3-D ideal MHD stability of super dense core plasma in LHD

The characteristics of confinement properties and magnetohydrodynamics (MHD) stabilities of the super dense core (SDC) plasmas are intensively studied in the Large Helical Device (LHD) experiment. The SDC plasmas are produced in the configuration with outward shifted magnetic axis and rapid fueling into the core region by pellet injection. The SDC plasmas have high electron pressure due to the large electron density (ne-1021[m-3]) in the core region which induces a large Shafranov shift (Δ/aeff>0.5). It is useful to apply the production method of the SDC plasmas for obtaining high-beta plasmas with high pressure in the core region. In this study, the characteristics of the ideal MHD instabilities of the SDC plasmas are analyzed by using the 3-D ideal MHD code TERPSICHORE. The calculated equilibrium of the typical SDC plasma with the central beta of β0=5[%] solved as the result of the HINT/HINT2 codes shows that a deep magnetic well depth appears around the core region due to the large Shafranov shift. The Mercier criterion (DI) shows that the plasma is stable all over the plasma region and consequently the low-n interchange modes do not appear. Although the typical SDC plasma is stable to the interchange mode, it is worthwhile to investigate the other pressure profiles and the magnetic field configurations to know the role of the MHD instabilities on the experimentally obtained SDC plasmas. In the experiment, the SDC plasmas cannot be produced in the configuration with inward shifted magnetic axis. Such configuration tends to be MHD unstable, in which the core resonant interchange modes become unstable in case of relatively low-beta plasma with finite pressure gradient in the core region. We will investigate how the MHD instabilities affect the production of the SDC plasma with steep pressure gradient in the core region in the configuration with inward shifted magnetic axis. (author)