Zhiwen Wu

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.

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.

Study of breakdown in an ablative pulsed plasma thruster

Breakdown in ablative pulsed plasma thrusters (APPTs) must be studied in order to design new types of APPTs and measure particular parameters. In this paper, we studied a parallel-plate ablative pulsed plasma thruster that used a coaxial semiconductor spark plug. By operating the APPT about 500 times with various capacitor voltages and electrode gaps, we measured and analyzed the voltage of the spark plug, the voltage between the electrodes, and the discharge current. These experiments revealed a time delay (∼1–10 μs) between spark plug ignition and capacitor discharge, which may affect the performance of high-pulsing-rate (>10 kHz) and double-discharge APPTs, and the measurements of some of the APPT parameters. The delay time decreased as the capacitor voltage increased, and it increased with an increasing electrode gap and increasing number of ignitions. We explain our results through a simple theoretical analysis.

Study on the wall ablation of heated compound-materials into discharge plasmas based on a modified model

Velocity of ablation vapor near the surface of heated compound-materials strongly affects the kinetic layer parameters modeled and manifested in the Knudsen layer. This paper discussed overlooked physics and clarified inaccuracies in the expression of velocity at the outer boundary of the kinetic layer induced by discharge plasma. The changes of average molecular mass coupling with discharge current on mass and momentum conservation equations in plasma layer were considered when modifying the expression of this boundary velocity. Our assessment of these effects indicated that velocity of ablation vapor showed a downtrend as the ratio of average molecular mass at inner and outer boundaries of plasma layer increased, which plays a decisive role in reducing the ablation rate. Compared with single species fluid model, the modified model that applies to the pyrolysis of heated compound-materials showed 56% drop in Teflons ablation rate when plasmas were fully ionized.

Analysis of Teflon Pulsed Plasma Thrusters Using a Modified Slug Parallel Plate Model

The Pulsed Plasma Thruster (PPT) is a reliable, relatively simple and low cost electric propulsion thruster that has been used widely in space flight mission. Numerical simulation is an important part of the overall research and development process for PPTs. To provide physical insights and guidance toward design of consistently efficient PPTs, a modified slug model for parallel-plate PPTs is presented in this paper. This new model is formed on the basis of electromechanical model (i.e. slug model), which has been published by American Worchester Polytechnic Institute. Different from common slug model, this new model assumes that after the plasma slug is formed there is additional mass accumulated to the plasma as it accelerates down the discharge chamber and force within the PPT comes from an gas dynamics due to build up of pressure within the thruster chamber and a magnetic acceleration from the Lorentz force. So the modified slug model can calculate the ablated mass per discharge and can describe the electromagnetic acceleration and aerodynamic efficiency in the process of plasma flow. The late time ablation (LTA) is sublimation of propellant that takes place after the main discharge, due to the propellant, temperature being above its sublimation point, and LTA is considered as one of the main contributors for PPT’s low efficiency. Thus the new model also analyze the impact of LTA for the performance of PPT according to the velocity of decomposed mixture of neutral atoms, monomers and free-radical chains of molecules (“slurry”) after the main discharge. Finally, influences of variations of electric parameters and configuration parameters on thruster performance are studied by numerical simulation.

Characteristics of plasma properties in double discharge ablative pulsed plasma thrusters

Ablative pulsed plasma thrusters, the earliest electric space propulsion devices, create highly transient plasmas in short discharges that are expelled to create thrust. In recent years, the double-discharge ablative pulsed plasma thruster design has been proposed to improve the low-thrust efficiency. In this study, optical emission spectroscopy was applied to investigate the plasma properties in different regions and energy distributions. The electron temperature and electron density of the plasmas are derived and discussed. This study provides a physical mechanism for double-discharge pulsed plasma thrusters.

Numerical Investigation of Self-Ignition Characteristics of Solid-Fuel Scramjet Combustor

The self-ignition characteristics of a solid fuel under supersonic flow have been investigated theoretically and numerically. Time-dependent two-dimensional axisymmetric compressible Navier–Stokes equations and species transport equations are solved numerically. Turbulence closure is achieved using the shear stress transport k-ω model. Polymethylmethacrylate fuel and a global one-step reaction mechanism are used in this study. The reaction rate is determined by finite-rate chemical kinetics with the turbulence–chemistry interaction modeled by an eddy-dissipation model. The numerical results generally agree with the experimental data in the published literature. The flame spread and pressurization during the self-ignition of polymethylmethacrylate in the combustor have been studied. The effect of inlet flow conditions and the geometry of the combustor on self-ignition behavior have been analyzed. Three stages of flame spread (namely, heat accumulation, secondary recirculation zone self-ignition, and orderl...

Numerical Simulation of a Water Propellant Pulsed Plasma Thruster

Water, an al ternati ve propellant, was consi dered to be a key point to i mprove the performance of pulsed pl asma thruster(PPT). To date, the effecti veness of PPT using water propellant had been testified by experi mental methods. First of all, combined with the available Tefl on PPT numerical si mulation, computational model was set up for water propellant PPT. Then a comparison was made wi th the experi mental results from the Uni versity of Tokyo. It showed that big di fferences were existed between the experi mental results and the simulation results. The pri mary reason was that the operati on process was simulated as first order completel y i onization which was an i deal situation. Fi nally several cases were calcul ated to discover that every initial energy corres pondingly had a sufficient ablation mass. And different supplied mass coul d meet different tasks.

Parametric Study of an Air-Breathing Electric Propulsion for Near-Space Vehicles

An air-breathing electric propulsion system is regarded as a potential alternative propulsion system to the propeller system for near-space vehicles. The goal of this paper is to study the performa...