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The temperature of a plasma generally starts at about 11,000οC, but present day laboratory plasmas can be a thousand times hotter. Particles in those hotter plasmas move at velocities exceeding 300,000 meters per second. These tempera- tures are comparable to those in the interior of our Sun. No known material could survive prolonged direct contact with such plasma. Fortunately, plasmas respond well to electric and magnetic fields. A magnetic channel can be constructed to both heat and guide a hot plasma so that the plasma never touches the material walls. Power requirements increase very rapidly as the exhaust velocity increases. While solar power remains viable for robotic cargo missions in the Earth-Moon environment, for human transportation it is difficult to envision any other option than nuclear electric power. This is especially true for missions beyond Mars, where the power of the Sun is relatively low. A number of low-power plasma rockets are already in use. The best known and oldest technology is the ion engine, which uses a metal grid to extract and accelerate ions from a relatively low temperature and low density plasma discharge. The ion jet must be neutralized by a spray of electrons ejected from a neutralizing gun at the rocket exhaust; otherwise the spacecraft would build up a negative electric charge that would ultimately pull the ions back in. Ion engines produce exhaust velocities up to 70 kilometers per second using Xenon propellant. A variation of the ion engine is the Hall Effect Thruster, which does away with the grid, replacing it with a magnetically rigidized cloud of electrons. This technique reduces the erosion and heating of rocket components that are directly exposed to plasma bombardment, a major problem in plasma rocket engineering. Left: A Hall Effect Thruster. Right: The jet from a Hall Effect Thruster. In this type of engine, ions are ejected by an electric field inside the device. The plasma density in ion and Hall thrusters is generally low due to physics constraints. Therefore, their power density is also low. This implies that high-power systems of those types would need to be physi- cally large. Other systems under development, such as the Magneto Plasma Dynamic Thruster (MPD), the Variable Specific Impulse Magneto Plasma Rocket (VASIMR), and the Pulsed Inductive Thruster (PIT) can reach much higher power densities. All offer comparative advantages and disadvantages, and intensive research and development is currently underway to test and deploy them. The low-power Hall Effect Thruster is presently being used in commercial satellites for small “station keeping” maneuvers, that is, for maintaining the satellite’s position. For primary propulsion, considerable research is being carried out on alternate high-power radio-frequency-driven systems. One such concept is VASIMR, which is being developed by the author and is depicted on page one. This engine can operate with a number of alternate gases and does not rely on rare and expensive propellants such as Xenon. In this engine, ions are guided and accelerated out the nozzle by magnetic fields. In the future, plasma rockets could be valuable in a number of ways. For example, low-power rockets could play an important role in maintaining the orbits of space stations. With sufficient available power, a plasma engine could also be used to nudge an incoming asteroid away from a collision path with Earth, or even to preposition it for mining operations. Indeed, in the more distant future, fusion-driven plasma rockets could be instrumental in carrying us far beyond our fragile planet, possibly ensuring the survival of the human species. References and Suggested Reading: Franklin R. Chang-Diaz, "Plasma Propulsion for Interplanetary Flight," Thin Solid Films, Vol. 506-507 (May26, 2006): 449-453. F. R. Chang Díaz, "The VASIMR Rocket," Scientific American 283, no. 5 (2000): 90-97. Eric J. Lerner, “Plasma Propulsion in Space,” The Industrial Physicist,” (October 2000): 16-19. Text: Franklin Chang Diaz Editors: Paul Rivenberg, Gerald Rogoff Images: NASA; Busek Co. Inc.; Princeton Plasma Physics Laboratory For more information: Call Toll Free 1-877-PLASMAS (752-7627) • E-mail: CPS@plasmacoalition.org Visit our website at http://www.plasmacoalition.org 022007 ©2007 Coalition for Plasma SciencePDF Image | Plasmas from the Coalition for Plasma Science Plasma Propulsion for Space Flight
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