Creating Hybrid Plasmas With Off-Axis ECCD for Radiating Divertor Studies in DIII-D

A long duration, high density, high power hybrid scenario has been developed for use in radiative divertor studies in DIII-D. Using 11.2 MW of co-NBI power and 3.4 MW of ECCD, with a total injected energy of up to 56 MJ, high performance hybrid plasmas with $\beta_{\mathrm{N\thinspace }}=$ 3.7 and H$_{\mathrm{98y2\thinspace }}=$ 1.5 were created. The hybrid plasmas were fully non-inductive at densities of n $\approx $ 4.2 \texttimes 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$ with central ECCD, but the EC deposition needed to be moved to $\rho \quad =$ 0.45 to avoid the right-hand cutoff when the density was raised to n $\approx $ 5.8 \texttimes 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$ for radiative divertor studies. Although moving the EC deposition to $\rho \quad =$ 0.45 had the effect of dropping $\tau_{\mathrm{E}}$ by 10{\%}, the energy confinement time increased with higher density like $\tau_{\mathrm{E\thinspace }}\propto $ n$^{\mathrm{0.4}}$, allowing high beta to be maintained. While the plasma current profile displays the usual self-organizing properties of hybrids -- an anomalously broad profile with q$_{\mathrm{min\thinspace }}$\textgreater 1 -- local current drive can still have a measurable effect on stability, either positively or negatively. For example, hybrid discharges with radial ECH deposited at $\rho \quad =$ 0.45 proved to be more robustly stable to n $=$ 1 modes (can be either a 1/1 or 2/1 mode) than similar discharges with co-ECCD at the same location. Interestingly, the large 1/1 mode had almost no effect on energy confinement but strongly degraded particle confinement; thus this mode needed to be suppressed to achieve the high pedestal densities required for radiative divertor studies.