Experimental investigation of flute-type electrostatic turbulence

Low frequency flute-type electrostatic fluctuations are investigated experimentally in a magnetized plasma column. The fluctuations are generated by the Kelvin-Helmholtz instability due to an azimuthal E₀*B₀/B₀² velocity shear at the edge of the column. Correlation measurements demonstrate a coupling between this instability and gradient drift instabilities located at a radial position with the strongest density gradient. Cross-correlation measurements of density and potential fluctuations demonstrate that the instability saturates by shifting the phase between these two quantities to a value differing from the one predicted by a linearized analysis. Also the dynamic properties of the turbulence were investigated by perturbing the time stationary turbulent fluctuations by an external signal. The turbulent fluctuations give rise to an anomalous plasma flux across the magnetic field lines. This flux is determined by measurements of density and velocity correlations, where the randomly varying velocity is identified with the fluctuating E*B₀/B₀² velocity. An explicit value for the effective bulk plasma diffusion coefficient is obtained from the measurements. The variations in turbulent plasma flux due to an externally injected signal were investigated. The turbulent diffusion may actually decrease in a transient time period.