Measurement of the mechanical behaviour of the LHC beam screen during a quench

The LHC (Large Hadron Collider) beam vacuum system will be equipped with a so-called beam screen. This screen, operating between 5 K and 20 K, is coaxially placed inside the beam pipe to reduce the beam induced heat load to the superconducting coil. The racetrack shaped stainless steel screen is perforated to provide vacuum pumping and has an inner layer of 50 μm high purity copper to limit the machine impedance. The beam screen needs to resist the induced Lorentz forces during a resistive transition (quench) of the magnet. This paper reports the measurements carried out on a 15.6 metre long screen tested in aperture, I, of the twin aperture prototype LHC main dipole magnet MBP201. The quench induced deformation of the beam screen cross-section, the angular displacement and the temperature increase after a quench have been measured with instrumentation developed for this test. Optical transducers, not influenced by the magnetic field, allowed measuring the deformation of the cross section with high resolution and precision. This gave an insight into the mechanisms that govern the behaviour of the beam screen during a quench. Finally, some "quench rings" positioned inside the beam screen were used to check the thickness of the beam screen wall where plastification would occur