Superdense core mode in the Large Helical Device with an internal diffusion barrier

In reduced recycling discharges using a local island divertor in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)], a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with ~5×10^20 m^?3 and temperature of 0.85 keV is maintained by an internal diffusion barrier (IDB). The density gradient at the IDB (r/a~0.6) is very high, and the particle confinement time in the core region is ~0.4 s. Because of the increase in the central pressure, a large Shafranov shift up to ~0.3 m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.