Measurement of Momentum Compaction Factor via Depolarizing Resonances at ELSA

Abstract

Measuring beam depolarization at energies in close proximity to a depolarizing integer resonance is an established method to determine the beam energy of a circular accelerator. This technique offers high accuracy due to the small resonance widths. Thus, also other accelerator parameters related to beam energy can be measured based on this method. This contribution presents a measurement of the momentum compaction factor with a high precision of 10. It was performed at the 164m stretcher ring of the Electron Stretcher Facility ELSA at Bonn University, which provides a polarized electron beam of up to 3.2GeV. MEASUREMENT PRINCIPLE Precise knowledge of key accelerator parameters is important for the analysis of numerous beam dynamics measurements and user experiments as well as to adjust the accelerator model. One essential parameter is the beam energy. An established method to precisely determine the beam energy in a storage ring is based on spin polarization measurements. If both a polarized beam and polarimetry are available, a decrease of polarization at certain, theoretically well known beam energies can be observed. The characteristic energy of such a depolarizing resonance can be determined experimentally by measuring polarization at various storage energies around the expected resonance. The small energy width of these resonances allows for an energy measurement with a precision of ∆E/E . 10. Building on this procedure, other accelerator parameters can be measured with likewise precision, if they are related to beam energy. One example is the momentum compaction factor. Recently, it was measured at the ELSA stretcher ring at Bonn University using a polarized electron beam. DEPOLARIZING RESONANCES In a flat circular accelerator (without solenoids) the stable spin axis, also known as invariant spin axis, usually points in vertical direction due to the strong vertical guiding fields of the bending magnets. The spins of revolving electrons precess around this axis γa times per turn according to the Thomas-BMT equation [1]. The spin tune Qspin = γa is given by the gyromagnetic anomaly a = (gs − 2)/2 of the electron and the Lorentz factor γ and thus depends linearly on beam energy. Only the projection of the polarization on the invariant spin axis can be preserved, since the finite energy width of the beam implies a spin tune spread, that leads to a spin spreading in the precession plane. ∗ Work supported by DFG † schmidt@physik.uni-bonn.de 0m 5m 10m 15m Source pol. e− Q1 Q2 Q3 M90 Q4 Q5 Q6 Q7 KD Q8 LINAC 2 26MeV Source of pol. e Q7 Q6 M2 M1 Q5 Q4 Q3 EKS Q2 Q1 LINAC 1 20MeV QT1 QT2 irradiation area INJSEPT M1 M2 B1 K1 M3 B2 S1 M4 VC1 S2