Under the assumption of quasi-neutrality (i.e. charge density difference between ions and electrons is neglected), the flow of plasmas is often modeled by the ideal MHD equations. Although, the ideal MHD equations have been successfully employed in modeling and simulating plasma flows, this model is derived by ignoring the Hall effect and treating plasma flows as single fluid flows. These effects are very important for many applications, e.g. space plasmas, Hall current thrusters, field reversal configurations for magnetic plasma confinement and for fast magnetic reconnection.

Two-fluid ideal plasma equations are a generalized form of the ideal MHD equations in which electrons and ions are considered as separate species. The design of efficient numerical schemes for the these equations is complicated on account of their non-linear nature and the presence of stiff source terms, especially for high charge to mass ratios and for low Larmor radii. In this work, we design entropy stable finite difference schemes for the two-fluid equations by combining entropy conservative fluxes and suitable numerical diffusion operators. Furthermore, to overcome the time step restrictions imposed by the stiff source terms, we devise time-stepping routines based on implicit-explicit (IMEX)-Runge Kutta (RK) schemes. The special structure of the two-fluid plasma equations is exploited to design IMEX schemes in which only local (in each cell) linear equations need to be solved at each time step. Benchmark numerical experiments are presented to illustrate the robustness and accuracy of these schemes.