Investigation of Rapid Adiabatic Passage for Controlling Collisional Frequency Shifts in a Caesium Fountain Clock

We explore the method of rapid adiabatic passage for collisional frequency shift measurements in the primary caesium fountain clock PTB-CSF2 at the Physikalisch-Technische Bundesanstalt (PTB). This method can be used to vary the local density distribution of the atom cloud by exactly a factor of 2. We investigate the performance of the method for different parameter ranges and obtained agreement between experimental and Monte-Carlo simulation results. When used for collisional frequency shift measurements, the rapid adiabatic passage method results in a systematic uncertainty contribution of less than 1% of the collisional shift value. Implications of this method for the systematic uncertainty of CSF2 are discussed.