Discharge lamps for Rb atomic clocks: The role of rf-power

Rubidium atomic clocks that fly on GNSS satellites employ the light from rf-discharge lamps for atomic signal generation and detection. In this application, the performance of the atomic clock and the capabilities of the navigation system depend sensitively on the lamp´s stability. In order to better understand the mechanisms that might lead to instability in these lamps, and hence the satellite clocks, we studied the optical emission from a Rb/Xe lamp as a function of the lamp´s rf-power. Surprisingly, we found that the electron density in the plasma was essentially independent of increases in rf-power above its nominal value (i.e., ldquorf-power gainrdquo), and that the electron temperature was only a slowly varying function of rf-power gain. The primary effect of rf-power was to increase the temperature of the neutrals in the plasma, which was manifested by an increase in Rb vapor density. Assuming that increased Rb density proportionally increases the lamp´s light intensity, we predict that the sensitivity of GNSS-quality Rb clocks to changes in lamp rf-power, |party/part[DeltaP_rf/P_rf]|, is on the order of 1.0 times 10^-14/%.