Non-inductive Hybrid Scenario—Transport and Turbulence at Reduced Plasma Torque

As the neutral beam injection (NBI) torque is lowered in steady-state hybrid plasmas via counter-beam injection, increased turbulence and thermal transport is observed, particularly in the ion channel. These discharges require $\textit{P}_{co-NBI}{=11}$ MW and $\textit{P}_{ECH}{=3}$ MW to achieve zero surface loop voltage. As the beam torque is reduced from ${\sim}{8.5}$ N-m to ${\sim}{4}$ N-m with $\beta_{N}\sim3$ and $\textit{q}_{95}\sim6$, the global confinement decreases from $\textit{H}_{98y,2}$ of ${\sim1.5}$ to ${\sim1.2}$. Local transport analysis using TRANSP shows that the lower torque discharges have increased ion thermal diffusivity across the whole profile and increased electron thermal diffusivity localized to the $\rho{=0.7}$ region. Similarly, Doppler Backscattering shows increased density fluctuations at intermediate wavenumbers at the lower torque. However, fast-ion transport caused by off-axis fishbones favorably decreases from $\sim0.7{m^2{/s}}$ to ${\sim}0.1{m^2{/s}}$ as the torque is lowered, partially offsetting the thermal transport reduction. These measured changes in turbulence and transport are being compared to plasma simulations using TGLF/GYRO to better predict the confinement of future steady-state hybrids that will be primarily RF-heated.