Topology bifurcation of a magnetic flux surface in magnetized plasmas

Abstract

Bifurcation phenomena in the radial propagation of a heat pulse driven by modulated electron cyclotron heating are observed in the Large Helical Device. There are two patterns of heat pulse propagation observed in the flat temperature region. One is a bi-directional slow heat pulse propagation observed in the fast magnetic shear drop and the other is a fast heat pulse propagation observed in the slow magnetic shear drop. This bifurcation in the heat pulse propagation suggests the topology bifurcation of a magnetic flux surface in the plasma.The transition from regular to chaotic motion is very often represented as a topological bifurcation of a torus in phase space [1]. Islands are formed in nested tori by resonant perturbations, and if global stochasticity sets in, turbulence-driven transport (such as superdiffusion) can occur [2]. This topological bifurcation has been considered to happen in hightemperature toroidal plasmas, e.g. during disruption in tokamaks [3,4], by a large deformation such as a spontaneous transition from the stochastic state to helical equilibrium in reversed-field pinch (RFP) plasmas [5][6][7], or at the plasma edge as seen in the edge-localized-modes mitigation

Impact of 3D magnetic field structure on boundary and divertor plasmas in stellarator/heliotron devices

“…The 3D effects have been intensely investigated so far in various devices and these investigations have made significant progress in the understanding of transport features [7,8,28,30,31,32,33,34,35]. The modifications of transport are caused by the stochasticity of field lines or by the 3D deformation of magnetic flux tubes [36,37]. The outermost region is called the laminar zone [38] or edge surface layers [1], where parallel correlation along field lines still persists to some extent, i.e., K L (Kolmogorov length) > C L (connection length to divertor plates).…”

Recent ECRH/ECCD experiments aiming for higher density and temperature operations in the LHD

“…At the bottom of the temperature drop (3.7 s), i/2p=1/2 is realized with gentle gradient (weak magnetic shear) at the core region and the flattening of the electron temperature around the e-ITB is observed as shown in Fig.10 (c)-1. It was reported that the stochastization can occur in the core region when the weak magnetic shear is realized near i/2p=1/2 and causes the flattening of the density and temperature profiles [19][20][21][22]. It was also reported that the required ECRH power to form and sustain the e-ITB is higher when the stochastic region exists in the core region of the target plasma [15].…”

Plasma Physics Researches in Asia Pacific Region

“…By comparing with detailed model calculations it was shown that the generation of the seed island could be attributed to toroidal coupling effects leading to the generation of 2/1 perturbation from a 1/1 central mode or that of 3/2 from a 2/2 harmonic of the central mode. A novel technique to experimentally distinguish the topological distinctions between a magnetic island, a partially stochastic island and a fully stochastic region was reported in [22]. The technique rested on the basic idea of propagating a heat pulse (using a modulated ECH source) and measuring the propagation speed of the pulse as it traversed the radial extent of the plasma.…”