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From skimming the Wikipedia article on ion thrusters, I notice that xenon is frequently (though not exclusively) used as the reaction mass in systems that have actually been deployed - Deep Space 1, Hayabusa, SMART-1, and Dawn, for example.

What are the properties of xenon that make it attractive for use in ion thrusters? I imagine that its inertness is certainly helpful, but in that case, why xenon as opposed to, say, krypton or some other noble gas?

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Xenon is the heaviest non-radioactive elemental inert gas. The added mass allows for denser packing at less pressure. The mass is one of the limiting factors, so having a more dense gas helps tremendously.

The limiting factor relates to the mass of the propellant. Essentially, a heavier mass allows for more momentum to come from the overall system. The mass will take longer to accelerate, allowing more momentum to be exerted on the particle. Radioactivity could cause all kinds of issues, as could something that would be reactive. Elemental is easier because it's easier to manipulate, and as you have to make the gas ionic, if it's not elemental it will have a much higher potential to react with something. Thus, it is more efficient to use a heavy elemental non-radioactive inert gas.

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  • $\begingroup$ So the pressure on the vessel that contains the reaction mass is a limiting factor here? That comes as a bit of a surprise. Roughly speaking, how much pressure are we talking about here? $\endgroup$
    – senshin
    Commented May 1, 2015 at 21:35
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    $\begingroup$ @senshin Put another way, the higher pressure the gas needs to be stored under, the stronger the container you need for it. (Assuming you're not using reactive or radioactive) Stronger containers generally weigh more, and weight decreases efficiency. $\endgroup$
    – Patrick M
    Commented May 2, 2015 at 2:07
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    $\begingroup$ @PatrickM Ah, now I get it. By using a gas with a high molar mass, you increase the mass required for the vessel, and that's a bad thing. $\endgroup$
    – senshin
    Commented May 2, 2015 at 21:58
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    $\begingroup$ Increase the mass of the vessel, without actually gaining anything useful as a result. $\endgroup$
    – PearsonArtPhoto ♦
    Commented May 3, 2015 at 0:10
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    $\begingroup$ Why not use mercury, which is liquid and so does not need a pressure vessel for storage? It also has a higher mass, and is not highly reactive. $\endgroup$
    – jamesqf
    Commented Jul 22, 2015 at 18:22
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According to the wikipedia page on ion thrusters:

Ionization energy represents a very large percentage of the energy needed to run ion drives. The ideal propellant for ion drives is thus a propellant molecule or atom that is easy to ionize, that has a high mass/ionization energy ratio. In addition, the propellant should not cause erosion of the thruster to any great degree to permit long life; and should not contaminate the vehicle.

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    $\begingroup$ however, Xe has very high ionization energy, so it would be one of the worst choice by that argument en.wikipedia.org/wiki/Ionization_energy#/media/… $\endgroup$
    – Prokop Hapala
    Commented Aug 2, 2015 at 20:51
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    $\begingroup$ Xe has a lower ionization energy than other noble gasses. $\endgroup$
    – James K
    Commented Oct 23, 2015 at 12:25
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    $\begingroup$ @ProkopHapala Proportianally to its mass, xenon's ionization energy is low, and more mass makes more thrust, so for that same ionization energy cost, you get more bang for your buck. $\endgroup$
    – wedstrom
    Commented Apr 1, 2018 at 19:49
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Apparently they aren't even restricted to noble elements: This great paper from AFRL notes that bismuth is a good contender that has been demonstrated in Hall thrusters. The drawback with fuels that are not gases at very low temperatures is that:

  1. They have to be vaporized, which requires extra energy and engineering.
  2. If they are not gaseous at low temperatures then there is (unvalidated) concern that the ions could condense back onto the spacecraft.

For high thrust to power missions, bismuth has been demonstrated as a viable alternative Hall effect thruster propellant. Bismuth, with its high atomic mass (209 amu) and low ionization potential (7.3 eV) appears to have advantages for missions where high thrust at reduced specific impulse is advantageous, primarily for orbit raising missions. Bismuth’s main drawback is that the metal must be vaporized to be ionized and accelerated within a Hall effect thruster. The requirement for high temperatures (boiling point of 1,837K) require special engineering considerations compared to the relatively simple gas distribution systems used for xenon. In addition, the use of vapor as a propellant has tended to cause concern for spacecraft operators despite the assurances of thruster developers. The risk of metal redeposition from the propellant on solar arrays and sensitive instruments is a large concern that will strongly limit bismuth’s appeal to spacecraft designers.

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