Cesium Atomic Fountain Clocks at Nmij

We describe the recent progress in our cesium atomic fountains at National Metrology Institute of Japan (NMIJ). We have developed three atomic fountains: NMIJ-F1, NMIJ-F2, and a truncated atomic beam fountain. NMIJ-F1 has been the primary frequency standard with uncertainty of 4×10 -15 since 2004. Especially in the last 4 years, we have reported the data to Bureau International des Poids et Mesures (BIPM) 21 times by operating NMIJ-F1 due to the progress in the stability and the reliability of the whole system. The second atomic fountain, NMIJ-F2, is under construction to achieve uncertainty of less than 1×10 -15 . The optical molasses for generation of cold atoms is composed of six horizontal laser beams and two vertical laser beams with power of about 100 mW per beam. In addition, we proposed the truncated atomic beam fountain to achieve both a low collisional frequency shift and high frequency stability.