Experimental validation of universal plasma blob formation mechanism

Abstract

Anomalous plasma transport in the boundary region of a tokamak plasma is commonly associated with the formation and evolution of coherent density structures known as blobs. Recently, a theory for a universal mechanism of plasma blob formation has been put forward. It is based on a breaking process of a radially elongated streamer due to poloidal and radial velocity shears. The theory is well supported by two-dimensional and three-dimensional numerical simulation results but lacks experimental validation. In this work, we report the first ever experimental validation of this universal criterion by testing it against NSTX data on blobs obtained using the gas-puff imaging (GPI) diagnostic. It is found that the criterion is widely satisfied in most L-mode discharges and may explain the significantly larger number of blob events. We also validate the theoretical criterion against ADITYA Langmuir probe data taken in the scrape-off layer region.

The Chimera Revisited: Wall- and Magnetically-Bounded Turbulent Flows

“…A theory on blob creation based on the breakup of streamers due to velocity shear was experimentally validated in [124]. These streamers are located outside the separatrix, so in that sense, they are different from the streamers previously mentioned.…”

Fascinating physics at the edge of magnetic fusion devices

“…Such 'detachment' of the blobs could be explained by different mechanisms (e.g. see [6,7,30] and the references therein). However, the gas puffing imaging (GPI) diagnostic shows the presence of isolated density 'bumps', which may later propagate outward.…”

Impact of resonant magnetic perturbation on blob motion and structure using a gas puff imaging diagnostic on the HL-2A tokamak

“…Simulation results predicted that larger-scale blobs can maintain a stable structure for longer [29]. However, the blob birth process still remains an open issue [32].…”