The Role of Toroidal Rotation in the Very High Energy Confinement Observed in Super H-mode Experiments on DIII-D*

Experimental analysis and modeling of recent super H-mode experiments on DIII-D show that driven high toroidal rotation, not high pedestal pressure, plays an essential role in achieving very high energy confinement $H_{98y2}>$1.5. Understanding the mechanisms leading to improved confinement in the super H-mode experiments is essential to the ability to extrapolate to a future reactor. For fixed plasma shape and rotation, the energy confinement time for discharges analyzed here with different plasma current, density, injected power, is proportional to the $\tau_{E,98y2}$ scaling, i.e. the energy confinement quality $H_{98y2}$ is constant, despite different pedestal pressure (up to $\times$2), and plasma stored energies. For fixed plasma shape but different toroidal rotation, which varies according to injected neutral beam torque per particle, $H_{98y2}$ varies linearly with rotation, independent of pedestal pressure. A transient phase of very high confinement quality, $H_{98y2}\sim$2 (well above standard H-mode, $H_{98y2}\sim$1), is only achieved at very high level of core rotation, e.g. 400 km/s at $\rho$=0.4, and is independent of the pedestal pressure. At moderate rotation on DIII-D (similar to levels expected in ITER) very high super H-mode pedestal pressure yields a lower confinement quality improvement ($H_{98y2}\le$1.2) if no core MHD modes are present. Linear gyrofluid and nonlinear gyrokinetic transport modeling confirms that rotation-driven E$\times$B shear is responsible for confinement quality significantly above standard H-mode, and that E$\times$B shear turbulence stabilization is far stronger than EM stabilization, so-called hot-ion stabilization ($T_i$/$T_e$), or fast ions effects. Gyrokinetic simulations also show a potential approach to improve confinement at low rotation: higher impurity (carbon) gradient in the plasma core can efficiently suppress ITG turbulence, and improve confinement in the super H-mode scenario.