Magnetoplasmadynamic Thruster

The magnetoplasmadynamic (MPD) thruster (also called an MPD arc-jet) is a form of electric rocket which produces thrust through the interaction between a current flowing through a plasma and a magnetic field acting on the same plasma. Generally speaking, devices of this type are called electromagnetic thrusters.

Description of electromagnetic thrusters

The force in operation in electromagnetic thrusters is called a Lorenz force and the direction of thrust is perpendicular to both the current and the magnetic field. In physics, this is known a cross product, in this case J X B (read j-cross-b). Therefore electromagnetic thrusters are sometimes categorized as JxB thrusters or Lorenz force thrusters. Electromagnetic thrusters are generally considered to be the highest power electric thrusters, suitable only to situations in which larger nuclear power sources are providing the electricity. (100s of kilowatts to tens of megawatts, with economies of scale at increasing power levels.)

The categorization of MPD thrusters

MPD thrusters are considered to be the classic electromagnetic thrusters. They are the first thrusters to have demonstrated megawatt level outputs. The first ones operated by oblating a teflon rod at the cathode (the electrode forming the center of the thruster engine) and heating the teflon to a plasma, which then gets ejected by the Lorenz force mentioned above. Since then variations of that concept have been used with different materials. What remains the same is that the device works by running a current between a central cathode and an anode forming the wall of the thurst chamber.

The forces at work in an MPD thruster are both the Lorenz force mentioned above and also (parasitically) an arcjet mode, which is more prevalent at lower powers (kilowatts). An arcjet is an electrothermal engine in which the force is produced solely by the electric heating of a gas and ejection through gas expansion. At lower powers, then, where the arcjet thrust is much more present than the Lorenz force, the MPD thruster is classed as an electrothermal device. However, at the higher powers for which it is being designed, the electothermal force dissappears and the Lorenz force dominates. At these power levels the device is an electromagnetic thruster.

Variations

MPD thrusters are refered to as being either self-field or applied field depending on whether the presence of the magnetic field is self induced by the creation of the plasma or applied externally.

Limitations

The MPD thruster is subject to high corrosion because of the presence of electrodes in megawatt level electric-plasma discharges. For this reason, similar electrodeless Lorenz devices have taken a lead over MPD thrusters in more recent research. The two prime examples are the Pulsed Inductive Thruster (PIT) and the [Variable Specific Impulse MPD Rocket [(VASIMR)]].

Sources

(More to be added later) http://www.nasa.gov/centers/glenn/about/fs22grc.html