Microwave power related frequency shifts of NIM5

Several evaluations of the frequency shifts have been carried out after NIM5 was moved to the new NIM campus at Changping in 2011. The dominate frequency uncertainty was the microwave power related frequency shifts, which was mainly induced by the microwave leakage for NIM5. The experimental results show that fountains are more sensitive to the leakage field below the Ramsey cavity than above the cavity at the odd multiples of the optimal power. This effect can be reduced by trimming interferometric RF switch time, which makes atoms feel more symmetric leakage field during the ascending and the descending. The fractional frequency shift due to this effect is evaluated by alternatively running the NIM5 fountain between two timing modes. The overall uncertainty of NIM5 is smaller than 2e-15. The comparisons of NIM5 with other fountain clocks in 3 months are shown and consistent with this result.