Katsumi Ida (NIFS)


National Institute for Fusion Science


Magnetic island is a closed magnetic flux surface bounded by a separatrix, isolating it from the rest of the space with nested magnetic flux surface. The separatrix is called X-point, while the center of the magnetic island is called O-point. The magnetic island is identified by the flattening of temperature at the O-point due to a lack of heat flux not due to the enhancement of transport [1]. There are two types of bifurcation phenomena observed in magnetically confined toroidal plasma. One is a bifurcation of the magnetic topology between nested and stochastic fields [2]. The nested state is characterised by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse of electron temperature flattening. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasma during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed [3]. The other bifurcation is between the magnetic island with high turbulence level and that with low turbulence level [4]. Recently, turbulence spreading from X-point to O-point of the magnetic island has been identified in experiment [5]. The ExB flow shear, which is often observed at the boundary of magnetic island [6], is supposed to prevent this turbulence spreading. Therefore, interplay between the penetration of turbulence spreading into magnetic island and ExB flow shear at the boundary of the magnetic island causes the bifurcation of turbulence and transport states inside magnetic island [7]. In this presentation, the role of magnetic island on turbulence and transport, especially as a barrier of turbulence spreading, is discussed. The idea of turbulence spreading barrier gives a new insight to the space coupling of turbulent transport in magnetic fusion plasma.



[1] K. Ida, et al., Phys. Rev. Lett. 109 (2012) 065001

[2] K.Ida, et al., New J. Phys. 15 (2013) 013061

[3] K.Ida, et al.,  Nat. Commun. 6 (2015) 5816

[4] K.Ida, et al., Sci. Rep. 5 (2015) 16165.

[5] K. Ida, et al., Phys. Rev. Lett. 120 (2018) 245001.

[6] K. Ida, et al., Phys Rev Lett 88 (2001) 015002.

[7] K. Ida, Plasma Phys. Control. Fusion 62 (2020) 014008