Energy loss of Fe ions in He plasmas at different thermodynamic states

In this work, we analyze the thermodynamic states of the helium plasma and their influence on the stopping power calculations which are needed for obtaining the energy loss of the iron beams traversing them. The analysis is made in ranges of plasma free electron densities (10¹⁵–10¹⁹ cm⁻³) and temperatures (1–10 eV) of experiments with iron beams at 6 and 4.3 MeV/u energies. For this purpose, we use Saha-Boltzmann equations for local thermal equilibrium (LTE) and a collisional-radiative model for non-local thermal equilibrium (NLTE) in steady-state situation implemented in a computer code. For the highest temperatures and free electron densities, LTE and NLTE models provide quite similar results for the average ionization and ion abundances. When the opacity effects are taken into account in the NLTE simulations, the optically thick simulations provide fairly similar results to those of the LTE model. The plasma thermodynamic states have a direct impact on the calculation of the energy loss. The differences on the plasma stopping power between considering it in LTE or in NLTE may entail a 10% of the total stopping for the experiments analyzed in the electron density region of 10¹⁸–10¹⁹ cm⁻³. These differences can be around 27% for plasmas with smaller electron density of 10¹⁷ cm⁻³ and around 42% for plasmas with an electron density of 10¹⁵ cm⁻³. New experiments would be appreciated to be made in a future to corroborate the latest calculations.