Accelerating Structures for Superconducting Electron Linacs

A study to determine the optuimum slow wave structure for a superconducting linear accelerator is being conducted. Vacuum, thermal, and mechanical problems at cryogenic temperatures impose limitations on such structures. Ultimately, the maximum energy gradient in a superconducting structure is limited by the critical magnetic field or electric field breakdown. At present a ¿/2, biperiodic, standing-wave structure is favored. Calculations of the dispersion diagram and longitudinal electric field profile has been obtained from a coupled resonator, normal mode analysis. A comparison is made with room temperature measurements on stacked test cells. The effects of full end cells and of incorrectly tuned cells on the flatness of the electric field profile are considered. The relationship between cell tuning and the presence of a stop band in the dispersion curve is shown. Problems of coupling to a high Q superconducting structure and tuning the frequencies of sections in a multi-section linac are considered. Results are given for the Q, the dispersion diagram, and the energy gain for a 952 MHz lead-plated superconducting structure 5 feet in length operating near 2°K.