Plasma propulsion engine is a type of electric propulsion that generates thrust from a quasi-neutral plasma. This is in contrast with ion thruster engines, which generate thrust through extracting an ion current from the plasma source, which is then accelerated to high velocities using grids/anodes.
These exist in many forms. However, in the scientific literature, the term “plasma thruster” sometimes encompasses thrusters usually designated as “ion engines”.
Plasma thrusters do not typically use high voltage grids or anodes/cathodes to accelerate the charged particles in the plasma, but rather use currents and potentials that are generated internally to accelerate the ions, resulting in a lower exhaust velocity given the lack of high accelerating voltages.
Plasma engines have a much higher specific impulse (Isp) value than most other types of rocket technology. The VASIMR thruster can be throttled for an impulse greater than 12000 s, and hall thrusters have attained ~2000 s. This is a significant improvement over the bipropellant fuels of conventional chemical rockets, which feature specific impulses ~450 s.
With high impulse, plasma thrusters are capable of reaching relatively high speeds over extended periods of acceleration. Ex-astronaut Franklin Chang-Diaz claims the VASIMR thruster could send a payload to Mars in as little as 39 days, while reaching a maximum velocity of 34 miles per second (55 km/s).
This type of thruster has a number of advantages. The lack of high voltage grids of anodes removes a possible limiting element as a result of grid ion erosion. The plasma exhaust is ‘quasi-neutral’, which means that positive ions and electrons exist in equal number, which allows simple ion-electron recombination in the exhaust to neutralize the exhaust plume, removing the need for an electron gun (hollow cathode).
Such a thruster often generates the source plasma using radio frequency or microwave energy, using an external antenna. This fact, combined with the absence of hollow cathodes (which are sensitive to all but noble gases), allows the possibility of using this thruster on a variety of propellants, from argon to carbon dioxide air mixtures to astronaut urine.
Possibly the most significant challenge to the viability of plasma thrusters is the energy requirement. The VX-200 engine, for example, requires 200 kW electrical power to produce 5 N of thrust, or 40 kW/N. This power requirement may be met by fission reactors, but the reactor mass (including heat rejection systems) may prove prohibitive.
Another challenge is plasma erosion. While in operation the plasma can thermally ablate the walls of the thruster cavity and support structure, which can eventually lead to system failure.
Due to their extremely low thrust, plasma engines are not suitable for launch-to-Earth-orbit. On average, these rockets provide about 2 pounds of thrust maximum. Plasma thrusters are highly efficient in open space, but do nothing to offset the orbit expense of chemical rockets.
Plasma engines are better suited for interplanetary missions.
Many space agencies developed plasma propulsion systems, including the European Space Agency, Iranian Space Agency and Australian National University, who co-developed a double layer thruster.