REFRIGERATION FOR FORCED-COOLED LARGE SUPERCONDUCTING MAGNETS.
Forced-cooled superconductors are viewed as a promising alternative in the development of high field superconducting magnets for future fusion devices. The high current density cabled superconductor is protected against thermal instabilities by forcing (single phase) supercritical helium through the cable. The cryogenic cooling system for a forced-cooled superconducting magnet works as a refrigerator and a recirculator at the same time. The paper discusses the conceptual design of the cooling system for forced-cooled superconducting magnets with the overall objective of reducing the refrigeration costs. The discussion centers on systems in which the recirculator and refrigerator are designed and optimized separately, as well as on systems where the recirculator and refrigerator are optimized as a unit. The general conclusion of this article is that economic cooling systems must employ efficient cold pump recirculators in which the large flow, demanded by the forced-cooled superconducting magnet, is confined to the cold end of the refrigerating column. If the liquid helium pump efficiency is less than 40 percent, systems employing elevated temperature compressors are more economic.