Global and local characterization of turbulent and chaotic structures in a dipole-confined plasma

Abstract

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 (2002)] 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. © 2009 American Institute of Physics.