A recent major upgrade of the superconducting - Darmstadt linear accelerator (S-DALINAC) allowed to conduct the first operational tests in an energy recovery linac (ERL) mode. The ERL operating regime imposes strict requirements for many accelerator components as well as for the accelerated beam quality and the beam energy spread in particular. At present, a significant beam energy spread is being observed in the capture section of the S-DALINAC. In that regard, the capture section part of the S-DALINAC injector needs to be upgraded in order to reduce the beam energy spread growth.
In this work, the 6-cell reduced-beta cavity development process is discussed and a final cavity design is proposed. The SRF cavity design is not limited by electromagnetic field simulations, but also includes the longitudinal beam dynamics computations and the cavity mechanical model investigation. The main challenge was to find an optimal upgrade solution that would satisfy contradictory requirements with minimised investments costs. The results of multi-physics simulations and qualitative analysis that allowed to make decisions related to the SRF cavity design considerations are discussed. The suggested 6-cell SRF cavity is expected to be manufactured by the end of 2019. The cavity commissioning tests are planned in 2020
