Observation and identification of zonal flows in a basic plasma physics experiment

Abstract

The role of self-generated zonal flows (ZF) in transport regulation in magnetic confinement devices via its shear is a potent concept and a physics issue [P. H. Diamond, S-I Itoh, K. Itoh, and T. S. Hahm, Plasma Phys. Controlled Fusion 47, R35 (2005)]. A basic physics experimental study of ZF associated with ITG (ion temperature gradient) drift modes has been performed in the Columbia Linear Machine [R. Scarmozzino, A. K. Sen, and G. A. Navratil, Phys. Rev. Lett. 57, 1729 (1986)]. The difficult problem of detection of ZF has been solved via a novel diagnostic using the paradigm of FM (frequency modulation) in radio transmission. Using this and discrete short time Fourier transform, a power spectrum peak was found at an ITG (“carrier”) frequency of ∼120kHz and FM sidebands at a frequency of ∼2kHz. ZF was definitively identified with azimuthal (poloidal) and axial (toroidal) symmetry (k_θ≈0,k_‖≈0) and radially inhomogeneous (k_r≠0) flow structures in cylindrical plasmas. However, the stabilizing effect of ZF shear appears to be small and no significant isotopic effects are observed.