Faraday Grid Slot Optimization for Enhanced Coupling Efficiency in ICP RF Ion Sources

Faraday grids serve as protective barriers against ion bombardment of the dielectric vacuum vessel walls of Inductively Coupled Plasma (ICP) RF ion sources. Sputtering events within these sources must be mitigated by the grids to allow successful integration into Neutral Beam Injection (NBI) assemblies. Optimizing the slot geometry on the grids is crucial to achieving the best combination of high RF coupling efficiency and an acceptable level of protection for the dielectric.

Ten cylindrical copper Faraday grids with variable width, number, and angle of the slots were tested with the ICP ion source RISE (Radiofrequency Ion Source Experiment).  Each variable was evaluated with three grids, maintaining all other parameters constant to isolate the effect of each variable on coupling efficiency. A "Baseline" grid served as a point of convergence for all data, ensuring each grid differed by only one variable. We report here the results of experiments that explore the relationship between the physical dimensions of the slots and the coupling efficiency, both in relation to and independent of the exposed surface area.  The results of this experiment will aid in the development of ICP ion sources for implementation into the DIII-D NBI system.

* Work supported by US DOE under DE-FC02-04ER54698, and DE-AC05-00OR22725.