GROUND WORK SUPPORTING THE CODES BASED UPON THE FREQUENCY FLUCTUATIONMODEL

The development of the frequency fluctuation model (FFM) had two stron g motivations. First, there was interest to model line shapes accounti ng fbr 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 behav es approximately like a pseudo-molecule embedded into a thermal bath. As a result, the pseudo-system can be considered to have internal stat es connected to each others by collisions with the bath. This simple s tarting point has been translated into a powerful renormalization proc ess, 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 compu tation 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 betwe en FFM, ion dynamics and molecular dynamics simulation. Next we presen t new results motivated by highly accurate line shape measurements. Th e first example illustrates the low density domain found in tokamaks w here high-n hydrogen lines are useful for density diagnostic. The seco nd example is the hydrogen-like helium Paschen alpha (P-alpha) line ob served in high intensity discharges which illustrates the high density case. The P-alpha line embodies the activity of last ten years concer ning charged emitter spectroscopy and we believe this transition to be a real benchmark for line shape codes. (C) 1997 Elsevier Science Ltd. All rights reserved.