Kinetic Method for Quasi-One-Dimensional Simulation of Magnetic Nozzle Plasmadynamics.

Abstract

A novel technique was developed modeling two-dimensional magnetic field effects in a one-dimensional electrostatic particle-in-cell code. This quasi-one-dimensional formulation incorporates two-dimensional effects through inclusion of cross-sectional area variation and magnetic field forces. The new method is verified with a newly formulated set of test cases of a two-particle system, magnetic mirrors, and fully two dimensional simulations.

Magnetic nozzle physics and ion acceleration in low temperature plasmas were investigated with a test problem using these kinetic simulations. Effects of the density variation due to plasma expansion and the magnetic field forces on ion acceleration were investigated. Density variation only weakly affected ion acceleration. Magnetic field forces acting on the electrons were found to be responsible for the formation of potential structures which accelerate ions. Formation of a high energy ion beam is seen due to ion acceleration. Strongly diverging magnetic fields drive more rapid potential drops and the length of the radio frequency heating region was found to significantly affect the electron temperature profiles. Simulations were performed with both argon and xenon. For the same driving current, argon simulations demonstrated higher ion velocities while xenon simulations showed higher plasma densities.

Ion acceleration physics was investigated verifying that ion acceleration occurs due to potential structures established by the magnetic field forces on the electrons. Effects of anisotropic electron pressure tensors were also found to be important for determining an Ohm's law used to solve for the induced electric field which accelerates the ions. Bi-Maxwellian and non-Maxwellian velocity distributions were seen for the electrons along with the anisotropic temperatures, verifying the need for kinetic simulations. Electron thermodynamic relations (isothermal, adiabatic, polytropic, double adiabatic) were evaluated for a number of simulation results. Results from quasi-one-dimensional simulations were used to estimate thruster performance parameters such as specific impulse and thrust.

Simulations with parameters similar to the Helicon Double Layer Thruster were performed. Results from these simulations look encouraging for future device studies. Similar electron temperatures and normalized density profiles are seen in the experiments and simulations. Velocity and energy distribution functions for ions and electrons also show similar behavior to that measured in experiments.