Design and realization of the microwave cavity in the PTB caesium atomic fountain clock CSF1

At the Physikalisch-Technische Bundesanstalt (PTB), the caesium atomic fountain clock CSF1 was developed. One key element of it is its microwave cavity, which was designed to have a low transversal phase variation across the cavity opening. This usually is achieved by using a cylindrical cavity with a TE011 field mode, having a high intrinsic quality factor, and by feeding the cavity symmetrically via two apertures at opposite position in the cavity wall. In contrast to other solutions, the CSF1 cavity is tightly coupled to the microwave feeds (Qload approximately 2000). Therefore, detuning of the cavity resonance frequency has a reduced impact on the microwave field amplitude in the cavity compared to the case of weak coupling. Thus, a temperature stabilization of the cavity can be avoided. The extent to which the tight coupling may have an impact on the transversal phase distribution was studied. This question was solved analytically for a simplified cavity model. The results were applied to define the coupling geometry of the CSF1 cylindrical cavity in such a manner that the transversal phase distribution should become minimized. The key parameter is the electric length of the waveguides that constitute the junction to the coupling apertures. The theoretical studies are presented in some detail, and steps of the practical realization of the cavity are described.