Nuclear Fusion

For a fusion reaction to take place, the nuclei, which are
positively charged, must have enough kinetic energy to
overcome their electrostatic force of repulsion. This can
occur either when one nucleus is accelerated to high
energies by an accelerating device, or when the energies of
both nuclei are raised by the application of very high
temperature. The sun is an example of thermonuclear fusion
in nature.
 
Thermonuclear reactions occur when a proton is accelerated
and collides with another proton. These two protons then
fuse and form a deuterium nucleus which has a proton,
neutrino and lots of energy. Such a reaction is not self
sustaining because the released energy is not readily
imparted to other nuclei. Thermonuclear fusion of deuterium
and tritium will produce a helium nucleus and an energetic
neutron that can help sustain further fusion. This is the
basic principal of the hydrogen bomb which employs a brief,
controlled thermonuclear fusion reaction. 

Thermonuclear reactions depend on high energies, and the
possibility of a low-temperature nuclear fusion has
generally been discounted. Early in 1989 two
electrochemists startled the scientific world by claiming
to achieve a room-temperature fusion in a simple
laboratory. They had little proof to back up their
discovery, and were not credited with their so-called
accomplishment. The two scientists were Stanley Pons of the
university of Utah and Martin Fleischmann of the University
of Southampton in England. They described their experiment
as involving platinum electrodes, an electrochemical cell
in which palladium and platinum were immersed in heavy
water. 

Nuclear fusion is also what powers the rest of the stars in
the solar system. In a thermonuclear reaction, matter is
forced to exist in a plasma state, consisting of electrons,
positive ions and very few neutral atoms. Fusion reactions
that occur within a plasma serve to heat it further,
because the portion of the reaction product is transferred
to the bulk of the plasma through collisions. In the
deuterium-tritium reaction the positively charged helium
nucleus carries 3.5 MeV. The neutron escaped the plasma
with little interaction and , in a reaction, could deposit
its 14.1 MeV in a surrounding lithium blanket. The neutrons
activity would breed tritium and also heat as an exchange
medium which could be used to produce steam to turn
generator turbines. However, the plasma also loses thermal
energy through a variety of processes: conduction,
convection, and electromagnetic radiation 

Energy also escapes in the reaction through line radiation
from electrons undergoing level transitions in heavier
impurities, and through losses of hot nuclei that capture
an election and escape and confining field. Ignition occurs
when the energy deposited within the plasma by fusion
reactions equals or exceeds the energy being lost. In order
to achieve ignition, plasma must be combined and heated.
Obviously, a plasma at millions of degrees is not
comparable with an ordinary confining wall, but the effect
of this incompatibility is not the destruction of the wall
as might be expected. 

Although the temperature of a thermonuclear plasma is very
high and the power flowing through it may be quite large
the stored energy is relatively small and would quickly be
radiated away by impurities if the plasma touched a wall
and began to vaporize it. A thermonuclear plasma is thus
self-limiting, because any significant contact with the
vessel housing causes its extinction within a few
thousandths of a second. Therefore, plasma must be
carefully housed and handled while it is occurring.
 
Most of the research dealing with fusion since 1950 has
used magnetic fields to contain the charged particles that
constitute a plasma. The density required in
magnetic-confinement fusion is much lower than atmospheric
density, so the plasma vessel is evacuated and them filled
with the hydrogen-isotope fuel at 0.0000000. 

Magnetic-field configurations fall into two typed: open and
closed. In an open configuration, the charged particles,
which are spiraling along magnetic field lines maintained
by a solenoid, are reflected at each end of a cell by
stronger magnetic fields. 

Present day mirror machines retard this loss by using
additional cells to set up electrostatic potentials that
help confine the hot ions within the central solenoidal
field. In a Closed reaction, the magnetic-field lines along
which charged particles move are continuous within the
plasma. This closure has most commonly taken the form of a
toros, or doughnut shape, and the most common example is
the tokamak. In a tokmak the primary confining field is
totoidal and is produced by coils of surrounding the vacuum
vessel. Other coils cause current to flow through the
plasma by induction. This toroidally flowing current wraps
itself around the plasma. 

The poloidal magnetic field, at right angles, that stronger
toroidal field, acting together, yield magnetic field lines
that spiral around the torus. This spiraling ensures that a
particle spends equal amounts of time above and below the
totoidal midplane, thus canceling the effects of a vertical
drift that occurs because the magnetic field is stronger on
the inside of the torus than on the outside. Additionally,
a certain type of plasma called Tokmak plasma can be heated
to temperatures of 10-15 million k by the current flowing
in the plasma. Imagine how quick one could broil chicken.
In less than half a second, a chicken would be golden brown
and tender; ready for dinner. 

At higher temperature the plasma resistance becomes too low
for this method to be effective, and heating is
accomplished by injecting beams of very energetic neural
particles into the plasma. These ionize, become trapped,
and transfer their energy to the build plasma through
collisions. Alternatively, radio frequency waves are
launched into the plasma at frequencies that resonate with
various periodic particle motions. The waves give energy to
these resonant particles, which then transfer it to the
rest of the plasma through collisions. 

Another approach to fusion pursued since about 1974, is
termed inertial confinement. A small pellet of frozen
deuterium and tritium are compressed to a very high
temperature and densities in a process analogous to what is
accomplished by bombarding the pellet from all sides,
simultaneously with a really intense laser. This causes the
pellet to vaporize and, by mechanical reaction, it imparts
inwardly directed momentum to the remaining pellet core.
The inertia of the inwardly driven pellet material must be
sufficient to localize the power of -9 seconds required to
get significant energy release. 

. The minimum confinement condition necessary to achieve
energy gain in a deuterium-tritium plasma is that of the
product of the density in ions per cubic cm and energy
containment time in seconds must exceed 6x10 -13th power.
This was attained for the first time in a hydrogen plasma
at the Massachusetts Institute of Technology in 1983. The
temperature required to ignite a fusion reactor is in the
range of 100-250 million k, several times the temperature
of the center of the sun. 

The goal of fusion is in effect, to produce and hold a
small star. It is a daunting and tedious research which is
considered to be the most advanced in the world.