Loading of a fountain clock with an enhanced Low-Velocity Intense Source of atoms

We present experimental work for improved atom loading in the optical molasses of a caesium fountain clock, employing a low-velocity intense source of atoms (LVIS) [Lu et al., Phys. Rev. Lett. 77, 3331 (1996)], which we modified by adding a "dark" state pump laser. With this modification the atom source has a mean flux of $4 \times 10^{8}$ atoms/s at a mean atom velocity of $8.6$ m/s. Compared to fountain operation using background gas loading, we achieved a significant increase of the loaded and detected atom number by a factor of 40. Operating the fountain clock with a total number of detected atoms $N_{\mathrm{at}}=2.9 \times 10^6$ in the quantum projection noise-limited regime, a frequency instability $\sigma_y\left(1\text{s}\right)=2.7 \times 10^{-14}$ was demonstrated.