Fast evaluation of the Fast-Ion Dα spectroscopy measurements at the ASDEX Upgrade tokamak

At the ASDEX Upgrade tokamak, a new fast method for the evaluation of the Fast-Ion (FIDA) diagnostic has been developed. Up to now, the analysis of FIDA data required forward-modelling with the FIDASIM code, whose long calculation times restricted the analysis to single time points within a discharge. We developed a faster forward model, which makes it possible to study the evolution of the fast-ion distribution over whole discharges. The computational time to evaluate a single time point has been reduced from minutes to near real-time. The new method allows the reproduction of the FIDA spectrum using the FIDASIM weight functions approach: the spectral intensity is obtained by convolving the fast-ion distribution function with a weight function. The latter can be seen as the product between a wavelength-independent FIDA intensity function, called , and an analytic probability of emission in a determined range. The computational times are strongly reduced by adopting a look-up table (LUT), in which the functions computed by FIDASIM are stored as a function of the most relevant plasma parameters, line of sight geometry and neutral components (beam components and halo). The LUT requires the previous knowledge of the neutral densities inside the tokamak, for this purpose, a fast model has been implemented within the RABBIT code. A detailed evaluation of the new method’s approximations is provided, highlighting the discrepancies identified relative to the FIDASIM reference.