Continuous energy measurement of the electron beam in the storage ring of Diamond Light Source with resonant spin depolarization

Electron spin polarization occurs naturally in an electron storage ring due to spin-flip synchrotron radiation. The equilibrium polarization level that is achieved in Diamond Light Source was studied theoretically and experimentally. A novel approach that enables measurements during user operation is presented, compatible with beams of small vertical emittance in the range of pm rad. An array of four custom-made scintillation detectors has been installed around the beam pipe downstream from the collimators to capture a significant fraction of the lost particles and witness the depolarization process in the presence of a background loss rate. A new concept for detecting the depolarization resonance even when it coincides with higher-order betatron resonances was introduced. The momentum compaction factor was measured to be in very good agreement with the storage ring numerical model. In the end, we measured a good long-term stability of the beam energy during user operation time.