Maintaining a Resonance Condition of an RF Spin Rotator Through a Feedback Loop in a Storage Ring

TL;DR

This paper demonstrates a feedback system that stabilizes the resonance condition of an RF spin rotator in a storage ring, enabling precise control of spin precession for electric dipole moment measurements.

Contribution

It introduces a phase-lock feedback method to maintain resonance of an RF Wien filter with respect to spin precession in a storage ring, improving phase stability.

Findings

Achieved phase variation standard deviation of ~0.2 rad.

Successfully implemented during deuteron EDM measurement runs in 2018 and 2021.

Discussed differences between single-bunch and two-bunch beam configurations.

Abstract

This paper presents the successful application of a phase-lock feedback system to maintain the resonance condition of a radio frequency (rf) spin rotator (specifically, an rf Wien filter) with respect to a 120 kHz spin precession in the Cooler Synchrotron (COSY) storage ring. Real-time monitoring of the spin precession and the rf Wien filter signal allows the relative phase between the two to be stabilized at an arbitrary setpoint. The feedback system compensates for deviations in the relative phase by adjusting the frequency and/or phase as needed. With this method, a variation in phase relative to the demand phase with a standard deviation of $\sigma_{\Delta\varphi}\approx 0.2\mathrm{rad}$ could be achieved. The system was implemented in two runs aiming at a first direct measurement of the deuteron electric dipole moment in 2018 and 2021. In addition, the difference between a single-bunch beam affected by the spin rotator and a two-bunch system in which only one bunch is exposed to the spin rotator fields is discussed. Both methods have been used during these beam times. The ability to keep the spin precession and the rf fields synchronized is also crucial for future investigations of electric dipole moments of charged particles using storage rings.