Long‐term high‐vacuum maintenance in the sealed cryogenic cable envelope for superconducting telecommunication cables

Abstract

AbstractTo obtain the high performance of a cyrogenic cable envelope employing multilayer vacuum insulations, it is necessary to maintain the presence between layers under 10−2 Pa. To investigate the possibility of long‐term high‐vacuum maintenance of a sealed cryogenic cable envelope containing adsorbents, we have cooled a 2‐m cryogenic envelope to a liquid nitrogen temperature and made a degassing analysis and degassing rate measurements. We found that the main degassing component was N2 and the degassing rates were at 10‐2 to 10‐3 Pa. We have also cooled a coconut‐shell charcoal to the liquid nitrogen temperature and obtained the N2 adsorption amounts. Based on the above results, we obtained the following conclusion. In a cryogenic cable envelope with an external diameter 70 mm in which Al is used as an outer pipe and the nitrogen shield pipes are stored inside, if we can realize the ultimate pressure below 10‐3 Pa, it is possible to have a durability life of over 20 years at a pressure below 10‐2 Pa by placing about 1 g of coconut‐shell charcoal per 1 m on the circumference of the liquid nitrogen shield pipe.