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 fountain 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 above the cooling region (40 cm above the microwave cavity center). A first measurement of the homogeneity of the magnetic flux density yields 0.33 nT (rms), only 0.16% of the mean value of 0.205 muT used in normal operation. The inherent elementary noise contributions of the fountain and of a thermal beam atomic clock are compared in some detail.