Sastry Pamidi (The Florida State University)
Various high temperature superconducting (HTS) power system components have been demonstrated using liquid nitrogen as the cryogen. Gaseous helium is being explored as a viable option for cooling HTS power devices. One of the major advantages of using helium is the versatility of operating temperature of HTS systems that provides adaptable and compact power distribution networks. However, challenges in using helium gas as a cryogen exist due its weak dielectric strength and low thermal capacity compared to that of liquid nitrogen. Gaseous helium mixed with small volume fractions of hydrogen and neon are being explored as a means of enhancing the dielectric strength. In order to provide HTS power system with required cooling power, high pressure helium at a pressure of around 1 MPa is essential and the additives for helium gas need to preserve the thermal characteristics of pure helium gas. Data on cryogenic thermal properties of helium gas mixtures with the additives at high pressures are not available. Theoretical evaluation using mixing theory has not been verified experimentally at cryogenic temperatures and high pressures. In this study, heat capacity and thermal conductivity were measured experimentally for helium mixtures with up to 4 % volume fraction of hydrogen and neon at pressures higher than 1 MPa and temperature lower than 77 K. These thermal properties would be useful for identifying a gas mixture with optimal dielectric and thermal characteristics essential for using the mixtures as cooling media for HTS power applications. Results of both theoretical and experimental studies on thermal characteristics of gaseous helium mixtures are presented.
Dr Jin-geun Kim (Florida State University)