Jean-Michel Rey

The Iseult/Inumac Whole Body 11.7 T MRI Magnet Design

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. Regarding the large aperture and field strength, this magnet is a real challenge as compared to the largest MRI systems ever built, and is then developed within an ambitious R&D program, Iseult, focus on high field MRI. The conservative MRI magnet design principles are not readily applicable and other concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will thus be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to the high level of stored energy, about 340 MJ, and a relatively high nominal current, about 1500 A, the magnet will be operated in a non-persistent mode with a conveniently stabilized power supply. In order to take advantage of superfluid helium properties and regarding the high electromagnetic stresses on the conductors, the winding will be made of wetted double pancakes meeting the Stekly criterion for cryostability. The magnet will be actively shielded to fulfill the specifications regarding the stray field.

Thermal conductivity measurements of epoxy systems at low temperature

We have developed a specific thermal conductivity measurement facility for solid materials at low temperature (LHe and LN2). At present, the Measurement of Thermal Conductivity of Insulators (MECTI) facility performs measurements on epoxy resin, as well as on bulk materials such as aluminum alloy and on insulators developed at Saclay. Thermal conductivity measurements on pre-impregnated fiber-glass epoxy composite are presented in the temperature range of 4.2 K to 14 K for different thicknesses in order to extract the thermal boundary resistance. We also present results obtained on four different bonding glues (Stycast 2850 FT, Poxycomet F, DP190, Eccobond 285) in the temperature range of 4.2 K to 10 K.

HTS Magnets: Opportunities and Issues for SMES

SMES (superconducting magnetic energy storage) devices are an attractive solution for pulsed current sources. However, the specific stored energy is moderate and has to be improved. High temperature superconductor (HTS) materials offer opportunities in terms of current carrying capabilities as well as mechanical strength. We report the optimization of solenoids with regards to mechanical stress and current density constraints. 20 kJ/kg requires current densities above 150 MA/m2 and stresses of about 400 MPa for energies in the range of 5 to 50 MJ. The optimization code makes possible the study of the influence of different parameters such as the maximum allowable magnetic field. A method is proposed to discriminate space of solution. The best configuration is achieved keeping in mind the peculiar properties of HTS tape (which impact protection of SMES), and the specifications of the application. Parallel to these theoretical approaches, we are developing the magnet technology by first realizing small magnets we test under very high fields (up to 18 T) to apply the large mechanical stresses.

HTS Insert Magnet Design Study

Future accelerator magnets will need to reach higher field in the range of 20 T. This field level is very difficult to reach using only Low Temperature Superconductor materials whereas High Temperature Superconductors (HTS) provide interesting opportunities. High current densities and stress levels are needed to design such magnets. YBCO superconductor indeed carries large current densities under high magnetic field and provides good mechanical properties especially when produced using the IBAD approach. The HFM EUCARD program studies the design and the realization of an HTS insert of 6 T inside a dipole of 13 T at 4.2 K. In the HTS insert, engineering current densities higher than 250 under 19 T are required to fulfill the specifications. The stress level is also very severe. YBCO IBAD tapes theoretically meet these challenges from presented measurements. The insert protection is also a critical because HTS materials show low quench propagation velocities and the coupling with the magnet makes the problem even more challenging. The magnetic and mechanical designs of the HTS insert as well as some protection investigation ways will be presented.

2-D/3-D quench simulation using ANSYS for epoxy impregnated Nb/sub 3/Sn high field magnets

A quench program using ANSYS is developed for the high field collider magnet for 3-D analysis. Its computational procedure is explained. The quench program is applied to a one meter Nb/sub 3/Sn high field model magnet, which is epoxy impregnated. The quench simulation program is used to estimate the temperature and mechanical stress inside the coil as well as over the whole magnet. It is concluded that for the one meter magnet with the presented cross section and configuration, the thermal effects due to the quench is tolerable. But we need much more quench study and improvements in the design for longer magnets.

