Operation of the 56 MHz superconducting rf cavity in RHIC with higher order mode damper

A 56 MHz superconducting rf cavity was designed and installed in the Relativistic Heavy Ion Collider (RHIC). It is the first superconducting quarter wave resonator (QWR) operating in a high-energy storage ring. We discuss herein a design of the cavity and its key components and the cavity operation with Au+Au collisions, and with asymmetrical Au+³He collisions. The cavity is a storage cavity, meaning that it becomes active only at the energy of the experiment, after the acceleration cycle is completed. Without beam, the cavity reached 1.93 MV and a Q₀ of 3.0×10⁸ after helium conditioning. The cavity voltage was limited at 300 kV with beam operation due to heating in the Higher Order Mode (HOM) coupler. With the cavity operating at 300 kV, an improvement in luminosity was detected from direct measurements, and the bunch length has been reduced. The uniqueness of the QWR necessitated development of an innovative design of the higher order mode dampers with high-pass filters, and a distinctive fundamental mode damper that enables the cavity to be transparent to the beam during acceleration.