In Search of the Elusive Zonal Flow Using Cross-Bicoherence Analysis

Abstract

We show that the modulational instability growth rate of zonal flows is determined directly from the quasilinear wave kinetic equation. We also demonstrate the relation between zonal-flow growth and the cross bispectrum of the high-frequency drift-wave-driven Reynolds stress and the low-frequency plasma potential by explicit calculation. Experimental measurements of the spatiotemporal evolution of the spectrum integrated bicoherence at the L-->H transition near the edge shear layer indicate a modification in the nonlinear phase coupling, which might be linked to the generation of sheared ExB flows.

Long-distance correlation and zonal flow structures induced by mean E×B shear flows in the biasing H-mode at TEXTOR

“…Similar ZFs have also been seen in spontaneous H-mode. [6][7][8][9]19,20 Our results appear thus to support the significance of ZFs and LDC in improved-confinement regimes.…”

“…5 Such ZFs might thus also be important ingredients in regulating turbulence and hence take part in explaining the transition from low to high ͑L-H͒ confinement regimes. [6][7][8][9] Investigations into ZFs have been intensified recently, both theoretically and experimentally as reviewed in Refs. 10 and 11.…”

A New Paradigm for 3D Collisionless Magnetic Reconnection

“…It is interesting to note that such process is reminiscent of the zonal flow creation (Drake et al, 1992) observed in tokamak in relation with the L-H transition (Diamond et al, 2000).…”

Experimental Study of Nonlinear Processes in Edge Turbulence of Toroidal Plasmas

“…For example, an interpretation of bispectral functions has been discussed in Refs. [15,22]. In a previous article [19], the bispectral analysis of the edge fluctuation data in the JFT-2M tokamak [23] (limiter configuration) has been reported.…”

“…This analysis has been applied to various plasma experiments [12][13][14][15][16][17][18][19][20] and has contributed much to the understanding of, e.g., the nonlinear interaction among drift wave turbulence, the Reynolds-stress drive for zonal flows, the onset of the high-confinement mode (H-mode) transition, and so on. In addition, the reality of the zonal flow has been confirmed experimentally [21], and the need for bispectral analysis has increased.…”