Scientists have long worked toward the goal of using thermonuclear fusion
reactions to produce useful power. The most promising two reactions involve the
isotopes of hydrogen:
The reaction that occurs with the greatest probability and at the lowest
temperatures involves fusing a deuterium nucleus with a tritium nucleus to form
a helium (He4) nucleus, along with an energetic neutron that helps
sustain further fusion. The products contain 17.6 million
Fusion reactions are easily induced by using a charged
At thermonuclear temperatures, matter can exist only in the primarily ionic
plasma state. Fusion reactions occurring in a plasma heat it further,
because the portion of the reaction energy that remains with the electrically
charged reaction products 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 escapes the plasma with little interaction
and, in a reactor, could deposit its 14.1 MeV in a surrounding lithium blanket.
This would breed tritium and also heat an exchange medium (such as helium),
which could be used to produce steam to turn generator turbines. However, the
plasma loses thermal energy through conduction,
A plasma at millions of degrees is not compatible with an ordinary confining wall, but the effect 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 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. Any significant contact with the vessel housing it causes the plasma's extinction within a few thousandths of a second.
Since the early 1950s, most fusion research has used magnetic fields to confine the charged particles constituting a plasma. The density required for this is much lower than atmospheric density, so the plasma vessel is evacuated, then filled with the hydrogen-isotope fuel at 0.000001 times that density. Magnetic-field configurations fall into two types: open and closed. In an open configuration the charged particles, which are spiraling along magnetic field lines maintained by a
In closed configurations, the magnetic-field lines along which charged
particles move are continuous within the plasma. This closure has most commonly
taken the form of a torus, or doughnut shape. The most common example is the
Tokamak plasmas can be heated to temperatures of 1015 million K by the current flowing in the plasma. At higher temperatures the plasma resistance becomes too low for this method to be effective, and heating is accomplished by injecting beams of very energetic neutral particles into the plasma. These ionize, become trapped, and transfer their energy to the bulk 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 transfer it to the rest of the plasma through collisions.
Experiments are also under way in which radio-frequency waves are used to push electrons around the tokamak to maintain the plasma current. Such noninductive current drive allows the tokamak pulse to outlast the time limits imposed by the fact that, in a transformer-driven tokamak, the plasma current lasts only as long as the current in the secondary coils is changing. When the secondary coils reach their current limits, confinement is lost, and the plasma terminates until the transformer can be reset (a matter of at least seconds). Although the plasma in an inductively driven tokamak is pulsed, the electricity produced would not be, because the thermal inertia of the neutron-capturing blanket would sustain steam generation between pulses. By allowing longer pulse or steady-state plasma operation, however, radio-frequency current drive could lessen the thermal stresses.
Another approach to fusion, termed inertial confinement, aims to compress a capsule of deuterium and tritium to very high temperatures and densities in a process analogous to what occurs in a hydrogen bomb. The compression is accomplished by bombarding the capsule from all sides at once with an intense pulse of
The minimum confinement condition needed to achieve energy gain in a deuterium-tritium plasma the so-called Lawson criterion is that the product of density in ions per cm3 and energy containment time in seconds must exceed 6 X 1013. This was first attained in 1983 in a hydrogen plasma at the Massachusetts Institute of Technology (MIT). In 1986 the Tokamak Fusion Test Reactor (TFTR) at Princeton University's Plasma Physics Laboratory demonstrated the existence of the theoretically predicted "bootstrap current," which arises from a dynamo effect within the plasma. This means that most of the electric current needed to sustain the poloidal component of the magnetic field perhaps can be supplied at little or no cost by the plasma itself. Beginning in 1993, the TFTR became the first fusion device to run routinely with a deuterium-tritium fuel mixture, for which the energy confinement proved significantly better than for a deuterium-only plasma. In 1994 the amount of power released in its pulses reached 10.7 million watts, and the temperatures reached were more than 40 times those at the Sun's core.
If simple scattering by collisions between isolated pairs of particles were the only mechanism by which energy was lost, then achieving the quality of confinement necessary for an electricity-producing fusion reactor would be a daunting but straightforward undertaking. The reality has been that early fusion experiments encountered rates of energy leakage too large to permit success. Progress occurred during the 1970s and 1980s as successive generations of tokamaks led to a better understanding of how the excess leakage, called "anomalous" diffusion, could be reduced by varying such factors as the dimensions of the plasma torus and the magnitude of the electric current flowing within it, and by finding better modes of operation. In the 1990s physicists began intensive explorations of the possibility of further improving confinement by carefully tailoring the distribution across the plasma of the electric current flowing within it. It was discovered in 1995 that by reversing the change in pitch, or shear, of the helical magnetic-field lines that hold the plasma, a core region could be created inside the shear reversal in which the unwanted leakage of energy and particles was reduced to very low levels.
Much research has yet to be done, however, before fusion power reactors become a reality. Japan, Russia, the European Union (EU), and the United States agreed in 1992 to work on the design of an International Tokamak Experimental Reactor capable of reaching ignition in the early part of the 21st century. In the meantime, South Korea is building a tokamak with superconducting coils that will be capable of maintaining high-temperature plasmas for pulses of up to 300 seconds. With the completing of the TFTR experiments at Princeton University in 1997, work is proceeding there on a spherical tokamak, to study the reactor-relevant physics of low-aspect-ratio plasmas (toroids for which the ratio of the major to minor radii is significantly less than 2). Work also continues on tokamaks at General Atomics in San Diego, Calif., and at MIT in Cambridge, Mass. Japan and the EU each have a large tokamak as well as smaller confinement devices.
The goal of fusion is, in effect, to make and hold a small star. Such a goal is daunting. That it is pursued even so is an indication of the benefits success could bring. Besides providing an almost inexhaustible fuel supply, fusion is environmentally benign. The resulting ash is harmless helium, the afterheat in the reactor structure would be much less than in a fission reactor, and the heat would be distributed through a greater thermal mass. Also, because fusion is not a chain reaction, it cannot run out of control. Any perturbation would cause the plasma to extinguish itself. It would also be difficult to produce nuclear-weapons materials surreptitiously at a fusion plant. Because no fissionable material should ordinarily be present, it would be a simple matter to detect characteristic gamma rays. Present levels of support for research are aimed at building a demonstration fusion power plant early in the 21st century.
Bibliography: Conn, R., et al., "The International Thermonuclear Experimental Reactor," Scientific American, April 1992; Fowler, T. K., The Fusion Quest (1997); Fujiwara, M., and Wan, Y., eds., The Frontier of Physics in Fusion-Relevant Plasmas (1998); Glanz, J., "Common Ground for Fusion," Science, Aug. 6, 1999; Panarella, E., ed., Current Trends in International Fusion Research (1997); Riordon, J., "Fusion Power from a Floating Magnet?" Science, Aug. 6, 1999; Roth, J., Introduction to Fusion Energy (1986); Szirmay, L. V., Nuclear Fusion Reactor Design Fundamentals (1992); Velard, G., et al., eds., Nuclear Fusion by Inertial Confinement (1993); Wesson, J., and Campbell, D., Tokamaks, 2d ed. (1997).