Hydrogen maser at temperatures below 1 K

We describe the design and operation of a cryogenic hydrogen maser for use at temperatures below 1 K. The maser incorporates a cooled atomic hydrogen beam and cylindrical dielectrically loaded resonant cavity with liquid-He-covered walls to prevent hydrogen recombination and dephasing of the atoms' oscillating dipole moment upon wall collisions. We have observed maser oscillation from approximately 350 to 575 mK at power levels up to 5×{}10^-13 W, and have measured the maser's line Q and frequency as a function of temperature and helium-film thickness.