Transition from Zonal Flows to Streamer like structures and associated edge Fluctuations

Abstract

We report experimental observations of a controlled transition from a zonal-flow (ZF) dominated regime to a coexistence regime of ZFs and streamers, and finally to a streamer-dominated state in a linear magnetized plasma column. The controlling parameter is the ion-neutral collision frequency. At low collisionality (2 x 10^-5 mbar), the plasma turbulence is dominated by coherent ZFs (600-700 Hz) that are nonlinearly driven by drift-wave fluctuations. With increasing collisionality (5 x 10^-4 mbar), the ZF growth is reduced and streamers emerge through nonlinear coupling of neighboring drift modes mediated by a mediator mode. At high collisionality (2 x 10^-3 mbar), ZFs are strongly damped and the turbulence becomes streamer-dominated. For each of these turbulent states, the corresponding edge fluctuations transition from coherent, symmetric to intermittent, asymmetric fluctuations with enhanced low-frequency content and larger spatial scales that can result in convective transport. Our results demonstrate the possibility of selective excitation of ZFs and streamers by regulating their collisional damping and establish the ion-neutral collision frequency as an effective control knob for regulating turbulent structures and edge transport in magnetized plasmas.