"The combined results from a variety of tokamaks around the world have produced an impressive set of achievements. Neutral beams and a variety of radio-frequency heating methods can provide tens of megawatts of heating power for creating high-temperature plasmas. Experimental devices have produced ion temperatures as high as 45,000 electron volts and densities of approximately 1020 particles per cubic meter, sufficient for fusion reactors. An important overall measure of physics performance is the 'triple product' of the peak ion density, the plasma energy confinement time and the peak ion temperature. A triple-product value of 7 x 1024 electron volt-seconds per cubic meter is required for an ignited, deuterium-tritium reactor. JT-60U, a large Japanese tokamak, has already achieved triple products of 1.3 x 1024 electron volt-seconds per cubic meter.
"The nominal fusion power of ITER is 1,500 megawatts; it represents a framework for a full-size fusion power reactor, though it is not designed to produce electricity. An extrapolation of the present knowledge of tokamaks indicates that commercial fusion reactors will be rather large and expensive. Fortunately, ongoing research programs are revealing ways to improve substantially the performance of tokamak reactors. These promising new directions include higher fusion power densities, and hence smaller reactors; development of 'transport barriers' in the plasma, leading to improved energy confinement and smaller sizes; self-driven plasma currents that permit steady-state operation and low recirculating power; and the development of advanced divertor concepts to provide particle control and heat removal over long reactor lifetimes.
"The rate of progress in the fusion program is consistent with the level of resources being devoted to it. Actual funding has been much less than anticipated during the detailed planning drawn up in the 1970s and 1980s. Present levels of funding in the U.S. ($244 million in fiscal year 1996) are not sufficient to keep pace with the earlier plans. As a result, the U.S. is unfortunately passing its traditional leadership in magnetic fusion to Europe and Japan.
"In August 1996, the U.S. Department of Energy issued a Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program. The nation's previous strategy was a schedule-driven development program to prove fusion to be a technically and economically credible energy source, with the goal of an operating, demonstration power plant by about 2025. In a climate of severe budgetary constraints, however, that strategy became highly unrealistic. In an attempt to stay as close as possible to the goal-oriented schedule, the fusion program has concentrated almost all of its available resources on the tokamak concept, virtually eliminating support for alternative approaches and for basic plasma science. Despite impressive scientific progress, the program continues to receive insufficient resources.
"The new strategy emphasizes an international effort aimed at advancing the scientific knowledge base needed for the development of an economically and environmentally attractive fusion energy source. To be a credible partner in this long-term quest, the U.S. needs a vigorous domestic program in fusion science and technology. At a constant level of funding, the restructured U.S. program will be able to focus on fusion's underlying scientific foundations and will enable the nation to take the lead in selected areas of expertise as part of the international effort to develop fusion energy.
"The restructured U.S. program will strive to remain a credible partner in the international fusion program that includes both ITER and many smaller projects in all areas of fusion science and technology. Given the high projected cost of creating a burning physics experiment and given that the U.S. now funds only about one sixth of the world research effort, a strategy based on international collaboration on fusion energy research and development can be highly cost effective.