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

Abstract

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.

Turbulent fluctuations during pellet injection into a dipole confined plasma torus

“…The fluctuations have a power spectrum that is dominated at low-frequencies by global modes with long wavelengths and decay at higher frequencies with a power-law. 27 Local turbulent transport is intermittent, consisting of inward and outward moving plasma filaments that are field-aligned and can move radially at speeds reaching 10% of the sound speed. 28 The PDF of the radial particle transport is non-Gaussian with high kurtosis and skewness.…”

“…The fluctuations observed in the large LDX experiment resemble the fluctuations observed in a smaller magnetic dipole experiment where the spatial and temporal structures of the fluctuations are characterized as chaotically rotating flute-like modes. 27,28 While not on-topic to the investigations reported in this paper, the largest low-frequency mode is axisymmetric (m ¼ 0) with a frequency near 100 Hz. This global mode is described in Ref.…”

“…Bounceaveraged drift-resonance couples energy between particles and waves and is essential to understanding radial transport of energetic electrons. 53 Nonlinear turbulent cascade couples turbulent energy from smaller to larger scales, 27 and the linear growth rate cannot be used to determine the saturated turbulent spectrum. Although the measured broad-band driftresonant fluctuations justify a quasilinear representation of transport, 19,21 a fully self-consistent model of interchange and entropy mode driven transport requires nonlinear simulations.…”

“…27,44,48 Before and after pellet injection, when long time records of turbulence are measured, the ensemble power spectrum, ensemble cross coherence, and ensemble cross phase can be accurately calculated using standard methods (e.g., Refs. 27 and 48).…”

“…With a central heating source, the central plasma pressure increases and becomes unstable when the pressure profile gradients, @p=@w, exceed the usual MHD condition, 6 Àd ln p=d ln dV $ c ¼ 5=3. Broadband electrostatic interchange turbulence develops consisting of many global modes with chaotic amplitudes and phases 27 that cause intermittent transport. 28 Although the energy flux is always outward, theory and simulation show that the radial particle and temperature flux can be either inward or outward depending upon the ratio of temperature to density gradient scale lengths, g d ln T e =d ln n. When g ¼ ðc À 1Þ ¼ 2=3, the MHD condition of marginal stability to pressure-driven interchange motion coincides with the "stationary" profile, where both @ðndVÞ=@w % @ðpdV c Þ=@w % 0.…”