Ion Dynamics and ICRH Heating in the Exhaust Plasma of The VASIMR Engine

Details Ion Dynamics and ICRH Heating in the Exhaust Plasma of The VASIMR Engine Ion Dynamics and ICRH Heating in the Exhaust Plasma of The VASIMR Engine

Jan 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.824b&link_type=abstract

IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,

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The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high power, radio frequency-driven magnetoplasma rocket, capable of Isp/thrust modulation at constant power. The plasma is produced by an integrated helicon discharge. However, the bulk of the plasma energy is added in a separate downstream stage by ion cyclotron resonance heating (ICRH.) Axial momentum is obtained by the adiabatic expansion of the plasma in a magnetic nozzle. Exhaust variation in the VASIMR is primarily achieved by the selective partitioning of the RF power to the helicon and ICRH systems, with the proper adjustment of the propellant flow. A laboratory simulation of the 25 kW proof of concept VASIMIR engine has been under development and test at NASA-JSC for several years. Experimentally, high density, stable plasma discharges have been generated in Helium, Hydrogen, Deuterium, Argon and Xenon. This paper will review the plasma diagnostic results obtained in 2000-2002 in a continuing series of performance optimization and design development studies. Available plasma diagnostics include a triple probe, a Mach probe, a bolometer, a television monitor, an H- photometer, a spectrometer, neutral gas pressure and flow measurements, several gridded energy analyzers (retarding potential analyzer or RPA), a surface recombination probe system, an emission probe, a directional, steerable RPA and other diagnostics. Reciprocating Langmuir and Mach probes are the primary plasma diagnostics. The Langmuir probe measures electron density and temperature profiles while the Mach probe measures flow profiles. Together this gives total plasma particle flux. An array of thermocouples provides a temperature map of the system. Ion flow velocities are estimated through three techniques: Mach probes, retarding potential analyzer, and spectroscopic measurements. During 2000-2002, we have performed a series of experiments on the VASIMR apparatus with several objectives, to explore the parameter space that optimizes helicon operation, to learn to operate the apparatus in high power modes and to demonstrate ICRH heating of the plasma. For example, we have studied the effect of the strength of the mirror magnet in the ICRH region on the performance of the helicon without any ICRH heating. When the RPA and the Mach probe are put in the same plane, the inferred ion bulk flow speeds agree within 20%, as do the forward vs. backward saturation current ratios. When the RPA is placed ~40 cm downstream from the Mach probe, the inferred speeds increase by at least a factor of 2. We attribute this apparent acceleration to a combination of adiabatic (magnetic mirror) acceleration and ambipolar electrostatic forces. In another series of experiments, we investigated the effect of introducing a minor gas constituent into the helicon discharge. The effect was startling and unexpected. The introduction of hydrogen into the helium discharge reduced the ionization efficiency of the helicon by nearly an order of magnitude. At the same time, a high-energy tail appeared in the ion velocity distribution function. It appears to us most likely that this tail is comprised of protons flowing at speeds approaching 150 km/s. The thrust is low in this mode, but the specific impulse is quite high. The same effect occurs for deuterium- hydrogen mixtures. ICRH heating experiments have shown the strongest effect for 2nd harmonic heating of helium. Continuing efforts to improve ICRH antenna coupling will be discussed. We have also begun to investigate the details of the velocity space distribution function in the plasma.

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