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A plasma propulsion engine is a type of electric propulsion that generates thrust from a quasi-neutral plasma. This is in contrast to ion thruster engines, which generate thrust through extracting an ion current from plasma source, which is then accelerated to high velocities using grids/anodes. These exist in many forms (see electric propulsion). Plasma thrusters do not typically use high voltage grids or anodes/ cathodes to accelerate the charged particles in the plasma, but rather uses currents and potentials which are generated internally in the plasma to accelerate the plasma ions. While this results in a lower exhaust velocities by virtue of the lack of high accelerating voltages, this type of thruster has a number of interesting 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 ion and electrons exist in equal number, which allows simply ion- electron recombination in the exhaust to neutralise the exhaust plume, removing the need for an electron gun (hollow cathode). This type of thruster often generates the source plasma using radio frequency of microwave energy, using an external antenna. This fact, combined with the absence of hollow cathodes (which are very sensitive to all but the few noble gases) allows the intriguing possibility of being able to use this type of thruster on a huge range of propellants, from argon, to carbon dioxide, air mixtures to astronaut urine.
Contents
- Helicon double layer thrusters
- Magnetoplasmadynamic thrusters
- Pulsed inductive thrusters
- Electrodeless plasma thrusters
- Hall effect thrusters
- VASIMR
- References
Plasma engines are better suited for long-distance interplanetary space travel missions.
In recent years, many agencies have developed several forms of plasma-fueled engines, including the European Space Agency, Iranian Space Agency and Australian National University, which have co-developed a more advanced type described as a double layer thruster. However, this form of plasma engine is only one of many types.
Helicon double layer thrusters
Helicon double-layer thrusters use radio waves to create a plasma and a magnetic nozzle to focus and accelerate the plasma away from the rocket engine. A Mini-Helicon Plasma Thruster, ideal for space maneuvers, runs off of nitrogen, and the fuel has an exhaust velocity (specific impulse) 10 times that of chemical rockets.
Magnetoplasmadynamic thrusters
Magnetoplasmadynamic thrusters (MPD) use the Lorentz force (a force resulting from the interaction between a magnetic field and an electric current) to generate thrust - The electric charge flowing through the plasma in the presence of a magnetic field causing the plasma to accelerate due to the generated magnetic force. The Lorentz force is also crucial to the operation of most pulsed plasma thruster
Pulsed inductive thrusters
Pulsed inductive thrusters (PIT) also use the Lorentz force to generate thrust, but unlike the magnetoplasmadynamic thruster, they do not use any electrode, preventing their erosion. Ionization and electric currents in the plasma are induced by a rapidly varying magnetic field.
Electrodeless plasma thrusters
Electrodeless plasma thrusters use the ponderomotive force which acts on any plasma or charged particle when under the influence of a strong electromagnetic energy density gradient to accelerate both electrons and ions of the plasma in the same direction, thereby able to operate without neutralizer.
Hall effect thrusters
Hall effect thrusters (also called stationary plasma thrusters SPT) combine a strong localized static magnetic field perpendicular to the electric field created between an upstream anode and a downstream cathode called neutralizer, to create a "virtual cathode" (area of high electron density) at the exit of the device. This virtual cathode then attracts the ions formed inside the thruster closer to the anode. Finally the accelerated ion beam is neutralized by some of the electrons emitted by the neutralizer. Serial production of Hall effect thruster started in Soviet Union in the 1970s. One of the early variants, SPT-100 is now produced under license by European Snecma Moteurs under the name PPS-1350. Similarly BPT-4000 and PPS-5000 are closely related to SPT-140. SPT-290 has a thrust of 1.5N, 5-30 kW power and specific impulse 30 km/s, efficiency 65% and weight 23 kg.
VASIMR
VASIMR, short for Variable Specific Impulse Magnetoplasma Rocket, uses radio waves to ionize a propellant into a plasma. Then, a magnetic field accelerates the plasma from the rocket engine, generating thrust. The VASIMR is being developed by Ad Astra Rocket Company, headquartered in Houston, TX. A Nova Scotia, Canada-based company Nautel, is producing the 200 kW RF generators required to ionize the propellant. Some component tests and "Plasma Shoot" experiments are performed in a Liberia, Costa Rica laboratory. This project is led by former NASA astronaut Dr. Franklin Chang-Díaz (CRC-USA).
The Costa Rican Aerospace Alliance has announced development of an exterior support for the VASIMR to be fitted outside the International Space Station. This phase of the plan to test the VASIMR in space is expected to be conducted in 2016. A projected 200 megawatt VASIMR engine could reduce the time to travel from Earth to Jupiter or Saturn from six years to fourteen months, and from Earth to Mars from 6 months to 39 days.