Transient transport analysis of improved confinement plasmas in LHD

In LHD, the thermal diffusivity depends on temperature rahter than on the temperature gradient, which is in contrast to tokamaks in which the depend ece on the temperature gradient is stronger. Therefore the thermal diffusivity driven by transient transport with heat propagation is similar to the steady state transport analysis given by the so-called power balance analysis in LHD. The transient transport analysis, as well as the steady-state transport analysis have been done for the plasmas with improved confinement in LHD using cold pulse propagation with a plastic pellet, heat propagation with modulated ECH and transport analysis between repetitive pellets. There are two types of improved modes observed in LHD. One is an electron internal transport barrier (ITB), which is characterized by peaked electron temperature profiles associated with the transition from ion root (negative Er) to electron root (positive Er) when electron cyclotron heating is applied to a low density NBI plasma. The other is a pellet enhanced confinement mode, which is characterized by a peaked electron density profile and a high global energy confinement time (1.5 times that for a gas puff discharge) sustained by repetitive pellets in the high density regime. Both the steady-state and transient transport analysis applied to these improved modes clearly show a significant reduction of thermal diffusivity, in the central region (r/a<0.3) for the ITB plasmas, and near the edge for the pellet enhanced mode. The temperature and temperature gradient dependence of the thermal diffusivity are investigated at the formation of the electron-ITB. A negative temperature dependence of the thermal diffusivity is observed inside the electron-ITB, while it has a positive dependence for the pellet enhanced mode. The role of rational surfaces on transport is also discussed for these confinement improved mode plasmas. (Author)