Investigation of mode coupling in a magnetized plasma column using fast imaging

Abstract

Spectral analysis and bicoherence analysis are applied to fast camera data in order to study the nonlinear mode coupling in the low-β cylindrical magnetized plasma device Mirabelle [T. Pierre et al., Rev. Sci. Instrum. 58, 6 (1987)]. The camera provides a measurement of electron plasma density fluctuations in a complete plasma column cross-section. By extraction of virtual probe data, bicoherence in the frequency domain is obtained and compared to probe signal bicoherence. Two-dimensional camera measurements make it possible to compute the mode number bicoherence as well as the related amplitude weighted phase coherence B⋆, which conserves time resolution. Investigation of temporal evolution of this quantity and power spectra shows typical sequences where a summed normalized B⋆ peak on a specific mode precedes a gain in spectral energy. The assumption of an energy transfer is confirmed by the value of the phase of B⋆. In addition to temporal dynamics, bidimensional data make it possible to study the localization of nonlinear couplings. Highest values are detected at a radial position where most of the present modes coexist. Fast camera imaging is a powerful, nonperturbative diagnostic providing two-dimensional time-resolved measurements. An adapted utilization of these data helps to get a better insight in drift wave turbulence and coupling mechanisms.