Wall locking and density limit disruptions

Two-dimensional simulations are presented of the locking of tearing modes to resistive walls in tokamaks during the early phase of density limit disruptions. Two different trigger mechanisms for the wall locking have been considered. In the scenario believed to be most relevant for density limit disruptions, the m=2/n=1 tearing mode becomes unstable while still rotating and develops a rotating magnetic island. When the mode lock occurs, wall stabilization is lost and the magnetic island grows leading to disruption. Factors determining the increase in island size resulting from the mode lock is discussed. Another trigger mechanism is when the tearing mode becomes linearly unstable by decreasing wall stabilization because the rotation is slowed down due to changes of the equilibrium. (authors)