Modeling and Designing Modifications to the DIII-D NBI System to Minimize Beam Reionization*

The NBI system plays a crucial role in delivering power and driving plasma current in the DIII-D tokamak. Over decades, modifications to the NBI system have been made to increase the power and to achieve advanced control over the neutral beams that are injected into the plasma. At this stage further increases in the overall power delivered to the plasma are limited by the problem of beam reionization in the drift duct region of the beamline. Reionization is caused by collisions between the beam and the background gas. Once reionized, the particles are then subject to forces from the strong fields in the drift duct.  Many can strike the drift duct, causing damage and reducing the injected beam power.  In this study, we investigate the gas dynamics of the neutral beamline and propose a modification to help minimize reionization and thus increase the power injected into the plasma. We accomplished this by utilizing SolidWorks and MolFlow (software packages) to model the gas dynamics and obtain pressure profiles along the beamline. We then modify this model to include Non-Evaporable Gas (NEG) pumps in the vicinity of the drift duct to analyze the effects on pressure profiles along the beamline. We propose the addition of the NEG pumps may provide an effective method for lowering reionization which would allow the NBI system to effectively deliver more power to the plasma.