A Compact Hydrogen Maser with Exceptional Long-Term Stability

A compact passive hydrogen maser with many unique features, including a significant reduction in size over previous hydrogen masers, is described. It uses the passive mode of operation, thereby permitting use of a small microwave TE011 cavity which is dielectrically loaded by a low-loss alumina (Al2O3). The cavity is 14.6 cm outside diameter and 13.7 cm high, and weighs only 4.4 kg. The unloaded-cavity Q factor is about 6000. With a conventional source, hexapole state selector, and four magnetic shields, the volume of the entire hydrogen maser resonator package is only about 20 l. The FEP-coated quartz bulb which is common in other masers has been replaced by a similar coating on the inside wall of the cavity. This has yielded a simpler design and more rugged hydrogen maser package. The technique for the application of liquid emulsion FEP is discussed. The cavity and attached endcaps compose the vacuum envelope, thus allowing use of a single vacuum system. The dimensional stability of the ceramic cavity under barometric changes is sufficiently within the range of the electronic cavity servo that a second vacuum system is not needed. For temperature control, a single oven is located in the magnetic shield nest. The most recent stability data compiled from four compact hydrogen masers built at NBS is presented. Comparisons are made against the cesium standard NBS-4 and a time scale comprised of nine commercial cesium standards (denoted as 8s).