18 How do you get the plasma hot enough for fusion to occur?
Description
This article is from the Fusion FAQ, by Robert F. Heeter heeter1@llnl.gov with numerous contributions by others.
18 How do you get the plasma hot enough for fusion to occur?
Much of the answer given below is taken from the PPPL World-Wide Web
homepage (which is public domain). PPPL material is in "quotations".
"In an operating fusion reactor, part of the energy generated will
serve to maintain the plasma temperature as fresh deuterium and
tritium are introduced. However, in the startup of a reactor, either
initially or after a temporary shutdown, the plasma will have to be
heated to 100 million degrees Celsius. In current tokamak (and
other) magnetic fusion experiments, insufficient fusion energy is
produced to maintain the plasma temperature. Consequently, the
devices operate in short pulses and the plasma must be heated afresh
in every pulse."
There are several methods for heating plasmas. These include Ohmic
Heating, Neutral Beam Injection, Magnetic Compression,
Radio-Frequency Heating, and Inertial Compression. Each of these is
discussed below.
1: Ohmic Heating
"Since the plasma is an electrical conductor, it is possible to heat
the plasma by passing a current through it; in fact, the current that
generates the poloidal field also heats the plasma. This is called
ohmic (or resistive) heating; it is the same kind of heating that
occurs in an electric light bulb or in an electric heater."
"The heat generated depends on the resistance of the plasma and the
current. But as the temperature of heated plasma rises, the
resistance decreases and the ohmic heating becomes less effective. It
appears that the maximum plasma temperature attainable by ohmic
heating in a tokamak is 20-30 million degrees Celsius. To obtain
still higher temperatures, additional heating methods must be used."
2: Neutral-Beam Injection
"Neutral-beam injection involves the introduction of high-energy
(neutral) atoms into the ohmically -- heated, magnetically --
confined plasma. The atoms are immediately ionized and are trapped by
the magnetic field. The high-energy ions then transfer part of their
energy to the plasma particles in repeated collisions, thus
increasing the plasma temperature."
3: Magnetic Compression
"A gas can be heated by sudden compression. In the same way, the
temperature of a plasma is increased if it is compressed rapidly by
increasing the confining magnetic field. In a tokamak system this
compression is achieved simply by moving the plasma into a region of
higher magnetic field (i.e. radially inward). Since plasma
compression brings the ions closer together, the process has an
additional benefit of facilitating attainment of the required density
for a fusion reactor."
4: Radiofrequency Heating
"In radiofrequency heating, high-frequency waves are generated
by oscillators outside the torus. If the waves have a particular
frequency (or wavelength), their energy can be transferred to the
charged particles in the plasma, which in turn collide with other
plasma particles, thus increasing the temperature of the bulk
plasma."
5: Inertial Compression
This is similar to magnetic compression in that decreasing the gas
volume causes the temperature to rise, but in the inertial approach
the compression is achieved by using laser or particle beams to
heat the outer layer of a target pellet; the outer layer vaporizes
and the pressure that the vaporized layer exerts back on the
core of the pellet accelerates the plasma inward on itself, and
the inertia of the imploding atoms in the pellet allows the pellet
to be compressed (for a very short time), and thus heated.
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