For years, the superconducting rf community has explored coaxial coupling configurations for fundamental power couplers (FC) and high order mode power couplers (HOM). The coaxial coupling method has vast advantages compared with ordinary welded-on couplers. It totally eliminates coupler kicks and it is detachable and easy to clean. In many applications, it is simply the only choice, but experiments at JLAB and DESY have only reached 8 MV/m with a coaxial coupler.
Our experiment is a major breakthrough in the coaxial coupling technique. In this first cold test, we reached 26 MV/m (no hard quench limit) with a quarter-wave detachable coaxial coupler. This is also a demonstration of the highest field gradient ever reached with a superconducting joint.
The realization of the coaxial coupling method and high field superconducting joints will significantly advance the design for the next generation of superconducting rf cavities.
This research is performed by Yi Xie of Euclid Techlabs LLC in collaboration with V. Yakovlev, N. Solyak and the rest of the Fermilab SRFD group. The coupler was fabricated by J. Rathke (AES Inc.) The project was carried out under DOE SBIR Grant DE-SC0002479 for Euclid Techlabs LLC, Principal Investigator A.Kanareykin.
Left:The quarter-wave coaxial coupler joint installed on a 1.3 GHz single-cell TESLA-type SRF cavity using a superconducting Conflat flange made of NbZr. Right:The measured quality factor Q0 as a function of the accelerating field Eacc. Further baking treatments for eliminating multipactor are ongoing. These treatments are expected to increase both Q0 and Eacc.