Long term magnetic performance of the steel concrete dipoles in LEP

The steel-concrete cores of the LEP bending magnets were built of regularly spaced steel laminations, the spaces being filled with cement mortar. The effects of compressive stresses were studied on models and the long term behaviour has been monitored during operation of the LEP machine over a period of four years. The requirements for stability and reproducibility of the magnetic field have increased in step with the development of the accelerator and its particle detectors. After the initial ageing in the LEP tunnel, the most important parameter was the temperature coefficient. The temperatures of a number of magnet cores are therefore continuously monitored and corrections are applied to the indicated value of particle momentum as measured by NMR and a flip coil in a reference dipole connected in series with the bending magnets. This reference magnet is in turn calibrated periodically by a direct measurement of flux variations in a loop mounted in the lower poles of all bending magnets installed in the tunnel.< >