A bunch lengthening RF cavity for Aladdin

A 202 MHz aluminum RF cavity has been constructed for the Aladdin synchrotron light source at the University of Wisconsin-Madison. The cavity operates on the fourth harmonic of the ring RF frequency and will be used to lengthen the electron bunches in order to increase beam lifetime. The cavity body is machined from a thick aluminum plate with a single penetration for the coupling loop which operates in vacuum. A cover plate at the gap end contains an annular tuner that is moved concentric with the beam axis. The entire inner surface of the cavity is copper plated to increase shunt impedance. The cavity will be initially operated in the beam driven mode with the tuner adjusted by computer to provide optimum bunch lengthening as the beam decays. In the future, active RF drive may be employed to allow a constant bunch length to be maintained at all times. A beam derived RF reference is proposed along with use of RF feedback to avoid instabilities. Results of the first storage ring operation using the new cavity are presented