Speaker
Description
We investigate tearing modes driven by current density gradient in collisionless tokamak plasmas by using the electromagnetic gyrokinetic simulation code ORB5. Two aspects of the dynamics of magnetic island due to the tearing mode, its width and rotation, are studied by simulations for flat and finite-gradient profiles of density and temperature. The evolution of the width (rotation) is elucidated by simulations for flat (finite-gradient) profiles because of small (large) rotation-speed in the absence (presence) of background diamagnetic effects. For flat profiles,
the initial saturation width of nonlinearly driven magnetic islands is related to the linear growth rate of the tearing mode; however, large islands in the initial saturation phase are prone to current density redistribution that reduces the island width in the following evolution. In addition, island-induced E × B and diamagnetic sheared flows develop
at the separatrix, able to destabilize the Kelvin-Helmholtz instability (KHI). The KHI driven turbulence enhances a strong quadrupole vortex flow that reinforces the island decay, resulting in a strong reduction of the island width in an eventual steady state. This strong reduction of the island width is enhanced by trapped electrons. For finite gradients
profile, the tearing mode rotates usually in the electron diamagnetic direction, but can change direction when the ion temperature gradient dominates the other gradients. The reduced growth of the tearing mode by diamagnetic effects results in a moderate island size, which remains almost unchanged after the initial saturation. At steady state,
strong zonal flows are non-linearly excited and dominate the island rotation, as expected from previous theoretical and numerical studies. When β is increased, the tearing mode is suppressed and a mode with the same helicity but with twisting parity, coupled with the neighboring poloidal harmonics, is destabilized, similar to the kinetic ballooning
mode.
