On structure formation and the transition to turbulence in a magnetized plasma column.

摘要

We present experimental results from an Argon plasma column that makes a controlled transition to a turbulent state as the axial magnetic field ( B) strength is increased. For intermediate values of B (300–500 G) coherent waves extending radially through the plasma column are observed. These are identified as Kelvin-Helmholtz (KH) modes driven by a velocity shear layer. At 450G we observe the onset of fluctuation energy which is localized to the radial density gradient. These fluctuations are identified as arising from the resistive drift wave (DW) instability. From measurements of the bicoherence we infer that three-wave interactions lead to the transfer of energy between triads of both types of modes. Consequently, the spectral range of fluctuation activity in general is broadened, in both density and potential. A temporally-coherent but spatially extended (‘quasi-coherent’ or QCM) mode develops between 600 and 800 G. We hypothesize that is a vortex chain resulting from the nonlinear evolution of the combined KH and DW instabilities. This coherent structure dominates the nonlinear interactions with the other spectral components and significantly reduces the outward radial transport. At a critical magnetic field (∼800 G) this coherent structure breaks up and the radial fluctuation-induced transport accordingly increases. This outward transport is apparently quenched by an azimuthally symmetric shear flow near r ∼ 4cm. The equilibrium component of the observed shear flow is a structure that is consistent with theoretical predictions of plasma self-organization (Hasegawa & Wakatani 1987). This shear also contains a significant large-scale fluctuating component with small azimuthal wavenumber. The evolution of the total energy spectrum E(k), measured for the first time in a laboratory experiment, is consistent with a transfer of part of the fluctuation energy to large spatial scales. In fact the final plasma state is considered to be consistent with many aspects of turbulence and self-organization in 2D fluids generally, and of collisional drift turbulence in particular.