Cryogenic Magnetic Field Monitor System in the SNS Neutron EDM Experiment*

The existence of a permanent neutron electric dipole moment (nEDM), d_n, would be a violation of charge-conjugation and parity (CP) symmetry. The current world limit for the neutron electric dipole moment is d_n < 1.8×10^-26 e·cm (90% CL). The nEDM experiment at the Spallation Neutron Source (nEDM@SNS) targets to improve this limit by two orders of magnitude, d_n ~ 10^-28 e·cm. For the suppression of systematic effects in the experiment induced by magnetic field gradients, the magnetic field non-uniformities inside of the experimental region need to be accurately monitored and regulated. A magnetic field monitoring system, which contains 39 single-axis cryogenic fluxgate magnetometer probes at discrete locations, has been designed and constructed to provide a first-pass measure of the field gradients inside of the experimental region using nondisruptive measurements of the components of the magnetic field. This talk will discuss the construction of the field monitor system, and results from initial tests of the system.