Axial-oriented and azimuthal-distributed grooves are formed on channel walls of a Hall thruster after the engine undergoes a long-term operation. Existing studies have demonstrated the relation between the grooves and the near-wall physics, such as sheath and electron near-wall transport. The idea to optimize the thruster performance with such grooves was also proposed. Therefore, this paper is devoted to explore the effects of wall grooves on the discharge characteristics of a Hall thruster. With experimental measurements, the variations on electron conductivity, ionization distribution, and integrated performance are obtained. The involved physical mechanisms are then analyzed and discussed. The findings help to not only better understand the working principle of Hall thruster discharge but also establish a physical fundamental for the subsequent optimization with artificial grooves.

Hall thruster with grooved walls

A type of Hall thruster without wall losses is designed by adding two permanent magnet rings in the magnetic circuit. The maximum strength of the magnetic field is set outside the channel. Discharge without wall losses is achieved by pushing down the magnetic field and adjusting the channel accordingly. The feasibility of the Hall thrusters without wall losses is verified via a numerical simulation. The simulation results show that the ionization region is located in the discharge channel and the acceleration region is outside the channel, which decreases the energy and flux of ions and electrons spattering on the wall. The power deposition on the channel walls can be reduced by approximately 30 times.

https://iopscience.iop.org/article/10.1088/0022-3727/49/46/465001/pdf

Computer simulations of Hall thrusters without wall losses designed using two permanent magnetic rings

Hall thrusters provide efficient space propulsion by electrostatic acceleration of ions. Rotating electron clouds in the thruster overcome the space charge limitations of other methods. Images of the thruster start-up, taken with a fast camera, reveal a bright ionization period which settles into steady-state operation over 50 μs. The cathode introduces azimuthal asymmetry, which persists for about 30 μs into the ignition.

/pdf/fast-camera-imaging-of-hall-thruster-ignition-49lda1w96z.pdf

Fast Camera Imaging of Hall Thruster Ignition

Two types of high frequency electromagnetic resistive instabilities are found to occur in a Hall thruster plasma in the presence of collisions and ionization, out of which one of lower growth rate (called lower magnitude high frequency instability (LMHFI)) is sensitive to the axial component of the wave vector. The effects of ionization, collisions, and electron drift velocity on the growth rates of these instabilities are studied in greater details. The LMHFI grows faster in the presence of ionization but shows weaker dependence on the electrons' E→×B→ drift, contrary to the case of other instability (called higher magnitude high frequency instability) which is sensitive to the azimuthal wave number and collisions.

High frequency electromagnetic resistive instability in a Hall thruster under the effect of ionization

Influence of hollow anode position on the performance of a hall-effect thruster with double-peak magnetic field

The Debye sheath has a significant effect on the performance of Hall thrusters. The dynamic characteristics of the two-dimensional sheath is investigated using the 2D-3V particle-in-cell method in this paper. The numerical results show that while the sheath exhibits the one-dimensional stability when the electron temperature is relatively low, it behaves as a two-dimensional (both in time and space) oscillating characteristic when the electron temperature is high. Moreover, it is found that the oscillating frequency is the same order as the electron plasma frequency and the spatial wavelength is equal to the length of the electrostatic wave.

Effect of electron temperature on dynamic characteristics of two-dimensional sheath in Hall thrusters

An experiment has been made to investigate the effect of curved magnetic fieldtopology on near wall conductivity in the ion acceleration region of Hall thrusters. The experimental results show that the electron current due to near wall conductivity is of the minimum in the case of focused topology and increases in the cases of both less-focus and over-focus topologies. This finding cannot be explained properly by the magnetic mirroreffect, which is the one and only reported effect related to the magnetic field curvature so far. Based on the analysis of interaction between the plasma and the wall, a new physical effect is proposed. The difference of magnetic fieldtopology causes different electric potential distribution, leads to different ion flux to the wall, results in the change of sheath property and secondary electron emission, and finally affects the electron current due to near wall conductivity. This effect is further justified by the agreement between the experiment and simulation which is performed with a particle-in-cell model. Therefore, we conclude that the ion flow injection is a significant effect to near wall conductivity in the scope of curved magnetic fieldtopology besides the magnetic mirroreffect. Moreover, we find that the focus topology of magnetic field is favorable to obtain a high thruster performance from both the ion acceleration aspect and the electron conduction aspect and so is useful practically for thruster optimization.

Experimental and theoretical study on effects of magnetic field topology on near wall conductivity in a Hall thruster

It was found through the experiments made with an Aton-type Hall thruster that some of the propellant was ionized in the buffer chamber by “quick electrons” This ionization is called “preionization” to discriminate it from the ionization in the discharge channel The effect of preionization on low frequency oscillation was experimentally studied by changing the electric field intensity in the buffer chamber The relationship between low frequency oscillation and preionization ratio was investigated through numerical simulation using a one-dimensional quasineutrality hydrodynamic model The results obtained indicate that the amplitude of low frequency oscillation decreases as the preionization ratio increases It was found through the analysis and numerical simulation of the physical process of low frequency oscillation that the positive feedback of electron density was the main cause of low frequency oscillation The increase of preionization ratio decreases the amplitude of the feedback variation thereby reducing the amplitude of low frequency oscillation

Effect of preionization in Aton-type Hall thruster on low frequency oscillation

In order to study the physical mechanism of an oscillation newly discovered by the Harbin Institute of Technology Plasma Propulsion Lab (HPPL) in the range of hundreds of kHz to several MHz, Hall thrusters with different magnetic coils are studied by changing one of the following three parameters: discharge voltage, anode flow and coil current, directly measuring the coil current and measuring plasma oscillations related to coil current oscillation with the Langmuir probe. Experimental results indicated that in the discharge process of a Hall thruster the broadband turbulence of the Hall current causes an unstable spatial magnetic field and this field causes the magnetic circuit to resonate as an equivalent high level resistance?inductance?capacitance (RLC) network. As the response of the network, the oscillation of the coil current has a large oscillating component at the natural frequencies of the network. Also, the oscillation of coil current has an effect on the discharge process at the same time, so that they reach a self-consistent equilibrium state. As a result of such a coupling, both coil current and the discharge current exhibit their oscillating component at the natural frequencies of the magnetic circuit. It is therefore concluded that the newly discovered oscillation is caused by the coupling between the magnetic circuit and the discharge circuit.

https://iopscience.iop.org/article/10.1088/0963-0252/16/4/010/pdf

Experimental study on the physical mechanism of coupling oscillation: a newly discovered oscillation in Hall thrusters

Scaling Design and Experimental Study on Hall Thrusters with Curved Magnetic Field

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