Hall-effect thrusters, which are electrostatic devices based on an E B plasma discharge, have successfully been used as satellite propulsion systems for the last few decades. However, the presence of anomalous electron cross-field transport is still poorly understood, and involves complex and strongly coupled mechanisms such as azimuthal electron drift instabilities and intense secondary electron emission (SEE) from the thruster walls. The present work focuses on the relative importance of these two phenomena. We use a 2D particle-in-cell/Monte Carlo collision model configured to simulate the radial-azimuthal directions near the thruster exit plane. A constant radial magnetic field and axial electric field are imposed, and electron drift instabilities are observed in the azimuthal (E B) direction. A simplified SEE model is implemented and an extensive parametric study is performed to directly determine the effect on electron transport. It is found that, for the operating conditions used in our simulations, SEE enhances the near-wall electron mobility by a factor 2, while reducing the bulk plasma mobility by about 20% (due to electron cooling). However, the dominant contribution to anomalous electron transport is still observed to be caused by electron drift instabilities driven by the E B discharge configuration. SEE modifies the electron mobility profile, but the spatially-averaged value remains relatively constant. Three different operating regimes are identified depending on the SEE rate value: two that are stable, and a third which shows an oscillatory behaviour. In addition to electron transport, the kinetic simulations give new insight into the plasma sheath formation at the radial walls, and comparison with typical analytical sheath models demonstrate important differences.
The evolution of the electron mean energy in the pre-sheath and the sheath of a low pressure plasma bounded by two planes is investigated with 1D particle in cell simulations. We observed that the electron mean energy is not constant in the sheath, but instead decreases significantly from the bulk towards the wall. From the simulations, a polytropic state law is proposed, allowing us to close the fluid equations for the electrons without the isothermal hypothesis. A comparison between the fluid model and the simulations show that the non-isothermal sheath model is more accurate than the isothermal model. The impact of the electron mean energy variation on the potential sheath drop and the electron particle and heat flux is evaluated.
Plasma transport in magnetized discharges is a long-standing problem because it strongly depends on instabilities whose properties and influence on the plasma transport are difficult to predict. A magnetized plasma column is investigated with 2D PIC simulations in the plane transverse to the magnetic field, at gas pressures between 3 and 12 mTorr and magnetic field intensities between 0 and 40 mT. At high magnetic field, instabilities develop and rotate in the diamagnetic drift direction. It is shown theoretically and by simulation that the magnetic field confinement is destroyed by the instability. Predictive formulas of the main parameters of the instability-enhanced plasma transport such as the edge-to-center plasma density ratio (or h factor), and the effective collision frequency are provided and successfully compared with the PIC simulations.
It is shown that the transport in low temperature, collisional, bounded plasma is enhanced by instabilities at high magnetic field. While the magnetic field confines the electrons in a stable plasma, the instability completely destroys the confinement such that the transport becomes independent of the magnetic field in the highly magnetized limit. An analytical expression of the instability-enhanced collision frequency is proposed, based on a magnetic field independent edge-to-center density ratio.
Edge-to-center plasma density ratios-so-called h factors-are important parameters for global models of plasma discharges as they are used to calculate the plasma losses at the reactor walls. There are well-established theories for h factors in the one-dimensional case. The purpose of this paper is to establish h factors in twodimensional (2D) systems, with guidance from a 2D particle-in-cell (PIC) simulation. We derive analytical solutions of a 2D fluid theory that includes the effect of ion inertia, but assumes a constant (independent of space) ion collision frequency (using an average ion velocity) across the discharge. Predicted h factors from this 2D fluid theory have the same order of magnitude and the same trends as the PIC simulations when the average ion velocity used in the collision frequency is set equal to the ion thermal velocity. The best agreement is obtained when the average ion velocity varies with pressure (but remains independent of space), going from half the Bohm velocity at low pressure, to the thermal velocity at high pressure. The analysis also shows that a simple correction of the widely-used 1D heuristic formula may be proposed to accurately incorporate 2D effects.
Hall-effect thrusters (HETs) operated with xenon are one of the most commonly used electric propulsion technologies for a wide range of space missions, including drag compensation in low Earth orbit, stationkeeping, and orbital insertion, as access to space becomes more affordable. Although anomalous electron transport, the electron drift instability (EDI), and secondary electron emission (SEE) have been studied experimentally and numerically in xenon-based HETs, the impact of alternative propellants is still poorly characterized. In this work, a two-dimensional particle-in-cell/Monte Carlo collision (PIC/MCC) code is used to model the (r − θ) plane of a HET operated separately with four different noble gases: xenon, krypton, argon, and helium. Models for electron induced secondary electron emission (SEE) and dielectric walls are implemented in order to investigate the coupling between the propellant choice and the radial thruster walls. For all conditions and propellants studied, an EDI and enhanced electron cross-field transport are observed. The frequency of the instability, as well as the electron mobility, are compared with analytical expressions from a recently developed kinetic theory. Confirming this theory, it is shown that while the frequency of the EDI depends on the propellant mass, the electron mobility appears to be almost independent of the propellant choice.
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