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LOW-TEMPERATURE PLASMAS 39 to be understood in the rotating plasmas utilized in plasma centrifuges, and further research in this area will be necessary to fully exploit their potential. In addition to isotope separation, the plasmas developed for plasma centrifuges can also be expected to be useful for other applications, including use in imploding, "z-pinch," plasma x-ray sources and in plasma switches. PLASMAS FOR ELECTRIC PROPULSION OF SPACE VEHICLES Space electric propulsion has been studied for the last three decades. Concepts that have been investigated include expanding electrically heated plasmas, accelerating plasmas with thrusters and plasma guns, accelerating ions, and laser propulsion. Research groups have included TRW in Redondo Beach, California; Avco Everett Research Laboratory in Everett, Massachusetts; the National Aeronautics and Space Administration (NASA) Lewis Research Center; and several university research groups. Electric plasma propulsion requires less fuel mass than chemical systems, potentially making launching of satellites and space exploration less expensive. Decreasing the weight of fuel and hence the overall payload could have a significant impact on the $9.5 billion currently spent annually for launches: $5 billion by the Department of Defense (DOD), $3 billion by NASA, and $1.5 billion by industry. A plasma propulsion device (an ion accelerator) has been tested and has worked successfully in space for 13 years. A key issue for any such device that is launched into space is its reliability and longevity in both its on-the-shelf and operating lives. This makes the use of electrodes problematic. TRW has been pursuing electrodeless thrusters, and these plasma accelerators potentially could satisfy the demanding reliability requirements of space qualifiable systems. Other areas that require research include a better understanding of the plasma, identification of the appropriate gas or fuel, and matching the electrical driver to the nonlinear plasma load. There is presently a pressing need for low power (e.g., of order 100 W) thrusters for long term maintenance of orbits. The requirements for interplanetary missions will require significantly higher-power thrusters. The development of space plasma propulsion systems also is synergistic with other applications. For example, plasma accelerators can be used in plasma processing and in the simulation of space plasmas to determine the chemistry of these reactive media on satellites. MAGNETOHYDRODYNAMICS The branch of magnetohydrodynamics (MHD) of interest here is that concerned with plasmas at low temperatures (2000â10,000 K) and high pressures (1â10 atm). Most of the current work in this area is engineering development rather than scientific research. These efforts have focused primarily on electrical power generation, with some effort directed toward space vehicle thrusters. The power