Pursuit of high performance, small ELM, high-q_min plasmas with stronger shaping

Fusion pilot plant steady-state designs such as CAT-DEMO consider operation with q_min > 2 and q₉₅ ~ 5–6.5, but at higher β_N (~3.5–4.5) than has been achieved experimentally in DIII-D high q_min (q_min > 2) plasmas. These are not limited by ideal or resistive wall modes, and lower order rational q surfaces are excluded to prevent most deleterious instabilities; however, operation at high β_N (> 3.5) is still often stifled by tearing modes. Maintaining q_min > 2 is another challenge, requiring significant off-axis current drive. Recent experiments used upgraded electron cyclotron current drive (ECCD) capabilities and the new Stage 1 “Shape and Volume Rise (SVR)” divertor to improve both MHD stability and current drive. Prior to the SVR, ECCD was launched from the top of the machine into a typical double null DIII-D shape. Though EC current was more broadly deposited than ECCD injected from the low field side (LFS), it was often not absorbed. In subsequent experiments, the higher elongation and triangularity SVR shape was used with LFS ECCD only. We examine and compare these experimental results to IPS-FASTRAN predictions, which anticipated higher pedestal pressure and broader pressure profiles, stronger wall stabilization of kink modes, and higher q₉₅ for higher bootstrap fraction.