Currents in, Forces on and Deformations/Displacements of the LHC Beam Screen Expected during a Magnet Quench

Due to the field asymmetry of the LHC dipoles, the magnetic field integral calculated from the centre of the aperture to the outside of the cold mass does not vanish. During a magnet quench this asymmetry generates an electromotive force and thus a current with a resultant lateral force on the beam screen. This induced force was observed indirectly when measuring the deformation of the beam screen cross-section during a quench using a precision displacement transducer, which will be described. The transducer (based on optical gratings) was developed specially to study the beam screen deformation in cryogenic environments and high magnetic fields. The results of the measurements are compared to calculations and to direct measurements of the induced voltage along the current path. An estimation of the forces exerted on the cold bore by the beam screen and of the actual current induced in the beam screen will be given.