Deconvolving the roles of E×B shear and pedestal structure in the energy confinement quality of Super H-mode experiments*

Recent experiments on DIII-D have validated previous analysis results suggesting that high plasma toroidal rotation, not high pedestal pressure, plays the essential role in achieving very high energy confinement quality (H_98y2>1.5) in super H-mode plasmas. Quasi-linear gyrofluid and nonlinear gyrokinetic transport modeling showed that the contribution from rotation in the core E×B shear is by far the main mechanism responsible for H_98y2  well in excess of standard H-mode. H_98y2 ~ 1.2 was predicted without E×B shear in high pedestal super H-mode plasmas. The new experiments confirm H_98y2~1.2 in high pedestal discharges with low E×B shear obtained by reducing the injected neutral beam torque at constant power. Conversely, high rotation shear discharges maintain very high confinement quality (H_98y2 >2), despite ~40% lower pedestal obtained by reducing the plasma triangularity. These results are consistent with previous simulations of the ITER Baseline Scenario using a super H-mode pedestal [1], showing that the predicted energy confinement quality is not above standard H-mode even at the highest pedestal pressure.

[1] W.M. Solomon, et al., Phys. Plasmas 23 (2016) 056105