Sheared-flow induced confinement transition in a linear magnetized plasma

S Zhou,T A Carter,W W Heidbrink,R Mcwilliams,D Schaffner,S Vincena,B Friedman,H Boehmer

doi:10.1063/1.3677361

Abstract

A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential ({delta}n/n{approx}e{delta}{phi}/kT{sub e}{approx}0.5) are observed at the plasma edge, accompanied by a large density gradient (L{sub n}={nabla}lnn{sup -1}{approx}2cm) and shearing rate ({gamma}{approx}300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (V{sub bias}) on the obstacle and the axial magnetic field (B{sub z}) strength. In cases with low V{sub bias} and large B{sub z}, improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by ExB drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller B{sub z}, large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m=1 and large radial correlation length dominates themore » outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.« less

Highlights

Modification of the plasma edge turbulence and turbulent transport has attracted continuous interest since the discovery of the high-confinement mode (H-mode), where a strong and sudden change in plasma characteristics, leading to improved energy confinement, is observed
The plasma edge turbulence and the associated turbulent transport are modified in three different ways: (a) alter the bias voltage Vbias that is applied on the annular obstacle (0 V–250 V), (b) change the axial magnetic field strength Bz (600 G–1800 G), and (c) switch the plasma species
A previous study on the modification of the Large Plasma Device11 (LAPD) edge turbulence by biasing the vacuum vessel8 shows that the amplitude and shearing rate of the edge flow are crucial parameters in the confinement transition

Summary

Introduction

Modification of the plasma edge turbulence and turbulent transport has attracted continuous interest since the discovery of the high-confinement mode (H-mode), where a strong and sudden change in plasma characteristics, leading to improved energy confinement, is observed. Many experiments and theories ascribed the improved confinement to the formation of the steady sheared flows, which can stabilize plasma turbulence and form a transport barrier. The sheared flow in plasmas can be spontaneously generated by nonlinear interaction of the edge turbulence (zonal flows), or induced by externally applied radial electric fields (E Â B drift). It was first demonstrated in the Continuous Current Tokamak (CCT) experiments that a sharp transport barrier, accompanied by an H-mode like state, is triggered by applied radial electric fields. Many experiments have used external bias to study modification of the plasma turbulence and transport.

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