Modeling source terms for hydrogen isotope ion–atom collisions in fluid simulations using the Boltzmann collision integral

Ion–atom interactions significantly affect ion transport in divertor regions of fusion devices. For divertor plasma simulations, neutral fluid models have been developed, offering advantages in both computational speed and favorable convergence properties. Despite this, discrepancies exist among fluid simulation codes in their treatment of source terms for ion–atom collision processes. These differences stem from the choice of atomic cross-section datasets and the specific modeling strategies adopted for hydrogen isotope ion–atom collisions. This study systematically evaluates these differences and establishes more accurate and consistent modeling approaches based on the Boltzmann collision integral (BCI). Using the BCI, the characteristics of ion transport in velocity space due to ion–atom collisions are analyzed, with particular attention to the roles of scattering angles and differential cross sections (DCSs). Furthermore, a BCI-based formulation for energy exchange is proposed, and several analytical expressions are benchmarked against direct BCI evaluations. Our findings also indicate that widely used charge exchange (CX) cross-section data may no longer reflect the latest theoretical or experimental insights. Based on this assessment, the use of the latest DCS data and BCI-consistent analytical expressions is recommended to enhance the fidelity of divertor plasma modeling.