Improved confinement regimes by the control of reversal of toroidal magnetic field in the TPE-RX reversed-field pinch plasmas

Improved plasma performances have been achieved in a reversed field pinch (RFP) experiment on toroidal pinch experiment-RX (TPE-RX, major/minor radii = 1.72 m/0.45 m, plasma current < 0.5 MA) by the pulsed poloidal current drive (PPCD) and the quasi-single helicity (QSH) state, which were obtained by controlling the reversal of a toroidal magnetic field. A steady high-density state has been obtained during the PPCD period with a single ice pellet injection, which indicates the marked (up to ten fold) increase in the particle confinement time (τp) in PPCD compared with that in the case of ordinary shots. Plasma electron temperature increases during the PPCD period, and poloidal beta increases at least three fold higher than that in the case of an ordinary shot. It has been also found that better plasma performance is observed in the QSH state than in the multi helicity (MH) state, both of which alternatively appear in a single shot under particular operating conditions. Meanwhile, it has been demonstrated for the first time that the QSH state can be obtained with an almost 100% probability by applying a small positive pulse to the reversed negative toroidal magnetic field. The result of the high-power (∼ up to 2 MW) NBI experiment in which the NBI System has been connected to the torus and whose injection experiment has just been started, will be presented. Magnetized plasma flow (MPF) injection has been for the first time demonstrated in RFP plasma. MPF injection shows increases in the electron density and in the toroidal flux, and the reductions in the density pump-out at the start-up phase and in the one-turn toroidal voltage at the flat-top phase are also observed. (author)