Neutral particle analysis in MeV energy range and relative role of He+ anti C5+ ions in fast proton neutralization in ICRF and combined ICRF/NBI-heated JT-60U plasmas

A neutral particle analyser of MeV energy range has been used to measure atomic hydrogen fluxes in ICRF and combined ICRF+NBI-heated JT-60U plasmas. Successful application of a pulse height analysis system to separate the particle signal from a noise produced by DD neutrons and gamma-rays is demonstrated. The energy dependence of the neutron sensitivity and energy resolution of the MPA detectors are comparable with calibration data. Charge exchange on He+ and C5+ ions is considered as the dominant process for neutralization of fast ICRF-driven protons. A simplified model based on analysis of the steady-state ion balance equation system is applied to estimate He+ and C5+ target densities. Model calculations are used to interpret the observed difference for the relatively low (E < 0.4 MeV) and high (E > 0.6 MeV) energy parts of the atomic hydrogen spectrum. It is confirmed that the model calculations can well explain the dependence of the atomic fluxes on the relative toroidal angle between the injected beam and the analyser, and the energy dependence of the Bur ratio between 'active' (ICRF+NBI) and 'passive' (ICRF alone) phases. The role of the charge-exchange target effect in the plasma density scan is also discussed.