Conditioning Process of the Helicon Radiofrequency Traveling Wave Antenna

The helicon system has been operational in the DIII-D tokamak since 2021 with a 476 MHz klystron injecting up to 1.2 MW of RF power into a comb-line traveling wave antenna through co-axial transmission lines. Each time the DIII-D vacuum vessel is vented, the antenna needs to undergo a conditioning phase to restore it to its full operational level. An unconditioned antenna is subject to nonlinear dissipative processes such as multipactor-induced plasmas within the stripline, vacuum feedthrough, in-vessel co-axial line, or antenna components that can absorb and/or reflect incident RF power, causing an RF pulse train to run short due to a perceived arc and a loss in RF power before it reaches the antenna. Progress in conditioning is quantified with three metrics: an increase in the total RF on-time; an increase in the total power reaching the antenna; and a decrease in the fraction of reflected power. Previous conditioning phases have demonstrated an exponential rise in RF on-time with # of plasma shots and a corresponding increase in the fraction of applied power reaching the feed module of the antenna. Improvements to the system such as pressurized transmission lines and anti-multipactor coating of components have led to an improvement in conditioning recovery following a vent.