Beyond BOLSIG+: Monte Carlo simulation of electron and ion swarms to obtain transport and rate coefficients for plasma modeling

It is common practice in low-temperature plasma modeling to calculate the electron transport and reaction rate coefficients from electron-neutral cross-section data by means of an electron Boltzmann equation solver, based on some approximate form of the kinetic theory of charged particle swarms. This paper explores the validity of these calculations and introduces MCIG, a Monte Carlo simulation code that is being released as a complement to BOLSIG+, a popular Boltzmann solver. MCIG provides rigorous reference values of the transport coefficients and rate coefficients under swarm conditions, without invoking the approximations used by most Boltzmann solvers, such as two-term Legendre expansion of the velocity distribution function. It can handle ions as well as electrons and allows for constant and oscillating electric and magnetic fields, pulsed and steady-state swarm configurations, non-zero gas temperature, superelastic collisions, and anisotropic angular scattering. The physical and methodological principles of MCIG are described in detail, including original techniques to handle non-conservative collision processes and obtain statistical error bars for all results. Subsequently, MCIG is used to quantify the accuracy of the two-term approximation for a wide range of gases and reduced electric fields, to illustrate differences between alternative definitions of the transport coefficients used in swarm data analysis and to explore the possible effects of anisotropy of scattering for electrons and ions. The Appendix gives analytical expressions of the transport coefficients used as a verification benchmarks.