Benchmark of neutral beam simulation codes with motional Stark effect for applications in ITER

Abstract Motional Stark effect (MSE) measurements have been useful for characterizing the magnetic field structure in several tokamaks over the past decades. The Stark splitting in the emission spectrum of injected high-energy neutrals carries information about both the magnetic field strength and direction, although only the directional information is extracted in most measurements. There are several codes capable of simulating the spectrum emitted by neutral beams with the MSE included, aiding the design and utilization of beam emission spectroscopy or MSE diagnostic systems. In this paper, we compare two of such codes, namely Simulation of Spectra by M. v. Hellermann and CASPER based on Cherab & Raysect, both expected to play an important role in the synthetic-diagnostic toolbox of ITER. The basis of the benchmark is an ITER scenario representative of the machine’s baseline performance, with emphasis on the relation between spectral components contributing to the background and the MSE spectrum itself. It was found that after carefully matching the inputs of the codes, the MSE simulations show generally good agreement, apart from a known issue regarding the sigma-to-pi line ratios. In terms of the two examined background components, bulk-ion charge exchange shows an overall good agreement, while the line radiation of the SOL region mirrors some fundamental differences in the modeling.