Kazuo Uematsu

Improvement of Three-dimensional Monte Carlo Code PHITS for Heavy Ion Therapy

The general purpose particle and heavy ion transport code, PHITS, was modified for improved analysis of dose distribution in carbon therapy systems. We added two new functions into PHITS, one for an energy dispersion calculation and the other for transport in an AC magnetic field, which enabled 3-dimensional modelling of a carbon therapy system for the first time. With this code we calculated the dose distribution in a carbon therapy system, and these results showed good agreement with experimental data. This improved version of PHITS is a valuable tool for the design of carbon therapy aperture or for the estimation of the dose distribution in treatment planning.

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.