Design of a higher harmonic RF system for the Advanced Light Source

We report on the design and fabrication of a third harmonic radiofrequency (RF) system for the Advanced Light Source (ALS) to be used for lengthening the bunch and increasing the Touschek-dominated beam lifetime. We plan to i nstall five single-cell 1.5 GHz copper RF cavities in one-half of an ALS st raight section with a predicted increase in the lifetime by a factor of 3. Each RF cell is designed to sustain a maximum voltage of 125 kV with a powe r dissipation of 5 kW. We present measurements made on an aluminum cavity m odel characterizing the RF properties of cavity such as the cavity R/Q and higher-order modes (HOMs). In particular, resonances in the cavity tuners w ere studied in order to avoid heating of the tuner bellows. Initial measure ments of the copper cavities indicate a Q value of 21 000, resulting in a s hunt impedance of 1.69 Mn per cell. (C) 2000 Elsevier Science B.V. All righ ts reserved.