Communications and navigation systems, computer networks, and precise scientific investigations all require a reliable frequency standard. The National Institute of Standards and Technology (NIST) provides this standard for the United States [1]. The atomic clock that serves as a primary frequency standard at NIST is an atomic fountain clock called NIST-F1 [2]. NIST-F1 is the first atomic fountain clock to be built at NIST and has been in operation since 1997. NIST-F1 is designed to measure the energy splitting of the hyperfine ground states of the ~(133)Cs atom with a very high degree of accuracy. This hyperfine energy splitting, which defines the second, is regularly measured with an uncertainty of one part in 10~(15). Such a measurement typically requires about a 2 weeks of data acquisition. The next generation of fountain clock is currently being designed at NIST and is in the early stages of testing. Efforts are underway to reduce the necessary systematic corrections so that the measurement of the hyperfine frequency can be made with a higher degree of accuracy. One modification to the apparatus will be to cool the drift region to cryogenic temperature, thereby reducing the correction for the black-body radiation frequency shift to a negligible level. An additional modification will be to reduce the atom density during the measurement while increasing the atom throughput in order to reduce the cold-collision frequency shift. This will be accomplished by reducing the dead time with a multi-ball toss scheme.
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