A recirculating cryogenic hydrogen maser

This paper reports on the design, theory, and initial testing of a new cryogenic hydrogen maser in which hydrogen (H) atoms circulated back and forth between a microwave-pumped state-selecting source and a small split-ring maser cavity. Initial tests observed no measurable fluctuations in the maser frequency at the level of 6 × 10
⁻¹³
for measuring times between τ = 1 s and 300 s.