Kinetic Landau-fluid closures of non-Maxwellian distribution plasmas

Non-Maxwellian plasmas are in fact fairly common in the laboratory, space, and astrophysical environment, and yet most of the theoretical studies so far on heat flux closure are performed under the near-Maxwellian assumption. In this work, new kinetic Landau-fluid closures are derived for non-Maxwellian distribution plasmas. A special static case (i.e., zero mode frequency) is first considered for plasmas with cutoff Maxwellian distribution. In the strongly collisional regime, this model reduces to Braginskii’s local heat flux model; while in the weak collisional regime, the heat flux becomes non-local and recovers the Hammett–Perkins model when the value of the cutoff velocity approaches infinity. Comparison of the thermal transport coefficient for Maxwellian, cutoff Maxwellian, and super-Gaussian distributions shows that the reduction of the high-speed tail particles leads to the corresponding reduction of the thermal transport coefficient across the entire range of collisionality, more reduction of the free streaming transport toward the weak collisional regime. In the collisionless limit, \chi approaches zero for the cutoff Maxwellian and the super-Gaussian distribution but remains finite for Maxwellian distribution. Interestingly, \chi is complex if the cutoff Maxwellian distribution is asymmetric, and Im(\chi) yields an additional streaming heat flux in comparison with the symmetric cutoff Maxwellian distribution. The derived Landau-fluid closures are general for fluid moment models, and applicable for the cutoff Maxwellian distribution in an open magnetic field line region, such as the scape-off-layer plasmas, the thermal quench plasmas during a tokamak disruption, and the solar corona, and the super-Gaussian electron distribution function due to inverse bremsstrahlung heating in laser-plasma studies.