SPRG Seminars
April 26, 2011:
" The VASIMR Project: An Industrial Application of Auroral Physics and New Opportunities for Simulation Studies "
Gar Bering, U. Houston
The VAriable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high power electric spacecraft propulsion system that uses a helicon discharge to generate plasma. The plasma is leaked though a strong magnetic mirror to the second stage, where it is energized by an RF booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. In the experiments reported in this paper, the booster uses ~0.5-0.7 MHz waves with up to 170 kW of power.
The single pass ion cyclotron heating (ICH) produced a substantial increase in ion velocity. Pitch angle distribution studies showed that this increase took place in the resonance region where the ion cyclotron frequency was roughly equal to the frequency on the injected rf waves. Downstream of the resonance region the perpendicular velocity boost is converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR. Results from high power Helicon only and Helicon with ICH experiments are presented from the VX-200 using argon propellant. A two-axis translation stage has been used within a new 150 cubic meter cryo-pumped vacuum chamber to survey the spatial structure of plasma parameters, momentum flux and magnetic perturbations in the VX-200 exhaust plume.
For the first time, the thruster efficiency and thrust of a high-power VASIMR prototype have been measured with the thruster installed inside a vacuum chamber with sufficient volume and pumping to simulate the vacuum conditions of space. The ionization cost of argon propellant was determined to be 87 eV for optimized values of RF power and propellant flow rate. 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. This work paves the way for design and eventual operation of a VASIMR engine in orbit on-board the International Space Station. Ad Astra Rocket Company is planning to fly a plasma rocket experiment as a major element of the company's “Aurora” electric power and propulsion test platform on the International Space Station (ISS) in 2014.
The Aurora platform will support a dual-jet magnetic quadrupole 200 kW version of the VASIMR plasma rocket (the VF-200). It will consist of two 100 kW parallel plasma engines with opposite magnetic dipoles, resulting in a near zero-torque magnetic system. The system will be available for basic plasma physics research in parallel with the testing of the VF-200 engine as a high power electric propulsion system. The Aurora package would become a National Plasma Physics Laboratory suitable for plasma physics studies in an open, wall free near-Earth orbital laboratory environment.
For example, the VF-200 high performance helicon plasma sources and ion acceleration stages will simulate conditions in the solar corona during solar flares by creating and controlling plasma jets in expanding magnetic field geometries. Such a study would measure quantities in the plasma flow with the goal of measuring magnetic reconnection and transport phenomena that may be similar in nature to that taking place in solar flares, CMEs and heliospheric topics of interest. This work would elucidate the effects inherent in a 3-d magnetic system and produce some of the physics occurring in the solar flares or in the post initiation phase.
The Aurora package is privately funded, and will provide only the instrumentation required to validate the VF-200's performance as a propulsion system. Little detailed exhaust plume monitoring instrumentation will be provided by AARC. An ISS arm deployed instrument package similar to the Plasma Diagnostics Package used on STS-3 in conjunction with the OSS-1 experiment and STS-51F in conjunction with Spacelab 2 has been proposed to NASA. The proposed instrumentation package will carry Langmuir probes, an RPA, dc magnetometer, plasma wave detectors, Faraday cups, electrostatic analyzers, solid state energetic particle telescope and Ar II and broadband imagers. Opportunities for collaborative plasma physics experiments at the AARC facility and using the ISS facilities will be discussed.