Transport regulation and energy transfer dynamics during zonal flow formation

Abstract The dynamical mechanism by which zonal flows (ZFs) regulate turbulent transport is experimentally investigated on the basis of the density-potential coupling and energy transfer functions. It is shown that the occurrence of intermittent ZF events deteriorates the temporal anti-correlation between the turbulent particle flux <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">Γ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> and Reynolds stress <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">ℜ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> , causing both to decrease simultaneously despite their opposing dependence on the density-potential coupling inferred from the cross-phase <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> . A two-stage mechanism in the ZF regulation of Γ is identified: Γ is initially governed by changes in α nφ and then maintained by decreasing cross-amplitude as the phase recovers. The transition between these stages is associated with an idle period of kinetic energy transfer, during which the shear flow is shown to be powered by the transfer of energy from intermediate density scales. Hence, the transport suppression mechanism appears to shift from a cross-phase driven reduction of Γ to a turbulence regulation regime where energy is redistributed from drift-wave scales to the macroscopic potential.