Coupling simulations for high field side lower hybrid antenna on DIII-D

Calculations show that high field side (HFS) launch of lower hybrid (LH) power represents an integrated current drive solution that both improves core physics (higher efficiency/proper location) and mitigates plasma material interaction/coupling issues. An HFS LHRF system has been developed for DIII-D to demonstrate these benefits, which represents the first design for an operating tokamak. The RF performance of the antenna was simulated by COMSOL coupled to ALOHA for the plasma response. Simulations predict low reflected power for a range of anticipated plasma conditions in the HFS scrape off layer. A vacuum gap of 0.5 mm at the antenna mouth results in a slight increase of power reflection coefficients (from <1% to <5%) and decrease of directivity (from 70% to 60%) as compared to cases without the vacuum gap. Experimental results from the final weeks of the 2018 DIII-D run campaign indicate that a 0.5-1.0 mm recess depth provides sufficient protection of the molybdenum structures from the plasma. This result motivates recessing the radiating molybdenum waveguides a similar distance behind the local graphite protection limiters.