Dependence of the L-Mode scrape-off layer power fall-off length on the upper triangularity in TCV

Abstract

This paper reports on experimental observations on TCV with a scan in upper triangularity , including negative triangularity, focusing on the power fall-off length in L-Mode. The upper triangularity is scanned from −0.28 to 0.47. Smaller is measured at the outer divertor target for decreasing together with higher edge temperature leading to increased confinement. This effect is observed for both magnetic drift directions for discharges in deuterium and helium. In helium larger values are observed compared to deuterium. The power fall-off length at the inner divertor target has a non-monotonic behaviour with changing triangularity. The largest values are around . The ratio increases for decreasing for positive triangularity and is approximately constant for negative triangularity. is compared to available scaling laws. Partial agreement is only observed for a scaling law containing a proxy for at ASDEX Upgrade (Sieglin 2016 Plasma Phys. Control. Fusion 58 055015). Extending this scaling to TCV and using at suggests that is independent of machine size . Possible explanations for smaller for decreasing is a reduction in turbulence or a direct effect of increasing .