Coherent vortical structures in two-dimensional plasma turbulence

A laboratory experiment was carried out in order to study the nonlinear saturated stage of the cross-field electrostatic Kelvin-Helmholtz instability in a strongly magnetized plasma. The presence of large vortex-like structures in a background of wide-band turbulent fluctuations was demonstrated by a conditional sampling technique. The formation of the structures was interpreted in terms of an inverse cascade, i.e. as the combined result of small scale structures generated by the instability and a subsequent coalescence of small vortices into larger ones. This interpretation was supported by numerical simulations. The importance of the large scale structures for the turbulent plasma transport across magnetic field lines was analyzed in detail.