First, what is a plasma propulsion? Plasma propulsion is a technology that the propellant is in an ionized, or plasma state. To achieve a plasma state, the gas is ionized by using electrical, nuclear, or any non-combustible energy transfer methods. Most satellites using plasma propulsion ionizes the gas electrically. The temperature of the plasma ranges from room temperature to 5000 K and above. Plasma Propulsion normally takes form in one of these three categories:
For this article, we will concentrate on thermal plasma propulsion. It is the simplest and common form of early plasma propulsion.
In general, thermal engines are rockets that the temperature of the propellant increases from the heat transfer of a power plant or hot element. The incoming propellant is cold, and non-reactive, and after several passes through a heating source, the gas is heated and ionized. Thermal engines have been theorized and worked on since the 1970s with NASA’s Nuclear Engine for Rocket Vehicle Application (NERVA).
NERVA operation is fairly simple: it starts from liquid hydrogen flows through a nuclear reactor. The nuclear reactor cools as the temperature of the liquid hydrogen increase. After several passes through this heat exchanger, the temperature of the liquid hydrogen is sufficient enough for propulsion. NERVA, and other thermal engines are not combustion engines as, no oxidizers are involved and no ignition for combustion
Electrothermal thrusters heat the incoming cold gas from electrical power sources, either DC or RF. Though in comparison to other plasma thrusters, this has the lowest specific impulse. The specific impulse is determined by the pressure differential. The higher the temperature, the higher the specific impulse. Heating the plasma electrically is called Joule, or Ohmic, Heating. In order to find the total Joule Heating power input to the gas, it is simply the product of the voltage and current.
There are three types of electrothermal thrusters: resistojets, arcjets, and RF thrusters. Resistojets utilize resistors as heating elements for ionization. However, resistojets have a low efficiency because heating the gas for ionization requires a large power source. The efficiency of a resistojet is further limited when the gas passes through the resistor and cools the resistor.
An arcjet thruster is a DC-based electrothermal thruster that uses two electrodes and a high potential difference to ionize the gas. The ionization is caused by an electrical arc created between the electrodes. There is a considerable amount of research into arcjet thrusters. One of the reasons this technology is popular is its simplicity. The technology requires only a cathode tip, an anode nozzle, and a high‑voltage power source.
RF thrusters are sought to be the best option for electrothermal propulsion. The power required to heat the gas is significantly lower than an arcjet and the configuration can be electrode-less. The amount of power required to sufficiently heat the gas depends inversely on the input frequency. RF-based electrothermal thrusters are a better alternative to DC-based electrothermal thrusters because of their lower power requirement. RF thrusters use an alternating current frequency with a range typically between 10 kHz and 10 GHz. The most common frequency used for producing plasma in the RF region is 13.56 MHz. RF thrusters require shielding to carefully isolate the RF power plant from the rest of the system and controllers.
Next article, we will discuss electrostatic propulsion and it works. Any questions, feel free to email us at email@example.com.