Research Help

> Hello all,
> I am new to this list. I am a graduate student at the University
> of North Dakota in the Space Studies department (www.space.edu). I
> am currently working on my masters thesis option and looking for any
> research help or suggestions. My topic is a permanant human mining
> colony for the mining of Helium-3 for fuel in fusion. Other
> aplications would follow.
>
Not necessarily the kind of help you are looking for, but take a
careful look at the helium-3 fusion fuel cycle. You'll see that it's a
little sketchy as far as the claims for aneutronic fusion go. To fuse
D-He3 you need a plasma in thermal equilibrium (this is a necessity for
fundamental reasons - nonthermal schemes turn out not to work because
the non-fusing scattering cross section is so much bigger than the
cross section for fusion: this means that if you have a beam impinging
on a target you end up losing most of the beam to elastic scattering
and not doing fusion (which is intrinsically inelastic). In thermal
equilibrium, any particle scattered from a given velocity vector into
another vector is matched by another particle scattering from the
second velocity vector back into the first: this is what defines
thermal equilibrium - in an average sense the total number of particles
with any given velocity vector remains constant when averaged over
times which are long relative to the interparticle collision time) <-
yup, that whole thing was a parenthetical comment :-)

Anyway, back to the necessity for thermal equilibrium: this means you
have Deuterium and He3 ions knocking around with the same energy.
You'll have D-D collisions, He3-He3 collisions, and D-He3 collisions.
The latter is what you want. The D-D collisions that fuse will produce
neutrons, which you don't want. Unfortunately, unless you go to
implausibly high temperatures (1000 KeV, or 10,000,000,000 degrees
Kelvin (yup, ten billion)) the cross section for D-D fusion is an order
of magnitude larger than that for D-He3, so the fusion reactions will
be dominated by D-D unless you go to a fuel mix that is almost entirely
He3, which drives the D-He3 fusion rate down as well, since nearly all
collisions are now He3-He3. Thinking purely in terms of collisions,
this isn't a big deal, but all the time you are sitting there waiting
for a D-He3 collision, the plasma is radiating furiously, losing energy
from the fusion core (~10 million degrees) to the reactor wall (~1000
degrees, which seems pretty hot, but as far as the core is concerned
might as well be absolute zero). The upshot is that with D-He3 you need
nearly equal proportions, so you get maybe a 50% reduction in neutrons
relative to D-D, at the cost of a hugely expensive fuel. The 50%
reduction has no real effect on the cost of the reactor - you save on
replacement of plasma facing components as they become radioactive
(only need to replace half as often), but the cost of a reactor isn't
driven by the plasma facing component replacement. You'll still need
tons of shielding.

The main point is that D-He3 isn't a good fuel cycle for terrestrial
applications. It might turn out to be good for fusion propulsion
applications, where you can afford to ignore the neutrons and the
savings in shielding mass due to the lower neutron flux might be worth
it, but I think that even so things sway in favor of just dealing with
the additional neutron flux and using a D-D fuel cycle.

It's too bad that the enthusiasm for D-He3 turned out to be poorly
founded, but basically nobody takes it seriously as an aneutronic fuel
cycle anymore. There are people who think that the 50% or so reduction
in neutron flux is worth it, but I don't think they are taking into
account the increased cost (even from lunar sources) of the fuel in
doing the trades. Actually, to be brutally honest, I think anyone
trying to predict the cost of a commercial fusion power plant is
deluding themselves. There is a real possibility that fusion will never
be able to compete with other sources of power. Fusion is *hard* - I
work on it in my day job, and I believe it can be done, but the sense I
get from talking to my fellow fusion researchers is that they are not
taking seriously the ongoing development of alternatives. Long before
it's economically viable to mine lunar He3 we'll have SPS. If we ever
see commercial fusion I believe it will be either D-D (and just deal
with the neutrons) or p-B11, which has no neutron producing side
reactions but is insanely hard even by the standards of a field that
has been trying for 40 years and has only managed to reach 60% of an
artificially low goal (which is where things currently stand - 60% of
"break even" defined in terms that include only the power used to heat
the plasma and exclude the megawatts required to power the magnets).

I think it's still worth writing a thesis on lunar He3 production - the
point is to learn, not to write a business plan. You should, however,
avoid harboring illusions about the future market for He3 - it may come
into being, but fusion is extremely unlikely to be the driver unless
there is a major breakthrough in dealing with side reactions. My advice
is to focus on the colony aspect rather than the mining. That's
something with general application, so you're more likely to see some
of your ideas actually implemented someday.

Hope this helps,
......Andrew

Dr. Andrew Case, PhD.
Institute for Research in Electronics and Applied Physics,
University of Maryland, College Park
"It is seldom that liberty of any kind is lost all at once."
- David Hume