July 31, 1998

The June 19 issue of Science magazine contains a letter from Alexander J. Glass questioning the likelihood that the tokamak concept will lead to a commercial power plant. Glass spent most of his career in lasers and inertial confinement fusion at the Lawrence Livermore National Laboratory but for several years was U.S. ITER Home Team Leader. His present affiliation and address is Bay Area Regional Technology Alliance, Fremont, CA 94538-2209 (//www.barta.org/). His letter is as follows:

"The debate over the future of the International Thermonuclear Experimental Reactor (ITER) (J. Glanz, "Requiem for Heavy-weight at Meeting on Fusion Reactors," News & Comment, 8 May, p. 818), unfortunately, does not go far enough. The real issue is not how much money should be invested in the next large tokamak, but whether any further investment in tokamak confinement is warranted at this time.

"The tokamak has been the main approach to magnetic confinement fusion since its inception almost 50 years ago. During the intervening half century, great progress has been made in understanding the physics of toroidal confinement and in translating that understanding into improvements and innovations in tokamak design. Although tokamak design is still based on empirical scaling laws, confidence in these laws has been strengthened by a wealth of experimental data. Numerous reviews of the ITER design have concluded that if the machine is built to the ITER design specifications, there is little doubt that it can achieve its scientific goal of a sustained thermonuclear burn.

"This statement reflects both the triumph and the tragedy of fusion research, because it also implies that if a tokamak is significantly smaller than the ITER design, it will not achieve a sustained thermonuclear burn and thus will not provide the basis for a power-producing reactor. The scientific community needs to re-examine the premise on which the public was originally asked to support fusion research, namely, that it would lead to the development of a practical, power-producing technology. In light of today's knowledge, it is highly unlikely that further development of the tokamak will lead to that outcome."

COMMUNITY RESPONDS:

Fusion researchers Richard Hazeltine (Univ. of Texas), Gerald Navratil (Columbia Univ.), William Nevins (LLNL), Miklos Porkolab (MIT), Ned Sauthoff (PPPL), Thomas Simonen (General Atomics) and Weston Stacey (Georgia Institute of Technology) have written a response. Scheduled to appear in the 7 August issue of Science, it reads as follows:

"In your June 19 issue Alex Glass incorrectly asserted that the tokamak must be as large as ITER to achieve sustained thermonuclear burn and that this boded unfavorably for the tokamak as a potential power plant concept. In fact, recent advances worldwide indicate that smaller tokamak power plants should be feasible. In current, few-second experiments, heat transport within the plasma has been reduced by suppressing turbulence, plasma pressure limits have been increased, the potential for steady state operation by naturally driven currents has been demonstrated, and concerns about handling exhaust heat have been mitigated. Integrating and extending these advances toward steady state is now a focus of international tokamak research. With such advanced tokamak physics, a smaller ITER, now being pursued internationally, would likely produce sustained plasma burn, and the tokamak power plant of the future becomes much more attractive.

"After several decades of intensively competitive research on a broad range of magnetic fusion concepts, the tokamak emerged and matured into the only concept that is ready to produce the plasma conditions required to explore the next fusion science frontier, the physics of sustained burning plasmas. This is not to say that other concepts should not be investigated. On the contrary, they should be pursued to further optimize fusion power systems; physics understanding is transferable both ways. Our conclusion is that a strong experimental and computational tokamak research program with a healthy alternative concept program is the optimum fusion research strategy. We strongly support the international construction of a tokamak burning plasma experiment as soon as possible. Fusion research is ready to enter the burning plasma era."

A SMALL POINT: Glass states, "The tokamak has been the main approach to magnetic confinement fusion since its inception almost 50 years ago." This is not correct. The promise of the tokamak was not demonstrated until the late 1960's and did not become the main approach in countries other than the USSR until the early 70's. The tokamak has been the main approach, worldwide, since that time, i.e., about 25 years.