Discharges with improved plasma confinement in a high pinch parameter region (0 -2) have been discovered in the TPE-1RM2O reversed field pinch experiment. These discharges become possible with careful control of the waveforms of the plasma current and the toroidal magnetic field after intensive discharge cleaning and are called the 'Improved High Theta Mode' (IHTM). In these discharges the increase of electron density is distinct when compared with low 0 cases (-1.55). The electron temperature increases slightly with 0 but the ion temperature remains almost constant. Consequently, the poloidal beta increases, from 8% at 0 N 1.55 to 18% at 0 N 2.0, and the energy confinement time increases from 0.2 to 0.4 ms. The fluctuation level of the magnetic field corresponding to the dynamo activities is reduced by about a factor of 3 in IHTM, which may explain this improvement. Possible causes of IHTM are discussed. It is found that the m = 1 mode rotates continuously during IHTM, while it always stops rotating just before a large soft X ray emission crash occurs.

Edge plasma fluctuations are studied with inserted triple Langmuir probes and magnetic coils in the TPE-1RM20 reversed-field pinch [Yo Yagi et al.]. l\vo-point measurements show that density and potential fluctuations have relatively low mode numbers (m <3, n <40). High coherence (y=0.5) with magnetic field fluctuations and similar mode spectra suggest that density and potential fluctuations are mainly caused by electromagnetic turbulence. Broadband magnetic fluctuations are dominated by m =0, low-n modes and internally resonant m = 1 and m =2 modes. A coherent (j=20-30 kHz) m =0, low-n mode is also observed. Particle flux driven by electrostatic electric field fluctuations is 50%-100% of total flux obtained from D a line intensity measurement. Low-frequency fluctuations (j<100 kHz) give the main contribution to the total flux. Electrostatic fluctuation driven electron energy flux is only of the order of 10% of total nonradiative power loss.

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.

Paper IAEA-CN-56/C-4-3]. The dominant modes are m= l/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 a= 130-170 kHz and f4= 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 (e > 1.6). The first type is triggered by a single, wall-locked m= l/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 = l/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= l/n=7-9 modes and the appearance of large m =0/n = l-5 modes. Mode rotation continues through the event, similar to the normal machine operating condition.

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.