Fast ion confinement in negative triangularity plasmas on the TCV tokamak

Abstract Negative triangularity (NT) plasma configurations have been extensively studied in the Tokamak á Configuration Variable (TCV), and have displayed enhanced heat confinement for thermal electrons and ions. Herein, the question of fast-ion confinement is addressed. Limited NT and positive triangularity (PT) plasmas are compared. The plasma volume and position are kept the same between the discharges to match the neutral beam heating deposition, isolating the effect of triangularity. These plasmas were MHD-quiescent with quasi-stationary conditions, minimising additional MHD-induced transport. The upgraded TCV Fast Ion Loss Detector (FILD) and the fast neutron detector were used as the main diagnostics to probe the lost and confined fast-ion population. The FILD signal was higher for NT, suggesting higher fast-ion losses. The neutron production was, however, higher in the NT case, suggesting a higher confined fast-ion population. Modelling was able to reproduce the main experimental observations well, using neoclassical transport alone. The slowing-down fast-ion content was computed to be higher in the NT case, with fast-ion orbit-following indicating that this shape changed the relative number of fast-ion losses arriving at the FILD, while the total number of fast-ion losses to the wall remained similar. The discrepancy in the confined fast-ion population was mainly attributed to the charge-exchange fast-ion losses, which are significantly higher for the PT case.