Noble-Gas Loss in Alkali Rf-Discharge Lamps and Its Possible Dependence on Electron Temperature

The optically pumped, vapor-cell atomic clock is a work horse of precise timekeeping, finding applications onboard global navigation satellites as well as at cellular communications base stations. At the heart of the device is the relatively simple alkali rf-discharge lamp, which enables the production of the atomic-clock signal and the sensing of the atoms' response to resonant microwaves. In the lamp, electrons extract energy from an rf-field via elastic collisions with noble-gas buffer atoms; the energetic electrons then ionize Rb atoms; finally, resonant light for optical pumping is produced through Rb
⁺
/electron recombination at the lamp's glass walls. Unfortunately, recent research has shown that alkali rf-discharge lamps slowly lose their buffer gas, giving rise to a life-limiting mechanism for the rf-discharge lamp and hence the atomic clock. Here, we review the literature on buffer-gas loss in alkali rf-discharge lamps and discuss a likely mechanism for the process. We then discuss the dependence of the discharge's electron temperature on rf-frequency, and how this might be used as a critical test of the proposed mechanism.