What is an Ion Thruster?

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Jul 29, 2022

An Ion Thruster is a form of electric propulsion that works on the concept of ionization. This process involves the extraction of ions from the plasma to produce thrust. The ions extracted are accelerated by using electric fields that generate many ion jets together called the ion beam that produces the required thrust for the spacecraft. The process of ionization is done with the help of electron bombardment or Electron Cyclotron Resonance (ECR) which uses high-frequency radiation (such as microwaves) and a large magnetic field. 

Ionization is the process of adding or removing electrons to produce ions. In most thrusters, ionization is usually done by electron bombardment in which a high-energy electron collides with a propellant atom releasing energy from the propellant atom resulting in a positively charged ion.  The gas produced by the result of the collision is called plasma which consists of positive ions and negative electrons and is easily affected by electric and magnetic fields. 

The electrons in the ion thruster are generated by the cathode through a process called Thermionic Emission. These electrons are attracted to the walls of the discharge chamber which is positively charged. The neutral propellant is injected into the discharge chamber, where the electrons bombard the propellant to produce positively charged ions and release more electrons. These electrons are prevented from reaching the discharge channel walls with the help of high-strength magnets so as to increase the chances of ionization.

At the end of the ion, the thruster is two grids, one which is positively charged (screen grid) with thousands of precise holes and the other which is negatively charged (also known as the accelerator grid). The positive ions reach the first grid to which a very high positive potential is applied and is configured to force the discharge plasma to reside at a high voltage. As ions pass the first grid they are accelerated to very high speeds from the accelerator grid as an ion beam, which produces thrust.

One of the most common propellants used in ion propulsion is xenon which is inert and has a high storage density as well as high atomic mass. Since Xenon is easily ionized it generates a desirable amount of thrust.

Ion thrusters are used for a wide variety of missions such as keeping communications satellites in the proper position (station-keeping) to propelling spacecraft throughout our solar system. They also require significantly less propellant for a given mission compared to chemical propulsion.