An experimental and theoretical study of the ECR plasma engine
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The process of Electron-Cyclotron-Resonance (ECR) plasma acceleration has several potential applications including use as a new type of electric space propulsion device designated the...
| Summary: | NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
The process of Electron-Cyclotron-Resonance (ECR) plasma acceleration has several potential applications including use as a new type of electric space propulsion device designated the ECR plasma engine. The ECR plasma engine is interesting due to its theoretical promise to deliver a combination of improved efficiency, specific impulse, power-handling capability, length of life, or operational flexibility relative to other electric propulsion devices now being developed. Besides its possible application in electric propulsion, the ECR plasma engine might be useful for beamed-energy propulsion or fusion propulsion. Related devices are used in the semiconductors field for plasma etching.
This study includes theoretical modeling and a series of experimental measurements. The theoretical work was focused in two areas. The first area involved the development of a collisionless, steady-state, axisymmetric model of a cold flowing plasma separating from a diverging magnetic field. This model suggests that beam divergence can be an important loss mechanism for plasma propulsion devices that use magnetic nozzles, but that the use of optimized field geometries can reduce divergence losses to acceptable levels. We suggest that future research be directed at confirming theoretical predictions made using the axisymmetric model of beam separation.
The second area of theoretical investigation involved the development of a steady-state, quasi-one-dimensional model that provides theoretical predictions of plasma density, electron temperature, plasma potential, ion energy, engine specific impulse, efficiency, and thrust. The quasi-one-dimensional model consists of a system of five first-order, nonlinear, ordinary differential equations. The boundary conditions required to solve the system of equations are relationships between the ambient neutral gas density, the plasma density, the two components of the electron temperature, and the position at which the plasma passes through the ion-acoustic Mach number 1. The model was used to solve two classes of problems that are thought to bound the conditions under which the ECR plasma accelerator operates. The first class of problem is based on the assumption of negligible conductive heat flow within the plasma. The second class of problem is based on the assumption that electron thermal transport along magnetic field lines is so large that the component of the electron temperature along magnetic field lines is isothermal. The model can be used to simulate accelerator operation in space or in the presence of a vacuum system with finite tank pressure. Measurements of plasma conditions in a working research device confirm the general features of the quasi-one-dimensional theory.
The experimental apparatus constructed to study ECR plasma acceleration consists of a vacuum facility, a 20-kW microwave power supply, and an ECR plasma accelerator. In tests of the facility we have measured microwave input power, reflected power, propellant flow rate, and vacuum-tank static pressure. The working ECR plasma research device uses argon propellant gas with 2.12-GHz microwave radiation at power levels of up to a few kilowatts. Among the plasma diagnostics employed in this research are a gridded energy analyzer, a Faraday cup beam-density analyzer, Langmuir probes, emissive probes, and a diamagnetic loop. With these diagnostics, we have measured plasma potentials of up to 70 eV and electron temperatures of up to 35 eV. Measurements of accelerated-ion kinetic energy show a direct relationship between ion energy and peak plasma potential, as predicted by theory. Indirect measurements indicate that the plasma density in the existing accelerator is on the order of [...].
We now understand previously unexplained losses in converting microwave power to jet power by ECR plasma acceleration as the result of diffusion of energized plasma to the metallic walls of the accelerator. Our theory suggests that future researchers should attempt to reduce the influence of these diffusion losses by increasing the cross-sectional area of the accelerator. It may be possible to reduce line radiation losses due to electron-ion and electron-atom inelastic collisions below levels estimated by past researchers through careful accelerator design. Minimizing inelastic collision losses will place a limit on the maximum thrust density that can be achieved using argon and other non-hydrogenic propellant materials. High thrust density may be achievable using propellants that are isotopes of hydrogen because once ionized, these species exhibit negligible inelastic collision effects. Deuterium is arguably the best candidate for achieving both high efficiency and high thrust, but will only be effective at specific impulses of over about 10,000 lbf s/lbm.
We expect that efficient ECR plasma engines can be designed for use in high specific impulse spacecraft propulsion at power levels ranging from a few kilowatts to tens of megawatts. The maximum theoretical efficiency of converting applied microwave power to directed jet power in this device can be more than 60 percent. The achievable total efficiency of converting direct-current electric power to jet power in a propulsion system based on the ECR plasma engine will probably be considerably less. |
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