D. Tommasini

Heat Transfer in an Enhanced Cable Insulation Scheme for the Superconducting Magnets of the LHC Luminosity Upgrade

The next generation of superconducting magnets for the interaction regions of particle colliders, as well as for fast cycled accelerators, will be confronted with large heat loads. In order to improve the evacuation of heat from the Nb-Ti coil towards He-II bath, a porous (enhanced) all-polyimide cable insulation scheme was proposed recently. The first results were promising, featuring a larger permeability to helium with respect to existing schemes under low compressive stress. In this paper we present an extended experimental study of heat transfer through the Enhanced Insulation into He-II bath, and comparison to the standard LHC insulation, at different levels of applied pressure. The thermal coupling between adjacent cables was investigated, as well as the impact of a localized heat deposition versus a distributed one. The results of this study show that, up to high pressure levels, the enhanced insulation scheme can provide a major improvement of heat transfer compared to the standard scheme used in the main LHC magnets.

The 16 T Dipole Development Program for FCC

A key challenge for a future circular collider (FCC) with centre-of-mass energy of 100 TeV and a circumference in the range of 100 km is the development of high-field superconducting accelerator magnets, capable of providing a 16 T dipolar field of accelerator quality in a 50 mm aperture. This paper summarizes the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.

Cable Insulation Scheme to Improve Heat Transfer to Superfluid Helium in Nb-Ti Accelerator Magnets

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.

Impact of coil deformations on field quality in the Large Hadron Collider main dipole

In superconducting accelerator magnets the coils are usually pre-stressed in order to avoid conductor movements induced by electro-magnetic forces. In this paper we use a finite element mechanical model of the main LHC dipole to evaluate the coil deformations determined by the pre-stress and their impact on magnetic field quality. The model explains the origin of the offsets between the nominal multipole values and those measured at room temperature in prototype and pre-series dipole magnets. We also present an experiment carried out to analyze the impact on field quality and coil stresses of coil azimuthal spacers (pole shims). A 1 m long dipole collared coil has been re-assembled several times with pole shims of different thickness and the field components have been measured each time. Experimental data are compared to numerical computations based on the mechanical model. One finds that variations of shim thickness induce not only a change of the azimuthal coil length, but also a different pattern in coil deformations. A good agreement is found between measurements and simulations.

Electrical and Mechanical Performance of an Enhanced Cable Insulation Scheme for Superconducting Magnets

New polyimide cable insulation schemes improving the cooling of Nb-Ti superconducting coils were recently developed to face the severe heat loads at which the next generation of superconducting accelerator magnets will work. In order to qualify the new insulation, a test campaign was realized to assess both its electrical and mechanical features with respect to the standard LHC insulation. The electrical tests assessed the dielectric strength and inter-turn leakage current to be satisfactory. The mechanical tests investigated the insulation thickness under load and the stress relaxation at ambient temperature, thus providing essential information for the magnetic and mechanical design of the final focusing magnets for the LHC upgrade phase I.

Status report on the LHC main magnet production

The LHC ring will contain 1232 main dipole and 382 main quadrupole double aperture magnets. All main magnets are superconducting and employ Nb-Ti/Cu Rutherford type cables operated at 1.9 K. The dipole production has reached the equivalent of almost three octants of cold masses and nearly four octants of collared coils. The quadrupole production has reached 75 cold masses and over 150 bare magnets. The ramping up of large scale magnet production has posed several challenges which will be discussed, like: the coil size uniformity, coil pre-stress control, cold mass welding technique and the geometrical shape issues. The magnetic measurement results at warm will be presented together with their usage for the quality control in the production. The common features and differences of the three dipole producers will be discussed. The latest version of the production schedule will be presented.

Performance of five and six block coil geometries in short superconducting dipole models for the LHC

A series of similar one meter long superconducting dipole models for the LHC is being manufactured and tested since 1995 for exploring design variants and assembly parameters. Until the end of 1997 all magnets of this series were based on a coil geometry subdividing the conductors in five distinctive winding blocks. In order to cope with new requirements of magnetic field distribution and coil design flexibility, one additional block has been added in the beginning of 1998. A significant number of models of both types have been built and tested, some of them re-built in a different version, adding up in more than 40 models tested so far. The paper reviews the performance of these two different coil designs in terms of manufacture, training behaviour and temperature margins as well as mechanical behaviour and magnetic field quality.

Design, Manufacture and Measurements of Permanent Quadrupole Magnets for Linac4

Compact quadrupole magnets are required for the CCDTL (Cell-Coupled Drift Tube Linac) of Linac 4, a 160 MeV linear accelerator of negative hydrogen ions which will replace the old 50 MeV proton Linac2 at CERN. The magnets, of an overall physical length of 140 mm and an aperture diameter of 45 mm, are based on Sm2Co17 blocks and can provide an integrated gradient of up to 1.6 Tesla. The magnetic field quality is determined by 4 ferromagnetic pole tips, aligned together with the permanent magnets blocks inside a structure made in a single piece. Tuning bars allow to individually trim the magnetic flux provided by each pole, to correct possible differences between blocks and to modify the field gradient intensity within about 20% of the nominal value. The paper describes and discusses the design, manufacture and magnetic measurements of a first prototype magnet.

Construction of the CERN Fast Cycled Superconducting Dipole Magnet Prototype

CERN is pursuing a small scale R&D on a fast cycled superconducting dipole magnet (FCM) of interest for the upgrade plan of the LHC accelerator complex. The FCM dipole prototype being built has a number of novel features if compared to other magnets for similar applications. In this paper we describe the magnet design, and its expected performance, focusing especially on the novel features (magnetic circuit, mechanical supports, cooling) and on the details of the manufacturing procedure (coil winding and impregnation, joints, instrumentation and quench protection).

STEADY-STATE HEAT TRANSFER THROUGH MICRO-CHANNELS IN PRESSURIZED HE II

The operation of the Large Hadron Collider superconducting magnets for current and high luminosity future applications relies on the cooling provided by helium-permeable cable insulations. These insulations take advantage of a He II micro-channels network constituting an extremely efficient path for heat extraction. In order to provide a fundamental understanding of the underlying thermal mechanisms, an experimental setup was built to investigate heat transport through single He II channels typical of the superconducting cable insulation network, where deviation from the macro-scale theory can appear. Micro-fabrication techniques were exploited to etch the channels down to a depth of ~ 16 im. The heat transport properties were measured in static pressurized He II and analyzed in terms of the laminar and turbulent He II laws, as well as in terms of the critical heat flux between the two regions.