Innovative mechanical design and development of single spoke resonators with microphonics control for the next generation of superconducting particle accelerators

Frequency detuning of accelerating structures is a technical challenge for the next generation of superconducting linear particle accelerators operating in continuous wave (CW) or pulsed regime with a low beam current. In such machines, any drift in the operating frequency (few tens of Hz) would get the cavities seriously detuned from their resonant frequencies, causing phase errors that negatively affect the beam. To address this issue of frequency detuning, an innovative methodology was developed to design and fabricate a new generation of low-beta superconducting single spoke resonators (or cavities) with a very low sensitivity to microphonics. The liquid helium containment vessel surrounding the Single Spoke Resonator of type 1 (SSR1) was designed such that the fluctuations of the helium bath do not cause substantial radio-frequency detuning. The structural design was carried out following the American Society of Mechanical Engineers (ASME) Code whenever possible. Alternative procedures were established to deal with any exception encountered and produce a level of safety consistent with that of the Code. The tuning mechanism for the active adjustment of the resonant frequency of the cavity was also designed and developed. Experimental tests were performed on fabricated prototypes to verify the validity of the design and fabrication approach. The systematic application of this design approach to other SRF cavities will allow consistent capital and operational costs savings, possibly totaling millions of dollars, for the next generation of superconducting particle accelerators, such as the Proton Improvement Plan-II (PIP-II) at Fermilab.