In reactor-grade tokamaks, it is important to investigate confinement properties of alpha particles. A double charge-exchange method using a high-energy probing beam is considered to be the most reliable one in diagnostic methods proposed for the measurement. In JT-60U, an alpha particle production experiment by D-3He ICRF heating will be performed to study the behavior of fusion product alphas. The alpha particle measurement is planned using a helium diagnostic beam (200 keV) and a mass-resolved neutral particle energy analyzer. The expected flux and spectral shape were evaluated by taking into account multistep ionization of helium beam atoms and neutralized alphas. The beam energy is lower than desirable for measuring fast confined alphas near the birth energy. However, by using the beam system, it has been found from the evaluation that we can investigate the confinement properties of fusion product alphas from the spectral shape. And also such a system using a present-day He beam is useful to diagnose behavior of confined alphas in reactor-grade tokamak such as ITER.
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