Method for Modeling Free Molecular Flow in the Presence of Particle Sources Inside the Computational Domain

A method for modeling free molecular flow in the presence of particle sources in the calculation domain is proposed. It can be used to model processes in the ionization chamber of a radio-frequency ion thruster or an ion source using molecular propellants that dissociate in a discharge. The method can also be used to calculate the parameters of the neutral plasma component in the ionization chamber of a radio-frequency ion thruster operating on ambient atmospheric gases. The approach is based on the boundary element method. The mathematical model does not take into account various excited states of atoms and molecules. It is assumed that the calculation domain contains neutral atoms and molecules, atomic and molecular ions of the propellant, and electrons. The propellant is fed into the ionization chamber in molecular form, and atoms are formed through interactions of the molecules with electrons in the discharge. It is shown that the presence of particle sources in the computational domain can be used as a boundary condition determining the particle flux incident on each element. An approximate method for estimating the effect of ionization and dissociation on neutral particle flows in the computational domain is described. Using the developed method, it is possible to obtain distributions of the concentrations of atoms and molecules arising from dissociation in radio-frequency low-pressure gas discharges. The method is verified by comparing the results of a test calculation of a nitrogen-fed RIT-10 thruster with published results of experimental studies. In most of the studied parameter range, the discrepancies between the calculated and experimental results were less than 20%.