The nonlinear coupling between gyroradius scale turbulence and mesoscale magnetic islands in fusion plasmas

Abstract

The interaction between small scale turbulence (of the order of the ion Larmor radius) and meso-scale magnetic islands is investigated within the gyrokinetic framework. Turbulence, driven by background temperature and density gradients, over nonlinear mode coupling, pumps energy into long wavelength modes, and can result in an electrostatic vortex mode that coincides with the magnetic island. The strength of the vortex is strongly enhanced by the modified plasma flow response connected with the change in topology, and the transport it generates can compete with the parallel motion along the perturbed magnetic field. Despite the stabilizing effect of sheared plasma flows in and around the island, the net effect of the island is a degradation of the confinement. When density and temperature gradients inside the island are below the threshold for turbulence generation, turbulent fluctuations still persist through turbulence convection and spreading. The latter mechanisms then generate a finite transport flux and, consequently, a finite pressure gradient in the island. A finite radial temperature gradient inside the island is also shown to persist due to the trapped particles, which do not move along the field around the island. In the low collisionality regime, the finite gradient in the trapped population leads to the generation of a bootstrap current, which reduces the neo-classical drive.

Interaction of turbulence with magnetic islands: effect on bootstrap current

“…[12], due to the use of the collisionless approximation, describe the limit D → ∞. It can also be seen that at sufficiently small diffusion coefficient the trapped particle distribution becomes flat.…”

“…Previous work using gyro-kinetic simulations has found that the temperature gradients through the island O-points are further modified by the action of drift-wave turbulence around and within the island [11,12,14,13]. These calculations, however, did not allow the direct calculation of the bootstrap current to determine its full effect.…”

“…[12] has revealed that the radial electron temperature gradient is resilient to the presence of the island. Closer inspection of the particle dynamics shows that the trapped and passing electron populations have differing responses.…”

“…Recent advances in computational power have enabled the first truly multi-scale selfconsistent simulations of turbulence to be performed [12,14,15]. Gyro-kinetic and fluid simulations [16,17,18,13,19] of the influence of large magnetic islands on driftwave instabilities and micro-scale turbulence have uncovered new physics.…”

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

“…These GKW simulations include electromagnetic effects on the small-scale fluctuations, but neglect the presence of the magnetic island. This implies that the estimates given above are to be understood as an upper limit on D, since inside an island the kinetic profiles are flattened and the drive for the turbulence is strongly reduced [43,44] (although this might be only partly the case for islands of about the marginal island size). These low values of the diffusion coefficient, in combination with the beam broadening discussed in the next subsection, suggest a relatively small role (again at the 10%-level) of transport effects on the determination of the CD profiles for the UL in ITER.…”