Quench current in conduction-cooled HTS magnets

The quench current of a superconducting magnet, I q , is the current at which a thermal runaway occurs. In trained LTS magnets I q can be estimated from a short sample critical current, I c , due to a steep electric field ( E )–current density ( J ) characteristic. Anisotropy and the slanted E ( J )-characteristic make the situation more complicated in HTS magnets. Furthermore, the I q of a conduction-cooled magnet depends strongly on the geometry of the thermal interface. Several criteria, such as the average electric field of 0.1 μV cm −1 and the maximum electric field of 1 μV cm −1 have been suggested for I q of an HTS magnet. However, in order to determine I q accurately a detailed stability analysis is required. In this paper different I q criteria for conduction-cooled HTS magnets are computationally compared at the operation temperatures of 4.2, 20 and 77 K. Computations are based on the I c data measured with a Bi-2223/Ag tape. 150 different solenoidal magnets having the wire length of 2, 5 and 10 km have been studied. The effect of the thermal interface geometry on I q has also been investigated. Rules of thumb for the quick estimation of I q at the given operation temperature are suggested.