Experimental characterization of broadband electrostatic noise due to plasma compression
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For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to inhomogeneous flows over a very broad frequency range. Such sheared flows are generated in the Earth's magnetosphere and intensify during active periods. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length (L-E) compared to the ion gyroradius (rho(i)) determines the character of the shear-driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column, a condition similar to that found in the natural boundary layers. The continuous variation of rho(i)/L-E and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. Broadband wave emission correlated to increasing/decreasing stress (i.e., rho(i)/L-E), a characteristic signature of a boundary layer crossing, is found under controlled and repeatable conditions. This result holds out the promise for understanding the cause and effect of the in situ observation of broadband electrostatic noise.