Uncertainty evaluation of the caesium fountain primary frequency standard NIM6

Abstract A new caesium (Cs) fountain clock NIM6 has been developed at the National Institute of Metrology (NIM) in China, for which a comprehensive uncertainty evaluation is presented. A three-dimensional magneto-optical trap loading optical molasses is employed to obtain more cold atoms rapidly and efficiently with a tunable, uniform density distribution. A heat pipe surrounding the flight tube maintains a consistent and stable temperature within the interrogation region. Additionally, a Ramsey cavity with four azimuthally distribution feeds is utilized to mitigate distributed cavity phase shifts. The Cs fountain clock NIM6 achieves a short-term fractional frequency instability of 1.0 × 10 −13 τ −1/2 at high atomic density, and a typical overall fractional type-B uncertainty is estimated to be 2.2 × 10 −16 . Comparisons of frequency between the Cs fountain NIM6 and other Cs fountain primary frequency standards through Coordinated Universal Time have demonstrated an agreement within the stated uncertainties.