Modification of turbulent transport with continuous variation of flow shear in the Large Plasma Device

TL;DR

This study demonstrates how continuous variation of flow shear in the Large Plasma Device can suppress turbulence and transport, revealing the relationship between shear rate and turbulence amplitude, with implications for plasma confinement.

Contribution

It introduces a method for continuous control of flow shear in LAPD and systematically studies its effect on turbulence and transport, providing new insights into shear suppression mechanisms.

Findings

Turbulent particle flux is suppressed at shear rates comparable to autocorrelation rate.

Flow shear can be reversed, affecting turbulence and confinement.

Suppression of turbulence correlates with amplitude reduction in low-frequency fluctuations.

Abstract

Continuous control over azimuthal flow and shear in the edge of the Large Plasma Device (LAPD) has been achieved using a biasable limiter which has allowed a careful study of the effect of flow shear on pressure-gradient-driven turbulence and transport in LAPD. LAPD rotates spontaneously in the ion diamagnetic direction (IDD); positive limiter bias first reduces, then minimizes (producing a near-zero shear state), and finally reverses the flow into the electron diamagnetic direction (EDD). Degradation of particle confinement is observed in the minimum shearing state and reduction in turbulent particle flux is observed with increasing shearing in both flow directions. Near-complete suppression of turbulent particle flux is observed for shearing rates comparable to the turbulent autocorrelation rate measured in the minimum shear state. Turbulent flux suppression is dominated by amplitude reduction in low-frequency ($<10$kHz) density fluctuations. An increase in fluctuations for the highest shearing states is observed with the emergence of a coherent mode which does not lead to net particle transport. The variations of density fluctuations are fit well with power-laws and compare favorably to simple models of shear suppression of transport.