Advanced Toroidal Facility Neutral Beam Injection: Optimization of Beam Alignment and Aperturing

The application of the existing Impurity Study Experiment (ISX-B) neutral beam injectors for the Advanced Toroidal Facility is studied. New techniques are required to handle the complicated stellarator geometry of both the vacuum vessel and the plasma. The power delivered to the plasma is found to be a strong function of the beam divergence but only a weak function of the beam focal length. Monte Carlo methods were used to follow the injected particles from the injector until they thermalized in the plasma. An aperture in the beam line is required to prevent excessive heating of the vacuum vessel by the injected beam. Shine-through can be a serious problem if very low density start-ups are necessary. Reasonable assumptions on beam divergence yield an estimate of over 1 MW of power absorbed by the plasma. Preliminary calculations indicate that there will be no excessive fast ion losses.