Multiscale interaction between a large scale magnetic island and small scale turbulence

Abstract

Multiscale interaction between the magnetic island and turbulence has been demonstrated through simultaneous two-dimensional measurements of turbulence and temperature and flow profiles. The magnetic island and turbulence mutually interact via the coupling between the electron temperature (T e ) gradient, the T e turbulence, and the poloidal flow. The T e gradient altered by the magnetic island is peaked outside and flattened inside the island. The T e turbulence can appear in the increased T e gradient regions. The combined effects of the T e gradient and the the poloidal flow shear determine two-dimensional distribution of the T e turbulence. When the reversed poloidal flow forms, it can maintain the steepest T e gradient and the magnetic island acts more like a electron heat transport barrier. Interestingly, when the T e gradient, the T e turbulence, and the flow shear increase beyond critical levels, the magnetic island turns into a fast electron heat transport channel, which directly leads to the minor disruption. PACS numbers:

Global gyrokinetic simulation of microturbulence with kinetic electrons in the presence of magnetic island in tokamak

“…It has been experimentally and numerically pointed out that the large magnetic island can generate strong shear flow around it, which substantially suppress the local turbulence and play an important role in the formation of the internal transport barrier (ITB). 6,15,32 Indeed, in our simulation, we observe that the vortex mode (d/ 1,2 ) can generate clear helical shear flow around the magnetic island. We define the helical flow as n ¼ mh À nf;…”

“…4 for the island case. The turbulence intensity can be represented by the ion density varianceñ 2 i ðrÞ calculated as…”

“…Magnetic islands in toroidally confined plasmas can be generated externally by resonant magnetic perturbations (RMPs) 1,2 or internally by magnetohydrodynamic (MHD) instabilities such as tearing modes or neoclassical tearing modes. 3,4 Magnetic islands can flatten the local pressure profile and modify plasma flow, thus affecting microturbulence and neoclassical bootstrap current.…”

Leading magnetic fusion energy science into the big-and-fast data lane

“…In practice we use a short-time Fourier transformation (STFT) which averages the Fourier coefficients obtained from FFTs calculated on slightly shifted time windows. Spectral quantities calculated off local T e fluctuations, such as the cross coherence or the cross phases, are used to identify macro-scale structures, so called magnetic islands, as well as micro-scale instabilities in the plasma [Cho17]. Understanding the physics resulting in magnetic islands is important for plasma confinement, and avoiding sudden loss of plasma control, known as a disruption.…”

Bifurcation phenomena in magnetically confined toroidal plasmas

“…The coupling between the transport bifurcation and the MHD bifurcation found as a bifurcation of the magnetic island state [19] is also discussed in section 4. The interplay between MHD instability and turbulent transport is recognized to be an important topic in toroidal plasmas [20,21]. In this review article, the various bifurcation phenomena observed in the toroidal magnetically confined plasma are reviewed and the physics mechanism for the bifurcation phenomena is discussed.…”