Properties of internal diffusion barrier in high density plasmas on large helical device

An experimental study is performed in order to extend the operational space of internal diffusion barrier (IDB) plasma, which was originally found in pellet fueled high density discharges with the local island divertor configuration, to an intrinsic helical divertor configuration in large helical device (LHD). It is revealed that the IDB can be reproducibly formed in intrinsic helical divertor configurations as in the local island divertor configuration. The IDB is efficiently formed by central pellet fueling in the outer shifted magnetic configuration (Rax > 3.7 m). Attainable core plasma density becomes higher as the magnetic axis shifts outward. Maximum core density reaches 1 x 1021 m-3. Central pressure exceeds 130 kPa, and therefore very large Shafranov shift (Δ/aeff ≥ 1/2) due to high central beta is observed even at high magnetic field (Bt - 2.54 T). (author)