Choice of design margins of superconducting magnets

摘要

One of the most important steps at the design stages of a superconducting (sc) magnet is a proper choice of current margins, i.e., of the ratio of certain critical value of a superconductor used to maximum corresponding value envisioned during normal operation of the magnet. Very often the ratio ng of the critical current at the operational magnetic field Bo to the transport current IQ at the same field is put to use as the characteristic margin. However, the only meaningful margin is the energy margin that is minimum energy (per unit of the superconductor length or of the magnet's winding volume) required to induce quench at given conditions. The energy margin depends mainly on the difference AT between the critical temperature TBI of the superconductor (at nominal values of the maximum field Bo and the transport current IQ) and the operational temperature To (usually equal to that of the coolant). In the simplest and practically important case of adiabatic windings, the interdependence between the energy margin and the temperature one is evidently single-valued. On the other hand, much-used "constant-field current margin" UB depends not only on the temperature margin AT, but also on Bo/Be ratio (Be is the critical field). As shown in the paper, it is preferable to characterize sc windings by "the load-line current margin" UBI, i.e., by the ratio of the critical current taken along the load line to the nominal transport current. The interdependence of AT and n_(BI) is practically single-valued. This statement is strict if the critical surface is approximated by a plane.