[FPSPACE] the He3 urban legend

[Stephane Chenard]

And what do the people whose job actually is to make nuclear fusion work
have to say about helium 3, hmm ? Is it the miraculous ingredient that would
finally make the fusion reactor work, if only we could mine it off the moon
?

It would be fun to look for more recent references, but for now this is what
the long-defunct Office of Technology Assessment had to say in 1989 in its
excellent report on nuclear fusion ("Starpower: The U.S. and the
International Quest for Fusion Energy", easily found on the web) on page 92:

"However, the D-3He reaction is much more difficult to start than the D-T
reaction. The minimum temperature required to ignite D-3He is several times
higher than that needed for D-T [deuterium-tritium]; the minimum confinement
parameter [the product of plasma density by confinement time] is about 10
times higher. Given that the requirements for igniting D-T have not yet been
experimetally achieved, attaining conditions sufficient to ignite D-3He is
considerably farther off."

Given that physicists are still struggling to make even a prototype D-T
fusion machine achieve anything like the performance needed for a reactor,
that doesn't exactly read like "if only we could get 3He". Are the
physicists interested at all in helium 3 then ?

"Today [in 1989] a resurgence of excitement about 3He comes with the
discovery that it is found in substantial amounts [...] on the moon".

So the "resurgence of excitement" did not come from new physics, but from
outside the physics community.

And that's not all. Says a footnote on page 91:

"Deuterium, being relatively scarce in a 3He-rich mixture, would be much
more likely to react with a 3He nucleus than with another deuterium nucleus,
making D-D reactions relatively rare. However, one consequence of this mode
of operation, in addition to minimizing neutron generation, would be the
lessening of output power since most of the 3He nuclei would be unable to
find D nuclei with which to react. Increasing the ratio of D to 3He to more
nearly equal proportions, therefore, would increase both the output power
and the neutron generation".

In other words, not only is 3He a lot harder to use, but it makes your
reactor less powerful. Its only advantage is "less neutrons", i.e. a cleaner
reactor and less radioactive waste, which is nice but would come at the
expense of power, and on top of that your clean reactor would not be content
with just a little dollop of 3He, it would need to be "3He-rich".

I doubt that the science has changed much since 1989, though perhaps the
technology has evolved to make helium 3 a bit more attractive.

However, it's been distressing in the past 17 years to see helium 3 bandied
about in space conferences and publications, and in some of the high-level
advocacy for going back to the Moon -- unless I missed something,
exclusively by space advocates, with never a fusion physicist seemingly
getting a say in the debate. You get the distinct impression that helium 3
advocates either are talking about things they know nothing about, or assume
that no one will bother to check and that there will never be a fusion
specialist in the audience anyway.

That would be vaguely reminiscent of the 1980s craze over space-grown
protein crystals and Shuttle-borne electrophoresis, which had the actual
protein crystal and electrophoresis specialists shaking their heads in
disbelief. Yes they took NASA's money when it came their way (that's only
human), but it didn't exactly buy the space industry a lot of respect, and
ultimately it mainly wasted money.

Certainly I don't claim to be a fusion physicist, but it would be
interesting and innovative to bring that community into the debate.