Optimization of an ion-optics system with “thick” electrodes for the diagnostic neutral beam injector of the TEXTOR tokamak

The optimization results of the ion-optics system in a neutral beam injector are considered which is developed for application of beam emission spectroscopy in the TEXTOR tokamak. The injector is capable to deliver a neutral beam of at least 1 A in hydrogen and deuterium at 50 keV for a pulse length of 5 s into the plasma. The simulation results with the AXCEL code for grids of 4 mm thickness are discussed. The ion current of 2 A extracted by a multihole grid system at a current density of 120 mA/cm² is generated by a rf driven ion source. The beamlet half angular divergence is below 0.6° for the optimized four-electrode ion optics system. The estimated temperature rise of the inertially cooled molybdenum grids with 150 holes and 58% transparency is max. 270 K and the related degradation in beam quality by thermal grid expansion is acceptable. © 1997 American Institute of Physics.