High-density plasma with internal diffusion barrier in the Large Helical Device

An attractive high-density operational regime which is a so-called internal diffusion barrier (IDB) has been discovered in a helical divertor configuration on the Large Helical Device (LHD). The IDB is characterized by steep density gradients and the plasma profile is divided by the IDB into a high-density core plasma and a low density mantle plasma. The IDB enables the core plasma to access the high-density/high-pressure regime. The attainable central density exceeds 1 x 10(21) m(-3) and the central pressure reaches approximate to 1.5 times atmospheric pressure. Core pellet fuelling is absolutely essential for the IDB formation and it is reproducibly obtained by employing intensive multiple-pellet injection. In the IDB core plasma, the particle diffusion coefficient is kept at a considerably low level, 0.05 m(2) s(-1), in spite of high-density and steep-density gradients whereas an inward particle convection velocity is not observed.