Development of high-homogeneity magnetic field coil for ¹²⁹Xe EDM experiment

We search for ¹²⁹Xe EDM by using an active nuclear spin maser. In this experiment, the amplitude of the maser oscillation signal is one of the most important parameters that eventually determine the frequency precision. The amplitude is proportional to the ratio of the transverse spin relaxation time T₂ to the effective longitudinal spin relaxation time (Formula presented.). In particular, for a spin maser of ³He (a co-magnetometer) for which (Formula presented.) typically reaches ∼50 h, a long T₂ is needed. T₂ depends on the homogeneity of the magnetic field which is applied with coils in order to keep the spins under precession. In the present report, we discuss on the design and construction of a new coil which provided a root-mean square (rms) field gradient of less than 5.0 μG/cm. The result of the field measurement has shown that the field gradient in the cell fulfills the target condition (Formula presented.), and in fact T₂ of ³He has been measured to be as long as 11,000 s.