Abstract
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 the cavity operation with Au+Au collisions, and with asymmetrical Au+He3 collisions. The cavity is a storage cavity, meaning that it becomes active only at the energy of experiment, after the acceleration cycle is completed. With the cavity at 300 kV, an improvement in luminosity was detected from direct measurements, and the bunch length has been reduced. The uniqueness of the QWR demands 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 bypassed during the acceleration stage.
Highlights
Relativistic Heavy Ion Collider (RHIC) was designed with accelerating and storage rf systems to operate at harmonic numbers h 1⁄4 360, and h 1⁄4 7 × 360 1⁄4 2520, which correspond to frequencies of 28.15 MHz and 197.05 MHz, respectively [1]
At the same longitudinal emittance, the full bunch length with the 56 MHz rf system may be suppressed to below the bucket size of the 197 MHz cavity. As it was shown from direct luminosity measurements during RHIC operation in 2014, the combination of the 56 MHz and 197 MHz rf systems raised the luminosity within the vertex of the detectors, which will be discussed in the later section of this paper
To avoid dangerously large amount of power from the fundamental mode being sent into the Higher Order Mode (HOM) coupler load, a Chebyshev high-pass filter was connected to the coupler loop
Summary
RHIC was designed with accelerating and storage rf systems to operate at harmonic numbers h 1⁄4 360, and h 1⁄4 7 × 360 1⁄4 2520, which correspond to frequencies of 28.15 MHz and 197.05 MHz, respectively [1]. The bucket length of the relatively high frequency storage cavities is 5.1 ns, which is not sufficient to accommodate the ion bunches without going through the rf gymnastics of bunch rotation, or rebucketing This rebucketing process causes an increase in longitudinal emittance due to nonlinearity and hardware complications. At the same longitudinal emittance, the full bunch length with the 56 MHz rf system may be suppressed to below the bucket size of the 197 MHz cavity As it was shown from direct luminosity measurements during RHIC operation in 2014, the combination of the 56 MHz and 197 MHz rf systems raised the luminosity within the vertex of the detectors, which will be discussed in the later section of this paper. The instability of the coupled bunch of the ring-cavity system was studied [8], and damping is provided to the higher order modes to the specified thresholds for passing through the transition energy and at store
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