The different evolution of plasma density voids and blobs in the edge region of a simply magnetized toroidal plasma

Abstract

The analysis of the anomalous transport in magnetised plasmas has pointed out the substantial contribution of intermittent burst events corresponding to positive density fluctuations propagating outwards to the edges of the devices. Much less attention was paid to negative density fluctuation structures, which could be named depletions or voids, whose propagation could contribute to the global anomalous transport, in particular, when a non-negligible diffuse plasma background is present in magnetized plasma device edges. Here, we discuss the results of an experimental investigation aimed to study the spatio-temporal evolution of density depletion structures propagating inwards in a magnetized plasma column simply magnetized toroidal device. Evidence of this behavior was gathered by a conditional sampling analysis applied to time series of plasma fluctuations. We observe that a significant fraction of void structures, after being ejected outside the main plasma column, gets pulled inwards and disappears by merging at the column edge. On the other hand, a smaller fraction as well as most of the positive density structures, after ejection, ends up advected towards the edges of the device and gets lost from the main plasma column. These behaviors testify the richness and complexity of density structure evolution in magnetized plasma turbulence.