THE CRYOGENIC HYDROGEN MASER: PROJECTED PERFORMANCE AND RECENT PROGRESS TOWARDS SPACEBORNE APPLICATIONS

The possibility of significant improvement in the frequency stabil-ity of hydrogen masers by operation at cryogenic temperatures I < 1 K) is discussed. The results of the Harvard/SAO collaboration to build and operate a cryogenic hydrogen maser are summarized, and ongoing work to include space-qualified technology in the cryogenic system is described. Finally, a short list is given of scientific applica-tions (on Earth and in space) of a cryogenic hydrogen maser of supe-rior frequency stability. Currently, roo111 temperature hydrogen masers can provide fractional frccluency stability close to lo-' ' for averaging intervals of about 104 seconds. Hydrogen maser operation at cryogenic temperatures is 0.5 K) may bring this stability into the 1 0 ~ ' ~ e g i o n, as both thermal noise and the line-broadenin