Accuracy evaluations of the Cs fountain primary frequency standard NIM5

The caesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology (NIM) of China. The NIM5 loads atoms in an optical molasses (OM) from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of about 1 Hz. Stabilizing the 9.19 GHz microwave frequency to the centre of the central Ramsey fringe, a typical fractional frequency stability of 3×10^-13(τ/s)^-1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties is typically less than 1.6×10^-15. The NIM5 operates for more than 300 days in a year with nearly continuous operation over 15 days. Comparisons of data among the NIM5 fountain clock and other 5 fountain frequency clocks were carried out in May, 2013 through Two-Way Satellite Time and Frequency Transfer (TWSTFT), also NIM5 was compared with UTC through UTC(NIM). The results demonstrated agreement within the stated uncertainties.