Effect of local ExB flow shear on the stability of magnetic islands in tokamak plasmas

The influence of local ExB flow shear on a relatively wide, constant-psi, magnetic island embedded in a large-aspect-ratio, low-beta, circular cross-section tokamak plasma is examined, using a slab approximation to model the magnetic geometry. It is found that there are three separate solution branches characterized by low, intermediate, and high values of the shear. Flow shear is found to have a stabilizing effect on island solutions lying on the low and high shear branches, via a nonlinear modification of the ion polarization term in the Rutherford island width evolution equation, but to have a destabilizing effect on solutions lying on the intermediate shear branch. Moreover, the effect is independent of the sign of the shear. The modification of island stability by local ExB flow shear is found to peak when the magnitude of the shear is approximately v(i)/L-s, where v(i) is the ion thermal velocity, and L-s the magnetic shear length.