FREE ESSAY ON FUSION

FUSION

Fusion reactions are inhibited by the electrical repulsive force that acts between two
positively charged nuclei. For fusion to occur, the two nuclei must approach each other
at high speed to overcome the electrical repulsion and attain a sufficiently small
separation (less than one-trillionth of a centimeter) that the short-range strong nuclear
force dominates. For the production of useful amounts of energy, a large number of nuclei
must under go fusion: that is to say, a gas of fusing nuclei must be produced. In a gas
at extremely high temperature, the average nucleus contains sufficient kinetic energy to
undergo fusion. Such a medium can be produced by heating an ordinary gas of neutral atoms
beyond the temperature at which electrons are knocked out of the atoms. The result is an
ionized gas consisting of free negative electrons and positive nuclei. This gas
constitutes a plasma. Plasma, in physics, is an electrically conducting medium in which
there are roughly equal numbers of positively and negatively charged particles, produced
when the atoms in a gas become ionized. It is sometimes referred to as the fourth state
of matter, distinct from the solid, liquid, and gaseous states. When energy is
continuously applied to a solid, it first melts, then it vaporizes, and finally electrons
are removed from some of the neutral gas atoms and molecules to yield a mixture of
positively charged ions and negatively charged electrons, while overall neutral charge
density is maintained. When a significant portion of the gas has been ionized, its
properties will be altered so substantially that little resemblance to solids, liquids,
and gases remains. A plasma is unique in the way in which it interacts with itself with
electric and magnetic fields, and with its environment. A plasma can be thought of as a
collection of ions, electrons, neutral atoms and molecules, an photons in which some
atoms are being ionized simultaneously with other electrons recombining with ions to form
neutral particles, while photons are continuously being produced and absorbed. Scientists
have estimated that more than 99 percent of the matter in the universe exists in the
plasma state. All of the observed stars, including the Sun, consist of plasma, as do
interstellar and interplanetary media and the outer atmospheres of the planets. Although
most terrestrial matter exists in a solid, liquid or gaseous state, plasma is found in
lightning bolts and auroras, in gaseous discharge lamps (neon lights), and in the crystal
structure of metallic solids. Plasmas are currently being studied as an affordable source
of clean electric power from thermonuclear fusion reactions. The scientific problem for
fusion is thus the problem of producing and confining a hot, dense plasma. The core of a
fusion reactor would consist of burning plasma. Fusion would occur between the nuclei,
with electrons present only to maintain macroscopic charge neutrality. Stars, including
the Sun, consist of plasma that generates energy by fusion reactions. In these "natural
fusion reactors" the reacting, or burning, plasma is confirmed by its own gravity. It is
not possible to assemble on Earth a plasma sufficiently massive to be gravitationally
confined. The hydrogen bomb is an example of fusion reactions produced in an
uncontrolled, unconfined manner in which the energy density is so high that the energy
release is explosive. By contrast, the use of fusion for peaceful energy generating
requires control and confinement of a plasma at high temperature and is often called
controlled thermonuclear fusion. In the development of fusion power technology,
demonstration of " energy breakeven" is taken to signify the scientific feasibility of
fusion. At breakeven, the fusion power produced by a plasma is equal to the power input
to maintain the plasma. This requires a plasma that is hot, dense, and well confined. The
temperature required, about 100 million Kelvins, is several times that of the Sun. The
product of the density and energy confinement time of the plasma (the time it takes the
plasma to lose its energy if not replaced) must exceed a critical value. There are two
main approaches to controlled fusion - namely, magnetic confinement and inertial
confinement. Magnetic confinement of plasmas is the most highly developed approach to
controlled fusion. The hot plasma is contained by magnetic forces exerted on the charged
particles. A large part of the problem of fusion has been the attainment of magnetic
field configurations that effectively confine the plasma. A successful configuration must
meet three criteria: (1) the plasma must be in a time-independent equilibrium state, (2)
the equilibrium must be macroscopically stable, and (3) the leakage of plasma energy to
the bounding wall must be small. A single charged particle tends to spiral about a
magnetic line of force. It is necessary that the single particle trajectories do not
intersect the wall. Moreover, the pressure force, arising from the thermal energy of all
the particles, is in a direction to expand the plasma. For the plasma to be in
equilibrium, the magnetic force acting on the electric current within the plasma must
balance the pressure force at every point in the plasma. 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. About one in every 3,000 water molecules contains a deuterium
atom. There is enough deuterium in the oceans to provide for the world's energy needs for
billions of years. One gram of fusion fuel can produce as much energy as 9,000 liters of
oil. The amount of deuterium found naturally in one liter of water is the energy
equivalent of 300 liters of gasoline. Tritium is bred in the fusion reactor. It is
generated in the lithium blanket as a product of the reactor in which neutrons are
captured by the lithium nuclei. A fusion reactor would have several attractive safety
features. First, it is not subject to a runaway, or meltdown, accident as is a fission
reactor. The fusion reaction is not a chain reaction; it requires a hot plasma.
