Development of a displacement sensor for the CERN-LHC superconducting cryodipoles

Abstract

One of the main challenges of the Large Hadron Collider (LHC), the particle accelerator under construction at CERN (the European Organization for Nuclear Research) in Geneva, resides in the design and production of the superconducting dipoles used to steer the particles around a 27 km underground tunnel. These so-called cryodipoles are composed of an evacuated cryostat and a cold mass, that contains the particle tubes and the superconducting dipole magnet and is cooled by superfluid Helium at 1.9 K. The particle beam must be centred within the dipole magnetic field with a sub-millimetre accuracy, this requires in turn that the relative displacements between the cryostat and the cold mass must be monitored with accuracy.Because of the extreme environmental conditions (the displacement measurements must be made in vacuum and between two points at a temperature difference of about 300 degrees) no adequate existing monitoring system was found for this application. It was therefore decided to develop an optical sensor suitable for this application.This contribution describes the development of this novel sensor and the first measurements performed on the LHC cryodipoles. (CERN) [1]. The LHC will be installed in a 27 km long underground tunnel in the area near to Geneva and will accelerate two counter-rotating colliding proton beams, each at a 7 TeV energy. This project is aimed at studying the interactions of the basic constituents of the matter at the multi-TeV energy level; this is supposed to enlighten our knowledge on Higgs particles and on the theory of Supersymmetry. High magnetic fields are required to bend the proton beams to the desired curvature radius; they will be provided by 1232 dipole magnets, which will generate a 8.3 Tesla field. Superconducting technology makes this possible. Paper submitted to Measurement Science and TechnologyThe 1232 superconducting main dipole magnets will be distributed over eight arcs and will operate in a superfluid helium bath at 1.9 K. The cold masses of 15 m length will be made up of coils of superconducting niobium-titanium alloy cable mantained in shape and place by austenitic steel collars. These in turn are contained within a magnetic circuit consisting of low-carbon steel laminations with non-magnetic steel end-laminations. A welded austenitic stainless-steel shrinking cylinder, made up of two welded half cylinders (shells), surrounds the whole and also acts as the liquid helium vessel. A cross section of the LHC main dipole is given in figure 1.

Deformations and Displacements of the LHC Superconducting Dipoles Induced by Standard and Non-Standard Operational Modes

“…Moreover the magnet extremity six degrees of freedom would have made hard to distinguish a deformation along one direction from tilt and from deformations in other directions. These constraints pushed us toward a tailor made solution developed in collaboration with the EPFL (Ecole Polytechnique Federale de Lausanne) and SMARTEC S.A. [3]. In fact, the original idea to overcome all the previous issues by mean of an optical device was eventually realized through the modification of a commercial measuring device used to monitor deformations in civil structures and already in production by SMARTEC S.A.. prototype dipoles that already formed the String1 half-cell, whereas the new one is made of pre-series dipoles of final design and construction.…”

“…To monitor the deformation of a mechanical structure generating an 8.3 T field at 1.9 K, insulated by thermal shields and housed in a cryostat, a specific measuring system has been developed at CERN in collaboration with external partners [3]. Magnet displacements and deformations were measured in operative phases during the test of the first LHC full-cell prototype called String2, schematically shown in Fig.…”

Structural Analysis and Validation of a Smart Pantograph Mast Concept

“…This simplification does not affect the results or the conclusions of the presented research, since the numerical outputs are the same for temperature and wind pressure sensors (i.e., scalar value for each instrumented point). Once the proof of the concept is established in this article, more sophisticated sensing systems for the mast could be designed based not only on FO pressure sensors (Pinet et al, ), but also on FO strain or displacement sensors (Glisic et al, ; Inaudi et al, ). The preference for FO sensors is based on their proven accuracy, resolution, versatility, and long‐term stability (Inaudi and Glisic, , ; Glisic and Inaudi, ).…”

Multi-wavelength fiber optic displacement sensing

“…In one type of measurement, the fiber illuminates the target with incoherent light and distance/displacement information can be determined from the intensity of light back reflected into the same fiber, and/or adjacent fibers [1][2] . In another approach, sometimes called "white light interferometry", partially coherent light is used, and the reflected light is mixed with light sent through an adjustable reference arm [3][4] ; here the distance is determined by the appearance of interference fringes, observable only when the arms balance to within the known (short) coherence length of the source. Accurate displacement measurements can also be made in the FMCW method, i.e.…”