Study of impurity C transport and plasma rotation in negative triangularity on the TCV tokamak

Carbon impurity transport is studied in the TCV tokamak using a charge exchange recombination diagnostic. TCVs flexible shaping capabilities were exploited to extend previous impurity transport studies to negative triangularity (delta < 0). A practical way of studying light impurity transport (like C, TCVs main impurity species due to graphite tiled walls) is to investigate the correlations between the impurity ion gradients that, in this study, highlighted significant differences between positive (PT) and negative delta (NT) plasma configurations. delta scans ( - 0.6 < delta < + 0.6 ) were performed in limited configurations, but displayed little correlation between C temperature, rotation and density gradients for positive delta. This stiff response for delta > 0 changes for negative delta, where the evolution of del(vtor) was accompanied by variations of del(nC) over a range of negative delta, showing that transport, in NT, is affected by velocity gradients. Similar delta scans were performed with additional NBH (Neutral Beam Heating), with power steps ranging from 0.25 MW to 1.25 MW, highlighting increased momentum confinement in negative delta. Finally, the evolution of intrinsic plasma toroidal rotation across linear to saturated ohmic confinement regime (LOC/SOC) transitions was explored at delta < 0, expanding previous studies performed in TCV for delta > 0 (Bagnato et al 2023 Nucl. Fusion 63 056006). Toroidal rotation reversal was not observed for delta < 0, despite clear LOC/SOC transitions, confirming that these two phenomena occur concomitantly only in a restricted number of cases and under specific conditions.