Target polarization effect on energy loss of O5+ ions near the Bohr velocity in low density hydrogen plasma

Energy loss of ions near the Bohr velocity in plasma is one of the important topics in intense heavy ions beam driven high energy density physics and inertial confinement fusion. Based on the ions-plasmas interaction experimental platform at HIRFL, this paper presents the new experimental energy loss results of 1.07 MeV (~66.9 keV/u) O5+ ions penetrating through a low-density partially ionized hydrogen plasma target (radio frequency plasma). A decrease of energy loss with free electron density increase is found, which is very different from our previous understandings. The new experimental results are discussed by considering the theoretical models which involves the charge screening of projectiles in the partially ionized plasma and the target polarization effect - Barkas correction term. In the charge screening aspect, the comparison between the momentum transfer under the Coulomb potential and the Debye potential is given, but due to the low ionization degree, the plasma screening effect seems not the main reason of the decrease of energy loss. In the target polarization effect aspect, in the Bohr velocity regime, the Barkas correction term can play a key role in the ion-atom collisions. Modeling the Barkas correction term on the proposed classical energy loss formula, the experimental data of ions in the gas target can be well fitted by the calculated value. In the partially ionized plasma, the frequent thermal electrons collisions can induce the atomic excitation of plasma target, correspondingly the Barkas correction term changes, where it decreases with the fraction of excited atoms increasing. As a result, the decrease of energy loss happens in our experiment. In the stopping of highly charged ions in a partially ionized low-density plasma, the collisions of ions and free electrons can produce an enhanced energy loss according to the previous studies, however, the target polarization effect, especially the atomic excitations, can significantly decrease the energy loss, which have been observed in our experiment. Therefore, the investigation of ions – partially ionized plasmas interaction should be further made and the Barkas correction could be a very important term.