Annual report of the Naka Fusion Research Establishment for the period of April 1, 1990 to March 31, 1991

R and D activities of the Naka Fusion Research Establishment, JAERI, are reported for the period from April 1, 1990 to March 31, 1991. Since the shutdown of JT-60 in November 1989, the reconstruction work of the JT-60 device was continued until the end of March 1991. In the JT-60 Upgrade, the poloidal field coils and vacuum vessel were renewed and the plasma current was planned to increase up to 6 MA with lower single null divertor. The divertor plates were designed to be toroidally continuous and to use high-heat-conduction C/C composite materials. Another objective of JT-60U is to facilitate tokamak experiments with deuterium as the working gas. In the JFT-2M program, a system for divertor bias experiments was brought into operation and initial experiments were started to study its effects on plasma discharges. Effects of ergodic magnetic limiter on H-modes were examined and stationary H-modes were obtained under the control of ergodic magnetic limiter currents. The DIII-D program was highlighted by the attainment of 11% beta with a double null divertor plasma. As for the fusion engineering research, development activities of the ceramic turbo-viscous pump and the surface insulation techniques for the tokamak in-vessel components are remarked in the vacuum technology area. In the high heat flux experiments with the JAERI Electron Beam Irradiation Stand (JEBIS), carbon-based materials and refractory metals were tested to evaluate surface erosion at plasma disruptions. The ITER Conceptual Design Activities, which began in April 1988 under the auspices of the IAEA, were successfully completed in December 1990. A lot of contributions to the program were made by JAERI people to support the design and R and D activities and to prepare a plan for the forthcoming Engineering Design Activities. (J.P.N.)