Improved confinement in a high pinch parameter region of the reversed field pinch plasma

Discharges with improved plasma confinement in a high pinch parameter region (Theta similar to 2) have been discovered in the TPE-1RM20 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 Theta cases (similar to 1.55). The electron temperature increases slightly with Theta but the ion temperature remains almost constant. Consequently, the poloidal beta increases, from 8% at Theta similar to 1.55 to 18% at Theta similar to 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.