Realized and planned improvements of the PTBs primary cesium clocks

Performance data for two primary cesium clocks are reported. Changes that have been made to improve the performance are described. One involves the use of a cavity with ring-shaped end parts in which the atoms interact with the microwave field in a region where its Pointing vector vanishes due to the symmetry. This configuration reduces the distributed phase shift, which seriously interferes with the determination of the end-to-end phase difference of the Ramsey cavity. Other changes involve the C-field correction, electronic system, and figure of instability.<>