Development of the megahertz planar laser-induced fluorescence diagnostic for plasma turbulence visualization Rev.When the neutral density increases sufficiently, plasma confined by a magnetic dipole field exhibits a transition to a high density, quasisteady state with complex turbulent behaviors. Experiments using the collisionless terrella experiment ͓B. Levitt, D. Maslovsky, and M. Mauel, Phys. Plasmas 9, 2507 ͑2002͔͒ used statistical tools and fast imaging to understand this turbulent state with respect to both global and local paradigms. Globally, the whole-plasma dynamics are observed using a unique high-speed imaging diagnostic that views the time-varying spatial structure of the polar current density. The biorthogonal decomposition for multiple space-time points is used to decompose the measured plasma dynamics into spatial and temporal mode functions. The dominant modes are long wavelength and radially broad with amplitudes and phases that are chaotic. The potential fluctuations are also found to be dominated by low azimuthal mode numbers. Locally, multipoint and multiple-time bispectral quantities are computed and used to estimate the linear dispersion and nonlinear structure coupling of a broadband of interacting fluctuations. The spectral power transfer is found to be from small to large scale in an inverse energy cascade. The energy spectrum displays a k −3 power law consistent with the enstrophy cascade in two-dimensional turbulence. In all cases, the fluctuations appear interchangelike and consistent with two-dimensional electrostatic interchange mixing. To the best of our knowledge, this is the first time when both local and global dynamics of turbulent interchange structures have been simultaneously measured in a strongly magnetized plasma.

Turbulence in plasma confined by a magnetic dipole is dominated by interchange fluctuations with complex dynamics and short spatial coherence. We report the first use of local current-collection feedback to modify, amplify, and suppress these fluctuations. The spatial extent of turbulence regulation is limited to a correlation length near the collector. Changing the gain and phase of collection results in power either extracted from or injected into the turbulence. The measured plasma response shows some agreement with calculations of the linear response of global interchange-like MHD and entropy modes to current-collection feedback. V C 2015 AIP Publishing LLC.

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