A Conformal Conducting Wall for Robust Stability of High $\beta_N$, Fully Noninductive Discharges in DIII-D

A conducting surface inside the DIII-D vacuum vessel, closer to the plasma, can increase the ideal-wall MHD stability limit above the high normalized beta ($\beta_N$) needed for 100\% noninductively-driven current at power plant relevant $q_{95}$. In discharges modeled with the planned heating/current drive upgrades, the required $\beta_N$ is as high as 5. This is roughly the calculated limit for n = 1 ideal-wall stability, even with a broad current density profile designed to couple well to the present conducting wall. Tearing and resistive wall modes will very likely limit $\beta_N$ to a value that is lower, but which is expected to scale with the ideal-wall limit. Conceptual designs for an axisymmetric wall that better matches the plasma shape raise the ideal-wall stability limited $\beta_N$ above 7. Analysis with VALEN of a 3-D wall model predicts $\beta_N\sim 6.4$. Increased stability margins are also expected for a wide range of DIII-D discharge scenarios even without a broad current density profile.