Laboratory Measurements Of Charge Exchange Spectra From Sulfur Ions
Résumé
X-ray emission from charge exchange (CX) collisions of solar wind heavy ions with interstellar neutrals (H, He) penetrating the solar system significantly contributes to the diffuse soft X-ray background in the 1/4 keV band (from 0.1 to 0.3 keV). Projectile solar wind ions include L-shell ions of sulfur, i.e. fluorine-like S7+ through carbon-like S10+. At present, there is no laboratory tested model available for interpreting the CX contribution to the 1/4 keV energy band. As a first step towards understanding these spectra we have used the Lawrence Livermore National Laboratory’s Electron beam ion trap (EBIT-I) and the NASA/GSFC EBIT Calorimeter Spectrometer (ECS) to measure high resolution X-ray spectra from sulfur ions produced by direct excitation (DE) and CX with SF6 molecules. The CX spectra show clear enhancement of transitions with high principal quantum numbers (n > 4) compared to the DE spectra where the n = 3 to n = 2 transitions are dominant. We discuss the results and the perspectives of extending this study to other highly charged ions (e.g. Fe, Mg, Si) and neutral targets (atomic H and He) as well as their impact on solar system SWCX studies. Part of this work was prepared by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.