[Reader-list] Fwd: fusion

[Nagraj Adve]

An email reply from our physicist friend at Princeton, M.V. Ramana, re
reportage about the fusion experiment. Very much worth reading.
Nagraj


I hadn't seen this article, but a former post-doc colleague of mine now
works at Livermore and I saw her over the Christmas break. So I had heard
about it. I did look through the article now, and also some of the
hilarious comments at the end.

The key paragraph to understanding this article is this: "The scientists
generated more energy from fusion reactions than they put into the nuclear
fuel, in a small  but crucial step along the road to harnessing fusion
power. The ultimate goal - to produce more energy than the whole experiment
consumes - remains a long way off, but the feat has nonetheless raised
hopes that after decades of setbacks, firm progress is finally being made."

As you may know, one can produce energy from nuclear reactions through two
means: fission and fusion. Fission involves a heavy nucleus breaking up
into two lighter nuclei and fusion involves two light nuclei coming
together. In either case, there is a difference between the initial
mass(es) and the final mass(es), which gets converted into energy through e
= mc^2.

Fission was the first to be observed in the laboratory. The problem with
fusion was to be able to bring together two nuclei against the repulsive
force exerted because the two nuclei are positively charged. The only way
to do it is to heat the gas to such incredibly high temperatures that the
two nuclei come close enough to overcome this repulsion. This was first
done in the hydrogen bomb where the radiation coming out of the first
fission stage compresses the lithium deuteride secondary.

The quest to generate electricity using fusion has to reproduce this sort
of energy production in a controlled fashion and without an atom bomb to
bring the nuclei together. There are two primary methods scientists have
tried. (There are a bunch of crackpot ideas, but I don't think they are
worth examining.) One is to use magnetic confinement. The other is by
shooting lasers at a pellet. Both these aim to compress matter for a very
brief period during which some fusion reactions occur and produce a large
quantity of heat. The experiment described here uses the latter, whereas my
neighbor, the Princeton Plasma Physics Laboratory specializes in the former.

In either case, there are three stages of challenges. The first is to have
enough fusion reactions in the target (the pellet or the confined gas) to
produce more energy than is put into the target (by the laser beam, for
example). If that is not met, of course, then one has a permanent
loss-making facility in energy terms. The second challenge is to produce
more energy than is used by the facility as a whole. So that includes all
the energy that goes into running the lasers and other equipment and so on
rather than just the amount fed into the target. The final material
challenge would be to put in all the equipment needed to convert the heat
into electricity and make that produce more electricity than is fed into
the system.
You can think of the first two as being "physics challenges" and the third
as an "engineering challenge"

After all these are solved, then there is an even more difficult challenge
- to make all of this economical.

The recent NIF experiment seems to have broken the first barrier. And the
scientists I know seem to think that the next big experiment, being planned
in France, would meet the second challenge. That would probably start
running sometime in the 2020s. My understanding that ideas about the third
and the economical challenge are still very preliminary and I don't think
it is worth trying to evaluate them. If one goes by history, the last could
be a killer as has been the case with fission power.

All in all, I tend not to think too much about fusion. It is too distant to
come into my horizon. As you know, my view on nuclear power in general has
been marked with scepticism. The primary reason for that sentiment less due
to a fundamental disagreement with the idea of fission energy, but the wild
and unfulfilled promises that proponents of that source of energy have made
over the decades.  What I find interesting, therefore, is trying to
understand actual technologies on the ground. Following in that vein, I
would be interested in studying fusion only well after the technology has
been developed, matured, implemented, and has had successes or failures.
Since that has not yet happened, I do not see much reason for engagement
with the literature on the subject.

Hope that helps.

Warm regards,
Ramana




----

http://www.theguardian.com/science/2014/feb/12/nuclear-fusion-breakthrough-green-energy-source