Numerical study for design of the passive stabilizer and its impact on MHD stability in the proposed KSTAR plasma

The effect of the passive plate stabilizer on the ideal magnetohydrodynamic (MHD) stability is numerically studied in order to guide its design in the proposed Korea Superconducting Tokamak Advanced Research (KSTAR). The parametric study is systematically performed by taking into account the two major variables: the plate’s distance from the plasma surface, and the poloidal (toroidally continuous) vacuum gap required for the access of neutral beam ports and other diagnostics near the outboard mid-plane. Extensive calculations are carried out on the plasma beta limits for low-n (n being toroidal mode number) MHD modes in several major operating regimes. The results lead to a practical and optimistic design point: plasma-wall separation ≈10 cm, and the outboard gap having a vertical separation of about 80 cm between the upper and lower plates. This is one of the choices that will make it feasible to achieve the advanced operation with bootstrap current fraction of over 80% at βn=5 in the reversed shear mode in KSTAR.