On the Electromagnetic Design of a $\beta _{g}=0.61$ 650-MHz Superconducting Radio Frequency Cavity

We present calculations for an electromagnetic design of a β_g = 0.61 multicell superconducting radio frequency cavity for the Indian Spallation Neutron Source project. The geometry parameters of the midcells are optimized using a step-by-step 1-D optimization technique. This is followed by the optimization of end cells, which is done to achieve the required field flatness and to avoid trapping of higher order modes. Calculations of the threshold beam current for the excitation of regenerative beam break-up instability excited by the dipole modes are also presented, which is followed by wakefield calculations and estimation of related effects. Specific aspects in these calculations, which are relevant for medium β_g cavities, are highlighted. Finally, studies are performed for the static and dynamic Lorentz force detuning, based on which the stiffness design of the cavity is optimized.