Comparative experimental study of coherent structures in a simple magnetized torus

This paper reports a comparative experimental study of large-scale coherent structures which are embedded in the turbulent fluctuations observed in a magnetized plasma device. The confining magnetic field is purely toroidal and has no rotational transform. Turbulent density fluctuations of two different plasma discharges are compared: thermionic discharge and inductive resonant wave heating. It turns out that fundamental features of spatiotemporal structures depend primarily on the plasma source and not on the magnetic field configuration. In the thermionic discharge plasma double-vortex structures and in the wave-heated plasma single-vortex structures are found. This can be attributed to the injection of mass and negative space charge in the case of thermionic discharge. The phase coherency is determined by the large-scale coherent structures. Phase shift of approximate to pi /2 between plasma potential and density structures changes in sign for the two different plasma sources.