Modeling Nonidealities in a Coaxial Transmission Line for the DIII-D Helicon System

The helicon system at DIII-D can provide 1 MW of power at 476 MHz to the DIII-D tokamak from a single klystron source. The RF power is transmitted through ~100 m coaxial transmission line formerly used at lower frequencies (~60-120 MHz). In an ideal system all the generated power is transmitted to the tokamak, but due to nonidealities such as insulating supports for the center conductor, some level of wave reflections are inevitable. At frequencies below ~100 MHz, these reflections are negligible, but at the frequency (~0.5 GHz) for the helicon system, they can be significant. Modifications of the coaxial components were made to address this issue; subsequent measurements have shown acceptably low wave reflection at 476 MHz. To verify these findings, the transmission line nonidealities, focusing on 90-deg bends and insulating supports, are being studied with COMSOL Multiphysics. Simulations of these components are compared to experimental results and the effects due to the insulating supports are investigated.