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The characteristics of the phase- and wall-locked mode found in a large-sized reversed-field pinch (RFP) machine TPE-RX [Y. Yagi et al. Plasma Phys. Controlled Fusion 41, 255 (1999)] are described in detail. The toroidally-localized radial magnetic field starts to grow after the setup of the RFP configuration in a current rising phase, reaching up to 2% of the poloidal magnetic field at the plasma surface, and stays at the same toroidal location throughout the discharge. The mode frequently locks to the thick shell gap position with a 20%–30% probability. The plasma–wall interaction is enhanced at the locked position where the thermal wall load is peaked by a factor of about 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 the rise time of the plasma current. Possible causes of the locked mode are discussed from the braking effect of the eddy current as well as from the halo current in the vacuum vessel.

Pulsed poloidal current drive (PPCD) (Sarff J S et al 1994 Phys. Rev. Lett. 72 3670) is conducted in a reversed-field pinch (RFP) machine, TPE-RX. The PPCD yields a twofold improvement of poloidal beta and energy confinement time. A quiescent phase is observed in the magnetic fluctuations, δb, during the PPCD. The result is discussed in terms of the change of the equilibrium configuration along the F -trajectory (F and are the reversal and pinch parameters, respectively). Representative mode amplitude is numerically simulated. The result indicates that a transient nature of the PPCD, where τ PPCD (characteristic time of the PPCD operation) τ D (0) (resistive diffusion time of the core) holds, allows a trajectory with a deeper F which yields a less turbulent configuration than shot-by-shot F -scans. It is shown that the improvement ratio of τ E approximately scales as δb −2 for five cases of the PPCD experiments in three RFP machines, including the present work in TPE-RX.

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