Theory of plasma blob formation and its numerical and experimental validations

Plasma ‘blobs’, which are localized enhanced density structures, have been widely observed in the turbulent edge and scrape-off layer (SOL) of several tokamaks. They are believed to contribute to the anomalous transport observed in these regions and hence have been studied extensively in recent years. While a great deal of theoretical and experimental work has been devoted to characterizing plasma blobs and their dynamics, the topic of mechanisms underlying the formation of blobs has received limited attention. A systematic and concerted effort has been made at the Institute for Plasma Research (IPR) to develop a generalised analytical criterion for blob formation and to test its validity against numerical simulations and experimental data. In this paper we present a consolidated account of this theoretical work including some comparisons with experimental data from TORPEX, NSTX and ADITYA devices.