Investigation of the distributed cavity phase shift in an atomic fountain

We report on measurements to characterize phase gradients inside the Cs microwave cavity in the SYRTE double fountain FO2. The clock frequency is measured as a function of the tilt of launch velocity with respect to gravity, as a function of the microwave power and when feeding the Ramsey cavity from either sides or symmetrically using the two microwave feedthroughs. These measurements are made with typical uncertainty below 3×10^-16 per point. They show clear non-vanishing results which can be compared to recent theoretical models of the phase distribution.