Why does the NASA NEXT engine use xenon? Why not use argon or krypton? Does it have to do with the thrust to power ratio or the specific impulse or something else?
This video explains it.
In short, it all boils down to golden middle between energy requirements, thrust and specific impulse. Ionizing lighter noble gases requires much higher potential - much more energy input, larger solar batteries, higher cooling requirements, circuitry and structure toughened against higher voltages and energies. While specific impulse would improve, it's already very high, meanwhile thrust is already low, and it would drop even further.
There are secondary advantages - containing xenon is easier, the large particles not as prone to filter through micropores of the tank and more mass storable in the same volume at the same pressure (and while propellant volume itself isn't that much of a concern, the tank for same pressure and higher volume is necessarily bigger - and heavier.) Xenon is not radioactive, like Radon, and being a noble gas is very non-reactive, safe to the structure. But its ionization potential is the primary reason - still reasonably low, while providing excellent performance.
Surely if currently achievable specific impulse appears to be insufficient for our goals, we'll move to lighter noble gases, but for now all destinations within the solar system are reachable on xenon engines, and the advantages of argon or krypton just don't justify the trade-offs.
$\begingroup$ Unfortunately the price of Xenon is very high because air contains very few Xenon. But as longs as the cost for Xenon is only a small part of the total mission budget, the price is not a real problem. $\endgroup$– UweMay 18, 2017 at 14:00
1$\begingroup$ This NASA presentation gives the 2015 Xenon status as 53 tons/year produced and electrical propulsion consuming as having consumed 10% of the prior 20 years output; and that NASA's consumption rate is high enough that they need to plan purchases far enough in advance that the market can provide them. (Might be more of interest there, I only read a few slides from it.) $\endgroup$ May 18, 2017 at 15:39
$\begingroup$ Very interesting @DanNeely. Another takaway, if you see a Xenon tube trailer and it is full, then you just saw ~16% of the yearly global production output. $\endgroup$– PhilMay 18, 2017 at 18:21
$\begingroup$ Very nice presentation from NASA. There is enough Xenon in the air for a very long future of ion thrusters. But only larger air separation plants could be extended for economical Xenon separation. Hopefully all new build large air separation plants are equipped with Xenon separation. But also Helium is an important gas for space technology and should be separated whenever possible. $\endgroup$– UweMay 18, 2017 at 19:34