Development of the cesium fountain frequency standard, NMIJ-F2

We have made much progress on NMIJ-F2, which is our second cesium fountain frequency standard aiming an uncertainty of <; 1×10^-15 as an immediate goal. The frequency stability is improved to 8.3×10^-14τ^-1/2 (τ: averaging time) by applying a cryogenic sapphire oscillator using a pulse-tube cryocooler as a local oscillator and optically pumping the atoms to the Zeeman sublevel mF = 0. Then, the homogeneous magnetic field in the interrogation region is obtained with magnetic shielding, a long solenoid coil, and two additional coils. The fractional frequency correction for the 2nd-order Zeeman shift is evaluated to be -165.5×10^-15. Moreover, the fractional frequency correction for the collisional shift (the frequency shift due to collisions between cold atoms) is measured to be (+3.3±0.5)×10^-15 by an extrapolation method.