Fermilab Main Ring low level RF system modifications for focus free transition beam tests

A novel idea for crossing transition energy has been proposed for study in the Fermilab Main Ring accelerator. The idea has been named focus free transition crossing and involves reducing the RF focusing force nearly to zero when the beam energy is near the transition energy by adding a third harmonic to the RF accelerating voltage, and then adjusting the accelerating phase angle to 90°. The modification of the accelerating voltage wave form shape and phase are accomplished by accurate program control of the amplitude and phase of the 53 MHz RF cavities and a recently installed 159 MHz cavity. The studies also require interrupting the normal LLRF system beam energy feedback loops in favor of a new energy control loop near the transition energy. This paper describes the functions of the LLRF system electronics recently installed to facilitate this control, and initial operational experience with the system