Effect of up-down asymmetric toroidal field ripple on fast ion loss in JT-60U

The effect of up-down asymmetric toroidal field ripple on fast ion loss was investigated experimentally in JT-60U deuterium discharges. The experiment was carried out for neutral beam (NB) injected fast ions and tritons from the d(d,p)t reaction. The loss of the NB injected fast ions was estimated from the decay of 2.5 MeV neutrons following a short pulse Do beam injection of 30 ms, and the triton loss was investigated from the decay of 14 MeV neutrons following 25 MW neutral beam injection (NBI) with a duration of 2 s. It was found that up-down asymmetric ripple did not enhance the NB injected ion loss compared with the ion loss in up-down symmetric ripple. The effect of the ion nabla B drift direction on the beam ion and triton losses was studied in up-down asymmetric ripple. No significant difference was observed in the loss for energetic ions between upward and downward nabla B drift discharges. The loss fraction of NB injected ions predicted by an orbit following Monte Carlo code agrees with the experimental fraction. (author). 25 refs, 8 figs, 2 tabs