Superimposed Electric/Magnetic "Dipole Moment Comparator" Lattice Design

Abstract

In contrast to a "single particle table-top trap", an essential feature of a storage ring "trap" is that $10^{10}$ or more particles can have their spins aligned in a polarized beam. This is a nunber of polarized particles large enough for the beam polarization to be detected externally, and fed back to permit external control of the beam polarization. The level of achievable spin control, though classical, not quantum mechanical, can be comparable to the control of one or a small number of polarized particles in a low energy trap. Motivated to investigate time reversal invariance, especially the detection of non-zero electric dipole moments (EDMs) this paper describes the design of a low energy storage ring having the superimposed electric and magnetic bending needed to "freeze" the spins of polarized beams. For electrons (of either sign) and protons the spins can be frozen with all-electric bending but, in general, superimposed electric/magnetic bending is required. Since constructive bending superposition in one direction implies destructive superposition in the other direction, counter-circulating beams must differ, either in particle type or momentum, in order for their orbits to be identical. For globally frozen spin operation the bunch polarizations remain constant relative to the momenta, for example remaining parallel to the circuating beam momentum vectors. With superimposed electric and magnetic bending, the globally frozen spin condition can be met over a continua (specific to particle type) of E/B ratios. When this condition is met, the out-of-plane, EDM-induced precession accumulates monitonically, which is obligatory for producing a measurably large EDM signal. As Koop has explained, the EDM signal will still accumulate if the polarization is allowed to "roll like a wheel" around a radial axis.