Ion orbit losses and tokamak improved confinement in negative and positive triangularity

Ion orbit losses through the plasma edge are a pervasive, but often neglected, component of tokamak edge confinement. Orbit losses, especially from near a plasma boundary X-point, are known to be be able to drive a near-edge annulus of negative radial electric field E_r. Losses are enhanced at higher temperature T_i. Strong radial shear of the ExB velocity reduces turbulent transport and allows a strong edge pressure gradient to form. Negative and positive triangularity (NT and PT) have very different edge ion orbits. PT maximizes particle trapping (mirroring reversal of the parallel velocity), leading to more and wider banana orbits, while NT minimizes mirroring. In some NT shapes, the orbit effects of strong NT at a single D-corner can strongly suppress H-mode onset for quite small geometric changes, when added to the other NT properties. PT plasmas with lower ``sub-H-mode'' levels of improved confinement tend to have reduced ion trapping and banana width; their weaker E_r and E_r shear make it hard to reach full H-mode. In nonaxisymmetriy, ion orbits in some stellarator helical fields could enhance X-point losses and maybe related to the widely observed density pumpout seen in tokamaks with small nonaxisymmetric magnetic perturbations.