Fusion Progress Report: A Milestone Achieved

If only we could produce fusion power in a controlled way, we would possess a virtually limitless supply of relatively clean and safe energy. Fusion represents the best hope humanity possesses to solve the problems of feeding, housing, and caring for the billions of additional people expected to populate our world in the next century. The problem for fusion researchers has been how to tame this most basic form of energy in the universe. Our sun and all other stars are fueled by fusion reactions, as are thermonuclear weapons. But igniting fusion in a controlled environment so that the high energy levels produced can be channeled safely into people’s homes has been another matter. In a fusion reaction, nuclei of low atomic weight, such as hydrogen, fuse and thereby form nuclei of higher atomic weight, such as helium. When this occurs, some of the mass of the original nuclei are lost and transformed to energy in the form of high-energy particles. To begin the process, thermonuclear fuel (deuterium and tritium) must be heated to temperatures in excess of tens of millions of degrees centigrade. This heating causes the nuclei to fuse on impact. At such temperatures, it is difficult to contain the fuel long enough to allow efficient thermonuclear burning. Owing to their large masses, stars can contain their high-temperature fusion reactions by tremendous gravitational fields. Unfortunately, that effect cannot be created in the laboratory. But researchers are getting closer. Progress in the field has come about through two general approaches to confine the fuel and thereby demonstrate controlled fusion reactions: magnetic confinement and inertial confinement. Magnetic confinement uses powerful magnetic fields to hold the super-hot fusion fuel, which is in the form of a plasma, for a few seconds. A number of fusion laboratories around the world use magnetic confinement, such as at the Joint European Torus project in Oxfordshire, UK, and at others in the Soviet Union, West Germany, and Japan. The chief research center using magnetic confinement in the United States is Princeton University’s Plasma Physics Laboratory. The second process, inertial fusion, makes no attempt to confine the’ fuel for such relatively long times. Rather, the heating and compression of a plasma takes place very rapidly, in billionths of a second, before the fuel has an opportunity to escape and cool. To heat and compress the plasma, inertial fusion directs intense laser beams or particle beams at the target containing hydrogen isotopes. If extreme density and temperature conditions are produced (fuel density greater than five times that of gold at a temperature beyond fifty million degrees centigrade), it is possible to produce many times more energy than that provided by the laser beams or particle beams. Two techniques can create inertial fusion: indirect (or hohiraum) drive and direct drive. Indirect drive converts beam energy to x-rays to drive the fuel capsule. U.S. weapons facilities at Lawrence Livermore, Los Alamos, and Sandia national laboratories are exploring this approach to inertial fusion. Direct-drive inertial fusion, on the other hand, bombards the spherical fuel pellets with radiation directly from laser or particle beams. This approach maybe more efficient in terms of energy expended than the indirect-drive method. The University of Rochester’s Laboratory of Laser Energet- ics (LLE) and the U.S. Naval Research Laboratory (NRL) are the main centers in the United States for carrying out direct-drive inertial fusion research. Attaining high-density fuel compression is a very important step in demonstrating the feasibility of inertial fusion. With sufficient beam energy, a high density compressed pellet will undergo thermonuclear ignition. Ignition means that high-energy charged particles created by the fusion reactions redeposit their energy in cold, unheated fuel and cause a burn wave to propagate which consumes the fusion fuel and produces prodigious amounts of energy. Inertial fusion that uses the direct-drive approach can produce high-density target compression and thermonuclear ignition conditions with less beam energy than that using the indirect-drive approach; however, it requires exceptionally precise illumination of the target surface by the beams to avoid nonuniform energy deposition. If a spherical fuel pellet is not uniformly driven, it cannot attain the high density required to produce ignition and massive multiplication of energy. The LLE is the only facility in the United States capable of conducting highly uniform and symmetric direct-drive implosion experiments. An in-depth review of the national inertial fusion program, conducted in 1986 by a National Academy of Sciences committee, concluded that, if the LLE program achieved 100-200 times liquid deuterium/tritium fuel density in compression experiments using its 2,000-joule OMEGA laser in 1988, it would support an upgrade of OMEGA to 30,000 joules. (A joule is a measure of energy equal to one watt lasting one second.) Such an upgrade could produce implosions in the range of thermo-nuclear ignition and would be a key element in the next major step toward inertial-fusion energy: the construction of a multi-megajoule laboratory microfusion facility capable of producing an energy multiplication factor of one hundred. This major milestone on the path to proving the feasibility of fusion energy production was passed in March 1988 with the successful demonstration of ultra-high density compression of laser-driven fusion pellets by the LLE. Experiments on the laboratory’s OMEGA facility (a 24-beam, blue light laser) produced a compression of deuterium and tritium fuel to a density nearly twice that of gold, the highest density measured yet in laser-fusion target experiments. The temperature achieved in these experiments was in the range of 5 to 10 million degrees centigrade. A committee of 10 scientific reviewers from Lawrence Livermore, Los Alamos, KMS Fusion; Inc., and the NRL was present to validate the results of the LLE experiments. The LLE and Lawrence Livermore are now joining in an effort to develop technology required for the laboratory microfusion facility which may be built in the 1990s. The planned upgrade of OMEGA may well serve as the test bed on which the relevant technology required for this giant step forward in fusion will be tried. Looking back, we would say that the success of the OMEGA experiments was the result of an intensive multi-laboratory effort which included: 1) advances in optical technology to produce a uniform laser irradiation distribution on the fuel capsules, 2) the development and implementation of precision diagnostics instrumentation to measure the extremely high-density fuel conditions, and 3) the development and application of sophisticated low-temperature technology to form uniform fuel layers in the fuel pellets. All in all, the future looks bright for fusion. Robert : L McCrory is director and John M. Soures is deputy director of the Laboratory for Laser Energetics, University of Rochester, Rochester, N.Y

Fusion Progress Report: A Milestone Achieved

Interested in reading more?

Become a Member of