June 23, 2005

    In the large tokamak device JT-60 in JAERI (Japan Atomic Energy Research Institute) [President Toshio Okazaki], an energy-saving operation mode (high-efficiency operation mode) for tokamak fusion power plants has been developed. The technical feasibility of the high-efficiency operation, in which a large fraction (75%) of the plasma current is maintained by a self-generated bootstrap current, has been confirmed for the first time. This result gives prospects of reducing the cost of electricity (COE) in fusion power plants.

    It is required to reduce the electric power for operation (station service power), in addition to raise the plasma pressure (temperature times density) to increase the fusion power, for reduction in the COE of fusion power plants. In tokamak devices, a plasma current is necessary to confine the plasma. In present-day long pulse operation of tokamaks, most of the plasma current is driven by injection of radio-frequency waves and/or neutral beams, which requires a large amount of electric power and thus increases the station service power. Therefore, to reduce the station service power, development of the high-efficiency operation mode, in which a large fraction of the plasma current is driven by a self-generated bootstrap current, had been requested.

    We have continued the development of the high-efficiency operation after recognizing its importance first in the world. In the previous studies, however, the sustained duration was limited by plasma instability, which was not controlled, and the plasma conditions (pressure profiles and current profiles) did not reach stationary conditions. Extending the plasma duration was one of great issues since it is important to confirm that the plasma with a large fraction of the bootstrap current reaches stationary conditions within some time.

    We successfully avoided the plasma instability by the control of the pressure profile through changing the plasma flow by using the JT-60 neutral beam injection system which has a variety of injection directions. As a result, we maintained a high-temperature plasma with a bootstrap current fraction of 75% for 7.4 seconds to confirm that the current profile and the pressure profile reached nearly stationary conditions, namely existence of the steady state, for the first time in the world. This result demonstrates the technical feasibility of the high-efficiency operation mode and gives prospects of reducing the COE of fusion power plants. Furthermore, it is expected to realize the high-efficiency operation in a burning plasma in ITER, based on the planned steady-state operation with a bootstrap current fraction of 50%. This result will be presented in an invited talk in the 32nd European Physical Society Plasma Physics Conference to be held from 27 June to 1 July in Spain.

Information T. Fujita Naka Fusion Research Establishment Japan Atomic Energy Research Institute Fax: 81-29-270-7419