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A mechanism for ambipolar ion acceleration in a magnetic nozzle is proposed. The plasma is adiabatic (i.e., does not exchange energy with its surroundings) in the diverging section of a magnetic nozzle so any energy lost by the electrons must be transferred to the ions via the electric field. Fluid theory indicates that the change in plasma potential is proportional to the change in average electron energy. These predictions were compared to measurements in the VX-200 experiment which has conditions conducive to ambipolar ion acceleration. A planar Langmuir probe was used to measure the plasma potential, electron density, and electron temperature for a range of mass flow rates and power levels. Axial profiles of those parameters were also measured, showing consistency with the adiabatic ambipolar fluid theory.

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Testing of the Variable Specific Impulse Magnetoplasma Rocket VX-200 engine was performed over a wide throttle range in a 150 m 3 vacuum chamber with sufficient pumping to permit exhaust plume measurements at argon background pressures less than 1 × 10 −3 Pa (1 × 10 −5 torr) during firings, ensuring charge-exchange mean free paths longer than the vacuum chamber. Measurements of plasma flux, radio frequency power, propellant flow rate, and ion kinetic energy were used to determine the ionization cost of argon and krypton in the helicon discharge. Experimental data on ionization cost, ion fraction, exhaust plume expansion angle, thruster efficiency, and thrust are presented that characterize the VX-200 engine performance over a throttling range from 15 to 200 kW radio frequency power. A semiempirical model of the thruster efficiency as a function of specific impulse indicates an ion cyclotron heating efficiency of 85 7%. Operation at a total radio frequency coupled power level of 200 kW yields a thruster efficiency of 72 6% at a specific impulse of 4900 300 s with argon propellant. A high thrust-to-power operating mode was characterized over a wide parameter space with a maximum thrust-to-power ratio of 51 5 mN∕kW at a specific impulse of 1660 100 s for a ratio of ion cyclotron heating radio frequency power to helicon radio frequency power of 0.7∶1.

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Recent exhaust plume measurements and plasma physics results are discussed related to the development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR ® ) VX-200 engine, a 200 kW flight-technology prototype. Results from high power Helicon only and Helicon with ICH experiments are presented from the VX-200 using argon propellant. Total VX-200 system efficiencies are presented from recent results with greater than 100 kW of RF power, and extrapolation from an empirical fit shows greater than 60% efficiency at less than 200 kW RF power. A two-axis translation stage has been used to survey the spatial structure of plasma parameters, momentum flux and magnetic perturbations in the VX-200 exhaust plume. These recent measurements of axial plasma density and ambipolar potential profiles, magnetic field-line shaping, charge exchange, and force measurements were made within a new 150 cubic meter cryo-pumped vacuum chamber and are presented in the context of plasma detachment. Recent results at 200 kW coupled RF power have shown a thruster efficiency of 72% at a specific impulse of 5000 s and a thrust of 5.7 N.2 VSWR plasma = voltage standing wave ratio of the ICH coupler, with plasma present VSWR vacuum = voltage standing wave ratio of the ICH coupler, with no plasma present W ICH = mean ion energy increase owing to ICH ω = angular frequency I.3 discuss an experimental investigation of the use of Ion Cyclotron Heating (ICH) to provide an efficient method of electrodeless plasma acceleration in the VASIMR ® engine. Particular emphasis in this paper will be placed on investigation of the spatial structure of the exhaust plume and recent advances in system performance.Research on the VASIMR ® engine began in the late 1970's, as a spin-off from investigations on magnetic divertors for fusion technology 12 . A simplified schematic of the engine is shown in Figure 1. The VASIMR ® consists of three main sections: a helicon plasma source, an ICH plasma accelerator, and a magnetic nozzle 3,13,14,15,16,17 . Figure 2 shows these three stages integrated with the necessary supporting systems. One key aspect of this concept is its electrode-less design, which makes it suitable for high power density and long component life by reducing plasma erosion and other materials complications. The magnetic field ties the three stages together and, through the magnet assemblies, transmits the exhaust reaction forces that ultimately propel the ship.The plasma ions are accelerated in the second stage by ion cyclotron resonance heating (ICH), a well-known technique, used extensively in magnetic confinement fusion research 18,19,20,21,22 . Owing to magnetic field limitations on existing superconducting technology, the system presently favors light propellants. However, the helicon, as a stand-alone plasma generator, can efficiently ionize heavier propellants such as argon and xenon.An important consideration involves the rapid absorption of ion cyclotron waves by the high-speed plasma flow. This process differs from the familiar ion cycl...