Zonal flow screening in negative triangularity tokamaks

Negative triangularity (NT) discharges exhibit reduced fluctuations and improved confinement, as compared to positive triangularity (PT). Zonal flow shear de-correlates the turbulence eddies. This, in turn, reduces the radial transport and improves confinement. So zonal flow is a natural 'suspect' responsible for improved confinement in NT. Neoclassical screening sets the strength of zonal flow shear for fixed drive and damping. Here, we compare zonal flow screening in PT and NT. Orbit calculations show that the radial excursions of trapped particles are reduced in NT, as compared to PT. Yet surprisingly, the neoclassical dielectric susceptibility actually increases with decreasing triangularity, such that the susceptibility is higher for NT than for PT. This is because the reduction in neoclassical polarization by shrinking the banana width is offset by the increase in neoclassical polarization by the enhancement of trapped fraction for NT. As a result, the zonal flow screening length is actually enhanced for NT, as compared to PT. Hence, the zonal flow residual is smaller for NT than for PT. Results from gyrokinetic simulations support the analytic calculations. This suggests that core transport improvement in NT cannot be attributed to simple elements of zonal flow physics.