First accuracy evaluation of NIST-F2

TLDR

NIST‑F2 is a second‑generation laser‑cooled caesium fountain primary standard with a cryogenic microwave cavity, developed at NIST and tested extensively since 2008. The paper presents the design of the physics package, laser and optics systems, and the methods used for accuracy evaluation. Operating at 80 K reduces blackbody radiation shift uncertainty by over 50×, the high‑Q Ramsey cavity lowers distributed cavity phase shift, and NIST‑F2 has been compared in‑house with a NIST maser and NIST‑F1. Nine in‑house comparisons since 2010 show a type B fractional uncertainty of 0.11 × 10⁻¹⁵, dominated by microwave amplitude effects, and a type A statistical uncertainty of 0.44 × 10⁻¹⁵ in the August 2013 evaluation.

Abstract

We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (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 shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>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 time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine 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. The type B fractional uncertainty of NIST-F2 is shown to be 0.11 × 10−15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44 × 10−15.