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Akihiro Ishizawa

Publications and source records attributed to Akihiro Ishizawa.

3 recordsLinked to original sources
arXiv2025

Is Turbulence able to Generate Magnetic Islands in Tokamaks? Gyrokinetic Simulations of Turbulence-Driven Magnetic Islands in Toroidal Geometry

We report a universal mechanism for turbulence-driven magnetic islands in fusion plasmas. Using gyrokinetic simulations of a linearly stable tearing mode in a large-aspect-ratio toroidal geometry under collisionless conditions, we demonstrate that micro-instabilities generate an $\boldsymbol{E}\times\boldsymbol{B}$ flow that drives magnetic field line reconnection. This process forms multiple small-scale islands along the resonant surface, which interact nonlinearly and eventually coalesce into large-scale magnetic islands. These islands are capable of significantly flattening the equilibrium profile across the island O-points and thus acting as a seed for their further neoclassical growth. Notably, the mechanism operates independently of the parity of the destabilizing micro-instability.

physics.plasm-ph↗
arXiv2024

Linear and Nonlinear Dynamics of Self-Consistent Collisionless Tearing Modes in Toroidal Gyrokinetic Simulations

We investigate tearing modes (TM) driven by current density gradient in collisionless tokamak plasmas by using the electromagnetic gyrokinetic simulation code ORB5. We elucidate the TM width by simulations for flat profiles, as the absence of background diamagnetic flows implies a small rotation-speed, while finite-gradients are included to investigate the TM rotation. For flat profiles, the initial saturation width of nonlinearly driven magnetic islands is related to the TM linear growth rate; however, large islands in the initial saturation phase are prone to current density redistribution that reduces the island width in the following evolution. Island-induced $\boldsymbol{E}\times\boldsymbol{B}$ and diamagnetic sheared flows develop at the separatrix, able to destabilize the Kelvin-Helmholtz instability (KHI). The KHI 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 process is enhanced by trapped electrons. For finite gradients profile, the TM usually rotates in the electron diamagnetic direction, but can change direction when the ion temperature gradient dominates the other gradients. The reduced growth of the TM by diamagnetic effects results in a moderate island size, which remains almost unchanged after the initial saturation. At steady state, strong zonal flows are nonlinearly excited and dominate the island rotation, as expected from previous theoretical and numerical studies. When the plasma beta is increased, the TM 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.

physics.plasm-ph↗