Formation of spiral structures of turbulence driven by a strong rotation in magnetically cylindrical plasmas

Abstract

A three-dimensional turbulence simulation is performed in order to understand the role of spiral structures observed in Kelvin-Helmholtz turbulence. The simulation is performed by introducing a vorticity source to drive the plasma rotation. By scanning the intensity of the vorticity source, a quasi-periodic formation of a spiral structure is obtained above a certain source intensity. The quasi-periodic oscillation can be seen in the fluctuation energy and the background, which exhibits a limit cycle oscillation. We clarify the roles of the spiral formation in the limit cycle oscillation: The dynamical change in the radial variation of the phase of the fluctuations causes a strong coupling of the fluctuations with the background through the transport of particles and momentum. The formation mechanism of the spiral structure is also studied. An instability due to the combination of the cylindrical effect and the flow inhomogeneity is consistent with the fluctuation which drives the spiral structure obtained in the simulation.