Accidental interruption of a plasma control system would extinguish the plasma and
terminate fusion. Second, the products of a fusion reaction are not radioactive; hence,
no long-term radioactive wastes would be generated. Neutron bombardment would activate
the walls of the containment vessel, but such activated material is shorter-lived and
less toxic than the waste products of a fission reactor. Moreover, even this activation
problem may be eliminated, either by the development of advanced, low-activation
materials, such as vanadium-based materials, or by the employment of advanced fusion-fuel
cycles that do not produce neutrons, such as the fusion of deuterons with helium-3
nuclei. Nearly neutron-free fusion systems, which require higher temperatures than D-T
fusion, might make up a second generation of fusion reactors). Finally, a fusion reactor
would not release the gaseous pollutants that accompany the combustion of fossil fuels;
hence, fusion would not produce a greenhouse effect. The fusion process has been studied
as part of nuclear physics for much of the 20th century. In the late 1930s the
German-born physicist Hans A. Bethe first recognized that the fusion of hydrogen nuclei
to form deuterium is exoergic (there is release of energy) and, together with subsequent
reactions, accounts for the energy source in stars. Work proceeded over the next two
decades, motivated by the need to understand nuclear matter and forces, to learn more
about the nuclear physics of stellar objects, and to develop thermonuclear weapons (the
hydrogen bomb) and predict their performance. During the late 1940s and early 1950s,
research programs in the United States, United Kingdom, and Soviet Union began to yield a
better understanding of nuclear fusion, and investigators embarked on ways of exploiting
the process for practical energy production. This work focused on the use of magnetic
fields and electromagnetic forces to contain extremely hot gases called plasmas. A plasma
consists of unbound electrons and positive ions whose motion is dominated by
electromagnetic interactions. It is the only state of matter in which thermonuclear
reactions can occur in a self-sustaining manner. Astrophysics and magnetic fusion
research, among other fields, require extensive knowledge of how gases behave in the
plasma state. The inadequacy of the then-existent knowledge became clearly apparent in
the 1950s as the behavior of plasma in many of the early magnetic confinement systems
proved too complex to understand. Moreover, researchers found that confining fusion
plasma in a magnetic trap was far more challenging than they had anticipated. Plasma must
be heated to tens of millions of degrees Kelvin or higher to induce and sustain the
thermonuclear reaction required to produce usable amounts of energy. At temperatures this
high, the nuclei in the plasma move rapidly enough to overcome their mutual repulsion and
fuse. It is exceedingly difficult to contain plasmas at such a temperature level because
the hot gases tend to expand and escape from the enclosing structure. The work of the
major American, British, and Soviet fusion programs was strictly classified until 1958.
That year, research objectives were made public, and many of the topics being studied
were found to be similar, as were the problems encountered. Since that time,
investigators have continued to study and measure fusion reactions between the lighter
elements and have arrived at more accurate determinations of reaction rates. Also, the
formulas developed by nuclear physicists for predicting the rate of fusion-energy
generation have been adopted by astrophysicists to derive new information about the
structure of the stellar interior and about the evolution of stars. The late 1960s
witnessed a major advance in efforts to harness fusion reactions for practical energy
production: the Soviets announced the achievement of high plasma temperature (about
3,000,000 K), along with other physical parameters, in a tokamak, a toroidal magnetic
confinement system in which the plasma is kept generally stable both by an externally
generated, doughnut-shaped magnetic field and by electric currents flowing within the
plasma itself. (The basic concept of the tokamak had been first proposed by Andrey D.
Sakharov and Igor Y. Tamm around 1950.) Since its development, the tokamak has been the
focus of most research, though other approaches have been pursued as well. Employing the
tokamak concept, physicists have attained conditions in plasmas that approach those
required for practical fusion-power generation. Work on another major approach to fusion
energy, called inertial confinement fusion (ICF), has been carried on since the early
1960s. Initial efforts were undertaken in 1961 with a then-classified proposal that large
pulses of laser energy could be used to implode and shock-heat matter to temperatures at
which nuclear fusion would be vigorous. Aspects of inertial confinement fusion were
declassified in the 1970s, but a key element of the work--specifically the design of
targets containing pellets of fusion fuels--still is largely secret. Very painstaking
work to design and develop suitable targets continues today. At the same time,
significant progress has been made in developing high-energy, short-pulse drivers with
which to implode millimeter-radius targets. The drivers include both high-power lasers
and particle accelerators capable of producing beams of high-energy electrons or ions.
Lasers that produce more than 100,000 joules in pulses on the order of one nanosecond
(10-9 second) have been developed, and the power available in short bursts exceeds 1014
watts. Best estimates are that practical inertial confinement for fusion energy will
require either laser or particle-beam drivers with an energy of 5,000,000 to 10,000,000
joules capable of delivering more than 1014 watts of power to a small target of deuterium
and tritium .