The role of edge current density on kink mode stability and its implication for magnetohydrodynamic activity associated with edge localized modes

The stability of a plasma equilibrium to the external kink mode is shown to be dependent on the details of the current density near the plasma edge and the value of the safety factor, q, at the edge. The buildup of the current density near the plasma edge is shown to decrease the shear in the safety-factor profile and lead to destabilization of the kink mode. Tokamak plasmas in the high confinement mode of operation have high current density near the edge and are known to exhibit activity referred to as edge localized modes. An important feature of this activity is intermittent bursts of magnetohydrodynamic (MHD) activity near the plasma edge. A model is proposed for the MHD activity in terms of the external kink mode. The role of the plasma geometry and equilibrium profiles is discussed.