>>13524251Ion engines don't work through a thermal process like NTR propulsion, though. In an ion engine you need to balance the electricity required to ionize each mol of propellant with the electricity you have leftover to accelerate the propellant, given your total power budget and the limitations of your ion engine's physical components. Heavy elements like mercury and xenon and especially caesium are generally a lot easier to ionize per mol, so almost all of the engine power budget is going towards accelerating the propellant vs just ionizing it in the first place, but since those ions are heavy they don't end up going that fast with a given electrical power. Lighter elements like krypton or argon are much harder to ionize, but much easier to accelerate, so those propellants tend to offer higher Isp at a reduced thrust output. I don't think anyone has tried ion propulsion using helium or hydrogen as propellants, because fully ionizing either of those would eat up a huge fraction of the power budget, and honestly the Isp offered even by xenon is more than enough for pretty much any application anyway. Before using helium or hydrogen would make sense, we would be optimizing electric propulsion to use propellants based on how cheap they are, rather than thrust or efficiency, simply because using H or He is difficult enough that for it to be viable, we would have necessarily invented the technology to be able to use argon, nitrogen, CO2, or even water very effectively.
I'll also throw out there that if you are using a hybrid nuclear electric thruster, your ionization energy comes for free because you flow your gas through a source of very powerfully ionizing radiation (ie, a fine plutonium 239 mesh firing out alpha particles), and since your ionization energy is free the use of higher Isp propellants becomes feasible, but again, the >6000 Isp offered by heavy xenon designs should be good enough for anyone.
