A Linear, Exponential-Discontinuous Scheme for Discrete-Ordinates Calculations in Slab Geometry

Presented here is a preliminary study of a strictly linear, discontinuous-Petrov-Galerkin scheme for the discrete-ordinates method in slab geometry. By ``linear'', we mean the discretization does not depend on the solution itself as is the case in classical ``fix-up'' schemes and other nonlinear schemes that have been explored to maintain positive solutions with improved accuracy. By discontinuous, we mean the angular flux $\psi$ and scalar flux $\phi$ are piecewise continuous functions that may exhibit discontinuities at cell boundaries. Finally, by ``Petrov-Galerkin,'' we mean a finite-element scheme in which the ``trial'' and ``test'' functions differ. In particular, we find that a trial basis consisting of a constant and exponential function that exactly represents the step-characteristic solution with a constant and linear test basis produces a scheme (1) with slightly better local errors than the linear-discontinuous (LD) scheme (for thin cells), (2) accuracy that approaches the linear-characteristic (LC) scheme (when the LC solution is positive), and (3) is positive as long as the first two source Legendre moments satisfy $|s_1| < 3 s_0$.