The ITER Project.

Letters from: [ Paul H. Rutherford ][1] [ Charles C. Baker ][1] Thomas H. Stix et al . take issue with the present course of the International Thermonuclear Experimental Reactor (ITER) initiative (Letters, 16 Feb., p. 891). ITER is a fusion research facility based on the highly successful “tokamak” approach to plasma confinement, now being designed under an international agreement executed in 1992 by the European Union, Japan, the Russian Federation, and the United States. ITER seeks to achieve, for the first time ever in controlled fusion research, a self-heated “burning” deuterium-tritium plasma. Plasma burn has been widely endorsed as a critically important and appropriate next step for fusion. Stix et al . question combining this step with three other “giant steps”: large size, long pulse, and superconducting magnets. The size of the ITER plasma is dictated by the requirement to provide sufficient energy confinement to reach the burning state; this is determined from size-scaling relationships derived from experiments on tokamaks with plasma volumes spanning two orders of magnitude; the largest of these tokamaks already operate at plasma densities and temperatures similar to those expected in ITER. Confinement in a tokamak is determined primarily by the magnitude of the plasma current: ITER's plasma current is about four times larger than that in the Joint European Torus. The step in pulse length is derived from the additional physics requirement to sustain the burn over times long compared with those characteristic of the evolution of plasma profiles. The step from copper to superconducting magnets has been found to be the most practical way of achieving the required pulse lengths. Superconducting magnets have already been used in tokamaks, including one of the size of Princeton's successful Tokamak Fusion Test Reactor. The long pulse and the superconducting magnets are also essential for ITER to advance fusion engineering objectives. The engineering features of ITER are generic to any fusion reactor; in ITER, they can be tested together in an integrated facility, made possible only by international partnership. Stix et al . also question the capability of the ITER design to achieve its stated technical objectives. Recognizing the need for broad technical consensus, a Technical Advisory Committee (TAC) was formed which, on a continuing basis, reports to the ruling ITER Council on the adequacy of the ITER design and its physics basis. The TAC's 16 members are drawn equally from all four partners, but they serve as individuals, independent of both the design team and their respective governmental agencies. Last July, the TAC completed a review of the “Interim Design” and, with unanimity, concluded that the design can fully meet ITER's mission and technical objectives. # {#article-title-2} The letters from Stix et al ., Ernesto Mazzucato, and William E. Parkins (16 Feb., p. 891) have two common themes, namely, that we should postpone or even cancel the ITER project and that we do not know how to make an attractive fusion reactor. Stix et al . and Mazzucato correctly note the tremendous progress that has been made in fusion research and that we are in a period where our expectations of continued progress are high. This is used as an argument to suggest that a commitment to the construction of ITER be postponed. I would argue just the opposite. We need to build on our excellent progress and take advantage of the unique opportunity offered by participation in ITER to move forward with fusion science and energy development. Projects like ITER are always faced with the idealistic argument that we should wait for better results. Such logic would have postponed most major undertakings that have lead to significant scientific and technological progress. The letters raise valid technical issues (including improved plasma confinement, control of disruptions, and engineering and materials development). These issues have been clearly recognized by fusion researchers, in general, and ITER, in particular. They are being responsibly addressed in the ITER research and development and base program activities. [1]: /lookup/doi/10.1126/science.272.5259.179e