Physics Issues for Extending the Pulse Length of High $f_{NI}$ DIII-D Discharges

The increase of the total available NB and gyrotron injected energies has enabled study of high $f_{NI}$ discharges with the high $\beta_N$ phase extended to $>$3~s. To minimize $n_e$ for maximum EC and NB CD, discharges were produced after a boronization. Typically $H_{98}\approx 1.5$, but the common decrease in $n_e$ in the later portion of the high $\beta_N$ phase correlates with reduced $\tau_E$. The broadly deposited ECCD appears to improve $n=1$ tearing mode stability, but a 2/1 or 3/1 mode is more likely with longer pulses as profiles are not stationary. Scaling of $J_{NI}$ with $B_T$ was studied to best match $J_{NBCD}$, $J$, and the $P_{beam}$ required for a given $\beta_N$. Short ELM-free phases, perhaps from improved H-mode pedestal stability at high $\beta_p$, and rapid fishbone-like bursts were found to affect discharge stability and capability for $\beta_N$ control.