A Key to Improved Ion Core Confinement in the JET Tokamak: Ion Stiffness Mitigation due to Combined Plasma Rotation and Low Magnetic Shear
New experimental evidence indicates that ion stiffness mitigation in the core of rotating plasmas, observed previously in JET, results from the combined effect of high rotational shear and low magnetic shear. Ion stiffness in the outer plasma region is found to remain very high irrespective of rotation. Dedicated experiments in plasmas with different q profiles and rotation levels point to a larger effect of rotation in reducing stiffness when the core q profile is made flatter. The results have implications for the understanding of improved ion core confinement in hybrid plasmas or Internal Transport Barriers, both characterized by high rotation and low magnetic shear. Experimental evidence in these scenarios is discussed. Simulations indicate that the physics be-hind these results may lie in the ITG/TEM turbulence behavior at the transition between fully developed turbulence and zonal flows quenching. These findings point to the need for future devices of achieving sufficient rotational shear and capability of q profile manipulation to reach improved ion core confinement, which is an essential feature of Advanced Tokamak operation.