Operation and Improvements of a Cesium Beam Standard Having 4-Meter Interaction Length

Some factors affecting a long cesium beam standard's accuracy are discussed and the results of two year's operation are given. Increasing the interaction length reduces the linewidth and thus the errors in measuring the line center, as long as the signal-to-noise ratio is sufficient. Errors due to nonlinearity of FM used in servo locking a crystal oscillator to the resonance are below 1×10
^-11
; other servo errors are smaller. The described variable C-field measuring technique and demagnetizable Armco iron shields reduce errors due to C-field uncertainty to 1.2×10
^-12
. Cavity phase shift effects are below 1.5×10
^-11
. Standard deviation of single measurements with 4.6 sec averaging time is 2.7×10
^-11
under good conditions. Considering all effects, including those of microwave power and of cavity tuning, the stated figure of accuracy is ±3×10
^-11
. Two year's VLF comparisons show agreement to about ±10
^-10
with respect to USFS.