Rabi Pulling and Long-Term Stability in Cesium Beam Frequency Standards

It is well known that the long-term fractional frequency stability of commercial cesium beam frequency standards ranges from better than to worse than even within a group of standards of the same model, but no clear description of how this may happen has been given. Experimental measurements on high-performance cesium beam standards are reported which show a direct correlation between long-term frequency stability, long-term stability of the microwave power, and power sensitivity. The latter is shown to vary with the C-field (the dc magnetic field enveloping the microwave cavity) in a way that suggests Rabi pulling as the main-effect coupling microwave power changes to frequency changes. The long-term frequency stability of different standards was measured at various values of the C-field, corresponding to different power sensitivities. The flicker floor level was found to be proportional to the power sensitivity for each setting. This was true all the way down to the low range, in a non-temperature-stabilized environment. At about this level, a limitation appears to be introduced by temperature sensitivity not related to micro-