Stability of Super Dense Core plasmas in the Large Helical Device

Recent experiments [N. Ohyabu et al, Phys. Rev. Lett, in press] using pellet injection into reduced-recycling discharges in the Large Helical Device have yielded Super Dense Core (SDC) plasmas with very peaked density profiles, high central density $\sim $ 4.5 x 10$^{20}$ m$^{-3}$, and improved confinement. We have examined ideal MHD stability of these SDC configurations the using the 3-D COBRA stability code [R. Sanchez et al, Comp. Phys. Comm \textbf{141}, 55 (2001)]. These calculations show that the core region inside the zero-shear radius has direct access to second stability, i.e., the stability margin increases with $\beta$. Outside the zero-shear radius, the plasma becomes unstable to ballooning modes at average $\beta \sim $3-4{\%}. Of course, resistive versions of the modes are expected to appear at lower $\beta$. These MHD effects may play a role in improving core confinement, and may also provide a useful mechanism to constrain the plasma pressure in the outer plasma region and thus help maintain the favorable SDC state.