A Magneto-Plasma Dynamic (MPD) thruster (MPDT) is a type of electrically powered spacecraft propulsion that generates thrust by using the Lorentz force (the force exerted on a charged particle by an electromagnetic field). It is also known as the Lorentz Force Accelerator (LFA) or (mostly in Japan) the MPD arc jet. In general, a gaseous material is ionised and then fed into an acceleration chamber, where magnetic and electrical fields are generated with the help of a power source. The particles are then propelled out through the exhaust chamber by the Lorentz force created by the interaction between the current flowing through the plasma and the magnetic field (which is either externally applied or induced by the current). Unlike chemical propulsion, there is no fuel combustion. Specific impulse and thrust increase with power input, as with other electric propulsion variations, while thrust per watt decreases. MPD thrusters are classified into two types: appliedfield and self-field. Magnetic rings surround the exhaust chamber to generate the magnetic field in applied-field thrusters, whereas a cathode extends through the middle of the chamber in self-field thrusters. The MPD thruster is composed of two metal electrodes: a central rod-shaped cathode and a cylindrical anode that surrounds the cathode. A high-current electric arc is struck between the anode and cathode, just like in an arc welder. As the cathode heats up, electrons are emitted, which collide with and ionize a propellant gas to form plasma. In theory, MPD thrusters could generate extremely high specific impulses (Isp) with an exhaust velocity of up to and beyond 110000 m/s, which is three times the value of current xenon-based ion thrusters and approximately 25 times better than liquid rockets. MPD technology has the potential for thrust levels of up to 200 newton (N) (45 lbF), which is far and away the highest for any form of electric propulsion and nearly as high as many interplanetary chemical rockets. This would allow electric propulsion to be used on missions that require quick delta-v manoeuvres (such as capturing into orbit around another planet), but with much greater fuel efficiency.
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