Abstract-In superconducting magnets operating at high heat loads as the ones for interaction region of particle colliders or for fast cycling synchrotrons, the limited heat transfer capability of state-of-the-art electrical insulation may constitute a heavy limitation to performance. In the LHC main magnets, Nb-Ti epoxy-free insulation, composed of polyimide tapes, has proved to be permeable to superfluid helium, however the heat flux is rather limited.After a review of the standard insulation scheme for Nb-Ti and of the associated heat transfer mechanisms, we show the existence of a large margin available to improve insulation permeability. We propose a possible way to profit of such a margin in order to increase significantly the maximum heat flux drainable from an all polyimide insulated Nb-Ti coil, as it is used in modern accelerator magnets.

In superconducting magnets, the energy deposited or generated in the coil must be evacuated to prevent temperature rise and consequent transition of the superconductor to the resistive state. The main barrier to heat extraction is represented by the electric insulation wrapped around superconducting cables. In the LHC, insulation improvement is a key point in the development of interaction region magnets and injector chain fast-pulsed magnets for luminosity upgrade; the high heat load of these magnets, in fact, is not compatible with the use of current insulation schemes. We review the standard insulation schemes for NbTi and Nb 3 Sn technology from the thermal point of view. We implement, in an analytical model, the strongly nonlinear thermal resistances of the different coil components including the permeability to superfluid helium of Nb-Ti insulations, measured during the LHC main dipole development. We use such a model to compare Nb-Ti and Nb 3 Sn technologies by taking into account their specific operating margin in different working conditions. Finally, we propose an insulation scheme to enhance the heat transfer capability of Nb-Ti coils.

Abstract-This paper presents the results of a comprehensive analysis, from the geometric point of view, of the pre-series LHC dipoles. The progressive change of the imposed magnet shape has been monitored from the first assembly stage until after the cold test. Data concerning the error on sagitta, extremity positions and sextupolar corrector positions are provided for the pre-series magnets. Implications of aligning out-of-tolerance dipoles by the extremities are also discussed.Index Terms-Corrector positions, dipole alignment, dipole shape.

The detailed shape of the 15 m long superconducting LHC dipole cold mass is of high importance as it determines three key parameters: the beam aperture, nominally of the order of 10 beam standard deviations; the connectivity of the beam-and technical lines between magnets; the transverse position of non-linear correctors mounted on the dipole ends. An offset of the latter produces unwanted beam dynamics perturbations. The tolerances are in the order of mm over the length of the magnet. The natural flexibility of the dipole and its mechanical structure allow deformations during handling and transportation which exceed the tolerances. This paper presents the observed deformations of the geometry during handling and various operations at CERN, deformations which are interpreted thanks to a simple mechanical model. These observations have led to a strategy of dipole geometry control at CERN, based on adjustment of the position of its central support (the dipole is supported at three positions, horizontally and vertically) to recover individually or statistically their original shape as manufactured. The implementation of this strategy is discussed, with the goal of finding a compromise between conflicting requirements: quality of the dipole geometry, available resources for corrective actions and magnet installation strategy whereby the geometry tolerances depend on the final magnet position in the machine.

Abstract-The Large Hadron Collider (LHC) superconducting dipole cold mass is a cylindrical structure 15 m long, made of a shrinking cylinder which contains iron laminations and collared coils. This structure, weighing about 28 ton is horizontally bent by 5 mrad. Its shape should be preserved from the assembly phase to the operational condition at cryogenic temperature. Hence an accurate comprehension of the mechanical behavior of the cold mass is required. In particular the flexural rigidity in horizontal and vertical directions represents one of the foremost properties to be aware of. To determine the flexural rigidity, deformations of the cold mass induced by the self weight have been measured and compared with the predictions of an analytical structural model. A particular care has been taken in reducing the experimental error by an appropriate fitting procedure.Index Terms-Optical interferometry, structural engineering superconducting magnet.