Effect of Source and Sink on Heat Transport in the SOL

Parallel electron heat flux in the SOL is determined via fully kinetic simulations using the one-dimensional particle-in-cell code PARASOL. The heat flux in fluid simulations is usually approximated in the collisional limit by the Spitzer-Harm heat flux qSH and in the collisionless limit by the one-way free-streaming heat flux qFS adjusted by the coefficient αe. Though αe is defined at the collisionless limit, in practice it is used generally and taken to be a constant ∼0.1. We survey the dependence of αe with respect to many plasma parameters: collisionality, source and sink relative position, source model, and sink radiation rate. Increasing radiation and the size of the radiation region generally increases αe. Without Langevin heating and electron radiation, αe is found tobe non-monotonic with collisionality. This effect is traced to the fact that in a collisionless plasma, the electron velocity distribution exhibits a high-energy tail, which implies that use of the one-way free-streaming heat flux is inaccurate. A new treatment for the free-streaming heat flux is proposed, wherein the symmetric bulk electron distribution is ignored, which makes αe ∼ 1 at all radiation rates for moderate and low collisionality. This method does not work for Langevin heating, which maintains a Maxwellian electron energy distribution (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)