Squids, snakes, and polarimeters: A new technique for measuring the magnetic moments of polarized beams

Abstract

Effective polarimetry at high energies in hadron and lepton synchrotrons has been a long standing and difficult problem. In synchrotrons with polarized beams it is possible to cause the direction of the polarization vector of a given bunch to alternate at a frequency which is some subharmonic of the rotation frequency. This can result in the presence of lines in the beam spectrum which are due only to the magnetic moment of the beam, and which are well removed from the various lines due to the charge of the beam. The magnitude of these lines can be calculated from first principles. They are many orders of magnitude weaker than the Schottky signals. Measurement of the magnitude of one of these lines would be an absolute measurement of beam polarization. For measuring magnetic field, the Superconducting Quantum Interference Device, or squid, is about five orders of magnitude more sensitive than any other transducer. Using a squid, such a measurement might be accomplished with the proper combination of shielding, pickup loop design, and filtering. The resulting instrument would be fast, nondestructive, and comparatively cheap. In addition, techniques developed in the creation of such an instrument could be used to measure the Schottky spectrum in unprecedented detail. We present specifics of a polarimeter design for RHIC, and briefly discuss the possibility of using this technique to measure polarization at high energy electron machines like LEP and HERA.

Radio-frequency polarimetry

“…The idea of the radio-frequency method of spin monitoring for a polarized beam circulating in an accelerator or storage ring was proposed in recent years [1][2][3]. It is based on a consideration that, if the coherent spin of the beam is declined from the equilibrium direction, ~ (vertical or, in general case, periodical along the beam orbit), then the electromagnetic field of the beam "observed" by a cavity located at a point of the orbit has to have a modulation with periodicity of the spin precession in the accelerator, which is different from the periodicity of the particle orbital motion.…”