The Origins of Accelerator Grid Current: Analysis of T5-Grid Test Results

Grid current levels were obtained at various operating points of a T5 ion thruster using flightmodel extraction grids. Because of the improved grid design and precision fabrication, better analysis of the dependence of grid current on operating point could be performed. The decel current can be explained largely in terms of impingement by charge exchange ions. However, accel current levels are found to have a major component which is independent of charge exchange and rises sharply with beam current. A DSMC-PIC model was constructed which includes the effects of momentum transfer. ion-neutral charge exchange, Coulomb collisions, and double-charge ions. Using existing far-field data on the energy distribution of beam ions to fix the ion temperature. excellent relative and absolute agreement between theory and experiment has been achieved. The results suggest that ion temperature in the discharge plasma can be far higher than other ion engine modeling studies have assumed. about 4 eV in the present case. Ions with high radial velocity have relatively high probabilit) of impinging on the accel grid. and will do so with a higher sputter yield than the average charge exchange ion.