Reduction of ITG turbulent transport by zonal flows in helical systems

Full text: From the viewpoint of improving plasma confinement, it is very important to investigate effects of magnetic configuration on turbulence-driven zonal flows. In this work, gyrokinetic theory and simulation results are presented to show how the helical geometry and the ExB rotation affect zonal flows and ion temperature gradient (ITG) turbulent transport. Larger zonal-flow generation and turbulent transport reduction are predicted by the zonal-flow response theory, and confirmed by the gyrokinetic ITG turbulence simulation for the neoclassically optimized helical configurations which have ripple-trapped particles with slow radial drift velocities. Moreover, collisionless long-time evolutions of zonal flows are investigated for helical systems having the neoclassically-driven ExB rotation, which is distinguished from the microscopic sheared ExB zonal flows. For this purpose, all classes of particles in passing, toroidally-trapped, and helical-ripple-trapped states are considered. Besides, the transitions between the toroidally-trapped and helical-ripple-trapped states are taken into account while solving the gyrokinetic equation analytically by taking its average along the particle orbits. It is shown that, when the radial displacements of helical-ripple-trapped particles are reduced either by neoclassical optimization of the helical geometry lowering the radial drift or by strengthening the radial electric field Er to boost the poloidal rotation, enhanced zonal-flow responses are obtained. Under the identical conditions on the magnitude of Er and the magnetic geometry, using ions with a heavier mass gives rise to a higher zonal-flow response, and therefore the turbulent transport is expected to show a more favorable ion-mass dependence than the conventional gyro-Bohm scaling. (author)