Large coherent MHD modes are observed to reduce the neutralbeam current drive efficiency and 2.5 ,MeV neutron emission in DIII-Dby as much as ~65%. These modes result in large (width w≲20 cm for minor radius a≈60 cm),stationary, single helicity magnetic islands, which might causeanomalous deuterium beam ion losses through orbit stochasticity. Ananalytic estimate predicts that co-going, passing deuterons with E≳40 keV become stochastic at island widthscomparable to those in the experiment. A Hamiltonian guiding centrecode is used to follow energetic particle trajectories with thetearing mode modelled as a radially extended, single helicityperturbation. In the simulations, the lost neutral beam current driveand neutron emission are 35% and 40%, respectively, which isconsistent with the measured reductions of 40±14% and 40±10%. Several features ofthe lost particle distribution indicate that orbit stochasticity isthe loss mechanism in the simulations and strongly suggest that thesame mechanism is responsible for the losses observed in the experiment.
Numerical analysis of fast ions transport induced by magnetic islands in HL-2A