Evolution process of the mode wall-locking and phase-locking in a reversed-field pinch plasma

Wall-locking and phase-locking modes are studied in detail in a reversed-field pinch device, TPE-RX [Y. Yagi et al., Fusion Eng. Design 45, 409 (1999)]. These mode-locking phenomena arise from tearing instabilities. Wall locking means the stopping of mode rotations, and phase locking means the locking of the phases of multiple modes. Phase locking induces a toroidally localized enhanced magnetic amplitude. There are two types of mode-locking states in TPE-RX. One of them exhibits a clear phase-locked structure, while the other exhibits a weak toroidal localization. Both types show finite toroidal rotation during the current-rising phase of the discharge, and are eventually wall locked during the current flat-top phase. However, the rotation speeds are clearly different between the two types. Confinement properties are compared between the two types of mode-locking states. It is shown that the threshold for the mode amplitude necessary to wall lock the toroidal rotation, as well as the bifurcation of phase-locked structures, agrees with theoretical predictions [R. Fitzpatrick et al., Phys. Plasmas 6, 1168, 3878 (1999)]