Design and commissioning of third harmonic cavities at BESSY II

We report on the design and commissioning of passive third harmonic cavities that have been installed in BESSY-II, a third generation high brightness synchrotron light source. Four cavities were installed to lengthen the electron bunches and increase the beam lifetime which is dominated by large angle intrabeam (Touschek) scattering. During routine user operation the system is running stable in a low power mode resulting in a current dependent bunch lengthening of up to a factor of 1.4, resulting in a significant improvement of beam lifetime. We also observed longitudinal stabilization of the beam using the cavities without the use of longitudinal feedback.