Relation between different critical current criteria and quench current in HTS magnets

Abstract

In practice, the critical current of an HTS magnet is the current at which a thermal runaway (quench) occurs. The stability analysis required to determine the quench current, I q is often a time consuming numerical problem. Usually the short sample critical current, I c is measured by using some electric field criterion. In trained LTS magnets I q can be estimated from I c due to a steep electric field ( E)–current density ( J) characteristic. For HTS magnets the situation is more complicated due to the anisotropy and slanted E( J)-characteristics. In this paper the theoretical maximum of I q, I q max of conduction cooled HTS magnets is computationally compared with different critical current criteria at 20 K. Computations are based on the I c data measured with a Bi-2223/Ag tape. Two electric field criteria, 1 and 0.1 μV/cm, are applied to the magnets by investigating both the maximum electric field in a single turn of the coil and the voltage between the magnet poles. The critical currents obtained by these criteria are compared with I q max in several coil geometries of a solenoidal conduction cooled HTS magnet having the wire length of 2, 5 and 10 km. I q max is determined from the balance between the available cooling power and the total loss power generated inside the magnet. The objective is to enable a magnet designer to determine a safe operation current for an HTS magnet without performing a detailed stability analysis.