SMES Optimization for High Energy Densities

High critical temperature superconductors (HTS) bring a lot of opportunities for SMES (Superconducting Magnetic Energy Storage). The large current densities under very high fields and the mechanical strength of IBAD route ReBaCuO coated conductors are very favorable characteristics. Electricity storage still is an issue in general and SMES bring a very interesting solution for pulse current supplies especially if its energy density increases. The record for SC magnet is 13.4 kJ/kg today. We study how to enhance this value. One of the main limitations for the SMES energy density is the mechanical stress as shown i.a. by the viriel theorem, which links simply stress and energy. The current density is another limitation not only the critical characteristic. Indeed protection also plays an important part and often is the real limitation for LTS magnets. We optimized solenoids with mechanical stress and current density constraints. 20 kJ/kg requires current densities of the order of 400 and stresses of about 400 MPa. These values are compatible with YBCO data but pose protection difficulties, which should be perhaps rethought. The design and these protection issues are discussed.

HTS Dipole Insert Developments

Future accelerator magnets will need to reach a magnetic field in the 20 T range. Reaching such a magnetic field is a challenge only reachable using high temperature superconductor (HTS) material. The high current densities and stress levels needed to satisfy the design criterion of such magnets make YBaCuO superconductor the most appropriate candidate especially when produced using the IBAD route. The HFM EUCARD program is aimed at designing and manufacturing a dipole insert made of HTS material generating 6 T inside a Nb3Sn dipole of 13 T at 4.2 K. In the HTS insert, engineering current densities higher than 250 MA/m2 under 19 T are required to reach the performances. The stress level is consequently very high. The insert protection is also a critical issue as HTS shows low quench propagation velocity. The coupling with the Nb3Sn dipole makes the problem even more difficult. The magnetic and mechanical designs of the HTS insert will be presented as well as the technological developments underway to realize this compact dipole insert.

On-Surface Test of the ATLAS Barrel Toroid Coils: Overview

The Barrel Toroid (BT) provides the magnetic field for the muon detectors in the ATLAS experiment at CERN. The Toroid is built up from eight superconducting coils. Each coil consists of two 25 m times 5 m racetrack shape double pancakes impregnated and pre-stressed inside an aluminum coil casing. The 42-tons cold mass is cooled by forced-flow liquid helium circulating in aluminum pipes glued to its surface. The coils are tested on surface prior to their underground installation. The test program has started in September 2004 and finished in June 2005. This paper describes the test set up and various commissioning tests performed at the ATLAS Magnet Test Facility. It includes the aspects of test preparation, vacuum pumping, leak testing, cooling down, powering and warming up. The 8 coils have passed the tests successfully and have been assembled into the Toroid in the ATLAS cavern. The testing completes the production of the so far largest racetrack coils in the world

Kinetics of phase growth in the Cu-Sn system and application to composite Nb/sub 3/Sn strands

Nb/sub 3/Sn is the superconductor most used in the R&D of high field accelerator magnets by either the wind&react or the react&wind technique. In order to program the low temperature steps of the heat treatment, the growth kinetics of Cu-Sn intermetallics was investigated as a function of duration and temperature. The diffusion constants of /spl eta/, /spl epsiv/ and /spl delta/ phases between 150 /spl deg/C and 550 /spl deg/C were evaluated using Cu-Sn model samples. For an accurate data analysis, statistical and systematic errors were determined. Next the behavior of Internal Tin and Modified Jelly Roll Nb/sub 3/Sn composites was compared with the model predictions.

Manufacturing and integration progress of the ATLAS barrel toroid magnet at CERN

ATLAS is one of the two experiments dedicated the search of the Higgs boson, which will be installed on the LHC ring at CERN in 2006. The ATLAS barrel toroid air-core magnet (BT) is 20 m in diameter and consists of 8 superconducting coils, each one 25 m long and 5 m wide. After several years of technological development, the major concepts have been proved in 1999/2000 during the construction of the B0 prototype; a technological model for BT. The delivery by several European industrial companies of all the major components for BT is nearly finished. The eight BT coils are now being integrated at CERN. The paper presents a general overview of the component manufacturing and integration progress. A special emphasis is put on the major component delivery (conductor, double pancake windings, aluminum coil casing and cryostat) together with a description of the two phases of the integration process: integration of the windings into their coil casings and integration of the cold mass into the vacuum vessel. The integration of the windings in their coil casings will be completed in October 2003. The closure of the first cryostat is planned for the end of the year. The start of the first cold test and the assembly in the cavern is foreseen for the beginning of 2004.