Space Electric propulsion (EP) is defined as “any system that accelerates a propellant through the conversion of electric potential energy into kinetic energy”. It is used to adjust satellite trajectories and propel spacecrafts through the solar system. Broadly speaking, this energy conversion can be electrostatic, electrothermal, or electromagnetic based. It could also be a combination (e.g., electrothermal coupled with electromagnetic). Electric propulsion (EP) has a high fuel efficiency. Therefore, less fuel and propellant storage is required, making EP more suitable for smallsats. In this article we elaborate on the electrostatic propulsion systems.
What is Field Emission Electric Propulsion (FEEP)?
Field-emission electric propulsion (FEEP) is an electrostatic propulsion method used in satellites that rely on thrust through ionization. FEEP thrusters are based on liquid metal ionization and acceleration of the ions by a strong electric field. Satellites that rely on FEEP benefit from the various unique technical features of this novel electric propulsion technology. For instance, its compact size and propellant storage capacities makes it suitable for small satellites. In recent years, the number of small satellites in space have increased considerably, which makes the FEEP propulsion method an interesting opportunity for future missions in space.
The core principle of FEEP is related to the Field effect, which refers to utilizing a strong electric field to generate charged ions. The difference between the electrode and the liquid metal’s surface is balanced by the tension of the fluid, which deforms to a series of cone shapes. The intensity of the strong electric field initiates the formation of the propellant stream. This stream is expelled at high velocities through the emitter and accelerator. The neutralizer provides negatively charged ions to reduce the charge of the propellant stream and to protect the assembly.
Neutralization of propellant streams in FEEP devices can be realized through thermionic cathodes. These thermionic cathodes are essential for the desired thrust as electrons are emitted with high thermal energy to overcome the potential barrier of the emitter material and vacuum in space. Without the heating capabilities of the thermionic cathode, the emitter material cannot reach the required temperature to facilitate the FEEP thrust generating process. In addition, a cathode is required to neutralize the charged plasma field which otherwise would get attracted to the space vehicle shell and reduce or eliminate any thrust produced.
What is a gridded ION thruster?
The gridded ion thruster is a common design for satellites that rely on ion thruster technology. These thrusters are considered to be highly efficient low-thrust electric propulsion systems. The basic concept of a gridded ion thruster is acceleration of ions by electrostatic forces. The electric fields are generated by two electrodes at the end of the thruster. One electrode is charged with highly positive electrons, whereas the other is charged with highly negative electrons. The ions are generated in the opposite region of the electrode, which results in the attraction of ions out of the discharge chamber. As a result, a large number of ion jets forms the thruster’s ion beam and contributes to the satellites propulsion through space.
The electrons are generated by a hollow cathode which is located at the upstream center of the thruster. The fuel propellant is bombarded by these electrons, generating a flow of plasma, which is the key ingredient for electric propulsion of ion thrusters. Another cathode is required to neutralize the ion beam that is generated by the impact of electrons onto the fuel propellant. This neutralizing cathode is often located at the downstream side of the thruster where the ion beam is ejected. As the thruster ejects primarily positive ions, an equal amount of negative ions are required to neutralize the ion beam to avoid damage to the thruster.
The function of thermionic cathodes in gridded ion thrusters is therefore critical for successful navigation through space: On the one hand, the upstream cathode generates the electrons that are necessary to realize plasma for thrust; on the other hand, the downstream cathode neutralizes the ion beam and ensures that no external damage is caused by the ejected positive ions. Kamet can supply cathode heaters that meet the technical requirements for your gridded ion thrusters and its electric propulsion through space.
What about Hall effect thruster (HET)?
The hall effect thruster technology is based on the Hall effect principle and has been applied in space missions for over 30 years. A hall effect thruster (HET) is an electric propulsion device that uses electric and magnetic fields to create a plasma and expel ions at high velocity to generate thrust. The thruster accelerates propellant through an electric field. The source of thrust for HET thrusters most commonly contains inert xenon or krypton gas, which is required for the creation of ionized plasma in order to generate thrust. In comparison to other electrostatic thrusters, the hall effect thruster has significant advantages such as higher thrust, longer life, and it requires less power than other ion thrusters.
A hall effect thruster consists of an anode located at the upstream end of a discharge channel of ceramic walls and inert xenon or krypton gas injector. An external cathode provides electrons to neutralize the ion beam. The magnetic circuit of a hall thruster is composed of magnets or coils, located at the inner- and outer edges of the channel. It generates a field of increasing intensity along the channel. Electrons emitted from the cathode are trapped by the magnetic field lines, which reduces axial mobility and generates a strong outwardly directed electric field. The neutrals from the injector are ionized by electronic impact, and accelerated by the electric field which creates thrust.
In order to provide the required electrons for ionization, a cathode heater is used for the ignition, discharge, and neutralization of the ion beams. The heater is used in a hollow cathode and its heating performance is therefore of critical importance for the hall effect thruster’s propulsion through space.
Mineral insulated heaters for electrostatic propulsion systems
Kamet’s mineral insulated heaters are suitable for Aerospace applications. They offer high power, consistent output, and temperatures up to 1000 degrees Celsius and even up to 1600 degrees Celsius. Our heaters offer excellent properties due to the refractory metals that are used for this application, such as Tantalum. We can provide cathode heaters with very small diameters starting at 0.5 mm, and a minimal bend radius of three times the outer diameter. The connection between the cold and hot end is laser welded, therefore the diameter stays the same over the whole length of the heater.
We work closely with leading-edge companies in the field of mineral insulated heating solutions. Our cathode heaters are tailor-made to customer specific requirements and our partners have the capability, knowledge, and experience to bring your cathode heater from the drawing board to production. Moreover, our partners can conduct extensive in-house testing once the cathode heater is finished to verify whether the technical specifications are aligned with your requirements. This ensures the cathode heater’s suitability for your electrostatic thruster as well as the highest quality and reliability for its mission in space. We look forward to thinking along with you for any thermal challenges you may have.