First performance results of PTB's atomic caesium fountain and a study of contributions to its frequency instability

At the Physikalisch-Technische Bundesanstalt (PTB), an atomic caesium fount ain was constructed. Ramsey fringes with a full width at half maximum (FWHM ) of 0.86 Hz were obtained by launching the atoms to a height of 83 cm abov e the cooling region (40 cm above the microwave cavity center). A first mea surement of the homogeneity of the magnetic flux density yields 0.33 nT (rm s), only 0.16% of the mean value of 0.205 mu T used in normal operation. Th e inherent elementary noise contributions of the fountain and of a thermal beam atomic clock are compared in some detail.