Transient Thermoelectrical Behavior of MgB2 Cables in the Superconducting Links for the High Luminosity Upgrade of the LHC

Abstract: High-current superconducting links (SC-Link) consisting of multiple MgB2 cables are being developed for the cold powering of the high luminosity upgrade of the LHC (HL-LHC). The SC-Links are designed to work over a temperature gradient between 4.2 and 20 K cooled by flowing subcritical helium gas to provide up to 20 kA per cable to the new inner-triplets to gain a higher luminosity. The SC-Links have a compact configuration of six 20-kA cables for three magnet circuits and several lower current cables. Upon the quench of one magnet, the neighboring circuits are exposed to a transient field up to 0.3 T, which may induce significant magnetization and coupling current losses with the potential of quenching the cables in these circuits. In this paper, we present a thermoelectrical analysis of the transient electromagnetics of the cable system, the thermal stability of the cables in temperature gradient, and the effect of lateral cooling by the flowing helium gas on the quench behavior. Furthermore, the analysis was compared with a comprehensive experimental study of direct thermometric and electrical measurements of the transient magnetic losses at different temperatures (4-30 K) and field sweep rates (0.3-30 T/s) as well as quench measurements under different cooling configurations. Cables samples of different stabilization matrices and strand configurations were measured to provide insight for the overall optimization by compromising between loss reduction and cryogenic stabilization.