Metrological aspects of the on-metas continuous fountain standard

We report on the preliminary evaluation of two metrological aspects specific to a primary standard based on a continuous fountain of laser-cooled Cs atoms. This type of standards takes advantage of the reduced density of the cold atoms beam which should result in a negligible collisional frequency shift. We describe a new theoretical evaluation of the average atomic density; in present flux condition, the calculation gives an average density corresponding to a shift of 0.7·10^-17. With the present atomic source intensity and in the limit of zero transverse temperature, the estimated collisional shift would be <10^-16 for a short-term stability of1·10^-13τ^{- 1}2 /. On the other side, the continuous operation of the fountain requires to separate the light emitted by the source of cold atoms from the continuous atomic beam to be interrogated. Without light-trap, in addition to the expected light shift, the measured Ramsey fringes show a reduction of the contrast which can be reproduced by a model of loss of population of the F=4, m_F=0 hyperfine level due to optical pumping by the fluorescence light from the source. The model also reproduces the asymmetric damping of the measured Rabi oscillations. The present state of the accuracy budget is also given taking into account measured stray light intensities at cavity level with and without light trap.