Scaling laws for electron kinetic effects in tokamak scrape-off layer plasmas

Abstract

Tokamak edge (scrape-off layer) plasmas can exhibit non-local transport in the direction parallel to the magnetic field due to steep temperature gradients. This effect along with its consequences has been explored at equilibrium for a range of conditions, from sheath-limited to detached, using the 1D kinetic electron code SOL-KiT, where the electrons are treated kinetically and compared to a self-consistent fluid model. Line-averaged suppression of the kinetic heat flux (compared to Spitzer-Härm) of up to 48% is observed, contrasting with up to 57% enhancement of the sheath heat transmission coefficient, γe. Simple scaling laws in terms of basic SOL parameters for both effects are presented. By implementing these scalings as corrections to the fluid model, we find good agreement with the kinetic model for target electron temperatures. It is found that the strongest kinetic effects in γe are observed at low-intermediate collisionalities, and tend to increase (keeping upstream collisionality fixed) at increasing upstream densities and temperatures. On the other hand, the heat flux suppression is found to increase monotonically as upstream collisionality decreases. The conditions simulated include regimes relevant to current and future tokamaks.

Multi-machine benchmark of the self-consistent 1D scrape-off layer model DIV1D from stagnation point to target with SOLPS-ITER

“…The difference in upstream ion and electron temperatures is typical for low collision edge plasmas and expected to be present in reactor scale devices [32]. Interesting for reactors could then also be the impact of kinetic effects on SOL solutions [33]. However, we expect the omission of ion temperatures to have limited impact on the foreseen use of DIV1D for optimization and control.…”