Modelling of the NBI contribution to the neutron energy spectra for the ITER Vertical Neutron Camera

The ITER Vertical Neutron Camera (VNC) is a multichannel collimator system designed to measure the neutron emissivity profile and spectra. The energy spectrum measurements of neutrons made by the VNC will contribute to the reconstruction of the ion temperature, fuel ratio, and fast ions' distribution function. In this paper, the results of neutron spectra calculations for the ITER VNC are presented. The developed software allows for the simulation of particle spectra resulting from interaction of suprathermal ions with thermal background plasma. The method relies on direct reaction rate calculation and explicit fusion reaction kinematics modelling. The assessment of the D-T neutron spectra is carried out for D-NBI heated baseline D-T ITER operation scenarios. It is shown that neutron flux resulting from fast ("beam") D with thermal T ion interaction dominates at E_n> 16 MeV part of the spectra. Thus, it is possible to assess fuel ratio based on calculated "thermal-thermal" and "beam-thermal" neutron energy spectra. An ability to observe sawtooth-induced mixing via VNC is studied. Lower energy boundary for the observable part of the suprathermal ion distribution is derived.