The study of second-order Zeeman shift of the cesium fountain clock NTSC-F1

The second-order Zeeman shift is dominant frequency correction in cesium fountain primary frequency standard. That is evaluated based on the magnetic field distribution along atom clouds trajectory by tracking the center fringe of | F=3, mF=1>-| F=4, mF=1> transition. The correction of the second-order Zeeman shift could be calculated by Breit-Rabi formula. In the beginning of the fountain clock's operation, we could not observe the Ramsey fringes of | F=3, mF=1>-| F=4, mF=1> transition because of significant inhomogenous magnetic field. In this paper, we firstly measured the magnetic field distribution above the resonant cavity in the range of 45 cm by applying the low-frequency transition method, and to improve the flatten of magnetic map by adjusting the current of the C field coil and compensation coils. Then we evaluated the second-order Zeeman shift using the frequency of | F=3, mF=1>-| F=4, mF=1> transition. The experimental results show that the magnetic field distribution obtained by the low-frequency transition and Ramsey transition are basically the same. The second-order Zeeman shift of the C field is 54.7×10-15, and the uncertainty is 10-19 caused by the inhomogeneous C field, the uncertainty of the time-varying C field is 1.4×10-16 in 15 days.