MHD modes driven by strong ExB velocity shear in tokamaks

It is theoretically shown that a strong shear of the ExB velocity can drive electromagnetic instabilities in tokamak plasmas. The instability threshold is determined by the condition dVE/dr>SVA/qR for finite values of the magnetic shear S. Here VE=cEr/B is the cross-field velocity, VA the Alfven velocity, q the safety factor and R the major radius of the tokamak. The Mercier modes are considered as an example of instabilities driven by the velocity shear. Effects of ion diamagnetic drift velocity and shear of this velocity on the Mercier modes are analysed. A set of magnetohydrodynamic (MHD) equations for a numerical analysis of the ExB velocity shear effect on drift-MHD modes is derived. The theoretically obtained threshold is found to be comparable to the threshold of some electromagnetic perturbations observed in JET shear-optimized discharges. (author)