A. Devred

Status of the Next European Dipole (NED) activity of the Collaborated Accelerator Research in Europe (CARE) project

Plans for LHC upgrade and for the final focalization of linear colliders call for large aperture and/or high-performance dipole and quadrupole magnets that may be beyond the reach of conventional NbTi magnet technology. The Next European Dipole (NED) activity was launched on January 1st, 2004 to promote the development of high-performance, Nb/sub 3/Sn wires in collaboration with European industry (aiming at a noncopper critical current density of 1500 A/mm/sup 2/ at 4.2 K and 15 T) and to assess the suitability of Nb/sub 3/Sn technology to the next generation of accelerator magnets (aiming at an aperture of 88 mm and a conductor peak field of 15 T). It is integrated within the Collaborated Accelerator Research in Europe (CARE) project, involves seven collaborators, and is partly funded by the European Union. We present here an overview of the NED activity and we report on the status of the various work packages it encompasses.

High field accelerator magnet R&D in Europe

The LHC magnet R&D Program has shown that the limit of NbTi technology at 1.8 K was in the range 10 to 10.5 T. Hence, to go beyond the 10-T threshold, it is necessary to change of superconducting material. Given the state of the art in HTS, the only serious candidate is Nb/sub 3/Sn. A series of dipole magnet models built at Twente University and LBNL and a vigorous program underway at FNAL have demonstrated the feasibility of Nb/sub 3/Sn magnet technology. The next step is to bring this technology to maturity, which requires further conductor and conductor insulation development and a simplification of manufacturing processes. After outlining a roadmap to address outstanding issues, we review ongoing R&D programs in Europe, and we present the next European dipole (NED) initiative promoted by the European Steering Group on Accelerator R&D (ESGARD).

Finite Element Model to Study the Deformations of

The next European dipole (NED) activity is aimed at the development of a large-aperture, high-field superconducting magnet relying on high-performances Nb3Sn conductors. Part of the NED program is devoted to the mechanical study of a new generation of Nb3Sn wires and to predict and describe their behavior under the severe loading conditions of the cabling process. The deformation resulting from the cabling process was simulated through mechanical analyses by finite elements (FE). The ensuing results of FE analyses are presented, allowing the wire behavior under simple uni-axial loads to be described. They are compared to cross section micrographs of deformed wires.

{\rm Nb}_{3}{\rm Sn}

The stability of superconducting wires is a crucial task in the design of safe and reliable superconducting magnets. These magnets are prone to premature quenches due to local releases of energy. In order to simulate these energy disturbances, various heater technologies have been developed, such as coated tips, graphite pastes, and inductive coils. The experiments studied in the present work have been performed using a single-mode diode laser with an optical fiber to illuminate the superconducting strand surface. Minimum quench energies and voltage traces at different magnetic flux densities and transport currents have been measured on an LHC-type, Cu/NbTi wire bathed in pool boiling helium I. This paper deals with the numerical analysis of the experimental data. In particular, a coupled electromagnetic and thermal model has been developed to study quench development and propagation, focusing on the influence of heat exchange with liquid helium.

Wires for the Next European Dipole (NED)

Superconducting dipole and quadrupole magnets for particle accelerators and storage rings usually rely on saddle-shaped coils wound from Rutherford-type cables. When using Nb/sub 3/Sn, the small radii of curvature of coil ends impose that the winding be performed with un-reacted conductors and that the heat treatment needed for Nb/sub 3/Sn compound formation be applied on the whole coil upon winding completion. This results in very stringent requirements on the cable insulation system, which must be able to sustain the high-temperature cycle while retaining its mechanical strength and avoiding the formation of deleterious carbon compounds. Furthermore, it is also desirable to integrate the insulation system as a means of conferring a rigid shape to the manufactured coils, in order to ease subsequent handling and assembly and to protect the brittle material. We review here how various accelerator magnet R&D programs around the world are presently dealing with these issues.

