The work carried out under this project has achieved the following basic goals:
- To understand and eliminate all FOCS faults and disturbances that may have an influence greater than 10-15 on the relative frequency performance of the standard. To do this, a unique method for protecting the atoms from residual microwave disturbances has been successfully implemented and validated. It consists of a ferrite bell, placed around the zone of free evolution of the atoms, and allowing efficient absorption of the parasitic microwave carried by the shields of the coaxial cables for supplying the signal to the microwave cavity. Another method of transporting the microwave to the Ramsey cavity using optical fibers has been demonstrated. This innovative technique is also interesting for all other pulsed fountains currently used and will be the subject of further development and publication.
- To develop evaluation methods necessary for the establishment of the uncertainty budget of FOCS. The continuous operation of the fountain, and in particular its dual-passage through the microwave cavity, makes the assessment of phase gradient effects more complex than in the case of pulsed fountains, and new methods of specific analysis have thus had to be developed. They are based on a combination of analytical models and Monte Carlo simulations, using the phase distributions in the microwave cavity determined by finite element methods as key elements. The results of the measurements carried out are in excellent agreement with the results of the simulations, and made it possible to determine an upper limit of uncertainty associated with these effects of 1 x 10-15 (k = 1).
- To determine a detailed uncertainty budget of FOCS for all the most important contributions, and to demonstrate a relative uncertainty of frequency below 2 x 10-15 (k = 1).
