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, Wuerzburg (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 fθ=130--170 kHz and fφ=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 (Θ>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