plasma propulsion[′plaz·mə prə′pəl·shən]
The imparting of thrust to a spacecraft through the acceleration of a plasma (ionized gas). A plasma can be accelerated by electrical means to exhaust velocities considerably higher than those attained by chemical rocks. The higher exhaust velocities (specific impulses) of plasma thrusters usually imply that for a particular mission the spacecraft would use less propellant than the amount required by conventional chemical rockets. This means that for the same amount of propellant a spacecraft propelled by a plasma rocket can increase in velocity over a set distance by an increment larger than that possible with a chemical propulsion system. Plasma propulsion is one of three major classes of electric propulsion, the others being electrothermal propulsion and ion (or electrostatic) propulsion. See Electrothermal propulsion, Ion propulsion, Specific impulse
Pure electromagnetic acceleration
The most promising and thoroughly studied electromagnetic plasma accelerator is the magnetoplasmadyamic (MPD) thruster. In this device the plasma is both created and accelerated by a high-current discharge. The discharge is due to the breakdown of the gas as it is injected in the interelectrode region. The acceleration process can be described as being due to a body force acting on the plasma. This body force is the Lorentz force created by the interaction between the current conducted through the plasma and the magnetic field. The latter could either be externally applied by a magnet or self-induced by the discharge, if the current is sufficiently high.
Microscopically, the acceleration process can be described as the momentum transfer from the electrons, which carry the current, to the heavy particles through collisions. Such collisions are responsible for the creation of the plasma (ionization) and its acceleration and heating (Joule heating).
The collision processes invariably heat the plasma. If the gas particles are exhausted hot, they are dissipating energy in kinetic modes useless to propulsion since their thermal motion is random. Moreover, if the exhausted atoms are in an excited or ionized state, the fraction of the internal energy tied in these internal modes is also not available for propulsion. If a fraction of these translational and internal modes is somehow recovered, the plasma acceleration is called hybrid (electromagnetic-electrothermal). Hybrid acceleration is an active area of research and is the most promising alternative for surpassing the 40% efficiency level of magnetoplasmadynamic thrusters.
Few tests of plasma thrusters in space are known publicly outside Russia. Most of the flown plasma propulsion systems are of the pulsed solid-fed (Teflon- ablative) type launched in the 1970s for satellite attitude control.