Alex L., The Magnetoplasmadynamic Thruster, S.I.N 4-09

As a continuation of the articles “Hall Thruster” and “Ion Thruster”, the third installment will discuss the very latest of this type of technology, the magnetoplasmadynamic (MPD) thruster. Again, the article “New Dawn for Electric Rockets” by Edgar Y. Choueiri, published on page number 58 of the February 2009 issue of Scientific American was used as the source for data regarding the thruster. First a refresher about the basics of the technology: the magnetoplasmadynamic thruster is a type of electric plasma engine (EPE).
EPE’s in general have characteristics that incline them toward being used in applications where high velocity and/or maneuverability is desired. This type of engine produces little thrust, but can do so over a much greater period of time while requiring a smaller input of fuel; they are limited by this small amount of thrust, for they cannot escape the gravitational field of the earth. In its most simple form, the functioning of an EPE can be described thus: an electrical or magnetic field is generated, into which atoms of gas are introduced. These atoms are ionized creating ions and free electrons that are accelerated out of the engine.
An MPD engine is composed of a shorter hollow cathode placed within a longer cylindrical anode; there is open space between the cathode and anode. Lithium vapor is pumped through the center of the cathode and escapes into the anode, where an “azimuthal” (encircling) magnetic field ionizes the lithium. The same electrical force that was induced to create the magnetic field generates a Lorentz force that accelerates the lithium ions out of the engine. The statistics of the thruster are: flight-tested but not operational, input power is 100-500kW, exhaust velocity is 15-60km/s, thrust is 2.5-25N, and efficiency is 40-60%. This is the most promising of the EPE’s for it has the highest thrust density and allows for modulation of thrust output (throttling) causing more precise control to be realized.