Although
the theoretical background of fusion as a source of energy has been solidly
established, experiments on a smaller scale have not yet yielded a positive
energy balance (less total energy needed to keep the fusion process going than
comes out as a net result).
Fusion Energy Foundation (link)
Having mentioned fusion power elsewhere, I got curious about
the subject again. In a sci-fi novel I once projected a future in which
tokomaks (I called them takamaks, a variant spelling) were quietly supplying
the world’s energy until the ocean seemed to resist the continued extraction of
deuterium from its waters. A fictional speculation, to be sure. That was ten
years or more ago. So now I got curious. I wondered what had transpired since.
My interest, this time, was more conventional. I wanted to know the energy
balance of this technology.
I was curious for a reason. A couple of years ago, in a
free-lance assignment, I looked at ethanol production in this context and
discovered there that, if all energy used in making ethanol is actually counted,
ethanol production has a negative balance, meaning that inputs exceed the
energy actually recovered. In the fusion game, this balance is referred to as
Q. The output divided by the input is Q. A Q=1 therefore means that energy expended
is equal to the energy harvested. Anything higher than 1 is a gain, meaning
that the technology pays for itself in an “energy economy.” If lower than 1,
the technology isn’t justified in a purely physical sense. I put that phrase in
quotes because energy balance is not the same as economic balance. Ethanol is
cost effective, especially since it’s subsidized; from an energy point of
vantage it is a loser.
The leading fusion process attempts to fuse a deuterium atom
with a tritium atom. These are forms (isotopes) of hydrogen. Hydrogen has a
single proton and a lone electron. The overwhelming majority of hydrogen atoms
(99.98%) are of this simple type. Deuterium has a proton and a neutron both; a
minute amount of hydrogen in the ocean (0.015%) takes the form deuterium.
Tritium has a proton and two neutrons. For all practical purposes it does not
appear in nature, at least not for long. It is produced by cosmic rays. It can also
be produced by man using lithium as a starting medium. Anyway….
When you force a D and a T close enough together, they do
everything in their power to resist you. Why? They both carry positive charges
and repel. You have to exert energy of 0.01 million electron volts (MeV) to
overcome this resistance. When you succeed, you produce an atom of helium, a
single neutron that flies off as radiation, and 17.59 MeV of extra energy. This
entire reaction therefore requires the 0.01 MeV at the very last stage—by the
time you’ve really squeezed the atoms together—to yield the output, thus
Q=1759! That’s major, as it were.
You might wonder how much energy an MeV actually carries.
Thanks to Wikipedia, I can report that 4 MeV is the stupendous force with which
a single snowflake crashes down on a trembling concrete drive. Obviously, in
the case of fusion, we need a whole lot of Ds and Ts colliding and fusing
before we can talk about real power.
Now we know the maximum potential of this technology. It’s a
whole lot more attractive than that of petroleum which, as I remember, produces
something like a Q=45. And that ratio has built the modern world. So what has
fusion produced thus far in its roughly 60-year history. Developments began in
1950.
The best performance on record to date was delivered by the
JET program (Joint European Torus, based in Culham, Great Britain). It
delivered Q=0.64 for a few seconds, the short duration being characteristic of
these experimental machines. The output was 16 Megawatts, produced by the
expenditure of 25 MW. The record!
The ITER program, the biggest yet and still in the building
stage, expects to achieve Q=10 in short bursts and Q=5 in more sustained
production runs. ITER’s original name was International Thermonuclear
Experimental Reactor, but that name worried people what with scare-words like thermonuclear and experimental being so close together, so the long phrase is no
longer used. ITER’s fusion reaction is being raised now in Cadarache, in
France.
Now the projected Q is a long ways of 1759, of course, but
you have to account for the fact that long before we have the D and T close
enough together to give them that last, tiny, 0.01 MeV nudge, we have get them
into hailing distance first. That requires creating enormously hot plasmas
inside magnetic containers that require cryogenic cooling in the -273 Celsius
range. The cooling alone is very energy-intensive.
As for timing, the facility won’t be built for another eight
years or so—if all goes well. And the distance between 0.64 and 10 is great.
To wind this up, the conditions ideal for bumping the heads
of Ds and Ts is at the core of the sun where gravity does most of this work,
where D and T have nowhere to go, and very sensitive disturbances can’t simply
cause that great plasma up there simply to fizz out—like the light in our
tokomaks routinely does.
Re-post Note: This
post appeared first on April 9, 2009 in the Wordpress version of LaMarotte. I
have since deleted that entire blog to avoid having my work used by
advertisers. A selection of such posts will be reproduced here over time.