Uncertainty evaluation of the continuous cesium fountain frequency standard FOCS-2

The continuous atomic fountain primary frequency standard FOCS-2 is quite unique since all the atomic fountain clocks presently contributing to the international atomic time (TAI) are operating in pulsed mode [1]. Our alternative approach offers the possibility to operate with a high atomic flux without the collisional shift limiting the accuracy or the Dick effect limiting the stability when a quartz is used as local oscillator [2], [3]. Moreover, it contributes to the metrological diversity since the relative importance of the error budget contributors is different in the two types of fountains, notably for microwave cavity and density related effects. In this contribution we present the status of the metrological evaluation of the continuous fountain FOCS-2. Thanks to an innovative atomic beam source followed by an atomic state preparation stage, we reach a signal-to-noise ratio of 600√Hz and thus a relative frequency instability of 6 × 10
⁻¹⁴
T
^−1/2
. A detailed investigation of the frequency shifts which are specific to the continuous operation (light-shift from the source and cavity end-to-end phase shift) shows that the continuous fountain is compatible with an inaccuracy at the 10
⁻¹⁵
level or below.