New Configurations for High-Field Tokamak ICRF Antenna Couplers

We provide an overview of a project looking at ICRH coupling in high field tokamaks, where the SOL is narrow and dense. This regime is quite different from past practice, as the slow wave is essentially absent, and the narrow SOL implies little-to-no tunnelling. We show through simulation that the Faraday shields in this regime, while not needed for polarization, still serve to shield nearby structures from the strap voltage along the field line. With of course, unshielded RF sheaths resulting in undesirable impurity production via sputtering. We explore geometries that might allow for recessed or enclosed straps, or baluns geometries which might reduce the strap voltage, and its image on nearby structures, and thus lead to smaller RF sheaths.

We also look at more horn-like geometries which attempt to direct the wave into a beam-like pattern, that would also result in less field on adjacent structures, and lower RF sheaths. Such beam-like patterns were apparent, for the first time, in prior simulations of the proposed SPARC ICRH antenna configuration. We perform a theoretical analysis of these prior simulations, and look at additional simulations to test our understanding of the observed behavior. Our simulation efforts will also look at the effect of density fluctuations on the coupling, and how these will impact the local fields and RF sheath formation near the couplers.

Finally, we detaill future plans for more integrated RF / sputtering modeling, which we expect to apply to low temperature plasma processing.