Effect of the superthermal electrons on the heat flux through a magnetized sheath

Abstract

A one-dimensional model, in which the ionization and collision are omitted in the Debye sheath region, is applied to investigate the effect of the superthermal electrons on the heat flux through a magnetized sheath. For different temperatures and concentrations of the superthermal electron, and magnitudes and directions of the magnetic field, the profiles of plasma parameters including ion density and flow velocity perpendicular to the wall, the background and superthermal electron densities, and sheath potential in the presheath region are calculated. The variation of the plasma density and sheath potential drop at the Debye sheath entrance with the superthermal electrons and magnetic field modifies the particle and heat fluxes across the Debye sheath to the material surface. The sheath heat transmission coefficient can increase significantly even for a very small superthermal electron population. The dependence of the sheath heat transmission coefficient on the magnetic field angle decreases with the contribution of the superthermal electron in a strong magnetized sheath. When investigation of the heat flux including the superthermal electrons to a water-cooled W/Cu monoblack for the tokamak divertor, compared to the case of without superthermal electrons, it is found that the increase in both heat flux to the material surface and surface temperature of the material is mainly due to the enhancement of the sheath potential drop caused by the superthermal electrons, but the increase in the latter is not as pronounced as the former.