Characterization of magnetic configurations of LHD in terms of boundary shape and the direction of their improvements

LHD in Japan is the largest stellarator / heliotron device with superconducting magnetic coils (major radius is 3.9 m with 3 T magnetic field). It has been demonstrating good confinement properties comparable to major tokamaks. For the magnetic configuration control, the vertical field control is very effective with which the confinement characteristics such as the magnetic field curvature and trapped particle trajectories are largely changed. For high temperature discharge experiments or high density discharge experiments, two distinct configurations have been used respectively: inward shifted configuration and outward shifted configuration which are produced by selecting different vertical field. In this paper, these two configurations are analyzed in terms of Fourier components of boundary shape for the purpose of understanding them with general physical quantities rather than specific device parameters. The direction of improving magnetic configurations for better confinement properties will be discussed.