Turbulence-Driven Zonal Flow Dynamics in Gyrofluid Simulations

M. A. Beer and G. W. Hammett

APS DPP meeting, (1998).

We investigate the dynamics of small-scale turbulence-driven sheared ExB flows in nonlinear gyrofluid simulations. The importance of these zonal flows in the regulation of the turbulence was shown in our early simulations and has been widely confirmed. Most of these flows experience fast collisionless linear damping, but there is a residual non-Maxwellian component of the flow which is undamped and scales with r/R [1]. In our original treatment, we included collisionless damping terms which capture the fast collisionless damping of the damped components, but which do not accommodate the linearly undamped components. Gyrofluid and particle simulations show similar scalings with r/R, at least far from marginal stability, suggesting that nonlinear damping (turbulent viscosity) keeps the residual flows from growing to large amplitudes. However, closer to marginal stability particle simulations [2] have seen flows rise to large levels and shut off the turbulence. It is possible that the system has multiple equilibria, leading to bifurcations or intermittent behavior dependent on initial conditions. We continue our investigation of this issue with a combination of fluid and kinetic treatments, including modifications to the gyrofluid equations which attempt to retain the residual flow.

1. Rosenbluth, M. N., Hinton, F. L., Phys. Rev. Lett. 80 724 (1998).

2. Dimits, A. M., et al., Sherwood Fusion Theory Conference (1998).