Advancements in the NRC-FCs2 primary frequency standard

We have made improvements in the stability, accuracy, and performance of the NRC-FCs2 fountain clock. The dominant systematic effects have been re-evaluated. Optically-stabilized microwaves are used to improve the short-term stability, now reaching σ y = 3 × 10 − 14 τ − 1 2 . We have evaluated the distributed cavity phase shift using absorption imaging. This technique dramatically reduces the evaluation time and final uncertainty. We have re-evaluated biases due to microwave leakage and synchronous phase transients with higher accuracy. The total systematic uncertainty of NRC-FCs2 is now 1.1×10⁻¹⁶ in fractional frequency, a factor of 2 improvement over its most recent evaluation.