Successful FIDA diagnosis of fast ions from negative-ion neutral beams near their injection energy*

Although charge-exchange cross sections peak at relatively low energies (<100 keV), judiciously chosen sightlines that reduce the relative velocity between the injected neutrals and the fast ions can measure high-energy ions. New sightlines that use a negative-ion neutral-beam injection (NNBI) source as the active beam were installed at the Large Helical Device (LHD). Fast-ion D-alpha (FIDA) spectra of dominantly NNB-injected plasmas collected during the 22^nd LHD campaign resemble theoretical predictions. Data from the views are compared with standard views that use positive-ion neutral beams as the active source. The comparison confirms that diagnostic exploitation of the relative-velocity dependence provides a means to measure high-energy ions produce by the NNBI. Data during the hydrogen and deuterium operations are also compared, as well as the dependence of the signals on electron density.