Diamagnetic forces in superconducting magnets

We have previously reported damage to two large superconducting solenoids caused by axial expansion forces and radial compression forces. These forces, which can be greater than and are of opposite sense to the classical Lorentz forces, we have attributed to the diamagnetism of the composite superconductors. Some irregularities experienced in the operation of force cooled superconducting magnet systems appear to be readily understood by considering diamagnetic forces. It seems likely that several other previously unexplained occurences in the operation of large superconducting magnets will also eventually be attributed to this effect. We describe how diamagnetic properties of superconducting strips produce repulsive forces on opposite edges of the strips. At the end pancake and innermost layer of a solenoid, these forces are seriously unbalanced and may override the Lorentz forces. We discuss how the magnitude of these forces may be estimated and how their effects may be minimized. We suggest that the stress analysis of large superconducting magnets currently in operation or in design be reviewed to consider the relevance of diamagnetic forces to their safe operation.