13 What are the basic approaches used to heat and confine the plasma? (Fusion)
Description
This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
13 What are the basic approaches used to heat and confine the plasma? (Fusion)
(Or, what is magnetic confinement? Inertial confinement?)
There are three basic ways to confine a plasma. The first is
the method the sun uses: gravity. If you have a big enough
ball of plasma, it will stick together by gravity, and be
self-confining.
Unfortunately for fusion researchers, that doesn't work here on
earth. The second method is that used in nuclear fusion bombs:
you implode a small pellet of fusion fuel. If you do it quickly
enough, and compress it hard enough, the temperature will go way
up, and so will the density, and you can exceed the Lawson
ignition value despite the fact that you are only confining your
pellet for nanoseconds. Because the inertia of the imploding
pellet keeps it momentarily confined, this method is known as
inertial confinement.
The third method uses the fact that charged particles placed in
a magnetic field will gyrate in circles. If you can arrange the
magnetic field carefully, the particles will be trapped by it.
If you can trap them well enough, the plasma energy will be
confined. Then you can heat the plasma, and achieve fusion with
more modest particle densities. This method is known as
magnetic confinement. Initial heating is achieved by a
combination of microwaves, energetic/accelerated particle beams,
and resistive heating from currents driven through the plasma.
(Once the Lawson ignition value is achieved, the plasma becomes
more-or-less self-heating.) In magnetic confinement, the plasma
density is typically about 1E20 particles per cubic meter, and with
a temperature of about 1E8 kelvin, we see that ignition could be
achieved with a confinement time of about 4 seconds. (All these
numbers in reality vary by factors of 2 or 3 from the rough values
I've given.) Currently, magnetic-confinement reactors are about
a factor of ten short of the ignition value. (TFTR has an
energy confinement time of 0.25 seconds during its best shots.)
More information on these different approaches is given in the
sections that follow.
Continue to:
My Books
