This thesis investigates fundamental flows of resistive magnetohydrodynamics (MHD) by a new numerical tool based on the gas-kinetic method. The motivation for this work stems from the need to analyze the mechanisms of plasma detachment in the exhaust plume of the magnetoplasma rocket known as VASIMRR. This rocket has great potential for reducing the travel time for deep space exploration missions. However, it is very difficult to investigate detachment in ground-based experiments because this large-scale device can fully function only in a vacuum. This difficulty makes computational analysis and modeling an important part of the design and testing process. A parallelized Boltzmann-BGK continuum flow solver is expanded to include resistive MHD physics. This new code is validated against known solutions to MHD channel flows and new results are presented for simulations of a laminar round jet subject to a constant applied magnetic field as well as the diverging magnetic field of a current loop. Additionally, a parametric map is presented that outlines appropriate conditions required when using a fluid model for magnetic nozzle flows. The work of this thesis serves as an introductory step to developing a robust numerical ow solver capable of simulating magnetic nozzle flows and other plasmas that cannot be easily replicated in ground facilities.

The numerical simulation and modeling of plasma detachment from a magnetic nozzle is presented. The detachment problem is of key importance to the plasma-based propulsion concepts that employ a guiding magnetic field to control plasma flow and motivated by the needs of the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) project. The detachment of the plasma exhaust is required to produce directed thrust. In the present scenario plasma can stretch the magnetic field lines to infinity, similar to the solar wind. In order to extend the magnetic nozzle model beyond the limitations of analytic theory, a numerical code is developed to simulate steady-state kinetic plasma flows and to evaluate nozzle efficiency. The direct solution of a steady-state problem, as opposed to an initial value problem, eliminates the need to deal with transient phenomena that are of secondary importance for continuously operated plasma thrusters. The new simulation code is verified against the analytic results and then used to model the plasma behaviour for the conditions of the Detachment Demonstration Experiment (DDEX) at the NASA Marshall Propulsion Research Center, Huntsville, Alabama. / text

In order to enable comparison of Helicon ion thruster performance across different vacuum test facilities, an understanding of the effect of operating pressure on plasma plume properties is required. Plasma property measurements are compared for thruster operation at two separate vacuum facility operating pressures to determine the effect of neutral ingestion on Helicon ion thruster operation. The ion energy distribution function (IEDF), electron temperature, ion number density, and plasma potential are measured along the thruster main axis for a replica of the Madison Helicon eXperiment. Plasma property values recorded at the ‘high-pressure condition’ (3.0×10^(-4) Torr corrected for argon) are compared to values recorded at the ‘low-pressure condition’ (1.2×10^(-5) Torr corrected for argon) for thruster operation at 100 - 500 watts radio frequency forward power, 340 – 700 gauss source region magnetic field strength, and 1.3 - 60 sccm argon volumetric flow rate (0.039-1.782 mg/s). Differences in plasma behavior at the ‘high-pressure condition’ result from two primary neutral-plume interactions: collisions between accelerated beam ions and ingested neutrals leading to a reduction of ion energy and neutral ionization downstream of the thruster exit due to electron-neutral collisions. Electron temperature at higher operating pressures is lowered due to an electron cooling effect resulting from repeated collisions with neutral atoms. Results suggest that Helicon ion thruster plasma properties are greatly influenced when subjected to neutral ingestion.

Magnetic nozzles, like Laval nozzles, are observed in several natural systems and have application in areas such as electric propulsion and plasma processing. Plasma flowing through these nozzles is inherently tied to the field lines and must separate for momentum redirection or particle transport to occur. Plasma detachment and associated mechanisms from a magnetic nozzle are investigated. Experimental results are presented from the plume of the VASIMR® VX-200 device flowing along an axisymmetric magnetic nozzle and operated at two ion energies to explore momentum dependent detachment. The argon plume expanded into a 150m3 vacuum chamber where the background pressure was low enough that charge-exchange mean-free-paths were longer than experiment scale lengths. This magnetic nozzle system is demonstrated to hydrodynamically scale up to astrophysical plasmas, particularly the solar chromosphere, implying general relevance to all systems. Plasma parameters were mapped over a large spatial range using measurements from multiple plasma diagnostics. The data show that the plume does not follow the magnetic field lines. A mapped integration of the ion flux shows the plume may be divided into three regions where 1) the plume briefly follows the magnetic flux, 2) diverges quadratically before 3) expanding with linear trajectories. Transitioning from region 1→2, the ion flux departs from the magnetic flux suggesting ion detachment. An instability forms in region 2 driving an oscillating electric field that causes ions to expand before enhancing electron cross-field transport through anomalous resistivity. Transitioning from region 2→3 the electric field dissipates, the trajectories linearize, and the plume effectively detaches. A delineation of sub-to-super Alfvénic flow aligns well with the inflection points of the linearization without a change in magnetic topology. The detachment process is best described as a two part process: First, ions detach by a breakdown of the magnetic moment when the quantity |v/fcLB| becomes of order unity. Second, the turbulent electric field enhances electron transport up to a factor of 4±1 above collisional diffusion; electron cross-field velocities approximate that of the ions and depart on more centralized field lines. Electrons are believed to detach by breakdown of magnetic moment further downstream in the weaker magnetic field.

Recent advances in plasma-based propulsion systems have led to the development of electromagnetic Radio-Frequency (RF) plasma generation and acceleration systems, called Helicon Plasma Thrusters (HPT). One of the pioneer companies developing this new type of space propulsion is T4i Technology for Propulsion and Innovation s.p.a., with its cutting-edge project called REGULUS, among which this study has been performed. A crucial part of HPT systems is the acceleration region, where, by the means of a magnetic nozzle, the thermal energy of the plasma is converted into axial acceleration and, in turn, into thrust. This study is focused on the numerically simulation of the plasma dynamics in the acceleration stage, using Xenon gas. A three-dimensional full Particle-In-Cell (PIC) simulation strategy is used to simulate the plume in the magnetic nozzle. The code developed for the plasma simulation is based on the open-source software Spacecraft Plasma Interaction Software (SPIS). The code has been conveniently modified and improved, neutrals and collision processes were added to evaluate their impact on the plasma properties. The features added improved the validity of the results, now one step closer to the physical reality. The code has been proven to be an extremely versatile and powerful tool for optimization and adaptation to different mission scenarios. / De senaste framstegen i plasmaframdrivning har lett till utvecklingen Helicon Plasma Thruster (HPT) som kombinerar elektromagnetisk högfrekvent (RF) plasmakälla och ett accelerationssystem. En av företagen som är pionjärer i att utveckla denna nya framdrivningsteknik är T4i Technology for Propulsion and Innovation s.p.a., med dess banbrytande projekt REGULUS, som detta arbete bygger på. En viktig del av HPT-systemet är accelerationsområde där plasmats termiska energin omvandlas till axiell accelleration i en magnetisk dysa. Denna rapport fokuserar på numeriska modelleringen av plasmadynamiken accelerationsområdet vid användning av Xenongasen. En tredimensionell Particle-In-Cell (PIC) simulering används för att studera plasmautflödet i magnetiska dysan. Koden bygger på den öppna mjukvaran Spacecraft Plasma interaction Software (SPIS). Koden har modifierats och förbättrats, en neutral komponent samt kollisionsprocesser har lagts till och deras påverkan på plasmabeteende har studerats. Dessa nya element förbättrade giltigheten av simulerings-resultaten. Nu ett steg närmre den fysiska verkligheten. Koden är ett mångsidigt och kraftfullt verktyg för optimering och anpassning till olika användningsscenarier.