Operation Space of Off-axis Electron Cyclotron Current Drive at High Density on DIII-D Enabled by ECH Upgrade

The ECH upgrade on DIII-D will increase not only the torque free heating but also the operational density space on DIII-D. This study presents how the higher frequency gyrotrons would allow heating and off-axis current drive at higher density. The 1MW gyrotrons with triple frequency of 104/137/170 GHz will increase the density limit (except operating at 104GHz) compared to existing 110GHz gyrotrons. At B_T>=2.1T where super H-mode plasmas operate, 137 GHz X2 injection could provide heating with >50% higher density limit. With O2 injection, plasmas with high Greenwald density fraction (n_ped/n_GW ≥ 1) can be accessed by higher frequency ECH. 170 GHz X3 injection with even higher density limit could provide central heating at B_T=2-2.2T which will be very useful especially when central heating is required for impurity control on DIII-D. 137 GHz X2 injection has ~40% higher density limit than 110 GHz X2 injection for off-axis (ρ~0.6) ECCD in steady-state AT plasmas at B_T=2.1T. O2 injection can be utilized at even higher density. Owing to the longer wave-electron interaction path, at the same density, top launch ECCD [Xi Chen, et al. NF 2022] is predicted to generally provide higher ECCD efficiency in both X2 and O2 than outside launch. Top launch O2 injection has 25% to 50% of ECCD efficiency compared to outside launch X2 injection in DIII-D AT plasmas. A new top launch scheme has been installed on DIII-D and experiments have been conducted to verify its function.