The beam stopping cross-section in a toroidal system is enhanced by the interaction between injected neutral beams and circulating fast ions generated by the injection. The ionization rate due to the fast ion impact and the fast ion charge exchange is evaluated by a simplified three-dimensional model. This enhancement will be conspicuous in beam driven steady state operation of next generation tokamaks such as ITER and PER. If this effect were taken into consideration in the reactor studies, the optimum beam energy and the wall load due to the beam shinethrough would be changed.
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