Plasma propulsion – How It Works

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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 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. 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 May 2020, a team from the Institute of Technological Sciences at Wuhan University published a paper on a prototype plasma jet device they developed capable of lifting a 1kg (2.2lb) steel ball over a 24mm (one inch) diameter quartz tube. The thrust needed to achieve such lift is equivalent to the relative thrust of a commercial aircraft engine. In the design, pressurized air is injected into a chamber and subjected to over 1,000 degrees Celsius and microwaves to create an ionised plasma, which is then expelled to create propulsion.

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. The VASIMR method for heating plasma was originally developed during nuclear fusion research. VASIMR is intended to bridge the gap between high thrust, low specific impulse chemical rockets and low thrust, high specific impulse electric propulsion, but has not yet demonstrated high thrust. The VASIMR concept originated in 1977 with former NASA astronaut Franklin Chang Díaz, who has been developing the technology ever since. 

VASIMR is a type of electrothermal plasma thruster/electrothermal magnetoplasma thruster. In these engines, a neutral, inert propellant is ionized and heated using radio waves. The resulting plasma is then accelerated with magnetic fields to generate thrust. Other related electrically powered spacecraft propulsion concepts are the electrodeless plasma thruster, the microwave arcjet rocket, and the pulsed inductive thruster. Every part of a VASIMR engine is magnetically shielded and does not directly contact plasma, increasing durability. Additionally, the lack of electrodes eliminates the electrode erosion that shortens the life of conventional ion thruster designs.

The propellant, a neutral gas such as argon or xenon, is injected into a hollow cylinder surfaced with electromagnets. On entering the engine, the gas is first heated to a “cold plasma” by a helicon RF antenna/coupler that bombards the gas with electromagnetic energy, stripping electrons off the propellant atoms and producing a plasma of ions and free electrons. By varying the amount of RF heating energy and plasma, VASIMR is claimed to be capable of generating either low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust. The second phase of the engine is a strong solenoid-configuration electromagnet that channels the ionized plasma, acting as a convergent-divergent nozzle like the physical nozzle in conventional rocket engines.

A second coupler, known as the Ion Cyclotron Heating (ICH) section, emits electromagnetic waves in resonance with the orbits of ions and electrons as they travel through the engine. Resonance is achieved through a reduction of the magnetic field in this portion of the engine that slows the orbital motion of the plasma particles. This section further heats the plasma to greater than 1,000,000 K (1,000,000 °C; 1,800,000 °F) —about 173 times the temperature of the Sun’s surface. The path of ions and electrons through the engine approximates lines parallel to the engine walls; however, the particles actually orbit those lines while traveling linearly through the engine. The final, diverging, section of the engine contains an expanding magnetic field that ejects the ions and electrons from the engine at velocities as great as 50,000 m/s (180,000 km/h).

Potential applications of VASIMR

VASIMR has a comparatively poor thrust-to-weight ratio, and requires an ambient vacuum.

Proposed applications for VASIMR such as the rapid transportation of people to Mars would require a very high power, low mass energy source, ten times more efficient than a nuclear reactor.  In 2010 NASA Administrator Charles Bolden said that VASIMR technology could be the breakthrough technology that would reduce the travel time on a Mars mission from 2.5 years to 5 months. However this claim has not been repeated in the last decade.

In August 2008, Tim Glover, Ad Astra director of development, publicly stated that the first expected application of VASIMR engine is “hauling things [non-human cargo] from low-Earth orbit to low-lunar orbit” supporting NASA’s return to Moon efforts.

Mars in 39 days by VASIMR

In order to conduct an imagined crewed trip to Mars in 39 days, the VASIMR would require an electrical power level far beyond anything currently possible or predicted.

On top of that, any power generation technology will produce waste heat. The necessary 200 megawatt reactor “with a power-to-mass density of 1,000 watts per kilogram” (Díaz quote) would require extremely efficient radiators to avoid the need for “football-field sized radiators”.

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