Confinement improvement in a single helical state of the reversed field pinch plasma on TPE-1RM20

In high theta discharges of the reversed field pinch (RFP) plasma on TPE-1RM20, it is observed that the soft X-ray signal ramps up when the amplitude of the magnetic fluctuations concentrates into a single helical mode (poloidal mode number, m = 1, and toroidal mode number, n = 7). Even in an extreme case where the fluctuation level of this single mode becomes quite large, as large as that in ordinary cases with a factor of about five, the soft X-ray signal is still sustained at a high level. This result indicates that the single helical state can play an important role in the improvement of plasma in RFP. This assumption is confirmed by the results of the nonlinear magnetohydrodynamics (MHD) simulations of RFP plasma. The simulation shows that the single helical state can have a significant closed magnetic surface area. On the other hand, the large stochastic area of the magnetic field lines always appears in the case of the multi-helical modes of the magnetic fluctuations. The numerical results of time variation of the m=1 mode amplitudes with different n values are quite similar to those of experimental observation in a low theta discharge. (author)