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I've been looking at some more fusion stuff lately. (Plasma based
only, not the cold fusion stuff.)

Let me start with some bad news.

D-T fusion emits about 80% of its energy in the form of a highly
energetic neutron. D-D fusion emits about 40% of its energy in the
form of neutrons.

As most of you know, high neutron fluxes really suck. They embrittle
metals and cause most materials to become radioactive. The only way to
get the energy back from the reaction is by doing a heat engine, which
is Carnot limited to about 35% efficiency.

If D-T and D-D fusion are all that is available, it is difficult for
me to see any reason to prefer a fusion reactor over a Thorium
reactor. A thorium reactor would require little research. The fuel is
cheap and plentiful. There is little proliferation risk. There would
probably be less radiological waste from a thorium plant than from a
D-T fusion plant [3]. This argument is why I think funding for ITER
should be cut. We are pouring $B into research on something that we
almost certainly won't want even if/when they can get it to work.


Now some worse news.

Dr. Todd Rider's PhD thesis [1] offers a proof that none of the
proposed aneutronic fuels could work. He offers calculations showing
that Bremmstrahlung losses will exceed fusion power in any neutral
plasma. He further claims that this is true regardless of the
velocity distribution of the plasma. A nice summary and discussion is
here [2].

A simpler article by Dr. Rider [4] Fundamental Limitations On
Advanced-Fuel Fusion
.

Some folks have become upset and claimed various malicious motivations
behind Dr. Rider's PhD [6]. I don't buy that.

There is a plasma-physics PhD, Dr. Art Carlson, who is active on
wikipedia and at talk-polywell.org and has read and seems to believe
Rider's thesis. From looking through his postings for the last couple
of years, I believe that both Dr. Carlson and Dr. Rider are completely
on the level. Many folks are disappointed or upset that aneutronic
fusion may not be possible, but I doubt many are more disappointed
than the folks who spent years getting PhDs in the subject only to see
the field collapse around them. As it says in the first paragraph of
Dr. Rider's Thesis:

For the record, the author would like to apologize for apparently
killing some of the most attractive types of fusion reactors that have
been proposed. He advises future graduate students working on their
theses to avoid accidentally demolishing the area of research in which
they plan to work after graduation.



Possible good news:

There are at least two programs that are pursuing plasma based
aneutronic fusion despite Dr. Rider's thesis: DPF fusion and the
Polywell.

For the Polywell, Dr. Bussard [6] gets a very different calculation
for bremsstrahlung losses than Dr. Rider does. For a specific machine,
he shows a ratio of 12 for a particular design. While [7] was written
before Rider's Thesis and Dr. Bussard is now dead, there is anecdotal
evidence that the Thesis didn't dissuade Dr. Bussard [5]:

I was witness, in fact, in 1995-96, to Dr. Bussard thinking Rider had
actually found a fatal flaw in the idea. He dissappeared in the office
for a couple of days of furious analysis and calculation, and emerged
about the most jubilant I'd ever seen him. He'd discovered that not
only was Rider wrong, but the machine itself had held the built-in
cure all along, and would work better than the original model had
predicted. I believe that was the edge thermalization process that
"anneals" out any tendency the device has to Maxwellianize.


Unfortunately, Dr. Bussard doesn't seem to have written a paper or
left notes about what what he may have come up with that day.

Eric Lerner (DPF) points out that in very large magnetic fields, the
electron-ion scattering cross section is minimized, which cuts the
bremsstrahlung significantly [8]. The fields involved (5 giga-gauss)
can only be produced in extreme circumstances and confined regions.


So what does this all mean? To me it means don't invest in fusion
yet. If anybody comes along with a commercial fusion scheme, ask about
two things: neutrons and bremsstrahlung.

[1] http://dspace.mit.edu/handle/1721.1/11412)
[2] http://www.fusor.net/board/view.php?bn=fusor_future&key=...
[3] http://nextbigfuture.com/2006/10/accelerating-futures-idea-m...
[4] http://www.fusor.net/board/getfile.php?bn=fusor_future&a...
[5] http://www.fusor.net/board/view.php?bn=fusor_future&key=...
[6] http://www.fusor.net/board/view.php?bn=fusor_future&key=...
[7] http://handle.dtic.mil/100.2/ADA257895
[8] http://arxiv.org/abs/physics/0401126
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