Structures for Superconducting Electron Linacs and RF Separators

Some basic principles underlying the design of structures suitable for superconducting electron linacs and RF separators are discussed. It is shown that the shunt impedance can be written as the product of two factors, one related to the shape of the individual cavities in the structure, and the other to the field flatness. Typical standing-wave structures are discussed, with emphasis on the ratio of peak-to-average field. The problem of field flatness is analysed using a coupled chain of resonant circuits. Results are given for the variation in field flatness with loss and detuning for the ??/2 and ?? modes. Travelling wave resonators are also of interest, since they give the highest gradient for a given peak field. It is shown that such a resonator must be matched with great precision, or substantial power will flow in the reverse direction. Other factors which enter into the design of superconducting structures, such as coupling, thermal and mechanical stability, and beam break-up, are briefly considered.