The use of an injected neutral beam-either a dedicated diagnostic beam or the main
heating beams-to localize and enhance plasma spectroscopic measurements can be exploited
for a number of key physics issues in magnetic confinement fusion research, yielding
detailed profile information on thermal and fast ion parameters, the radial electric
field, plasma current density, and turbulent transport. The ability to make these
measurements has played a significant role in much of our recent progress in the scientific
understanding of fusion plasmas. The measurements can utilize emission from excited
state transitions either from plasma ions or from the beam atoms themselves. The primary
requirement is that the beam "probe" interacts with the plasma in a known fashion.
Advantages of active spectroscopy include high spatial resolution due to the enhanced
localization of the emission and the use of appropriate imaging optics, background
rejection through the appropriate modulation and timing of the beam and emission collection/detection
system, and the ability of the beam to populate emitter states that are either nonexistent
or too dim to utilize effectively in the case of standard or passive spectroscopy.
In addition, some active techniques offer the diagnostician unique information because
of the specific quantum physics responsible for the emission. This paper will describe
the general principles behind a successful active spectroscopic measurement, emphasize
specific techniques that facilitate the measurements and include several successful
examples of their implementation, briefly touching on some of the more important physics
results. It concludes with a few remarks about the relevance and requirements of active
spectroscopic techniques for future burning plasma experiments. (C) 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.3699235]
