The use of atomic and molecular data in fusion plasma diagnostics

Considerable attention in modern large fusion experiments is focused on neutral beam penetrated plasma and on divertor plasma. In the former, ion/atom reactions drive the population dynamics while in the latter, although electron processes predominate, there are significant ion/atom and atom/atom influences. Neutral atoms in beams, impurities in beam penetrated plasma and complex partially ionized impurities in divertor plasma find themselves in the collisional radiative regime where the simple stationary excitation/cascade picture of the coronal model is invalidated by secondary collisions. Thus atomic data needed in such fusion applications are not a few reaction cross-sections but large complete sets which support full collisional-radiative models. Also, experiment analysis does not make use directly of fundamental atomic data but rather effective coefficients deduced from such models. In the paper, we describe two cases. The first is neutral deuterium beam attenuation and beam emission and its linking to active diagnostic spectroscopy using beams. We describe how the diagnostic analysis and experimental data reduction are carried out efficiently using derived effective stopping and effective emission coefficients. Then we describe the organization of the fundamental ion/atom data and how collisional-radiative modelling generates the effective coefficients from them. This is an area in which long-term, intensive collaborative effort on both the fundamental data and the detailed spectral reduction is now bearing fruit. The second case is the more general task of modelling ionization state, power and impurity line emission for interpretation of divertor observations. We explain the matching of effective collisional-radiative coefficients to the divertor plasma models and of photon emission coefficients to the post-processing for particular spectrometers and lines-of-sight. Then, we illustrate how just one part of the substantial fundamental atomic data requirement--dielectronic recombination--must be prepared for the application. Finally, the generalized collisional-radiative model, which prepares the effective coefficients, is described and we explain how it gives proper attention to the dynamic nature and non-negligible density of the divertor plasma and why resolution of metastable states is a requirement. The Atomic Data and Analysis Structure, ADAS and the JET Joint Undertaking Experiment are used in illustration