High $\beta_N$ steady state scenario development on DIII-D

On \hbox{DIII-D}, on- and off-axis neutral beams and electron cyclotron heating have expanded access to a wide range of $q$-profiles. Plasmas have been sustained with $q_{min}=1.3-2.5$ to evaluate the suitability for high $\beta_N$, high performance steady state operation. Nearly stationary plasmas were sustained for two current profile relaxation timescales (3~s), with $q_{min}=1.5$, $\beta_N=3.5$, and performance that projects to $Q=5$ in ITER. The duration of the high $\beta_N$ phase is limited only by the available NBI energy. Low-order tearing modes are absent and the predicted ideal-wall $n=1$ kink $\beta_N$ limit is $>$4. To achieve a steady state, higher $\beta_N$ is needed to increase the bootstrap current. Higher $q_{min}$ decreases the required external current drive near the axis and can increase the stability $\beta_N$ limit. Experiments to produce $\beta_N =4-5$ and $q_{min}\geq 2$ with $B_T=1.75-2$~T were limited to $\beta_N<3.3$ by relatively low energy confinement (H$_{\rm 89}<2$) rather than tearing modes. Low H$_{\rm 89}$ is likely due to a combination of increased thermal transport at high $q_{min}$ (low poloidal flux), and depositing more power at larger radius. We will discuss upcoming experiments to achieve higher $\beta_N$ and improved confinement.