Diagnosis of bound electron density by measuring energy loss of proton beam in a partially ionized plasma target

Abstract

Partially ionized plasma contains the bound electrons aspect which have an impact on the instability of the plasma, among other things. The evolution process of bound electron density cannot be obtained from the popular method applying for diagnosis of the free electron density aspect, in which the existing optical diagnostics fail. In this paper, we introduce a high-precision experiment: the energy loss of a 100 keV proton beam penetrating through the partially ionized hydrogen plasma target was measured based on the platform of ion beam - plasma interaction at the Institute of Modern Physics, Chinese Academy of Sciences. The bound electron density is figured out according to the energy loss model of Bethe theory, the free electron density information obtained by laser interferometry diagnosis and the electron tempercture measureed by spectrum (T_e=0.68 eV; n_fe =2.41×10¹⁷ cm^-2). It is found that the bound electron density decreases with the plasma lifetime. The diagnosis of bound electron density by measuring energy loss of ion beam has the advantages of on-line, in-situ and high resolution, and provides a new way to solve the problem of the bound electron density in partially ionized plasma. A COMSOL simulation reveals that the high-temperature free electrons quickly spray out of the plasma area through a mechanical diaphragm, thus the total number of free electrons decreases. In order to maintain a relatively high degree of ionization in this plasma, in principle, more and more bound electrons are ionized to be free electrons, correspondingly the density of bound electrons decreases. The simulation result agrees well with our experimental data. Based on this finding, more detailed plasma target parameter is provied, which is helpful to deepen the understanding of the interaction process between ion beam and plasma. In future, more research work about lowly energy highly charged ions-plasma interaction will be introduced.