Plasma propulsion engine

A 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 (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 use currents and potentials which are generated internally in the plasma to accelerate the plasma ions. While this results in a lower exhaust velocity by virtue of the lack of high accelerating voltages, 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 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). This type of thruster often generates the source plasma using radiofrequency or 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.
Plasma engines are better suited for long-distance interplanetary space travel missions.
In recent years, many agencies have developed several forms of plasma propulsion systems,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.
Advantages.
Plasma engines have a much higher specific impulse ("I"sp) 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 about 2000 s. This is a significant improvement over the bipropellant fuels of conventional chemical rockets, with specific impulses in the range of 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. 
Certain plasma thrusters, such as the mini-helicon, are hailed for their simplicity and efficiency. Their theory of operation is relatively simple and can use a variety of gases, or combinations of gases as propellant.
These qualities suggest that plasma thrusters will be valuable to many mission profiles.
Drawbacks.
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. Design and materials advancement may solve this problem.
Due to their extremely low thrust, plasma engines are not suitable for launch-to-orbit on Earth. On average, these rockets provide about 2 pounds of thrust maximum. Plasma thrusters are highly efficient in open space, but do nothing to negate the launch expense of chemical rockets.
Plasma engines in use.
While most plasma engines are still confined to the laboratory, some have seen active flight time and use on missions. As of 2011, NASA, partnered with the aerospace company Busek, and launched the first hall effect thruster aboard the Tacsat-2 satellite. The thruster was the satellite's main propulsion system. Since then, the company has launched another hall effect thruster in 2011. More plasma thrusters are likely to see flight time as the technologies mature. In 2020, research on what a plasma jet could look like was published by Wuhan University.
Engine types.
Helicon plasma thrusters.
Helicon plasma thrusters use low-frequency electromagnetic waves (Helicon waves) that exist inside plasma when exposed to a static magnetic field. An RF antenna that wraps around a chamber of gas is used to create the waves and excite the gas. Once the energy provided by the antenna couples with the gas, plasma is created. Once plasma is formed, this is expelled at high velocity to produce thrust by using different acceleration strategies that requires different combinations of electric and magnetic fields of ideal topology. These thrusters are capable of using many different propellants, making them ideal for long term missions as they belong to the category of electrodeless thrusters. The simple design also makes it versatile in that it can be made out of simple materials such as a glass soda bottle.
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 electrodes, negating the erosion problem. 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.
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 the development of 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.