Target polarization effect on energy loss of O⁵⁺ ions near 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 ionbeam driven high energy density physics and inertial confinement fusion. Based on the ions-plasma interactionexperimental platform at HIRFL, this work shows the new experimental energy loss results of 1.07 MeV(similar to 66.9 keV/u) O5+ ions penetrating through a low-density partially ionized hydrogen plasma target (radiofrequency plasma). The decrease of energy loss with free electron density increasing is found, which is verydifferent from our previous result. The new experimental results are discussed by considering the theoreticalmodels which involves the charge screening of projectiles in the partially ionized plasma and the targetpolarization effect-Barkas correction term. For the charge screening , the comparison between the momentumtransfer under the Coulomb potential and that under the Debye potential is given, but due to the low ionizationdegree, the plasma screening effect seems not to be the main reason for the decrease of energy loss. For thetarget polarization effect , in the Bohr velocity regime, the Barkas correction term can play a key role in theion-atom collisions. Modeling the Barkas correction term based on the proposed classical energy loss formula,the experimental data of ions in the gas target can be well fitted by the calculated values. In the partiallyionized plasma, the frequent thermal electron collisions can give rise to the atomic excitation of plasma target,correspondingly the Barkas correction term changes: it decreases with the fraction of excited atoms increasing.As a result, the energy loss decreases in our experiment. In the stopping of highly charged ions in a partiallyionized low-density plasma, the collisions between ions and free electrons can produce an enhanced energy lossaccording to previous studies. However, the target polarization effect, especially the atomic excitations, cansignificantly reduce the energy loss, which is observed in our experiment. Therefore, the interaction betweenions and partially ionized plasma should be further studied, and the Barkas correction can be a very importantterm.