Mercury lamp studies in support of trapped ion frequency standards

The mercury linear ion trap frequency standard (LITS) [1] at JPL continues to advance with multiple applications. In particular, the outstanding long-term stability [2] and practicality of the ground-based clock have attracted significant interests for time-keeping and metrology. However, the mercury RF discharge lamp used for optical pumping and state detection may limit the ultimate stability performance of the clock [3-4], constraining even broader application. For mercury ion frequency standards, the operational lamp behavior is described by the ratio of useful light at 194nm and unwanted background light at 254nm (194/254). This ratio has been observed to depend on several factors Increasing the 194nm output decreases optical pumping times and an increase of the 194/254 ratio improves the clock signal-to-noise ratio (SNR). These improvements lead to an improvement in clock short-term stability and enable the use of an even broader range of local oscillators. We have carried out several experiments to unfold the relationship between the 194/254 and the fabrication factors: buffer gas pressure, lamp ID, and the quantity of mercury. The quantitative results may be used to improve the process of lamp fabrication for mercury ion frequency standards. The research here may also shed light on other lamp-based applications.