Recent Improvements In Ionization Costs and Ion Cyclotron Heating Efficiency in The VASMIR Engine

Abstract

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high power magnetoplasma rocket, capable of Isp/thrust modulation at constant power. The plasma is produced by a helicon discharge. The bulk of the energy is added by ion cyclotron resonance heating (ICRH.) Axial momentum is obtained by adiabatic expansion of the plasma in a magnetic nozzle. Thrust/specific impulse ratio control in the VASIMR is primarily achieved by the partitioning of the RF power to the helicon and ICRH systems, with the proper adjustment of the propellant flow. Ion dynamics in the exhaust were studied using probes, gridded energy analyzers (RPA’s), microwave interferometry and optical techniques. This paper will review 3 years of single-pass ICRH ion acceleration data. During this interval, the available power to the helicon ionization stage has increased from 3 to 20 kW. The increased plasma density has produced increased plasma loading of the ICRH antenna and significant improvements in antenna coupling efficiency and in ion heating efficiency. We explored the details of the ion dynamics in a deuterium exhaust plasma using ~19 kW of RF power to the helicon ionization stage and 1.3 kW to the ICRH acceleration stage. Owing to significant reductions in ionization cost, the total ion flux in the exhaust plasma is an order of magnitude greater than the flux obtained during the experiments that were reported as recently as November of 2004. In this high-density plasma, the available energy per ion is reduced compared to last year, but the booster efficiency of the ICRH process has increased. Ion energization of ~17 eV/ion has been demonstrated in this higher flux flowing plasma. This energy increase corresponds to a booster efficiency (ηb) of 67%, in agreement with model predictions. Results also confirm conversion of transverse ion motion to axial motion via conservation of the first adiabatic invariant.