Shinatora Cho

Kinetic particle simulation of discharge and wall erosion of a Hall thruster

The primary lifetime limiting factor of Hall thrusters is the wall erosion caused by the ion induced sputtering, which is predominated by dielectric wall sheath and pre-sheath. However, so far only fluid or hybrid simulation models were applied to wall erosion and lifetime studies in which this non-quasi-neutral and non-equilibrium area cannot be treated directly. Thus, in this study, a 2D fully kinetic particle-in-cell model was presented for Hall thruster discharge and lifetime simulation. Because the fully kinetic lifetime simulation was yet to be achieved so far due to the high computational cost, the semi-implicit field solver and the technique of mass ratio manipulation was employed to accelerate the computation. However, other artificial manipulations like permittivity or geometry scaling were not used in order to avoid unrecoverable change of physics. Additionally, a new physics recovering model for the mass ratio was presented for better preservation of electron mobility at the weakly magneticall...

Hall Thruster Channel Wall Erosion Rate Measurement Method Using Multilayer Coating Chip

Lifetime problem is extremely important for a Hall thruster because most missions require more than 10,000 hr operation. Hall thruster’s primary lifetime-limit is caused by channel wall erosion. Since conventional erosion measurement methods require long time operation, it is important to develop a fast erosion measurement method to investigate erosion problem and to reduce erosion rate. An erosion rate measurement method using multilayer coating chips is developed for this purpose. This method accelerates the channel wall reduction rate measurement by using small chips with alternate very thin coating layers. The objective of this study is to demonstrate the usefulness of this method. The relationship between magnetic flux density and channel wall erosion rate was examined, and the result was compared with that of conventional emission spectroscopy. In addition, axial distribution measurement of channel wall reduction rate was also conducted.

Study of electron transport in a Hall thruster by axial–radial fully kinetic particle simulation

Electron transport across a magnetic field in a magnetic-layer-type Hall thruster was numerically investigated for the future predictive modeling of Hall thrusters. The discharge of a 1-kW-class magnetic-layer-type Hall thruster designed for high-specific-impulse operation was modeled using an r-z two-dimensional fully kinetic particle code with and without artificial electron-diffusion models. The thruster performance results showed that both electron transport models captured the experimental result within discrepancies less than 20% in thrust and discharge current for all the simulated operation conditions. The electron cross-field transport mechanism of the so-called anomalous diffusion was self-consistently observed in the simulation without artificial diffusion models; the effective electron mobility was two orders of magnitude higher than the value obtained using the classical diffusion theory. To account for the self-consistently observed anomalous transport, the oscillation of plasma properties was speculated. It was suggested that the enhanced random-walk diffusion due to the velocity oscillation of low-frequency electron flow could explain the observed anomalous diffusion within an order of magnitude. The dominant oscillation mode of the electron flow velocity was found to be 20 kHz, which was coupled to electrostatic oscillation excited by global ionization instability.

Parametric Kinetic Simulation of an IHI High Specific Impulse SPT-Type Hall Thruster

A 1 kW class magnetic layer type Hall thruster designed for high specific impulse operation by IHI Corporation, Japan was modeled by a fully kinetic particle code. The measured maximum performance of the thruster was 64% in anode efficiency and 3,200s (3.1kW, 800V) in anode specific impulse. The thruster performance, wall heat loss and erosion, and the plasma property distributions were numerically investigated for the operation conditions ranged from 300V to 700V in discharge voltage, and 2mg/s to 4mg/s in xenon mass flow rate. Simulations with two numerical models: with and without the Bohm diffusion assumption were performed for each thruster operation conditions to characterize the uncertainty caused by the Bohm diffusion model. The simulation results were compared with the measured results, and exhibited excellent agreement with the maximum performance error of 20% for both models. It is suggested that as engineering tools, with and without Bohm simulations can be respectively used as the worst and best case analysis for the performance, heat load, and erosion.

External Discharge Plasma Thruster

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Lifetime Simulation of an SPT-Type Hall Thruster by Using a 2D Fully Kinetic PIC Model

A 2D3V Fully Kinetic Particle-In-Cell Direct-Simulation-Monte-Carlo simulation model was developed for lifetime simulation of SPT-type Hall thrusters. By introducing semiimplicit field solver, the fully kinetic computation was numerically stable without using artificial permittivity. The channel wal sheath was medeled self-consistantly, and the impact of numerical spatial resolusion on the sheath structure was investigated. In addition, the electron energy distribution function was directly simulated, and the result shows that the energy distribution is non-Maxwellian inside the wall sheath region because of the secondary electron emission from the channel wall ceramics. By using the developed fully kinetic simulation, the lifetime performance of two different magnetic field configurations of a 300 W class SPT-type Hall thruster was modeled by periodic channel geometry update according to the calculated channel wall erosion rate. The simulation result shows that the magnetic field confinement by introducing magntic shield can effectively improve the thruster lifetime performance by suppressing the ion wall loss.

Effect of thruster scaling on pre-sheath and ion-loss region in Hall thrusters

2-D plasma simulation of Hall thrusters based upon a hybrid particle-in-cell method has been conducted and the effect of thruster scaling on the ion-loss region and the degree of ions lost to the channel wall were investigated. The Bohm criterion was utilized for the boundary condition of the insulator wall and a magnetic field with plasma lens focusing was applied in the vicinity of the exit. The calculation results visualize the effects of pre-sheath and ion-loss region, which can be criteria for less-erosion designs. The results of thruster scaling show decreasing trends of the effects of ion-loss region and ion-loss fraction with increasing thruster size, which indicates that less erosion and better thrust efficiency are expected by thruster upsizing.

Supporting Structure of the LSD Wave in an Energy Absorption Perspective

In Repetitively Pulsed (RP) Laser Propulsion, laser energy irradiated to a vehicle is converted to blast wave enthalpy during the Laser Supported Detonation (LSD) regime. Based on the measured post‐LSD electron number density profiles by two‐wavelength Mach Zehnder interferometer in a line‐focusing optics, electron temperature and absorption coefficient were estimated assuming Local Thermal Equilibrium. A 10J/pulse CO2 laser was used. As a result, laser absorption was found completed in the layer between the shock wave and the electron density peak. Although the LSD‐termination timing was not clear from the shock‐front/ionization‐front separation in the shadowgraph images, there observed drastic changes in the absorption layer thickness from 0.2 mm to 0.5 mm and in the peak heating rate from 12–17×1013 kW/m3 to 5×1013 kW/m3 at the termination.