Magnetic field structure and confinement of energetic particles in the LHD

The LHD achieves high-performance plasma confinement by the coordination of magnetic surface region and chaotic field line layer. It is theoretically and experimentally shown that drift surfaces exist for highly energetic particles being extended over the last closed flux surface (LCFS) in the LHD. Those particles are considered as lost particles due to the loss-cone in the previous theories, where the analyses are limited inside the LCFS. The present theory predicts that the LHD has no loss-cone and that highly energetic particles confined over the LCFS exist. These are consistent with the LHD experimental results in both the ion cyclotron range of frequency (ICRF) heating experiments and the low magnetic field neutral beam injection (NBI) heating experiments. From particle orbit analyses and studies on the connection length of diverter field lines, it is also shown that plasma can exist in the chaotic field line layer located outside the LCFS in the LHD. The plasma in the chaotic field line layer is clearly detected by CCD-cameras in the LHD experiment. This ambient plasma might be expected to have the role of a kind of an impregnable barrier for the core plasma, which suppresses both the MHD instabilities and the cooling of the core plasma due to charge exchange processes. The line-tying effects of diverter field lines that are slipped out from the chaotic field line layer can also stabilize the ballooning mode and the vertical displacement events of plasma column. (author)