Performance of a DIII-D neutral beam ion source with a new accelerator grid

The DIII-D tokamak utilizes seven neutral beam ion sources for plasma heating and current drive. These ion sources have performed with high availability and reliability since 1987. However, components of the ion sources, especially the grids of the accelerator, have since developed hardware failures either due to old age or fabrication defects. Due to lack of the plasma grid with diamond shape rails, we have built a plasma grid with circular cross section rails and installed it in one of the ion sources. Tests were performed on this ion source (we will call it the new ion source) and results were compared with one of the original ion sources. The new ion source runs just as reliably as the original ion sources, though the operation window is slightly smaller and the value of the optimum beam perveance is about 4% less (4% less beam current). This new ion source has been used to inject neutral beams into plasmas of the DIII-D tokamak to support physics experiments. No operational problems or difficulties were encountered during more than 5 weeks of operation. Details of the new plasma grid and the operational results will be presented.