Fusion Science and Technology / Volume 58 / Number 1 / July/August 2010 / Pages 70-90
Chapter 3. Confinement and Transport / Special Issue on Large Helical Device (LHD)
Particle confinement processes are studied in detail on the Large Helical Device (LHD). Diffusion coefficients (D) and convection velocities (V) are estimated from density modulation experiments. The magnetic configuration and collisionality are widely scanned in order to investigate parameter dependences of D and V. To study the effect of the magnetic configuration, magnetic axis positions (Rax) are scanned from 3.5 to 3.9 m. This scan changes the magnetic ripples quite significantly, enabling the effects of neoclassical properties on measured values to be widely elucidated. Dependences of electron temperature (Te) and helically trapped normalized collisionality are examined using the heating power scan of neutral beam injection. It was found that generally larger (or smaller) contributions of neoclassical transport in the core region, where normalized position < 0.7, resulted in more hollow (or peaked) density profiles. The larger neoclassical contribution was found to be situated at a more outwardly shifted Rax for the same Te and for higher Te or lower h* at each Rax. However, it is to be noted that Rax = 3.5 m shows different characteristics from these trends, that is, a more peaked density profile at higher Te or lower h*. The edge ( > 0.7) diffusion and convection are dominated by anomalous processes. Measured edge turbulence shows a possible linkage.