As previously mentioned in Chapter 10, mode locking is a process by which the rotation of a slowly growing magnetic island chain in a tokamak plasma is braked due to electromagnetic interaction with a rigid, electrically conducting wall surrounding the plasma, causing the chain to eventually lock (i.e., become stationary in the laboratory frame) to a static resonant magnetic perturbation (e.g., an error-field) [2,3,5,6,9,14,15,17,20,22,24,27,28]. Locked magnetic island chains are strongly correlated with disruptions (i.e., sudden, catastrophic losses of thermal and magnetic energy) in tokamak plasmas .
In Chapter 10, we investigated the interaction of a rotating magnetic island chain with a conducting wall in the absence of a magnetic perturbation. In this chapter, we shall concentrate on the interaction of a rotating magnetic island chain with a static resonant magnetic perturbation in the absence of a wall. The generalization to the case where there is both a wall and a magnetic perturbation is straightforward .