Experimental Observation of Zonal Flow and Its Scalings in Axisymmetric Magnetic Field

Abstract

The anomalous radial transport generated by drift wave turbulence is a fundamental open physics question in magnetic confinement systems, both in modern tokamaks and current and next generation mirror machines. The role of self-generated zonal flows (ZF) in transport regulation via its shear is a potent concept and a physics issue. ZF are believed to be spontaneously excited by drift wave turbulence via Reynolds stress from small-scale fluctuations to large-scale flow.

A basic physics experimental study of zonal flows associated with ITG (ion temperature gradient) drift modes has been performed in the Columbia Linear Machine (CLM). The difficult problem of detection of ZF has been solved via a novel diagnostic using the paradigm of FM (frequency modulation) in radio transmission. We find a power spectrum peak at ITG (‘carrier’) frequency of ~120 kHz and FM sidebands at frequency of ~2 kHz. We have definitively identified ZF with azimuthal and axial symmetry (k_θ = 0, k_// ≈ 0) and radially inhomogeneous (k_r [not equal] 0) flow structures in cylindrical plasmas in uniform axisymmetric magnetic field. However, quantitatively, the stabilizing effect of ZF shear appears to be small and no significant isotopic effects are observed. The unique complementary roles of ion acoustic damping and ZF shearing in the saturation of ITG have been experimentally demonstrated using stabilizing and destabilizing feedback techniques. Theoretically ZF is supposed to be saturated via ν_ii. As this is very small both in tokamaks and CLM, we investigate the scaling ZF with in ν_in which can be significant in CLM.