Mode-locking phenomena in the TPE-RX reversed-field pinch plasma

The characteristics of the phase- and wall-locked mode found in a large-siz ed reversed-field pinch (RFP) machine TPE-RX [Y. Yagi Plasma Phys. Controll ed Fusion 41, 255 (1999)] are described in detail. The toroidally-localized radial magnetic field starts to grow after the setup of the RFP configurat ion in a current rising phase, reaching up to 2% of the poloidal magnetic f ield at the plasma surface, and stays at the same toroidal location through out the discharge. The mode frequently locks to the thick shell gap positio n with a 20%-30% probability. The plasma-wall interaction is enhanced at th e locked position where the thermal wall load is peaked by a factor of abou t 3 on average. The locked mode disappears in some experimental conditions. The probability for the locked mode to appear depends on the experimental conditions, especially on the filling pressure of the fueling gas and on th e rise time of the plasma current. Possible causes of the locked mode are d iscussed from the braking effect of the eddy current as well as from the ha lo current in the vacuum vessel. (C) 1999 American Institute of Physics. [S 1070-664X(99)00110-X].