It is theoretically shown that a strong shear of the E× Bvelocity can drive electromagnetic instabilities in tokamak plasmas. The instability threshold is determined by the condition dVE/ dr>SVA/qRfor finite values of the magnetic shear S. Here VE= cEr/Bis the cross-field velocity, VAthe Alfvén velocity, qthe safety factor and Rthe 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 E× Bvelocity 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.
Understanding the effect of sheared flow on microinstabilities