HIR: A collisional-radiative model for hydrogen ionization in plasma with anisotropic fast ions
The paper presents the HIR (Hydrogen Ionization Rates) software package developed for calculating the level populations of atomic hydrogen and the dynamics of its ionization in hot plasma, including the case of fast ions with an anisotropic distribution function. The code is based on a steady-state collisional-radiative model that includes electron and ion impact excitation, de-excitation, ionization, charge exchange, and spontaneous radiative transitions. It is demonstrated that stepwise ionization via excited states contributes up to 20% to the total ionization rate and requires including levels up to the principal quantum number N=8. For the GDML and GOL-NB mirror trap facilities, the effects of stepwise ionization on the mean free path of fast atoms are analyzed. It is shown that collisions with different plasma species are non-additive due to stepwise ionization, leading to a reduction in the mean free path by up to 7% at 40 keV. The anisotropy of the fast ion distribution is found to have a negligible effect (less than 1%) on the ionization dynamics in the GDML plasma. The ionization cost of hydrogen atoms in the GOL-NB plasma is calculated, demonstrating a strong dependence on plasma density due to competition between collisional and radiative de-excitation. The HIR package is openly available on GitHub.