Parallel heat flux and flow acceleration in open field line plasmas with magnetic trapping

Various simulations and experimental observations have suggested the importance of kinetic effects, such as particle orbital losses, the anisotropy of distribution functions, and the long mean-free-path of superathermal particles, in the tokamak edge region. The magnetic field strength modulation in a tokamak scrape-off layer (SOL) provides both flux expansion next to the divertor plates and magnetic trapping in a large portion of the SOL. In this work, the effects of magnetic trapping and a marginal collisionality on parallel heat flux and parallel flow acceleration are examined. The various transport mechanisms are captured by kinetic simulations in a simple but representative mirror-expander geometry. The observed parallel flow acceleration is interpreted and elucidated with a modified Chew-Goldberger-Low (CGL) model that retains temperature anisotropy and finite collisionality. We will also show that the use of sheath-boundary-condition in modelling tokamak SOL to be problematic since it simply prohibits the flow transition from subsonic to supersonic at the mirror throat far away from the divertor.