Fusion Science and Technology / Volume 49 / Number 2 / February 2006 / Pages 96-121
Technical Paper / JFT-2M Tokamak
The flexible mid-sized machine of JFT-2M has contributed to the understanding of the physics of improved confinement and the control of improved discharges using some innovative techniques. The improved confinement modes achieved during additional heating on JFT-2M were H-mode in both divertor and limiter configurations, improved L-mode, counter-neutral-beam injection, and pellet-injected H-mode. These improved modes are characterized by two improvements: (a) H-mode that has sharp density and temperature gradients at the edge and (b) other modes that have peaked density, temperature, and toroidal rotation profiles near the center. The improvement of pellet-injected H-mode achieved by central fueling was a combination of H-mode and core improvement with peaked profiles. The discovery of limiter H-mode had an impact on the physics understanding of H-mode and showed the formation of a transport barrier at a place without discontinuity of the magnetic field line topology. The appearance of edge-localized modes (ELMs) by applying ergodic fields was investigated, and it was clarified that n 4 helical components were effective in producing ELMs. Scrape-off-layer biasing had the effect of compressing neutrals at the divertor region. It would be understood that compressed neutrals at the divertor region might increase banana ion loss through charge exchange and increase the negative radial electric field inside the separatrix. This situation would reduce the H-mode power threshold. High-recycling-steady (HRS) H-mode could be reproducibly obtained by boronization using tri-methyl-boron. It was found that HRS appears at a pedestal collisionality of e* > 1.