Optimized collimator design and synthetic signals for the ITER Fast Ion Loss Detector

An improved geometry for the ITER Fast Ion Loss Detector (FILD) (i.e. Lost Alpha Monitor) collimator has been optimized to maximize the signal-to-noise ratio and resolution for the lost 3.5MeV fusion-born alpha particles. A new optimization method has been developed to be able to study a broad range of geometries and ensure the best performance through the entire velocity-space of interest. The collimator now presents advanced shaping, fitting the helical trajectories of the ions. Synthetic diagnostic simulations have been conducted with the FILDSIM code to evaluate its performance. These simulations provide the main parameters of the diagnostic, including the gyroradius and pitch angle resolutions, and the collimator factor. Synthetic signals are computed from ASCOT ion distributions to evaluate the optic and data acquisition system of ITER. A scan of the FILD insertion has been performed to evaluate the ion flux at different measuring positions. The synthetic signals in this work show that the new collimator geometry enhances both collimation and resolution, ultimately improving the signal to noise ratio by ∼ 70 %. These simulations incorporate the latest update on the optical system and multiple sources of noise: scintillator noise generated by gamma and neutron fluxes, camera-induced noise, neutronic noise on the camera sensor, optic transmission and field of view, and distortion due to the optical system.