Improvement of energy confinement in toroidal plasmas

There has been significant progress during this last decade on the energy confinement improvement in the toroidal plasma experiments, tokamaks and helical systems (stellarator and heliotron/torsatron). Improved modes in tokamak plasmas may be grouped into three types depend upon the enhancement factor of the energy confinement time and the pressure profile shape such as (A) H-mode with relatively steep edge pressure gradient, (B) extremely central peaked profile like super shot and high poloidal beta mode, and (C) VH-mode and high poloidal beta H-mode with intermediate shape. The case (C) might be regarded as an extension or a combination of the previously known improved modes (A) and (B). Enhancement factor of approximately four times over the L-mode confinement is reported commonly to the third type. H-mode and other improved modes have been discovered also in helical systems. Tokamak and helical system have different characteristics of magnetic topology, therefore confinement studies in both configurations are complementary each other. A brief survey of efforts aiming at a steady-state tokamak development, and ideas for a future advanced tokamak scenario including issues relevant to the physics understanding are also given. Stabilizing/destabilizing effects, on both MHD and microscopic turbulence, due to the current density profile as well as the flow velocity profile are found to be important areas to be addressed intensively. Further improved performance would be expected in a future helical system with well defined divertor configuration. (author) 66 refs