Damage by diamagnetic forces in large superconducting coils

Damages to a 75kG, 41 cm bore NbTi coil and to a 19 cm bore Nb3Sn coil are described. These are attributed to axially expansive and radially inwards forces produced by the diamagnetic properties of the composite superconductors. Qualitative explanations and approximate calculations explain the facts. Diamagnetic forces have been neglected previously in the design of superconducting coils. We have learnt that they can be important. We suggest methods of avoiding their effects. We question the validity of the traditional Lorentz force calculations for superconducting coils.