Turbulent transport across shear layers in magnetically confined plasmas

Abstract

Shear layers modify the turbulence in diverse ways and do not only suppress it. A spatialtemporal investigation of gyrofluid simulations in comparison with experiments allows to identify further details of the transport process across shear layers. Blobs in and outside a shear layer merge, thereby exchange particles and heat and subsequently break up. Via this mechanism particles and heat are transported radially across shear layers. Turbulence spreading is the immanent mechanism behind this process.

Estimation of the plasma blob occurrence rate

“…At low densities ( Fig. 1) a process is revealed, where blobs in and outside the confined region merge and thereby transfer density into the SOL [34]. This exemplifies the picture of turbulence spreading, where turbulent structures radially transport energy by merging and breaking up.…”

“…A physical explanation for blobs preferentially generated outside the confined region can be found in Refs. [34,41]. For the following procedure the generation area is defined by the smallest possible region in which 50 % of all trajectories originate from.…”

On the phase velocity in between weak and strong plasma edge turbulence

“…Furthermore, the spectra shown in the following are in a region with a moderate amount of shearing. Therefore, structures propagating in opposite directions, as often observed in experiments [38][39][40][41], are not observed here.…”

Origin and turbulence spreading of plasma blobs

“…15 This exemplifies the picture of turbulence spreading, where turbulent structures radially transport energy by merging and breaking up. 16 Even if turbulence spreading has been mostly studied in the context of zonal flow [16][17][18] and pinch 19 physics, it can be expected to be also important for the far SOL, where the mean gradients are found to be flatten out and hence cannot generate themselves any strong events in that region.…”

“…This corresponds to the cascade-like picture of turbulence spreading 16 recently investigated at the shear layer close to the LCFS in simulations and experiments. 15 In the SOL, turbulence spreading transports the free energy from the region 7 mm outside the LCFS into the far SOL. The generation location of the turbulent intensity transport corresponds to the position, where the radial electric field exhibits its maximum ( Fig.…”