Study of nonlinear energy transfer between drift wave turbulence and spontaneously generated sheared flows in a laboratory plasma

Experiments in a laboratory plasma are used to identify how small-scale turbulent structures give rise to large-scale sheared zonal flows. A new technique based on cross-bispectral analysis has been developed and applied to directly measure the nonlinear energy transfer rates between drift wave turbulence and sheared flows. In addition fast imaging is used to directly observe the turbulent structure dynamics. A combined study using both Langmiur probe arrays and fast visible light imaging shows that the sheared zonal flow is sustained by the emission of drift vortices in the central plasma which then propagate in a spiral trajectory, approach the shear layer, and then merge into the sheared flow, thereby transferring their momentum and kinetic energy to it. The shear flow is then amplified. The results are consistent with previous probe measurements of the turbulent Reynolds stress, and provide a detailed confirmation of the basic theoretical expectations for the turbulent drive of zonal flows in magnetized plasmas.