Influence of mortar-induced stresses on the magnetic characteristics of the LEP dipole cores

The steel-concrete cores of the Large Electron Positron colliding-beam accelerator dipole magnets are built of regularly spaced soft-magnetic-steel laminations, the spaces being filled with cement mortar. The effect of compressive stresses on the permeability and the coercivity of soft magnetic steel has been measured and the consequences for magnetic characteristics of the cores have been evaluated. By controlled straining of the aged cores in a hydraulic device, the tension in the mortar is brought locally to exceed the rupture limit. Thus, the compressive stresses in the steel laminations are partially relieved and the loss of bending strength in the dipoles is reduced. The excitation characteristics of the cores are measured by means of a fixed set of excitation windings and measuring coils into which the cores are moved after stress relieving. These measurements include determination of the equivalent surface areas of flux loops, which are embedded in the lower poles and will permit measurement of the field integral on the center line during LEP operation.