In this work, we investigate the influence of return-current electrons generated by two-stream instability on the average charge state of low-energy proton beams propagating through partially ionized hydrogen plasmas. A kinetic model is developed by coupling a parameterized beam distribution, a thermal-electron component, and a drifted return-current-electron component into the relative collision-rate formalism. The resulting relative speed distributions are then introduced into a multi-channel charge-exchange rate equation system that includes ionization, excitation, de-excitation, electron capture, radiative recombination, and spontaneous decay among the hydrogen states with principal quantum numbers n ≤ 3. This framework makes it possible to evaluate how collective effects modify microscopic atomic rates and, in turn, alter the equilibrium charge-state population of the beam.
The calculations show that, when two-stream instability becomes important, the ion-electron relative speed distribution is strongly reconstructed by the emergence of return-current electrons. Compared with the conventional thermal-electron model, the return-current component shifts a significant fraction of ion-electron collisions toward lower relative energies. As a result, part of the electron-impact ionization channel is suppressed because the corresponding collision energies fall below or closer to the ionization threshold, In contrast, the electron-capture rate increases only slightly, and this increase mainly comes from the broadening of the beam-ion velocity distribution rather than from the return-current-electron drift itself. This imbalance reduces the average charge state of the transmitted proton beam.
For typical plasma conditions with ionization degree 50%, electron temperature T_e = 10 eV, and electron density n_e = 1×10¹³ cm^-3, the inclusion of two-stream-instability-induced return-current electrons lowers the average charge state by about 11.9% at 30 keV and by about 2.2% at 100 keV. The effect weakens with increasing beam energy because ionization progressively dominates over capture in the higher-energy range. Calculations with varying beam density further show a two-stage reduction behavior: the charge-state decrease first grows rapidly with increasing n_b/n_e, and then becomes slower after the return-current-electron contribution approaches saturation. In comparison, the broadening of the beam-ion velocity distribution alone produces a much smaller correction to the total charge-exchange balance.
These results indicate that, for intense low-energy ion beams in plasmas, collective effects cannot be treated as a correction to transport only at the macroscopic level. Instead, they also reshape the microscopic collision-rate distribution and thereby modify the charge-state evolution itself. Since the stopping power scales approximately with the square of the projectile charge state, even a modest reduction in average charge state can lead to a noticeable change in energy deposition. The present study therefore provides a physically transparent framework for incorporating non-equilibrium electron distributions into charge-state calculations and offers useful guidance for ion-beam transport modeling in fusion and highenergy-density plasma environments.