Benchmark of a new multi-ion-species collision operator for $\delta f$ Monte Carlo neoclassical simulation

Abstract A numerical method to implement a linearized Coulomb collision operator in the two-weight δ f Monte Carlo method for multi-ion-species neoclassical transport simulation is developed. The conservation properties and the self-adjoint property of the operator in the collisions between two particle species with different temperatures are verified. The linearized operator in a δ f Monte Carlo code is benchmarked with other two kinetic simulations, a δ f continuum gyrokinetic code with the same linearized collision operator and a full-f PIC code with Nanbu collision operator. The benchmark simulations of the equilibration process of plasma flow and temperature fluctuation among several particle species show very good agreement between δ f Monte Carlo code and the other two codes. An error in the H-theorem in the two-weight δ f Monte Carlo method is found, which is caused by the weight spreading phenomenon inherent in the two-weight δ f method. It is demonstrated that the weight averaging method serves to restoring the H-theorem without causing side effect.