Field Flatness Tuning of Five-Cell SRF Elliptical Cavity

A basic study on the superconducting radio frequency (SRF) cavity with a geometrical beta of 0.42 has been performed for an extension of Proton Engineering Frontier Project (PEFP). As a prototype of the PEFP SRF cavity, a five-cell niobium cavity with the elliptical shape has been designed and fabricated. Before RF test in low temperature, we performed the field flatness tuning to make the field distribution in the cavity uniform along the beam axis. In general, the field distribution in the cavity is irregular because of several factors such as mechanical deformation during the cavity fabrication, the shrinkage due to the electron beam welding and so on. The field flatness in a multi-cell SRF cavity affects not only the net accelerating voltage but also the peak surface field, the Lorentz detuning effect and the coupling of the couplers. Therefore, the field distribution should be corrected after cavity fabrication. In this study, the field distribution was measured by using a bead-pull method and the field flatness tuning was performed by using first order perturbation theory based on the work done by Padamsee et al