12 Just how hot and confined do these plasmas need to be? (Fusion)
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This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
12 Just how hot and confined do these plasmas need to be? (Fusion)
(Or, what conditions are needed for controlled fusion?)
Basically, the hotter your plasma, the more fusion you will have,
because the more ions will be flying around fast enough to stick
together. (Although actually you can go *too* fast, and the atoms
then start to whiz by too quickly, and don't stick together long
enough to fuse properly. This limit is not usually achieved in
practice.) The more dense your plasma is, the more ions there are
in a small space, and the more collisions you are likely to have.
Finally, the longer you can keep your plasma hot, the more likely
it is that something will fuse, so duration is important too. More
importantly, the slower your plasma loses energy, the more likely
it is that it will be able to sustain its temperature from internal
fusion reactions, and "ignite." The ratio of fusion energy
production to plasma energy loss is what really counts here.
Hotness is measured by temperature, and as explained above, the
D-T fuel cycle (the easiest) requires temperatures of about 10 keV,
or 100,000,000 degrees kelvin. Density is typically measured in
particles-per-cubic centimeter or particles-per-cubic meter.
The required density depends on the confinement duration.
The Lawson product, defined as (density)*(confinement time) is a
key measure of plasma confinement, and determines what
combinations of density and energy confinement will give you
fusion at a given temperature. It is important to note that
what you must confine is the *energy* (thermal energy) stored
in the plasma, and not necessarily the plasma particles.
There's a lot of subtlety here; for instance, you want to
confine your fuel ions as well as their energy, so that they
stick around and fuse, but you *don't* want to confine the
"ash" from the reactions, because the ash needs to get out
of the reactor... But you'd like to get the *energy*
out of the ash to keep your fuel hot so it will fuse better!
(And it gets even more complicated than that!)
Regardless, it's true that for a special value of the Lawson
product, the fusion power produced in your plasma will just
balance the energy losses as energy in the plasma becomes
unconfined, and *ignition* occurs. That is, as long as
the plasma fuel stays around, the plasma will keep itself
hot enough to keep fusing.
A simple analogy here is to an ordinary fire. The fire won't
burn unless the fuel is hot enough, and it won't keep burning
unless the heat released by burning the fuel is enough to keep
the fuel hot enough. The flame continually loses heat, but
usually this loss is slow enough that the fire sustains itself.
You can accelerate the heat loss, however, by pouring water
on the fire to cool it quickly; this puts the fire out.
In fusion, the plasma continually loses heat, much as a fire
gives off heat, and if the plasma loses heat faster than heat
is produced by fusion, it won't stay hot enough to keep burning.
In fusion reactors today, the plasmas aren't quite confined well
enough to sustain burning on their own (ignition), so we get
them to burn by pumping in energy to keep them hot. This is sort
of like getting wet wood to burn with a blowtorch (this last analogy
is usually credited to Harold Furth of PPPL).
For the D-T fuel cycle, the Lawson ignition value for a temperature
of about 200,000,000 Kelvin is roughly 5E20 seconds-particles/m^3.
Current fusion reactors such as TFTR have achieved about 1/10th of
this - but 20 years ago they had only achieved 1/100,000th of this!
How can we improve the Lawson value of a plasma further, so we get
even closer to fusion ignition? The trick is to keep the heat in the
plasma for as long as possible. As an analogy to this problem,
suppose we had a thermos of coffee which we want to keep hot. We can
keep the thermos hotter longer by (a) using a better type of
insulation, so that the heat flows out more slowly, or (b) using
thicker insulation, so the heat has farther to go to escape, and
therefore takes longer to get out.
Going back to the fusion reactor, the insulation can be improved by
studying plasmas and improving their insulating properties by
reducing heat transport through them. And the other way to boost
the Lawson value is simply to make larger plasmas, so the energy
takes longer to flow out. Scientists believe it's technically
feasible to build a power-producing fusion reactor with high
Lawson value *Right Now*, but it would have to be large, so large
in fact that it would cost too much to be able to make electricity
economically. So we're studying plasmas and trying to figure out
how to make them trap energy more efficiently.
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