Coherent high-order mode beating in CESR sliding joints from multi-bunched stored electron and positron beams

Stored beams containing trains of bunches can excite trapped higher-order modes (HOM) in discontinuities such as sliding joints (SLDJTs). Due to the relatively high Q, the trapped fields excited by multiple bunches in a SLDJT can interfere constructively and a coherent enhancement may result at certain SLDJT openings for a given bunch spacing. This resonant HOM loss in the SLDJT causes abnormal heating and can pose an operational problem for CESR at high beam currents or at short bunch lengths. Calculations show resonance behavior at certain SLDJT openings depending on the bunch spacing within each train. Two types of experiments were performed to investigate the SLDJT resonant loss as a function of the SLDJT opening at both 14 ns and 28 ns bunch spacings. In one type of measurement, the temperatures of 100 SLDJTs with random openings were sampled at each bunch spacing. In another experiment, the temperatures of a selected group of SLDJTs were measured as a function of opening for both bunch spacings. The measurements showed maximum heating at the SLDJT openings consistent with the calculations. The beam current and bunch length dependence of the SLDJT temperatures was also measured. Measures have been taken to set the openings of all CESR SLDJTs sufficiently off resonance so that significant heating does not occur at full design current.