Charge exchange in ion-atom collusions relevant to fusion plasma diagnostics

Abstract

Total charge exchange and excitation cross-sections are presented for collisions between He(^2+) ions and helium atoms, the laboratory frame collision energy being in the range 16-800keV. Cross-sections from the helium ground state and the He(ls2s(^1)S) and He(ls2s(^3)S) metastable states are calculated. The calculations were performed using the semiclassical impact parameter model where wave-functions are expressed as an expansion of two-electron atomic basis states. Transfer channels included plane-wave translation factors. The transition He(^+)(n = 4) → He(^+)(n = 3) results in the emission of visible light (468.5nm) which is observed in the JET fusion device. Helium beam injection into the JET device introduces some of the atoms in initially excited states. Even a small fraction of metastable helium effects the observed 468.5nm spectrum significantly because of the very large charge exchange cross-sections. Accurate charge exchange cross-sections to the He(^+)(n = 4) states are required to analyse the spectroscopic data used in the diagnosis of the plasma. The cross-sections produced are therefore also presented in a form suitable for direct inclusion into the plasma analysis database. Where possible the current results are compared to previous calculations. Total charge exchange cross-sections from the He(ls2s(^1)S) state are in good agreement with the previous one-electron calculation. Charge exchange cross-sections from the triplet state are found to differ with those from the singlet. The results are discussed in the context of their usefulness in the diagnosis of fusion plasmas. Suggestions for further work are made.