Ground work supporting the codes based upon the frequency fluctuation model

Abstract

The development of the frequency fluctuation model (FFM) had two strong motivations. First, there was interest to model line shapes accounting for ion dynamics and second the inclusion of higher order radiative processes in plasmas was considered important for future development. The FFM relies on the hypothesis that the emitter-plasma system behaves approximately like a pseudo-molecule embedded into a thermal bath. As a result, the pseudo-system can be considered to have internal states connected to each others by collisions with the bath. This simple starting point has been translated into a powerful renormalization process, called FFM, resulting, a few years ago, in a fast line shape code called Pim Pam Poum (PPP) and more recently into a code for the computation of radiative redistribution. We present a few of the milestones in this evolution, starting with the motivations for choosing the FFM technique. In this part of the discussion a simple three level model will be used to provide a comprehensive explanation of the links between FFM, ion dynamics and molecular dynamics simulation. Next we present new results motivated by highly accurate line shape measurements. The first example illustrates the low density domain found in tokamaks where high- n hydrogen lines are useful for density diagnostic. The second example is the hydrogen-like helium Paschen α (P α) line observed in high intensity discharges which illustrates the high density case. The P α line embodies the activity of last ten years concerning charged emitter spectroscopy and we believe this transition to be a real benchmark for line shape codes.