Experimental observation of hysteresis of magnetic island dynamics during change of poloidal flow in a helical plasma

摘要

1. Introduction: Magnetic islands play key roles in toroidal plasma confinement from the viewpoint of MHD stability. In Tokamaks, for example, a seed island triggers, a neoclassical tearing mode, and its growth leads to serious deterioration of the confinement. On the other hand, in a helical device, magnetic islands intrinsically disappear as they are stabilized during a plasma discharge under certain conditions [1, 2] The nonlinear growth or suppression of the magnetic island during a discharge has been observed in the Large Helical Device (LHD) [1]. The resonant magnetic perturbation (RMP) coils make a vacuum magnetic island with m/n = 1/1 (here, m/n is poloidal/toroidal Fourier mode number) structure. Both magnetic island growth and suppression is seen with the two disparate plasma responses distinguished by a sharp boundary in the parameter space defined by the plasma P and collisionality at the rational surface(Fig.1). Generally, at low beta (0) and high collisionality (v), the plasma tends to make the island grow in width. However at sufficiently high p and/or low v, the plasma abruptly changes to a configuration with no island. Recently, the causal relations between the island dynamics and the plasma rotation attract attention experimentally [3] and/or theoretically [4, 5]., Experimental results showing that the poloidal rotation ω_(pol) increases prior to the island suppression regardless of its- direction have been. observed in LHD and TJ-II [3]. Theoretical studies have addressed this problem, in which the viscous torque and electromagnetic torque have key roles to explain the above phenomena in helical plasmas. The purpose of this study is to clarify the dynamics of the poloidal rotation during the island transition and its hysteresis.