Magnetoplasmadynamic thruster
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The Magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electric propulsion (a subdivision of spacecraft propulsion) which uses the Lorentz force (a force resulting from the interaction between a magnetic field and an electric current) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet. Generally, a gaseous fuel is ionized and fed into an acceleration chamber, where the magnetic and electrical fields are created using a power source. The particles are then propelled by the Lorentz force resulting from the interaction between the current flowing through the plasma and the magnetic field (which is either externally applied, or induced by the current) out through the exhaust chamber. Unlike chemical propulsion, there is no combustion of fuel. As with other electric propulsion variations, both specific impulse and thrust increase with power input. There are two main types of MPD thrusters, applied-field and self-field. Applied-field thrusters have magnetic rings surrounding the exhaust chamber to produce the magnetic field, while self-field thrusters have a cathode extending through the middle of the chamber. Applied fields are necessary at lower power levels, where self-field configurations are too weak. Various propellants such as xenon, neon, argon, hydrazine, and lithium have been used, with lithium generally being the best performer.
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[edit] Advantages
In theory, MPD thrusters could produce extremely high specific impulses (Isp) of up to and beyond 11,000 s (110 km/s exhaust velocity), triple the value of current xenon-based ion thrusters, and about 20 times better than liquid rockets. But perhaps the most impressive characteristic of MPD technology is thrust levels of up to 200 newtons (N) (45 lbf), by far the highest for any form of electric propulsion, and nearly as high as many interplanetary chemical rockets. This would allow use of electric propulsion on missions which require quick delta-v maneuvers (such as capturing into orbit around another planet), while having many times greater fuel efficiency.
[edit] Problems with MPDT
MPD thruster technology has been explored academically, but commercial interest has been low due to several remaining problems with the technology, namely extreme power requirements on the order of megawatts (MW) for optimum performance. Current interplanetary spacecraft power systems (such as radioisotope thermoelectric generators (RTGs)) are incapable of these power levels. Even NASA's Project Prometheus reactor will only be able to generate power in the hundreds of kilowatts range. Other problems with MPD include the degradation of cathodes which generate the magnetic fields. As a result, MPD thrusters have not yet been used as propulsion on any spacecraft, though a Japanese MPD test, the EPEX (Electric Propulsion EXperiment) was deployed on shuttle mission STS-72.
[edit] Research
Research on MPD thrusters has been carried out in the US, the former Soviet Union, Japan, Germany, and Italy. Experimental prototypes were first flown on Soviet spacecraft and, most recently, in 1996, on the Japanese Space Flyer Unit, which demonstrated the successful operation of a quasi-steady pulsed MPD thruster in space. Research at Moscow Aviation Institute, RKK Energiya, University of Stuttgart, ISAS, Centrospazio, Alta S.p.A.,Osaka University, University of Southern California, Princeton University's Electric Propulsion and Plasma Dynamics Lab (EEPDyL) (where MPD thruster research has continued uninterrupted since 1967), and NASA centers (Jet Propulsion Laboratory and Glenn Research Center), has resolved many problems related to the performance, stability and lifetime of MPD thrusters.
