Xenon

Xenon is a noble gas, much like krypton and argon. It is the most efficient choice for the fuel of most types of ion engines and may be used as well as an ionizing agent in a nuclear MHD generator. Xenon is quite expensive, at about $20 per liter (6g). There are about 45 billions tons of Xenon in Earth's atmosphere. Due to cost and availability concerns, argon may be a more practical choice than Xenon.

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