Plasma Confinement Studies in LHD

"The initial experiments of the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R=3.9 m, a = 0.6 m). Heating by NBI of 3 MW has produced plasmas with a fusion triple product of 8 x 10^18 keVm^-3s at a magnetic field of 1.5T. An electron temperature of 1.5 keV and an ion temperature of l.1 keV have been achieved simultaneously at the line-averaged electron density of 1.5 x 10^19 m^-3. The maximum stored energy has reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to =0.7%. Energy confinement times, reaching 0.17 s at the maximum, have shown a manner similar to the present scaling law derived from the existing medium sized helical devices, but improve on it by 50%. A distinguishing feature of a favorable dependence of energy confinement time on density remains in the present power density (~40kW/m^3) and the electron density (3x 10^19m^-3) regimes unlike L-mode in tokamaks. Temperatures of both electrons and ions as high as 200 eV have been observed at the outermost flux surface, which indicates a qualitative jump in performance from the helical devices to date. Spontaneously generated toroidal currents agree with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to finite-beta eff\u27ect has been well described by the 3-D MHD equilibrium analysis. An escape of particles from the core region leading to a hollow density profile has been observed in hydrogen plasmas, which is mitigated through core fueling with a pellet injection or in helium discharges.