Liqiu Wei

Stabilizing of low frequency oscillation in Hall thrusters

Numerical simulations and experiments have been conducted to investigate the effects of a filter on low frequency oscillation in Hall thrusters. With one-dimensional quasineutral hydrodynamic model, the effects of filter components are studied by way of simulation computations. The simulation results show that with proper filter parameters, low frequency oscillation can be stabilized. Further, an eigenvalue study of the linear stability has been performed and the stability conditions according to filter parameters are given. Finally, the theoretical analysis is validated qualitatively through experiments.

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

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 thereb...

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

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.

Characterization of coupling oscillation in Hall thrusters

The characterization of coupling oscillation in a Hall thruster is experimentally studied by varying magnetic flux density or discharge voltage to obtain the relationship between discharge parameters and coupling oscillation. The dispersion relation of coupling oscillation is deduced using a 2D collisionless quasi-neutral fluid model and the factors having their effects on coupling oscillation are obtained. Experimental results and theoretical analysis indicate that coupling oscillation increases with magnetic flux density, discharge voltage or coupling intensity coefficient. The instability has a very large wave number within a frequency spectrum ranging from hundreds of kilohertz to megahertz.

Visual evidence of suppressing the ion and electron energy loss on the wall in Hall thrusters

A method of pushing down magnetic field with two permanent magnetic rings is proposed in this paper. It can realize ionization in a channel and acceleration outside the channel. The wall will only suffer from the bombardment of low-energy ions and electrons, which can effectively reduce channel erosion and extend the operational lifetime of thrusters. Furthermore, there is no additional power consumption of coils, which improves the efficiency of systems. We show here the newly developed 200 W no wall-loss Hall thruster (NWLHT-200) that applies the method of pushing down magnetic field with two permanent magnetic rings; the visual evidence we obtained preliminarily confirms the feasibility that the proposed method can realize discharge without wall energy loss or erosion of Hall thrusters.

Effects of magnetic field strength on the low frequency oscillation in Hall thrusters

In order to study the effect of magnetic field strength on low frequency oscillation in Hall thrusters, experiments were carried out with different operating parameters. Experimental results show that the effect of magnetic field strength on the low frequency oscillation changes with operating parameters. In the decline zone of magnetoampere characteristic curve, low frequency oscillation increases with the increase of magnetic field strength at low mass flow rate, while decreases with the increase of magnetic field strength at high mass flow rate. With further experiments and numerical simulations, it is found that the change of electron current at low mass flow rate and the change of ion current at high mass flow rate account for the variations of low frequency oscillation. Finally, the physical analysis is performed.

Experimental study on low frequency oscillation in a plume of Hall thrusters

In order to study the low frequency oscillation (20?40?kHz) in a plume of Hall thrusters, the plasma parameter oscillations are measured with a Langmuir probe in axial and circumferential directions, and oscillations of different energy ions are measured with a multi-grid probe by varying the potential to repel selected ions. Experimental results indicate that the oscillation of plasma parameters in the plume exhibits the same frequency as a low frequency discharge current oscillation and is synchronous in the circumferential direction at the time scale of the low frequency discharge current oscillation. The time delay of the oscillation in different axial positions is related to the propagation of plasma density along the axial direction. The oscillation of Xe+ has the same frequency as the low frequency discharge current oscillation, but the oscillation of Xe2+ or a high-valence ion is more complex and scattered due to their instability in the ionization process. It is therefore concluded that the low frequency oscillation of plasma parameters in the plume is strongly related to the low frequency discharge current oscillation and the ionization process of a neutral atom.

Effect of oblique channel on discharge characteristics of 200-W Hall thruster

In an experiment involving a 200-W Hall thruster, partial ionization occurs in the plume area because of the extrapolation of the magnetic field. To improve the thruster performance, the concept of an oblique channel is proposed for improving the ionization degree in the plume area. Calculations performed using a Particle-in-cell (PIC) simulator and the experimental results both show that an oblique channel structure can reduce the wall loss. Compared with a straight channel under similar conditions of the discharge voltage and current, the ionization degree in the plume area, thrust, specific impulse, propellant utilization, and anode efficiency are improved by ∼20%. The oblique channel is an important design consideration for improving the partial ionization of the plume area in the thruster.

Effect of dielectric wall temperature on plasma plume in an argon atmospheric pressure discharge

In this letter, the effect of the dielectric wall temperature on the length and volume of an atmospheric pressure plasma jet (APPJ) is investigated using a single-electrode configuration driven with an AC power supply. To distinguish the APPJ status from the argon flow rate, the three modes, laminar, transition, and turbulent, are separated. When the dielectric wall is heated, the APPJ length and volume are enhanced. Also, the transition regions remarkably expand over a large range of flow rates. The results indicate that different factors contribute to the expansion of the transition region. The increase in the radial and axial velocities is the main cause of the expansion of the transition region to the low-velocity region. The expansion to the high-velocity region is dominantly induced by a change in the viscosity.

Experimental test of 200 W Hall thruster with titanium wall

We designed a 200 W Hall thruster based on the technology of pushing down a magnetic field with two permanent magnetic rings. Boron nitride (BN) is an important insulating wall material for Hall thrusters. The discharge characteristics of the designed Hall thruster were studied by replacing BN with titanium (Ti). Experimental results show that the designed Hall thruster can discharge stably for a long time under a Ti channel. Experiments were performed to determine whether the channel and cathode are electrically connected. When the channel wall and cathode are insulated, the divergence angle of the plume increases, but the performance of the Hall thruster is improved in terms of thrust, specific impulse, anode efficiency, and thrust-to-power ratio. Ti exhibits a powerful antisputtering capability, a low emanation rate of gas, and a large structural strength, making it a potential candidate wall material in the design of low-power Hall thrusters.