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Magnetoplasmadynamic thrusters


Magnetoplasmadynamic (MPD) thrusters have been under development since the late 1960's in Russia and in the US, where several variations of the primary concept were investigated (steady state vs quasi-steady, self field vs applied field). Research on low and medium power MPD thrusters has then gradually started in other countries, most notably in Germany, Japan and Italy.

Principle of operation

Schematic view of a magnetoplasmadynamic thruster

In a self field MPD thruster, the arc current created between a central cathode and an annular peripheral anode ionizes the propellant and induces an azimuthal magnetic field. In a manner somewhat reminiscent of dense plasma focus devices, the generated J×B Lorentz body force compresses and accelerates a quasi-neutral plasma along the central axis.
Because the self-induced magnetic field is only significant at very high power, low power MPD thrusters often resort to an externally applied magnetic field in order to enhance the acceleration process (applied field MPD thrusters).

MPD thrusters may operate in steady state mode or in pulsed, quasi-steady mode. The latter strategy is aimed at improving their efficiency via an increase of the instantaneous operating power, while keeping an average power requirement consistent with conventional spacecraft capabilities.

Typical capabilities and use

The great scalability of MPD thrusters allows them to cover an impressive range of operating parameters: from a few kW to several MW of power, from a few mN to several hundred N of thrust and from 1000 km/s to 10000 km/s of specific velocity. MPD thrusters, in particular self-field thrusters, are most efficient at high power and can perform with up to 40% of efficiency.

Despite its potential advantages, the development of MPD thrusters remains hindered by longstanding cathode erosion problems limiting their life to a few hundred hours at best. The only MPD thruster ever used in space is EPEX, a 1kW quasi-steady engine succesfully flown on the Japanese SFU satellite in 1995.

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