Feasibility of fast-alpha diagnostics with Pellet Charge Exchange

Characterization of fusion alpha particles is essential for the study of plasmas with significant alpha heating and, consequently, for the scientific success of ITER. Current diagnostic plans include gamma spectroscopy, collective Thomson scattering, charge exchange recombination spectroscopy, and a fast ion loss detector, although it remains unclear whether these methods will fully meet the requirements of this novel experiment. Pellet Charge Exchange (PCX) is an alternative approach in which alpha particles acquire electrons from the neutral cloud of an ablating fueling pellet through charge exchange, producing He+ ions and He atoms. The concept was previously tested in TFTR experiments, where escaping He atoms were analyzed using a neutral particle analyzer. In this work, a new optical approach is explored by modeling the emission of excited He+ ions produced in the charge exchange reactions. This signal could provide information on the density, energy, and distribution of fusion alpha particles along the pellet flight path, in the pedestal region of the plasma. The presented results are based on solving a system of rate equations for the n-resolved ionization states of alpha particles propagating through pellet clouds, with cloud parameters obtained from the Hydrogen Pellet Injection code HPI2. The model tracks the populations of ionization and excitation states of helium in one dimension, including the density of He+ (n = 4) states whose emission would be observed. Estimates of emission intensity and signal-to-background ratio based on expected bremsstrahlung radiation are also provided, identifying viable parameter ranges for PCX diagnostics.