APPLICATION OF HIGH TEMPERATURE SUPERCONDUCTIVITY TO ULTRASTABLE FREQUENCY SOURCES*

The advent of superconductivity at temperatures above 77 ~e lv in [ l] will make possible new capabilities in frequency sources for both ground based and space borne applica-tions. In this paper we assess the impact of these new materials on atomic frequency standards. As an example, an active hydrogen maser could use Josephson junctions and infinite conductivity to sense and shield magnetic fields to high precision while enabling a significant reduction in size and mass. Similar benefits are found in the use of a small, heavily loaded superconducting cavity for the maser. High temperature superconductors will also have ramifications for other frequency standards, and for technologies related to frequency sources such as distribution networks. High temperature superconductivity is discussed from a technologists point of view, with particular emphasis on the temperature dependence of various properties of su-perconducting shields, sensors, cavities, e t ~. While some aspects of superconductivity, such as shielding effectiveness, are essentially completely developed at temperatures only slightly below the critical temperature T,, others, such as microwave surface losses, require much lower temperatures. The most important aspects which relate to ultra-high stability appear to relate to the sensing and shielding of magnetic fields. Light weight and small size can result from effective shields and also from application of small persistent mode electromagnets in (e.g.) ion pumps. The temperature de-pendence of trapped fields is discussed, since, while external fields may be completely shielded, any field which may be necessary inside the shield is affected by variations in the penetration depth