Development of a Synthetic Diagnostic for 2D Beam Emission Spectroscopy Measurements at W7-X

Beam emission spectroscopy (BES) allows for density fluctuation measurements based on line emission at the intersection between a neutral beam and lines of sight. In particular, by using multi-channel BES systems, velocimetry techniques can be applied which allow for unique measurement capabilities to study phenomena such as Reynolds stress and zonal flows. The W7-X BES system utilizes two 4x8 avalanche photodiode (APD) arrays to measure ion-gyroscale turbulent transport. The system is predicted to be able to resolve minimum fluctuation levels of δn/n ≈ 0.5% at a 2 MHz sample rate. A synthetic diagnostic for the W7-X BES system has been developed so that GENE-3D simulation data can be directly compared with experimental BES data to validate the code in the future. The synthetic BES diagnostic utilizes point spread functions (PSF) incorporating finite lifetime effects, line of sight geometry, and magnetic field configuration using the pyFIDASIM code [1]. Comparisons between the raw GENE-3D data and synthetic fluctuation amplitudes, frequency spectra, correlation lengths, and power spectra are presented. Additionally, wavenumber spectra are presented to estimate the sensitivity of the BES system.

[1] X. Han et al 2024 JINST 19 P11004