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December 14, 2010

Pulsed field-reversed configuration (FRC) thrusters

Pulse Discharge Network Development for a Heavy Gas Field Reversed Configuration Plasma Device

A simple LRC circuit model is used to conduct a parametric study of the effects of charging voltage, capacitance, resistance, and inductance on the current waveform of a pulse forming network for field reversed configuration (FRC) plasma production. Using known waveforms from existing networks, estimates of realistic values of resistance and inductance are established for a base network model. Parametric modification of the base model is used to study the effects of each component of the discharge network. Results indicate that increasing charging voltage causes an increase in peak current, but does not effect rise or reversal times. However, increasing capacitance increases peak current and increases rise and reversal times. Further, optimum circuit parameters are determined for the design and construction of an FRC formation test article. Three main design criteria are used and are based on magnetic diffusion time, auto-ionization of background gas, and peak magnetic field strength. Results indicate that a pulse forming network with charging voltage of 25 kV and capacitance of 1 μF provides the widest range of resistance and inductance values such that the waveform meets the design criteria.


State of the Art of FRC (Field Reversed Configuration) Propulsion

Fusion applications of FRCs have been investigated for decades. The goal of these investigations was compression and heating of the FRC to achieve D-T or D-He3 fusion reactions. To achieve fusion temperatures during FRC creation, the plasma was non-adiabatically heated and large densities and temperatures created (e.g., ion
temperature Ti = 50-1000 eV, electron temperature Te = 50-250 eV, and average density n = 10^15 cm-3). Many of these experiments focused on increasing confinement time, overall dimensions, and plasma density. Further, FRCs
have been used to simulate high-temperature plasma instability events in tokomaks. Specifically, in Ref. 16 a conical device with a centerline-peaked pre-ionization plasma was used to create and accelerate FRCs. While it was not designed for propulsion, results indicated expelled plasma velocities of 1.2x10^5 m/s.
FRCs for space propulsion application have been previously investigated at the University of Washington, University of Alabama-Huntsville, and at the Air Force Research Laboratory (AFRL). These studies have mainly focused on lower energy FRC formation and translation with higher atomic mass gases. Specific results from
these investigations are described next.

Research performed at the University of Washington has investigated the use of FRCs for space propulsion and fusion, both individually and as a combined spacecraft system. Slough, et.al., have investigated the Propagating Magnetic Wave Plasma Accelerator (PMWAC) device for space propulsion and also an earth-to-orbit fusion plasmoid device. Both of these have similar operating principles. First, a FRC is created using the method shown in Figure 2. Then the FRC is accelerated using a magnetic wave created by a sequence of pulsed electromagnetic coils. If the device is only providing propulsion, then the accelerated FRC is expelled at high velocity. However, if fusion is desired, then the FRC is compressed to smaller diameter causing the temperature to increase to fusion levels. Power can then be extracted for use creating the next FRC and the process is repeated. Results showed an ejection velocity of at least 1.8x10^5 m/s for each deuterium plasmoid, which yielded a total impulse bit of 0.3 N-s.

The University of Washington in collaboration with MSNW LLC is also developing the Electrodeless Lorentz Force (ELF) thruster. The goal of the ELF device is to demonstrate efficient acceleration of a variety of propellants to high velocities (10-40 km/s) and operation at high power (e.g. >100 kW). The device is designed around a conical geometry with a rotating magnetic field current drive to ionize the gas and drive an azimuthal current to form an FRC.

Investigations at the University of Alabama-Huntsville and NASA Marshall Space Flight Center have centered on the Plasmoid Thruster Experiment (PTX). PTX produces plasmoids in an analogous fashion to that shown in Figure 2; however, a conical geometry is used instead of cylindrical. This geometry has benefits because the FRC creation and acceleration occur within the same step. Unlike the PMWAC developed by Slough, a traveling magnetic wave is not required to accelerate the FRC. Results have shown electron temperature and density of 7.6 eV, and 5.0x10^13 cm-3 for argon and 23 eV and 1.2x10^14 cm-3 for hydrogen. Exit velocities up to 2.0x10^4 m/s have been measured.

Recently AFRL has become interested in FRCs for space propulsion application. Specifically, the electric propulsion group at Edwards Air Force Base constructed an annular FRC device called XOCOT. The XOCOT project primary goal was to develop FRC-based plasmas at low power with long lifetime for propulsion applications.

The program investigated different charging energies, voltages, and timing, as well as multiple propellants and preionization techniques. Results showed multiple plasma formation and implosions are possible with densities and electron temperature on the order of 3.0x10^13 cm-3 and 8 eV, respectively.



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