Frequency offset system to feed RF power to accelerating cavity with keeping field uniformity.

We developed a novel system to feed RF power to a higher-order resonant mode (HOM)-damped cavity while maintaining field uniformity. An HOM-damped TM020 cavity has slots located at the node of the magnetic field of the accelerating mode to absorb the HOMs. The temperature of the ferrite absorbers should be kept low to maintain the damping performance of the HOMs. However, if the uniformity of the TM020 field distribution is broken, part of the power of the TM020 mode leaks into the slots, heats the absorbers, and degrades the absorbing capability of the HOMs. Therefore, it is important to maintain the field uniformity. However, the movement of the frequency tuner to resonate with the cavity during startup may cause field deformation. One solution to avoid this situation is to fabricate cavity dimensions such that the insertion of the tuner is small under the nominal RF feeding power. In this case, the resonant frequency at the cold start was higher than the designed value owing to the low cavity temperature. As a countermeasure, we added a function to the low-level RF system to shift the frequency of the RF signal fed to the cavity during startup. The system consists of a digital-to-analog converter (DAC) and an in-phase and quadrature modulator combined with dedicated firmware running on the field-programable gate array of the DAC module. This system was introduced in NanoTerasu, a Japanese synchrotron radiation facility, and has worked as expected.