NIST-F3, a Cesium Fountain Frequency Reference

A cesium fountain frequency reference known as NIST-F3 is under development at the National Institute of Standards and Technology (NIST). The fountain is intended to provide input for the NIST timescale and assist with evaluation of NIST’s primary and secondary frequency standards. Here, we describe details of the NIST-F3 apparatus and demonstrate a short-term frequency stability of 2.2 × 10?13/ ? ? for averaging times as long as 5 × 104 s (? is the averaging time in seconds). We also present an initial evaluation of the main frequency biases. Due to the particulars of the microwave cavity design employed in NIST-F3, the fountain exhibits a relatively large tilt sensitivity. We measure fractional frequency shifts of order 10?14/mrad when tilting the fountain. Despite this complication, we show that the tilt sensitivity can be managed with the standard techniques that have been developed for cold-atom fountains. This initial investigation suggests it will be possible to achieve our goal of a fractional frequency stability better than 0.5 × 10?15 for up to a few months of averaging time.