COHERENT MAGNETIC-FIELD FLUCTUATIONS AND LOCKED MODES IN A REVERSED-FIELD PINCH

Magnetic field fluctuations have been measured in the TPE-1RM20 reversed-field pinch [in Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 14th International Conference, 1992, Wurzburg (International Atomic Energy Agency, Vienna, 1992) Paper IAEA-CN-56/C-4-3]. The dominant modes are m = 1/n = 7-9, resonant near the axis. The fluctuations are coherent, corresponding to nearly continuous rigid body rotation with poloidal and toroidal frequencies of f(theta)= 130-170 kHz and f(phi)= 11-14 kHz, respectively. Discrete dynamo events (in which toroidal magnetic flux is generated) of two different types are observed at high values of pinch parameter (THETA > 1.6). The first type is triggered by a single, wall-locked m = 1/n = 7 mode. The slowing down of the mode rotation is in rough agreement with a simulation which assumes the effect of induced eddy current in the vacuum liner. In the subsequent relaxation event, the locked mode decays and the m = 1 spectrum cascades to higher-n modes. However, in some cases the saturated m = 1/n = 7 mode exists for periods up to t = 0.4 msec, resembling single helicity Ohmic states. The second type of event is characterized by phase locking of several m = 1/n = 7-9 modes and the appearance of large m = 0/n = 1-5 modes. Mode rotation continues through the event, similar to the normal machine operating condition.