The development and testing of a flow cooling system for high-temperature superconducting (HTS) magnets is described. The system includes a turbo-Brayton cryocooler, a magnet thermal interface, and a magnet thermal isolation and support system. The target application is the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). Turbo-Brayton coolers are well suited to such spacecraft applications, as they are compact, modular, lightweight, and efficient, with long maintenance-free lifetimes. Furthermore, the technology scales well to high-cooling capacities. The feasibility of using turbo-Brayton coolers in this application was proven in a design exercise in which existing cooler designs were scaled to provide cooling for the magnet sets required by 200 kW and 1 MW VASIMR engines. The performance of the concepts for the thermal interface and the thermal isolation and support system were measured in separate laboratory tests with a demonstration system built about a representative HTS magnet. Cooling for these tests was provided by a flow cooling loop comprising a compressor, recuperator and GM cryocooler, with the flow pressure, temperature, and mass flow rate selected to effectively simulate the turbo-Brayton operating condition. During system testing, the magnet was cooled below its design operating temperature of 35 K, and good thermal uniformity (<0.4 K) and low thermal loads (<0.5 W) were demonstrated.

In the Advanced Space Propulsion Laboratory (ASPL) helicon experiment (VX-10) we have measured a plasma flux to input gas rate ratio near 100% for both helium and deuterium at power levels up to 10 kW. Recent results at Oak Ridge National Laboratory (ORNL) show enhanced efficiency operation with a high power density, over 5 kW in a 5 cm diameter tube. Our helicon is presently 9 cm in diameter and operates up to 10 kW of input power. The data here uses a Boswell double-saddle antenna design with a magnetic cusp just upstream of the antenna. Similar to ORNL, for deuterium at near 10 kW, we find an enhanced performance of operation at magnetic fields above the lower hybrid matching condition.

-The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an open-ended, RF-heated, magnetic mirror-like plasma device. The three-stage system features an RF driven plasma generator/injector, an RF power booster and a hybrid magnetic nozzle. Its electrodeless design enables high power density. The system provides access to very high and variable thrust and exhaust velocities (3x10 – 3x10 m/sec) of interest in fast human and robotic interplanetary propulsion as well as efficient, highpayload orbit transfer capability. In its near-term form, the VASIMR is an electrically driven rocket, powered by solar or nuclear energy. However, its technology also paves the way for ignited plasma rockets powered by controlled thermonuclear fusion. The development of the VASIMR began at the NASA Johnson Space Center (JSC) in 1980 and has continued since. At present, a JSC-led, multi-center theoretical, experimental and systems engineering program is under way. This paper describes the development of the VASIMR from its early stages to the present, focusing on the main physics and engineering areas of interest. The latest theoretical and experimental results, mission applications, systems engineering, as well as the first space experiment being planned for this technology are discussed in detail.