Operation of the caesium fountain frequency standard NPL-CsF2 at the collisional shift cancellation point

We report on characterisation of a caesium fountain primary frequency standard NPL-CsF2 operating in a regime where the collisional frequency shift is nearly cancelled by controlling the clock state population ratio and other fountain parameters. We describe in detail the operation protocols possible in this case and their limitations and report on a long-term study of the stability of the parameters defining the cancellation point. Finally, we report on the frequency measurements where NPL-CsF2 was used as a reference, namely, a new measurement of the ⁸⁷Rb clock transition and several evaluations of the TAI step interval.