laws for the discharge current salient features, such as oscillation amplitude and dominant frequency. The gathered insight sheds light on the physical processes involved in the thruster discharge. At the same time, the possibility to model with simple functional laws the main oscillatory mode of Hall thrusters offers a unique aid to the optimization of thruster design and the evaluation of thruster performance during life.

On the onset of breathing mode in Hall thrusters and the role of electron mobility fluctuations

“…We now focus on the role of mobility on the self-sustenance mechanism of the breathing mode. The idea that mobility can play a dominant role in the instability originates from the observation that breathing mode can be suppressed, both in experiments [8,23] and in the present model (not shown here), by increasing the intensity of the magnetic field. Note that in the model, B r enters only via the mobility μ.…”

Investigation of the Effect of Magnetic Field and Propellant on Hall Thruster’s Stability via a 0D Model

“…Concerning the frequency, the linear stability analysis yields values in the range of 5-30 kHz, which are in line with those typically observed for the breathing mode instability in Hall thrusters. Moreover, the frequency tends to grow with B max , in agreement with previous experimental observations [6,8,33,34] and higher-order simulations [23] carried out by different authors and with different thrusters.…”

“…Interestingly, the tuned 0D model identifies a minimum value of B max below which the system becomes unstable. This behavior is coherent to the local-to-global mode transition that is typically observed in Hall thrusters operating at low magnetic fields, both in the experiments and in higher order simulations (see, for instance, [6][7][8]23]). Unfortunately, in the absence of dedicated experimental data for different magnetic field intensities, we could not perform a quantitative validation of these results.…”

“…The transition was observed to shift towards higher magnetic fields when the voltage and/or the mass flow rate were increased. More recently, Giannetti et al [8] presented the results of the processing of a large amount of thrust and current trace measurements to derive general data-driven scaling laws for current oscillations and thruster performance when the channel geometry and operating conditions of the thruster are varied. Different types of intrusive [9][10][11] and nonintrusive [12][13][14][15] diagnostic have also been used to characterize the longitudinal plasma dynamics in the channel and near plume during the breathing mode cycles.…”

An unstable 0D model of ionization oscillations in Hall thruster plasmas

“…Breathing mode is found ubiquitously in Hall thrusters, at least for some operating conditions, and appears to be a general feature of E × B discharges. Nevertheless, the specific characteristics of the oscillations vary greatly with the thruster design and operating condition [2,3]. The presence of this oscillatory mode has detrimental effects on the thruster performance and can cause coupling issues with the driving electronics.…”