Kinetic simulations of electron heat flux in the scrape-off layer

Abstract

Parallel electron heat flux qe|| on open field lines is studied with the 1D2V kinetic Vlasov-Fokker-Planck (VFP) code KIPP [15–17]. In order to assess the importance of kinetic effects on the scrape-off layer heat transport in a systematic way, a series of self-similar KIPP cases with the stagnation point and one target, logical sheath condition, parabolic ion velocity profiles reaching ion sound speed at the target were run. The variable parameter was upstream electron collisionality v* (collisionality scan cases) and for each collisionality two different numerical schemes of particle source were implemented for comparison. It shows no significant difference between the results with two different schemes of particle source for any scanned collisionality. In high collisionality cases, heat flux converges to Braginskii formula as expected, while in comparatively low collisionality cases transport coefficients deviate far away from classical value due to target cut-off effect implicating that fluid model is not applicable near the target.Enhanced radiation near the target, forcing Te drop by factor 10 for low upstream v* comparable to that expected at the separatrix in the ITER baseline H-mode scenario, was required in KIPP runs. Pairs of such cases with two different ways of radiation power sink, by modeling sink (a) evenly from all velocity space, and (b) only from high energy electrons, were compared. The simulation results show almost no difference between the two cases in the steady-state, despite the presence of a strong heat flux limiting upstream and enhancement downstream.