Beams, brightness, and background: Using active spectroscopy techniques for precision measurements in fusion plasma research

Abstract

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. V C 2012 American Institute of Physics. [http://dx.

X-ray imaging crystal spectroscopy for use in plasma transport research

“…In order to penetrate to the core, beams must be of order 1 MeV, much greater than the peak in the charge-exchange cross section for low-Z impurities. 34 A lower energy diagnostic neutral beam is planned for ITER, but core measurements will be limited by photon statistics, and the survivability of the plasma facing optics is a concern. In contrast, the plasma facing optic in ITER's planned crystal spectrometer 35 will be much farther removed from the plasma, reducing neutron flux and risk of plasma surface interactions, and by properly choosing the impurity species, the signal can be weighted to the core.…”

Energetic ion transport by microturbulence is insignificant in tokamaks

“…Later work 79 reviewed NBI (E b ¼ 1 MeV) in ITER and determined that expected NBCD profile modification was minimal. It is worth noting that work considered fractional beam energy components (E b =2 and E b =3) that arise in standard neutral beams from molecular deuterium, 80 but MeV beams in ITER will use negative neutral sources 81 that produce only the full energy component. The inclusion of lower energy beam ions should tend to increase the modeled effect of turbulence-induced diffusion.…”

Indirect measurement of poloidal rotation using inboard–outboard asymmetry of toroidal rotation and comparison with neoclassical predictions

“…Diagnostics relying on active charge exchange (CX) recombination [10,11] have the benefit of localized measurements, but, for instance, the energy dependence of the CX cross sections and the dependence of the rotationfree wavelength on plasma parameters can give rise to apparent velocity contributions that are difficult to compensate even with extensive modelling and/or costly diagnostic configurations [8,12].…”