A high field side multijunction launcher with aperture impedance matching for lower hybrid current drive in DIII-D advanced tokamak plasmas

A high field side (HFS) lower hybrid current drive launch scenario improves wave accessibility, has single pass damping at r/a ~ 0.6-0.8 and good current drive efficiency in DIII-D advanced tokamak discharges. The DIII-D experiment is an opportunity to validate HFS wave propagation, absorption and scrape-off layer benefits. A HFS multi-junction launcher is designed and simulated in COMSOL over a range of plasma edge conditions to evaluate n|| spectrum, directivity, and return loss. The COMSOL model utilizes a lossy anisotropic dielectric modeled with the cold plasma dispersion relation cross validated against ALOHA and Petra-M codes. The COMSOL model seamlessly includes a realistic plasma model and coupler that allows for rapid optimization of a single launcher module, while Petra-M allows more complex simulation of an eight-module array including curvature, and warm plasma effects. The resulting design utilizes a traveling wave poloidal power divider to minimize peak electric fields in the vacuum region of the coupler, and an internal aperture matching structure provides an impedance match to the plasma over a wide range of plasma density and density gradient edge conditions.