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.

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Fluctuations of magnetic fields and related magnetohydrodynamic (MHD) phenomena are investigated in the TPE-1RM15 reversed-field pinch experiment [Plasma Physics and Controlled Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 2, p. 453]. Mode analysis of fluctuations measured by multichannel coils reveals that nonlinear interactions between m=1 and m=0 modes, such as nonlinear coupling and phase locking, play significant roles during a dynamo event (i.e., the flux genertion process in the sustainment phase), resulting in transition from an unstable state to a stable state. Behaviors of these fluctuations are found to be toroidally asymmetrical due to strong nonlinearity. Study of the current ramping experiment shows that the inverse of global energy confinement time depends on the squared fluctuation level offset linearly, which is consistent with the prediction of the transport model based on the diffusion of stochastic field lines. By examining the dependence of the resistive part of the loop voltage on the fluctuation level, the input power to the electrons and ions are estimated to be about 70% and 30% of the total input power, respectively.

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