Recent results of the LPTF cesium fountain primary frequency standard

A Cs atomic fountain has been in operation at LPTF for more than one year. Ramsey fringes as narrow as 0.7 Hz are obtained. The short term fractional frequency stability is 2 10^-13 τ^1/2 , limited by the frequency noise of the local oscillator. Most of the classical sources of inaccuracy are considerably reduced in an atomic fountain, most being reduced to the 10^-16 level (in molasses operation). The new limiting effect is the spin exchange frequency shift due to collisions between cold atoms. The measurements we have performed show that it is possible to have a shift of 2 10^-13 with a potential stability of 10^-13 τ ^1/2. An evaluation of the major shifts and of the frequency stability limits in our fountain primary frequency standard are presented