| The interaction between the energetic particle beam and material is an important research field of nuclear technology.The research on application and basic physics is very important,and it also occupies a very important position in the development of new interdisciplinary subjects such as nuclear medicine and nuclear agriculture.Charge exchange is one of the important research directions,which can greatly promote the development of applied technologies such as nuclear fusion and material analysis.Graphite has very attractive properties,which can be used as the first wall material of the nuclear fusion tokamak device,as a moderator and structural material in the reactor,and can also replace cesium wrapped metal as negative ion enhancement material in the plasma to promote the long-term controllable regulation of negative ion sources.The charge transfer between particles and highly oriented pyrolytic graphite(HOPG)is mainly concentrated on the hydrogen atom of single electron,which is the model system of charge exchange research.However,the observed H-fraction in the experiment is relatively high,which is inconsistent with the prediction of the traditional resonant charge transfer model based on free electron gas approximation.Generally,in small angles and low energy collisions(<5 keV),only the formation of negative ions is observed,but no positive ions are observed.With the increase of incident energy,it can be predicted that the yield of positive ions will become more and more prominent.However,there is a lack of research on the charge exchange between multi-electron ions and HOPG at more than ten keV energy region and in-depth discussion on the competition mechanism between positive and negative ions.In this work,the experimental study on the formation of negative ions by C-,O-,and F-ions in grazing scattering on HOPG surfaces at 8.5–22.5 keV energy was carried out.The scattering angle was fixed to 8°,and the variation curves of negative-ion fraction with incident velocity(energy)and exit angle were measured.The three kinds of negative-ion fraction increased with the increase of incident velocity(energy)and decreased with the increase of exit angle.Among them,the F-fraction is the highest,and the C-fraction is the smallest,indicating that the three negative-ion fractions are consistent with the order of their electron affinity energy,respectively.The velocity(energy)dependence of F-ion fraction does not agree with the second type of parallel velocity effect,which is inconsistent with the theoretical prediction.We calculated the electron density of states of HOPG,and considered that the pseudo-band gap of HOPG had an important influence on charge exchange.The surface‘effective high work function'transformed the two types of parallel velocity effects,which was in good agreement with the experimental results.On the other hand,the angle dependence of negative ion formation is unexpected,which is different from the experimental results of most low-energy(keV)ion beams scattering from metal and semiconductor surfaces.This may be due to the interaction of surface‘effective high work function'and close collisions to form positive ions,which leads to the dominant electron capture of ions in the exit trajectory,that is,the larger the exit angle,the shorter the interaction time,and the lower the probability of negative ion formation.This work clarified the important influence of positive ions on the formation of negative ions,and qualitatively and reasonably explained the experimental results of negative-ion angle dependence.This work greatly improves the understanding of charge exchange process in more than ten keV energy region. |
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