Design Study of the Higher Harmonic Cavity for Advanced Photon Source Upgrade

A higher-harmonic cavity is planned for the proposed Advanced Photon Source (APS) multi-bend achromat (MBA) lattice to increase the bunch length, improve the Touschek lifetime and increase the single-bunch current limit. We have investigated a range of options including 3rd, 4th, and 5th harmonics of the main radio frequency (RF) system, as well as configurations with and without external RF power couplers. The current baseline is a single 4th harmonic superconducting cavity with adjustable RF couplers and a slow tuner which provide the flexibility to operate over a wide range of beam currents. The cavity is designed to provide 0.84 MV at 1408 MHz for the nominal 6 GeV, 200 mA electron beam, and 4.1 MV main RF voltage. In this paper, we discuss the harmonic cavity parameters based on analytical calculations of the equilibrium bunch distribution and make comparisons to other options.