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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 May 1 '15 at 21:35
  • $\begingroup$ Well, the inert gas law states that the pressure is proportional to the mass of the molecule. So for an equal mass of Xenon vs Krypton, you're talking about a 57% increase in pressure for the same mass of gas. $\endgroup$ – PearsonArtPhoto May 1 '15 at 21:37
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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 May 2 '15 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 May 2 '15 at 21:58
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    $\begingroup$ Increase the mass of the vessel, without actually gaining anything useful as a result. $\endgroup$ – PearsonArtPhoto May 3 '15 at 0:10
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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 Aug 2 '15 at 20:51
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    $\begingroup$ Xe has a lower ionization energy than other noble gasses. $\endgroup$ – James K Oct 23 '15 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 Apr 1 '18 at 19:49

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