Recent progress of development of cesium fountain primary frequency standard NMIJ-F2

For our cesium fountain primary frequency standard, NMIJ-F2, uncertainty for most of the dominant physical effects such as the second-order Zeeman shift, the blackbody radiation shift, and the collisional shift has been evaluated. As for the microwave power dependence shift, when continuous microwave is fed into the Ramsey cavity, a frequency shift of +5×10
^-15
is caused. The shift is greatly reduced to (+0.1±0.1)×10
^-15
by turning off the microwave only for 10 ms after Ramsey interrogation. Instead, the phase transient of the microwave due to the switch may cause a frequency shift but has not been evaluated yet. Excluding the shift due to the microwave switch and the generally non-dominant shifts such as the light shift and the Ramsey pulling, the tentative type B uncertainty is evaluated to be 0.59×10
^-15
. However, the frequency of NMIJ-F2 is different from International Atomic Time by 3 times the tentative uncertainty for the moment. We are considering that the problem will be solved by evaluating the phase transient due to the microwave switch.