Observation of coherent self-organized drift structures in a turbulent DC discharge

Abstract

In this study it is demonstrated that low-hequency turbulent spectra of a magnetized DC discharge consist of flute-type and drift waves. The existence of waves showing several characteristics of solitons or vortices are made plausible through the properties of propagation and amplitude conditions for self-binding. Self-organized phase-locked waves appear as well as spontanous transitions to coherent structures at velocities complementary to those of periodic linear modes. Spectra obtained from fluctuations of density and potential only agree for components belonging to periodic drift waves and slow drift soliton-like perturbations.In the regime of turbulence wave coupling occurs, whereas in the windows of order the coherent waves show no coupling with the remaining spectral contributions.The scenario of transition to turbulence is such that, at small magnetic inductions B, waves of short wavelengths establish a regime of order. With increasing B , broad spectra occur with modes of short and long wavelengths. Within this regime of turbulence windows of ordered soliton-like behaviour reappear. The present study extends a previous publication on forced harmonics and multimode spectra of drift waves by Kauschke and Schliiter.

Dynamics of particle flux in a cylindrical magnetized plasma

“…This structure was characterized by a spiral shape and had the main features of a drift wave instability (f = 1.2 kHz, m = 1, n = 1 with n the axial mode number). Such large structures have also been observed in other devices [33,34], where they have been compared with solitons, as they are similarly formed by different wave components that are phase-locked by their non-linear interactions. In the LMD-U an additional m = 0, f = 0.4 kHz mode was observed featuring large potential fluctuations and a finite radial wavelength and was called a zonal perturbation.…”

Transition to subcritical turbulence in a tokamak plasma

“…in figure 4( a ) there are two), counting them systematically is a not entirely trivial problem, which is often encountered in computer-vision and pattern-recognition applications. It has been considered before in the context of experimental measurements of turbulence (Kauschke 1999, Müller et al. 2005; see review of various techniques by Love & Kamath 2007).…”

“…While 2D structures are easily discerned by a human eye (e.g., in figure 4(a), there are two), counting them systematically is a not entirely trivial problem, which is often encountered in computer-vision and patternrecognition applications. It has been considered before in the context of experimental measurements of turbulence (Kauschke 1999;Müller et al 2005; see review of various techniques by Love & Kamath 2007). Structure counting can be reduced to an imagelabelling, or segmentation, problem by applying a threshold function to our density fluctuations: setting values below a certain percentile (here 75%) of the maximum amplitude to 0 and above it to 1.…”