Performance of the Four NRC Long-Beam Primary Cesium Clocks Cs V, Cs VIA, Cs VIB, and Cs VIC

Recent performance of the four laboratory primary cesium clocks at the National Research Council (NRC) is outlined and the results of both internal and external time scale comparisons described. Difficulties which have been experienced with these clocks, including failures in both the microwave resonance excitation systems and the vacuum systems are discussed. The results of seven years of primary clock operation for Cs V and over two years for the three Cs VI clocks show that all the clocks have operated well within their estimated accuracy and stability limits of ±1 × 10-13 for Cs V and ±1.5 × 10-13 for Cs VIA, Cs VIB, and Cs VIC.