When I was looking into electric rockets I found out that the propellants used were Helium, Nitrogen, Argon, Krypton etc. The above elements are noble gases (with the exception of Nitrogen) but I also found that Lithium, Hydrazine and Hydrogen were also being used as propellants which aren't inert per se. So what all can be described as the ideal qualities of a propellant in electric rockets?
Being inert is one of the most important properties you are looking for, especially in the electrostatic thrusters, where corrosion of the grid is what limits their operational life.
To make the propellant easy to store, it should ideally be a liquid. Hydrogen and helium, however, require cryogenic cooling to achieve that. Smaller tanks usually means a lower dry mass, so high density is preferable too.
Due to the tyranny of the rocket equation, a high exhaust velocity is desirable. That favours a low atomic mass. However, there are a few other things to consider in electrical engines that change this picture a little:
- Low thrust is the main drawback of electrical rocket engines, even though they have a very good exhaust velocity. For that reason, trading propellant efficiency for thrust is often desirable. That means using propellants with a higher atomic mass.
- Available electrical energy is also a limit. Preferably, the energy required to ionize the propellant should be low, so more energy can be used for the acceleration of the propellant instead. When a low ionization energy is required, the engine is also more easy to build overall, and can sometimes also be quite a bit lighter. A low ionization energy is the reason why xenon is a popular propellant.