Results on limitations in primary cesium standard operation

We report on the most recent design changes in our two primary cesium standards, their current operational use, results obtained, and limitations. NBS-4, the shorter device with an interaction length of L = 0.52 m, has been extensively used for many months as a clock. After improvements in the magnetic shielding and microwave feed, we have obtained σ
y
(1 week < τ < 2 weeks) = 7 × 10
⁻¹⁵
in a 10-Hz bandwidth for its frequency stability. NBS-6, the longer, more accurate device (L = 3.75 m), features a linewidth (
$\buildrel {\sim} \over &#60; 30$
Hz), which is believed to be the narrowest linewidth ever reported for a cesium device. NBS-6 has been operated to give a short-term stability σ
y
(1 s) = 7 × 10
⁻¹³
in a 10-Hz bandwidth and has capability of easy beam reversal. The current and past rates of the International Atomic Time (TAI) in terms of our primary cesium standards are reported and compared with the results of other laboratories. With NBS-6 we have calibrated the rate of the NBS time scale of an uncertainty of 0.9 × 10
⁻¹³
.