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Fusion

.. eating, 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.

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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 worlds 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 .

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