Space-time discretization of series expansion methods for the Boltzmann transport equation

The approximate solution of the Boltzmann transport equation via Galerkin-type series expansion methods leads to a system of first order differential equations in space and time for the expansion coefficients. This system is extremely stiff close to the fluid dynamical regime (for small Knudsen numbers), and exhibits a mildly dispersive behavior, due to the acceleration of waves by the external force (the electric field). In this paper a class of difference methods is presented and analyzed which represent a generalization of the well-known Scharfetter-Gummel exponential fitting approach for the drift-diffusion equations. It is shown that, by using appropriate operator splitting methods for the time discretization, one obtains stability properties which are only mildly dependent on the Knudsen number and essentially independent of the size of the electric field.