Thomas P. Heavner, Steven R. Jefferts, Jon H. Shirley, Thomas E. Parker, Elizabeth A. Donley, Neil Ashby, Stephan E. Barlow, Filippo Levi, Giovanni Costanzo
We report the first accuracy evaluation of NIST-F2, a second generation laser-cooled Cesium fountain primary standard developed at NIST with a cryogenic (Liquid Nitrogen) microwave cavity and flight region. The 80 K atom interrogation environment reduces the uncertainty due to the Blackbody Radiation (BBR) shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high Q (≅50,000) at this low temperature resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser timescale and NIST-F1 as part of in house accuracy evaluations. We report the results of 9 in house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems, and the accuracy evaluation methods. An uncertainty budget showing all known biases is presented. The most recent (July to August 2013) had a statistical (Type A) fractional uncertainty of 0.44×10-15 and a systematic (Type B) fractional uncertainty of 0.11×10-15. The Type B uncertainty is dominated by microwave amplitude dependent effects. The microwave amplitude effects were re-evaluated recently since the NIST-F2 microwave synthesizer was slightly modified resulting in a slightly larger uncertainty in this systematic.