While space advocates tend to support the use of helium-3 as a future fusion fuel, given the fact that its reaction with deuterium produces no free neutrons and thus keeps unwanted radioactivity to a minimum, present-day fusion research works with the reaction of deuterium with tritium, the latter being bred from lithium (though to start the reactor off tritium must be brought in from outside, supplied most likely from Canadian fission reactors).
I put the following question to the ITER project through their website: "How much more attractive would fusion be as a future power source if you had access to helium-3 in quantity, allowing you to work with the deuterium / helium-3 reaction rather than the deuterium / tritium one? In asking this, I am influenced by John S. Lewis, "Mining the Sky" (Addison-Wesley, 1996), who mentions ITER to make the point that fusion research is making good progress, but expects that fusion plants will eventually be fuelled with helium-3 extracted from the atmospheres of the giant planets."
Thank you to Bill Spears, who responded with the following discussion.
I see from your web site that you are thinking longer term. The question you ask is not so easy to answer in those terms. Baldly speaking, under the same confinement conditions the rate of energy production from the D+3He reaction is about 1/80th that from D+T, and its optimum operating temperature is about 6 times higher. For comparison the energy production rate from "catalysed" DD (i.e in which the D+T and D+3He "side" reaction energies are also included) is 1/50th and the optimum temperature 3 times that of D+T. So other things being equal, why would one choose D+3He in preference to catalysed DD, if one wants to get away from D+T?
The main advantage of D+3He is that it is neutron-free, so the reactants and the reaction products are not radiaoactive -- one just has to make use of the energy produced -- the other reactions activate surrounding materials, creating a waste problem, even if manageable. However given what we know today about plasma confinement, making up the above deficit will be difficult with the tokamak, and may be a problem with all magnetic confinement schemes, since they maintain a long term pressure balance between the burning plasma and the surrounding confinement.
It could be that inertial fusion (firing beams of light or particles at a stream of frozen fuel pellets) makes more sense in this case, and most interplanetary drive schemes call on this type of fusion as their base. The availability of the 3He fuel in space also encourages this thinking, as you illustrate. The differences in the reactions also narrow somewhat, as there is no long term pressure balance to be maintained (the pellets implode). It could be that with continued development in beam technology, you can make a net energy gain machine out of this route too.
For terrestrial energy production in the relative near term (like next 100+ years) I would argue that magnetic fusion represents the best course because the problems of making a net energy gain power station look soluble today. With inertial fusion, getting a net energy gain system appears more difficult today, although experiments are underway (e.g. NIF in US). It could be that the greater suitability of this route for space propulsion actually leads eventually to it being used for propulsion first and then, after technical evolution in space, supplanting magnetic fusion in the very long run (300+ years) as a terrestrial energy source.
I hope that clarifies the situation.
Bill Spears
Project Coordinator
ITER Garching Joint Work Site
Last revised 17 October 2003 / 34th Apollo Anniversary Year