Transport Barrier Properties near the Separatrix of the C-2W Field-Reversed Configuration

In the C-2W Field Reversed Configuration (FRC), a closed-fieldline FRC plasma is embedded in an (open-fieldline) mirror scape-off-layer plasma. Doppler Backscattering measurements show that the statistical properties of density fluctuations change significantly near the excluded flux radius. Radially elongated streamers dominate scrape-off layer transport where E×B flow shear is low (as also confirmed by a positively skewed probability density function (pdf) of density fluctuations, and in global gyrokinetic simulations) [1]. In contrast, strong local E×B flow exhibiting Zonal Flow characteristics, and short radial fluctuation scales dominate inside the excluded flux radius (confirmed by a negatively skewed pdf). Anti-correlation of the rms density fluctuation level and the dominant fluctuating (Zonal) E×B velocity shear is observed in the barrier layer, confirming that Zonal E×B shear limits local turbulence levels. Radial transport barrier formation in both hydrogen and deuterium plasmas maintains a core plasma state characterized by very low ion-scale fluctuation levels [2]. The dependence of turbulence and barrier properties on divertor bias is explored in bias termination experiments.

[1] C. Lau et al., Nucl. Fusion 59 066018 (2019).

[2] L. Schmitz et al., Nature Comm. 7 13860 (2016).