Additively Manufactured RF Launchers: Advantages, Challenges, and Future Prospects

A system for high field side launch of lower hybrid waves at 4.6 GHz is planned for DIII-D at n_||=^-2.7 and is expected to access improved efficiency current drive of 140 kA/MW at r/a=0.6-0.8. A novel multijunction launcher reduces circulating power to lower electric field below the 9.3 kV/cm multipactor threshold over a wide range of edge densities (n_e=10¹⁷ to 10¹⁸ m^-3) with a traveling wave power divider and by matching aperture impedance to the plasma edge.

Additive Manufacture (AM) with Laser Powder Bed Fusion resulted in fully dense, vacuum compatible Glenn Research Copper 84 alloy and allowed monolithic printing of poloidal power dividers that were unachievable with conventional machining. Pentagonal waveguides modified for AM self-support overhanging elements with a chamfered roof that maintains RF tuning properties and low return loss. Chemical mechanical polishing smoothed inaccessible internal AM surfaces for low RF loss. Laser welding joins launcher segments and brazing techniques add TZM limiters to grill apertures. Future launchers may be monolithically printed as complete units.