Steady-State, High-Performance Operation of DIII-D

The DIII-D AT program objective is to develop and demonstrate a steady-state scenario with performance that extrapolates to Q$>$5 in ITER. This year, we examine two aspects of AT optimization. Using the newly improved pumping, we have established a double-null, high triangularity AT reference plasma. Further optimizing this shape by varying squareness ($\zeta$) has shown that confinement appears to improve with reduced $\zeta$ and that there is an optimum $\zeta$ for MHD stability. Changes in pedestal and ELM conditions with varying $\zeta$ are being analyzed. We are also undertaking experiments to optimize the $q$ profile for AT operation. As the tools needed for maintaining a stationary high performance $q$ profile are not yet available, we concentrate on optimization under slowly varying transient conditions, using varying combinations of co- and counter-injection as well as B$_{\rm T}$ ramping to modify the current profile. The variables are $q_{min}$ ($>$2), $q_0$- $q_{min}$ (range 0-1), and $\rho_{q_{min}}$ ($>$0.5). Issues being addressed are the nature of the limiting instabilities ($n=2$ and 3 are predicted to dominate), the dependences of the $\beta$ limit and of $f_{NI}$ on $q_{min}$ and rotation.