Reduction of waveguide vacuum trips in CEBAF accelerating cavities with a combination ion pump and non-evaporable getter pump

Abstract

Pressure spikes in the fundamental power coupler waveguide are a significant source of trips during the operation of both original and refurbished superconducting radio-frequency cavities installed in the CEBAF accelerator. In its original configuration, two waveguides from a cavity-pair are connected to a single 32 l/s ion pump. In 2017 a new cavity-pair was installed in a refurbished cryomodule in which a NEXTorr®D200-5 combination ion pump and non-evaporable getter pump was installed for each waveguide. It is shown that hydrogen is the dominant residual gas in the waveguide and simulations with Molflow+ indicated that the new pumping scheme allows reducing the pressure at the end of the waveguide by a factor of ∼5, compared to the original configuration. The new cavity-pair has been operating in CEBAF for a total of 431 days and it only had a total of 3 trips due to vacuum spikes in the waveguide.