Zero collisional frequency shift operation of a caesium fountain clock

We report preliminary results from a Cs fountain primary frequency standard operated in a regime where the collisional frequency shift is zero. This shift is of opposite sign for the two clock states. The shift is cancelled by equalizing the contributions from the two clock states by adjusting the clock state composition during the microwave interrogation. Design details of, and modifications to, the fountain NPL-CsF2 are described. Attention has been paid to assuring long-term stability of the fountain parameters, especially the atom cloud size and the amplitude of the interrogating microwave field.