Daren Yu

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...

Effect of oscillating sheath on near-wall conductivity in Hall thrusters

The effect of an oscillating sheath on near-wall conductivity has been studied using a model of the incident electron motions with the characteristic parameters of the oscillating sheath quantified. The model predicted an increase in both the frequency of electron-wall collisions and the coefficient of secondary electron emission compared to the steady sheath regime. This effect indicates that near-wall conductivity is enhanced in the oscillating sheath regime.

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

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.

Numerical simulation for near wall conductivity effect on current profiles in the annular channel of hall-type stationary plasma thrusters

Instead of the slab channel model used in previous work, a cylindrical model representing the actual annular channel in Hall-type stationary plasma thrusters is introduced in this paper for the near wall conductivity (NWC) effect on the current profile. The electron dynamics process in the plasma, however, is still described by the same test particle method in current theories where electrons are randomly emitted into the plasma from the wall to simulate electron scattering on the insulating wall. Monte Carlo method is applied to solve the model. The numerical results show that the current density peaks due to NWC near the inner wall of the channel is different from that near the outer wall, a typical annular effect other than in the slab model where the two current peaks are the same, and the ratio of them is approximately the same as the ratio of the inner and outer radii. Besides, the integrated current in the thin layer near the wall grows linearly with the electric field and quadratically with the in...

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

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.

Effects of ionization distribution on plasma beam focusing characteristics in Hall thrusters

The relationship between ionization distribution and divergence of plasma beam in a Hall thruster is investigated using spectrum and probe methods. Experimental results indicate that the shift of ionization region towards the exit of channel causes the reduction of accelerating field and the enhancement of electron thermal pressure effect, which result in further deviation of equipotential lines to magnetic field lines and further increase in divergence of plasma beam. It is, therefore, suggested that to put the ionization region deep inside the channel and separate it from the acceleration region at the design, and development stage is helpful to improve the plasma beam focusing characteristics of a Hall thruster.

Characteristics of electron near-wall transport under two-dimensional dynamic sheath in a Hall effect thruster

It is demonstrated that the features of measured electron current profile in the near-wall region of a Hall effect thruster are mainly due to the substantive breakdown of electron Hall drifts caused by the azimuthal field of the two-dimensional dynamic sheath. This kind of cross-field diffusion shows its close connection with the anomalous electron transport.

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.

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.

The spatiotemporal oscillation characteristics of the dielectric wall sheath in stationary plasma thrusters

A two-dimensional particle in cell model is used to simulate the sheath oscillation in stationary plasma thrusters. The embedded secondary electron emission (SEE) submodel is based on that of Morozov but improved by considering the electron elastic reflection effect. The simulation results show that when the SEE coefficient is smaller than one due to the relative low electron temperature, one-dimensional static sheath can be found; as the electron temperature increase, the SEE coefficient approaches to one and temporal oscillation sheath appears; when the electron temperature increases so high that the SEE coefficient is beyond one, the sheath oscillates not only in time but also in space.