Search

The characteristics of particle transport in three different magnetic configurations are studied from density modulation experiments in the Large Helical Device (LHD). These three configurations are represented as different magnetic axis positions (R ax ) of the vacuum field. Experiments were carried out in a range of different heating powers for each configuration with almost constant density. The experimental values of particle diffusion coefficients (D) and particle convection velocities (V) are compared with neoclassical estimates. The value of D is found to be anomalously large compared to neoclassical values in both the core and edge in all configurations. At low collisionality, this anomaly tends downward. The core convection velocities are comparable with neoclassical estimates. In more-outward-shifted configurations, particle transport is enhanced. The electron temperature and electron temperature gradient are the determinate parameters for D and V, respectively, in each configuration. The effective helical ripple is one of the important parameters for particle transport in the LHD; however, other hidden parameters exist. The role of fluctuations in particle transport is investigated from turbulence measurements using a two-dimensional phase contrast interferometer. Three kinds of fluctuation having different locations, propagation direction, and peak wave number are observed. One of these, which exists in the outermost edge region and propagates in the ion diamagnetic direction in the laboratory frame, plays a possible role in edge anomalous diffusion. The amplitudes of ion diamagnetic fluctuation components are compared with the linear growth rate of the ion temperature gradient mode.

Long-pulse operation and high-energy particle confinement properties were studied using ion cyclotron range of frequency (ICRF) heating for the Large Helical Device. For the minority-ion mode, ions with energies up to 500 keV were observed by concentrating the ICRF heating power near the plasma axis. The confinement of high-energy particles was studied using the power-modulation technique. This confirmed that the confinement of high-energy particles was better with the inward-shifted configuration than with the normal configuration. This behavior was the same for bulk plasma confinement. Long-pulse operation for more than 2 min was achieved during the experimental program in 2002. This was mainly due to better confinement of the helically trapped particles and accumulation of fewer impurities in the region of the plasma core, in conjunction with substantial hardware improvements. Currently, the plasma operation time is limited by an unexpected density rise due to outgassing from the chamber materials. The temperature of the local carbon plates of the divertor exceeded 400 deg, C, and a charge-coupled device camera observed the hot spots. The hot spot pattern was well explained by a calculation of the accelerated-particle orbits, and those accelerated particles came from outside the plasma near the ICRF antenna.

A local island divertor (LID) experiment has begun in the Large Helical Device (LHD) to demonstrate improved plasma confinement, and fundamental LID functions were demonstrated in the sixth experimental campaign in 2002-2003. It was clearly shown that when an m/n = 1/1 island is generated by adding a resonant perturbation field to the LHD magnetic configuration, the particle flow is guided along the island separatrix to the backside of the island, where carbon plates are located on a divertor head. The particles recycled there are pumped out efficiently so that the line-averaged core plasma density is reduced by a factor of {approx}2 at the same gas puff rate, compared with non-LID discharges. Obvious improvement of the global plasma confinement was, however, not observed yet, because the discharge could not be optimized, due to a large amount of outgas from the divertor head to the core plasma. The size of the divertor head was found to be larger than the optimum one; hence, the core plasma impacted slightly on the core plasma-facing portion of the divertor head with which the core plasma was not expected to collide.

Recently, dramatic progress has been achieved in the study of helical systems with high-beta experiments. Discharges with more than 3% beta plasmas have been achieved in Large Helical Device (LHD) and Wendelstein 7-AS (W7-AS). Although magnetohydrodynamic (MHD) instabilities affect local pressure gradients, the global transport property does not seem to limit the achieved beta value in either device. We summarize the LHD high-beta properties in MHD stability, equilibrium, and transport, and we show the relationship between the experimentally achieved parameters and theoretical predictions. We contrast the LHD results with the W7-AS high-beta properties. In both devices, stationary discharges in the definitely MHD unstable region have not been observed. We mention the key issue for achievement of the beta values >5%.