Improved plasma performance on Large Helical Device

Since the start of the Large Helical Device (LHD) experiment, various attempts have been made to achieve improved plasma performance in LHD [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. Recently, an inward-shifted configuration with a magnetic axis position R_ax of 3.6 m has been found to exhibit much better plasma performance than the standard configuration with R_ax of 3.75 m. A factor of 1.6 enhancement of energy confinement time was achieved over the International Stellarator Scaling 95. This configuration has been predicted to have unfavorable magnetohydrodynamic (MHD) properties, based on linear theory, even though it has significantly better particle-orbit properties, and hence lower neoclassical transport loss. However, no serious confinement degradation due to the MHD activities was observed, resolving favorably the potential conflict between stability and confinement at least up to the realized volume-averaged beta of 2.4%. An improved radial profile of electron temperature was also achieved in the configuration with magnetic islands, minimized by an external perturbation coil system for the Local Island Divertor (LID). The LID has been proposed for remarkable improvement of plasma confinement like the high (H) mode in tokamaks, and the LID function was suggested in limiter experiments

Initial physics achievements of large helical device experiments

The Large Helical Device (LHD) experiments [O. Motojima, et al., Proceedings, 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437] have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas (R = 3.9 m, a = 0.6 m), which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection (NBI) has produced plasmas with a fusion triple product of 8 X 10^18 keV m^3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to = 0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics (MHD) instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas

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.