YO!—a time-of-arrival receiver for removal of helicity-correlated beam effects

A parity violation experiment, G⁰, at Jefferson Lab is sensitive to arrival time differences, at the target, of electron beams in the two helicity states. Instead of the Jefferson Lab standard 499 MHz beam structure, G⁰ uses a 31.1875 MHz structure where only 1 out of 16 microbunches contains electrons. Photon counters triggered by time-of-arrival at the target mandate that timing must be independent of delays associated with different orbits taken by the electrons in two helicity states. Corrections to the parity violating asymmetries due to any arrival time differences require the generation of a clean 31.1875 MHz trigger signal and phase matching this signal to the beam's arrival at the target. The time of arrival receiver, named the YO! receiver, has 10 kHz output bandwidth which is sufficiently larger than the settling time (500 μs) of the ≈30 Hz helicity flip. This enables the correction of each helicity bin for any orbit-induced timing inequalities. The device combines conventional receiver and DSP techniques for maximum sensitivity, bandwidth and flexibility and eliminates the 2π/n phase shifts associated with frequency dividers by means of a sampling phase detection scheme. This paper describes the performance of this device during bench testing, commissioning and in data taking phase of the experiment.