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.

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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 formationnamely, gravitational collapse, compression h …


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