The detection of fast oxygen ions in neutral beam-heated plasmas of the TJ-II stellarator using spectroscopy methods

During heating, by neutral beam injection, of magnetically confined plasmas in the TJ-II stellarator broad spectral structures, that straddle intense emission lines from multiply ionized oxygen ions, are observed by a normal-incidence spectrometer viewing a plasma region well separated from the neutral beam/plasma interaction volume. These features are seen to appear and to disappear shortly after neutral beam switch-on and switch-off, respectively. After excluding other possible origins it is demonstrated that their source is fast oxygen ions that originate as accelerated H2O and H3O ions in the ion sources of the neutral beam injectors. Next, discussions on how fast multiply ionized ions can perform multiple toroidal transits about this device, thereby giving rise to the spectral structures observed, are presented. Finally, implications of these emissions for spectral analysis and interpretation of spectra in hot magnetically confined plasmas are addressed.