Kinetic simulation of high I(sp) plasma thruster

Abstract

Two different numerical models are developed for the analysis of the VASIMR thruster. The first, zero dimensional model provides a calculation of the main parameters of the helicon hydrogen discharge. It solves a set of 12 coupled non-linear plasma chemistry equations with relevant boundary conditions. The second model is a fully kinetic model, which allows us to calculate distribution functions of the plasma species in the presence of various collisions, RF heating and non-uniform magnetic field. The simulation focused on resolving the energy Soss problem in current hydrogen experiments. Results of the helicon source simulation, using the first model indicate that Frank-Condon neutrals transfer 10-15% of input power to the wall. Lyman-a radiation accounts for 20-25% of input power. Molecular ions fraction is about 20% of the ion population. Electron and ion temperatures are 6eV and leV, respectively. Kinetic simulation shows that distribution functions of plasma species are not equilibrated. INTRODUCTION The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) Project'' is a high 1 plasma thruster that is expected to operate in a semi-collisional regime. Current hydrogen experiments' * are showing higher energy losses than helium discharges. Design of an efficient hydrogen thruster will require identification and removal of these energy losses. Current experimental information of plasma composition is limited, making it difficult to determine the energy loss processes. The numerical model developed here will provide an analysis tool for improving the thruster design and to support the experiment. This device combines three distinct areas i) plasma source, ii) plasma RF-heating cell and iii) magnetic nozzle, as shown in the Fig.l. The most probable propellant is Hj (or D2) to achieve maximum 1 , at a given mass fueling rate. There are a number of important collisional processes taking place in such a thruster. There is a set of inelastic collisions to break incoming molecular gas into neutral atoms, ionization and excitations of neutrals, and wall interactions. Next, there is a set of elastic collisions of potential importance: Coulomb, resonance charge-exchange, etc. The Knudsen number for the different elementary processes varies in a range from 0.1-10, which indicates that short the short mean free path limit fails, and a pure kinetic analysis is required. In the current work we present results from two models: A) 0-D non-linear plasma chemistry model for a helicon plasma source, and B) ID-2V Fokker-Planck model embracing ion heating and plasma exhaust region. Model A allows us to obtain crucial parameters of the helicon discharge plasma and neutral species composition, temperatures of all plasma components, energy/particle fluxes out of the plasma source and heat fluxes onto wail. Model B is more advanced. It takes into account ICRH heating of ions, Coulomb collisions, mirror force, ambipolar electric field. However, it assumes that there are no molecular ions or molecules in the system. We apply continuous Fokker-Planck approach, which gives a number of advantages when compared to standard PIC methods , but is also more computationally challenging. As a result of kinetic treatment via Model B we obtain distribution functions of the plasma species, which provide a compete picture of the operational regimes of the VASIMR thruster. MODEL A The Heiicon plasma discharge has proven to be a robust and efficient plasma source . The Helicon source consists of a dielectric (quartz) tube embraced by the helicon antenna, which launches several waves in the plasma. Electrons are heated through either collisional or Landau damping of the waves in the plasma. This process is not included self-consistently here; it is a subject of a separate study. In this model we assume that a certain power, W, is transmitted to the electrons. The electron temperature and density in the * MIT NED Researcher, also Professor at Moscow Inst. of Physics and Technology * Professor of Nuclear Engineering Department, MIT Copyright © 2000 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 1 American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. source are 6eV and 2x10 cm as measured in the ASPL experiments . is