A multiscale hybrid Maxwellian-Monte-Carlo Coulomb collision algorithm for particle simulations

Coulomb collisions in particle simulations for weakly coupled plasmas are modeled by the Landau-Fokker-Planck equation, which is typically solved by Monte-Carlo (MC) methods. One of the main disadvantages of MC is the timestep accuracy constraint t << 1 to resolve the collision frequency. The constraint becomes extremely stringent for self-collisions in the presence of high charge state species and for inter-species collisions with large mass disparities (such as present in Inertial Confinement Fusion hohlraums), rendering long-time-scale simulations prohibitively expensive or impractical. To overcome these difficulties, we explore a hybrid Maxwellian-MC (HMMC) model for particle simulations. Specifically, we devise a collisional algorithm that describes weakly collisional species with particles, and highly collisional species and fluid components with Maxwellians. We employ the Lemons method for particle-Maxwellian collisions, enhanced with a more careful treatment of low-relative-speed particles, and a five-moment model for Maxwellian-Maxwellian collisions. Particle-particle binary collisions are dealt with classic Takizuka-Abe MC, which we extend to accommodate arbitrary particle weights to deal with large density disparities without compromising conservation properties. HMMC is strictly conservative and significantly outperforms standard MC methods in situations with large mass disparities among species or large charge states, demonstrating orders of magnitude improvement in computational efficiency. We will substantiate the accuracy and performance of the proposed method with several examples of varying complexity, including both zero-dimensional relaxation and one-dimensional transport problems, the latter using a hybrid kinetic-ion/fluid-electron model.