Experimental Validation of a Filament Transport Model in Turbulent Magnetized Plasmas

In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices.

Figure 1

Tokamak geometry. A SOL filament has been represented along with the field line connecting it to the target plate (in red).

Figure 2

Size of filaments vs collisionality parameter, ${\mathrm{\Lambda }}_{\text{div}}$. Colors indicate different heating powers. Thin solid lines provided as a guide to the eye to mark the two regimes. In the inset, evolution of collisionality with edge density.

Figure 3

Filament scaling. Color code as in Fig. 2. Black dotted line indicates the regime separation, as observed in Fig. 2. Red and black lines indicate IN and SL regimes, as expressed in Eq. (4). Solid or dashed line indicates ${\tau }_i=0/{\tau }_i=3$. Blue solid line indicates the electromagnetic regime described in Ref. [12].

Figure 4

Density $e$-folding length scaling with divertor collisionality. Black, blue, and red circles represent $P_{\mathrm{IN}}$ as in Fig. 2. Purple circles correspond to seeded discharges [empty(solid) mean before (after) ${\mathrm{N}}_2$ puffing]. Light blue stars are data points from JET. Straight lines mark the two regimes as in Fig. 2.