Helicon Antenna Diagnostic and Testing Program for DIII-D Tokamak

A test program has characterized the rf response of a prototype high power helicon antenna module. A quarter length module made out of uncoated copper was installed in a vacuum chamber and fed via resonant circuit with commercial television amplifier providing 1 ms-long 7 kW pulses at 0.1% duty cycle. The high power helicon system will apply 1 MW to 30 full-length antenna modules [Nagy FST 2017], so the resonant circuit is designed to couple power into the quarter module to reach an electric field of 18 keV/cm, comparable to the full module. Multipactor is significant only at modest powers, and can be conditioned to a minimal level. The effects of a titanium nitride coating added to the copper were found to be minimal, no more effective in terms of standoff than well-cleaned copper, simplifying a material choice for the final product. Quarter module testing is occurring alongside instrumentation and diagnostic development. Arc detection, rf probe measurements, and potential optical and piggyback signal-mixing techniques now being developed for use with the high power system are discussed.