Life, line Q, spin-exchange tuning and stability of atomic hydrogen maser frequency standards

It is shown that the STSC (Sigma Tau Standards Corporation) design of the hydrogen maser may be expected to provide stable performance for periods of time measured in decades, without long-term interruption for recoating of the storage bulb, changing of pumps, or replenishment of hydrogen. In addition, instability or interruption due to degradation of line Q or systematic errors associated with the cavity automatic tuner was shown to be very small. The masers do have some initial change in frequency with time, and this is believed to be due to changes in the interaction of the hydrogen atoms with the wall of the storage bulb. The initial drift is very small, however, typically less than 2 × 10 ^-15/day, and it disappears with a time constant of between six months and one year. Thus, for most of the operational life of the maser, the long-term drift is so small as to be inconsequential for all present applications