Measurement of Transient Magnetic Fields of Interest to the Muon g-2 Experiment at Fermilab

The muon g-2 experiment at Fermilab has measured the magnetic moment anomaly of the muon by storing polarized muons in a magnetic storage ring and measuring the difference between the muon spin and cyclotron frequencies in combination with a measurement of the storage ring magnetic field.

Muons injected into the storage ring are put on a stored orbit using three pulsed “kicker” magnets, each consisting of two parallel metal plates, through which fast pulsed currents flow and produce ~200 G magnetic fields that turn on/off in roughly one cyclotron period (150 ns) . These pulsed fields induce eddy currents in the surrounding metal vacuum chamber, which persist during the muon storage interval and perturb the magnetic field in the ring. Determining the magnitude, time-dependence, and spatial dependence of these fast transient fields represents a significant challenge and source of uncertainty in the magnetic field determination.

Using a scale model of the kicker apparatus with a removable metal box representing the vacuum chamber, we characterize the spatial dependence of these eddy currents’ magnetic fields in the transverse plane and azimuthal directions of the toroidal muon storage volume. We also construct simulations to compare the measured distributions of both the magnetic field transients and induced eddy currents to theoretical predictions and analyze potential discrepancies, suggesting non-trivial models of current distribution within the kicker plates.