Design concept for the microwave interrogation structure in PARCS

In this paper we describe key aspects of the conceptual design of the microwave interrogation structure in the laser-cooled cesium frequency standard that is part of the primary atomic reference clock in space (PARCS) experiment. The PARCS standard uses balls of cold atoms launched in a pulsed beam configuration. The microwave interrogation will take place in two independent high-Q (∼20,000) cavities operated in the TE₀₁₁ mode. The cavities will be operated off resonance by several linewidths, with a resonant coupling structure delivering the microwave to the two cavities. One persistent problem related to the end-to-end phase shift has been the extreme temperature sensitivity of the phase of inside the cavities to that just outside the cavities. The end-to-end phase shift must ultimately be known to around 3 microradians, and stable long enough to allow measurement of the shift as well as to allow normal clock operation. Operating the cavities off resonance reduces this sensitivity more strongly than reducing the cavity Q.