Active fast ion charge exchange measurements using a neutral particle analyzer and multiple beam species in C-2W

In order to measure the fast ion using neutral particle analyzers (NPAs) in the low neutral density core region of a magnetic confinement fusion device, active change exchange measurements are often performed using a neutral beam (NB) as a charge-exchange (CX) target. One of the complications with this approach is that an NB injected as a CX target can also contribute to the total fast ion source. C-2W has a unique solution to this difficulty in that it is equipped with both eight NB injectors, which can inject beams of different particle species, and an electro-magnetic NPA (EM-NPA), which can measure multiple ion species simultaneously. This enables the active and passive fast ion CX components to be clearly distinguished. The decrease in amplitude of the CX spectra when a hydrogen NB is terminated was clearly observed by the EM-NPA in both hydrogen and deuterium channels. This reduction of observed fast ion flux was mainly caused by the diminished fast ion source, not crosstalk or a general reduction in fast ion confinement. As an example application of this technique on C-2W, fast ion behavior during a periodic density drop is explored. The large difference between the active and passive CX components of the EM-NPA signals clearly demonstrates the usefulness of the active fast ion CX measurement.