MHD modes driven by strong E x B velocity shear in tokamaks

It is theoretically shown that a strong shear of the E x B velocity can dri ve electromagnetic instabilities in tokamak plasmas. The instability thresh old is determined by the condition dV(E)/dr > SVA/qR for finite Values of t he magnetic shear S. Here V-E = cE(r)/B is the cross-field velocity, V-A th e Alfven velocity, q the safety factor and R the major radius of the tokama k. The Mercier modes are considered as an example of instabilities driven b y the velocity shear. Effects of ion diamagnetic drift velocity and shear o f this velocity on the Mercier modes are analysed. A set of magnetohydrodyn amic (MHD) equations for a numerical analysis of the E x B Velocity shear e ffect on drift-MHD modes is derived. The theoretically obtained threshold i s found to be comparable to the threshold of some electromagnetic perturbat ions observed in JET shear-optimized discharges.