Characteristics of the TPE Reversed-Field Pinch Plasmas in Conventional and Improved Confinement Regimes

We present the characteristics and experimental scaling laws of reversed-field pinch (RFP) plasmas, which are obtained from the recently established toroidal pinch experiment (TPE) database. The database contains information for approximately 1500 discharges, which cover two decades of RFP experiments at National Institute of Advanced Industrial Science and Technology under conventional operating conditions, consistently selected from TPE-1RM (R/a = 0.5/0.09 m), TPE-1RM15(R/a = 0.70/0.135 m), TPE-1RM20 (R/a = 0.75/0.192 m), and TPE-RX (R/a = 1.72/0.45 m, the present device), where R and a are the major and minor radii of plasma, respectively. We present the results of our study on increased confinement improvement and the physics in pulsed poloidal current drive (PPCD) discharges in the TPE-RX RFP device. A comparison of the improved energy confinement time, tau(E), with the reference scaling law of the TPE database is attempted. The result shows that tau(E) in PPCD agrees well with the TPE scaling because of the strong pinch parameter dependence in the TPE scaling law of tau(E). A potential improved confinement mode in the quasi-single helicity (QSH) state is also investigated in TPE-RX regarding the operation conditions where the QSH spontaneously appears, with a typical island structure observed in soft X-ray tomography.