Off-axis Neutral Beam Current Drive on DIII-D

Off-axis Neutral Beam Current Drive (NBCD) physics is being validated by DIII-D experiments using tilted beams. Off-axis NBCD broadens the current and pressure profiles to improve energy confinement and low-n kink-type ideal-MHD stability, enabling access to reactor relevant $\beta_N>4$ at fully non-inductive $f_{NI}=1$ conditions. The off-axis NBCD profiles measured by the magnetic field pitch angles from the motional Stark effect (MSE) diagnostic agree well with modeling using the orbit-following beam-slowing-down Monte Carlo code NUBEAM for H-mode plasmas in a range of discharge conditions without large-scale MHD activities such as Alfvén eigenmodes and sawteeth, showing that the off-axis NBCD efficiency does not decrease with deposition at large minor radius. Projecting to the Compact Advanced Tokamak (CAT) Pilot Plant shows an excellent current drive efficiency with the NBCD peak at $\rho > 0.6$, which aligns well with the high bootstrap current $f_{BS} > 0.8$ operation, maintaining a broad current profile with $q_{min} > 2$. We will report initial results from the NBCD measurement experiment using a newly available 2nd tilted beamline to advance physics understanding of off-axis NBCD for high $\beta_N$ steady-state scenario development.