Investigation of fast ion characteristics via neutron emission spectroscopy in NBI–ICRF third-harmonic heating plasmas on EAST

We present the first comprehensive multi-sightline neutron spectrometry measurements during synergistic NBI and deuterium third-harmonic ICRF heating on EAST. Using the time-of-flight enhanced diagnostic (TOFED) and three compact organic scintillator spectrometers, we observed a significant broadening in the measured spectra, indicating the acceleration of fast ions well beyond the injection energy. TRANSP simulations reveal a fast ion distribution extending to approximately (Formula presented) (Formula presented) with a marked pitch-angle asymmetry favoring co-current velocities. These simulations are validated by synthetic spectra generated via the GENESIS code and the spectrometers’ instrumental response functions, which show excellent agreement with measurements. Furthermore, a TRANSP-independent weight-function analysis of the spectral cutoffs confirms the pitch asymmetry of the energetic tail. Orbit-topology calculations using the Orbit Weight Computational Framework (OWCF) demonstrate that this asymmetry is determined by the fast ion loss boundaries on EAST. These results establish the capability of multi-sightline neutron spectrometry to unveil the phase-space distribution of radiofrequency-accelerated ions, serving as a vital diagnostic for MeV-range fast ion physics relevant to future burning plasma research.