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 q min -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.

It is becoming clear that tokamak anomalous transport is dominated by radially extended non-local modes which originate from strong toroidal coupling of rational surfaces in non-uniform plasmas. To aid in understanding the internal transport barrier (ITB) formed in reversed magnetic shear experiments, in addition to the well known shear flow effect, the article points out an important non-local effect and/or finite size effect which comes from the complex behaviour of the mode over a finite radial region around the minimum q (safety factor) surface. The non-local mode, which is characterized by its radial extent and the degree of tilting in the poloidal direction (Δr, θ0), changes its structure depending on the sign of the magnetic shear, and as a result such modes are weakly excited across the qmin surface. This leads to a discontinuity or gap which disconnects the phase relation in the global wave structure across the qmin surface. Once such a discontinuity (or gap) is formed, transport suppression occurs and therefore a transport barrier can be expected near the qmin surface. The existence of this discontinuity is confirmed through use of a toroidal particle simulation. It is also shown that whether such a discontinuity is efficiently established depends on the presence of the radial electric field and the related plasma shear flow.