Toroidal mode structure in weak and reversed magnetic shear plasmas and its role in the internal transport barrier

It is becoming clear that anomalous transport in an L-mode tokamak plasma is dominated by radially extended non-local modes in toroidal geometry. This indicates that various improved modes such as the internal transport barrier (ITG) formed in a recent reversed magnetic shear experiment can be achieved by suppressing such a global structure of the toroidal mode. We investigate the role of plasma shear rotation and negative/weak and also reversed magnetic shear on these non-local modes by employing our toroidal simulation code together with a non-local theory. From simulations, in addition to the overall reduction of the wave excitation in the entire negative magnetic shear region, we find that the weak or zero magnetic shear which appears near the minimum q (safety factor) surface breaks up toroidal coupling. This leads to a discontinuity (or gap) in wave excitation around the qmin-surface, leading to the emergence of the transport barrier. The plasma shear rotation which is observed in the experiment enhances the discontinuity so that the performance of the discontinuity is increased. We also investigate the structure of the self-generated radial electric field induced by such ITG modes and the related plasma shear rotation (referred to as the zonal flow) which influence the fluctuation level in the steady state. (author)