Physics of systematic frequency variations in hydrogen masers

The frequency stability of hydrogen masers for intervals longer than 10/sup 4/ s is currently limited by systematic processes. The physics of frequency-determining mechanisms internal to the maser that are susceptible to systematic variations, and the connections between these internal mechanisms and external environmental factors are discussed. From estimates of the magnitudes of systematic effects, it is found that the primary internal mechanisms limiting long-term maser frequency stability are cavity pulling, at the level of parts in 10/sup 15/ per day, and wall shift variations, at the level of parts in 10/sup 16/ to parts in 10/sup 15/ per day. Strategies for reducing systematic frequency variations are discussed.<>