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 . Bibliography: