The preliminary evaluation of the new fountain clock NIM6

In this paper, we introduce the design of our new fountain clock NIM6. Besides some improvements on the vacuum system, Ramsey cavity to reduce the Type B uncertainty, NIM6 collects atoms from a MOT loading optical molasses to get a better signal to noise ratio. The atom density will be more uniform compared to a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling after MOT to keep the collisional shift low. Cs atoms are collected in the lower MOT chamber for 0.5 second, and then the MOT magnetic field is turned down. Atoms are accelerated for 1 ms in a moving molasses, and further cooled for another 1.5 ms by adiabatically reducing the cooling beam intensities and red offsetting the frequencies.