The equilibrium
thus obtained has to be stable. A plasma is stable if after a small perturbation
it returns to its original state. A plasma is continually perturbed by random
thermal noise fluctuations. If unstable, it might depart from its equilibrium
state and rapidly escape the confines of the magnetic field (perhaps in less
than one-thousandth of a second). A plasma in stable equilibrium can be
maintained indefinitely if the leakage of energy from the plasma is balanced by
energy input. If the plasma energy loss is too large, then ignition cannot be
achieved. An unavoidable diffusion of energy across the magnetic field lines
will occur from the collisions between the particles. The net effect is to
transport energy from the hot core to the wall. This transport process, known as
classical diffusion, is theoretically not strong in hot fusion plasmas and is
easily compensated for by heat from the alpha particle fusion products. In
experiments, however, energy is lost from plasma more rapidly than would be
expected from classical diffusion. The observed energy loss typically exceeds
the classical value by a factor of 10-100. Reduction of this anomalous transport
is important to the engineering feasibility of fusion. An understanding of
anomalous transport in plasmas in terms of physics is not yet in hand. A
viewpoint under investigation is that the anomalous loss is caused by fine-scale
turbulence in the plasma. However, turbulently fluctuating electric and magnetic
fields can push particles across the confining magnetic field. Solution of the
anomalous transport problem involves research into fundamental topics in plasma
physics, such as plasma turbulence. Many different types of magnetic
configurations for plasma confinement have been devised and tested over the
years. This has resulted in a family of related magnetic configurations, which
may be grouped into two classes: closed, toroidal configurations and open,
linear configurations. Toroidal devices are the most highly developed. In a
simple straight magnetic field the plasma would be free to stream out the ends.
End loss can be eliminated by forming the plasma and field in the closed shape
of a doughnut, or torus, or, in an approach called mirror confinement, by
plugging the ends of such a device magnetically and electrostatically. In the
inertial confinement a fuel mass is compressed rapidly to densities 1,000
to10,000 times greater than normal by generating a pressure as high as 1017
pascals for periods as short as nanoseconds.
Near the end of this time period
the implosion speed exceeds about 300,000 meters per second. At maximum
compression of the fuel, which is now in a cool plasma state, the energy in
converging shock waves is sufficient to heat the vary center of the fuel to
temperatures high enough to induce fusion reactions. If the product of mass and
size of this highly compressed fuel material is large enough, energy will be
generated through fusion reactions before the plasma disassembles. Under proper
conditions, more energy can be released than is required to compresses, and
shock-heat the fuel to thermonuclear burning conditions. The physical processes
in ICF bear relationship to those in thermonuclear weapons and in star
formation—namely, gravitational collapse, compression heating, and the onset of
nuclear fusion. The situation in star formation differs in one respect: after
gravitational collapse ceases and star begins to expand again due to heat from
exoergic nuclear fusion reactions, the expansion is arrested by the gravity
force associated with the enormous mass of the star. In a star a state of
equilibrium in both size and temperature is achieved. In ICF, by contrast,
complete disassembly of fuel occurs. The fusion reaction least difficult to
achieve combines a deuteron (the nucleus of the deuterium atom) with a triton
(the nucleus of a tritium atom). Both nuclei are isotopes of the hydrogen
nucleus and contain a single unit of positive electric charge. Deuterium-tritium
(D-T) fusion requires the nuclei to have lower kinetic energy than is needed for
the fusion of more highly charged heavier nuclei. The two products of the
reaction are an alpha particle (nucleus of the helium atom) at an energy of 3.5
million electron volts (MeV) and a neuron at an energy of 14.1 MeV. (One MeV is
the energy equivalent of 10 billion Kelvin.). The neutrons, lacking electric
charge, is not affected by electric or magnetic fields within the plasma and can
escape the plasma to deposit its energy in a material, such as lithium, which
can surround the plasma. The electrically charge alpha particle collides with
the deuterons and tritons (by their electrical interaction) and can be
magnetically confined within the plasma. It there by transfers its energy to the
reacting nuclei. When this redeposition of the fusion energy into the plasma
exceeds the power lost from the plasma (by electromagnetic radiation,
conduction, and convection), the plasma will be self-sustaining, or “ignited.”
With deuterium and tritium as the fuel, the fusion reactor would be an
effectively inexhaustible source of energy. Deuterium is obtained from seawater.
