Effects of magnetic island on profile formation in flux-driven ITG turbulence

Abstract

Full-f gyrokinetic simulations of ion temperature gradient (ITG) turbulence in the presence of a magnetic island are performed. A newly developed method for evaluating the flux-surface average is implemented to treat adiabatic electrons inside the magnetic island precisely. A neoclassical simulation below the threshold for linear ITG instability shows that the density profile does not relax at the O-point, although the ion temperature profile is flattened there. This results from the force balance in the direction of the magnetic field between the pressure gradient related to ion parallel motion and the mean radial electric field. A flux-driven ITG turbulence simulation shows a quasi-periodic transport reduction due to interaction between the background temperature profile and the vortex mode, which is a nonlinearly generated mesoscale structure with the same mode numbers as the magnetic island. These results indicate that not only the parallel streaming but also the equilibrium electric field and turbulence contribute significantly to profile formation around a magnetic island.

Gyrokinetic simulation of magnetic-island-induced electric potential vortex mode

“…Such a large difference in the description of magnetic surfaces inside islands may cause zonal flow inaccuracy in ITG turbulence simulations, which plays an important role in ITG turbulence saturation and transport suppression. Besides, note that the averaged bootstrap current on the perturbed magnetic surface inside islands has a significant influence on the growth of magnetic islands [19]. Thus, this result may also be a valuable reference when studying the growth of neoclassical tearing modes (NTMs).…”