CS2: The PTB´s new primary clock

The design principles of the Physikalisch-Technische Bundesanstalt´s (PTB´s) new primary clock, CS2, are reported. Experimental results concerning C field homogeneity, RF power dependence, cavity phase difference, and its gradient are given. The theoretical frequency instability due to shot and thermal noise, σ(2, 1 s) = 2.7 × 10⁻¹², is compatible with the data so far obtained. The present uncertainty of CS2 is estimated to be 22 × 10⁻¹⁵. Frequency intercomparisons of the PTB´s two primary clocks, CSI, and CS2, averaged over one year yield a fractional difference of (3 ± 4) × 10^−l5.