Minimum Quench Energy and Early Quench Development in NbTi Superconducting Strands

During operation of the Large Hadron Collider at CERN, heat will be generated inside the coils of its superconducting magnets as a consequence of ramping of magnetic field, and of the interaction of lost beam particles with the magnet mass. Heat has to be transferred from the conductor into the He II coolant and removed from the magnet environment. During the LHC R&D stage, this transfer has been extensively studied on simulated coil segments at CEA/Saclay, and by analyzing dynamic behavior of short model magnets at CERN. Owing to the importance of efficient cooling for the design of future superconducting accelerator magnets, study of heat transfer has been restored at CERN and in frame of the Next European Dipole Collaboration. The article features two recently performed works: 1) Attempt to analyse archived high ramp rate quench data of 1-m-long LHC model dipole magnets of the 2nd generation. 2) Development of a method for direct measurement of heat transfer on segments of production LHC dipole magnet coils.

Insulation systems for Nb/sub 3/Sn accelerator magnet coils manufactured by the wind & react technique

The next European dipole (NED) activity is developing a high-performance Nb3Sn wire (aiming at a non-copper critical current density of 1500 A/mm2 at 4.2 K and 15 T), within the framework of the Coordinated Accelerator Research in Europe (CARE) project. This activity is expected to lead to the fabrication of a large aperture, high field dipole magnet. In preparation for this phase, a working group on magnet design and optimization (MDO) has been established to propose an optimal design. Other parallel work packages are concentrating on relevant topics, such as quench propagation simulation, innovative insulation techniques, and heat transfer measurements. In a first stage, the MDO working group has selected a number of coil configurations to be studied, together with salient parameters and features to be considered during the evaluation: the field quality, the superconductor efficiency, the conductor peak field, the stored magnetic energy, the Lorentz forces and the fabrication difficulties. 2-D magnetic calculations have been performed, and the results of this comparison between the different topologies are presented in this paper. The 2-D mechanical computations are ongoing and the final stage will be 3-D magnetic and mechanical studies.

Evaluation of the Transfer of Heat From the Coil of the LHC Dipole Magnet to Helium II

The Next European Dipole (NED) Collaboration is working to develop the technology of high field, dipole magnets for a future luminosity upgrade of the LHC at CERN. The challenge is to design and build a short prototype, large aperture (88 mm), dipole magnet to operate at 15 T using conductors. While the baseline design uses a cosine- winding configuration, a key part of the NED program has been the evaluation of other winding configurations. This paper reports on an evaluation of the helical dipole winding method and its applicability to the high field, large aperture magnets required for NED. The paper describes the development of magnetic models using the OPERA3D code. These models have been used to assess the field homogeneity, peak field performance, forces and conductor efficiency of the helical winding. The parameters developed from this analysis are compared with cosine- models. A conceptual winding geometry is presented and evaluated.

Comparison of 2-D Magnetic Designs of Selected Coil Configurations for the Next European Dipole (NED)

This paper presents a new finite element numerical method to analyse the coupling between twisted filaments in a superconducting multifilament composite wire. To avoid the large number of elements required by a 3D code, the proposed method makes use of the energy balance principle in a 2D code. The relationship between superconductor critical current density and local magnetic flux density is implemented in the program for the Bean and modified Kim models. The modeled wire is made up of six filaments twisted together and embedded in a low-resistivity matrix. Computations of magnetization cycle and of the electric field pattern have been performed for various twist pitch values in the case of a pure copper matrix. The results confirm that the maximum magnetization depends on the matrix conductivity, the superconductor critical current density, the applied field frequency, and the filament twist pitch. The simulations also lead to a practical criterion for wire design that can be used to assess whether or not the filaments are coupled.

An Evaluation of the Helical Winding Method Applied to the Next European Dipole Project

This paper deals with the two-dimensional computation of magnetization in an elliptic superconducting filament by using numerical and analytical methods. The numerical results are obtained from the finite element method and by using Beans model. This model is well adapted for Low T/sub c/ superconductor studies. We observe the effect of the axis ratio and of the field angle to the magnetic moment per unit length at saturation, and also to the cycle of magnetization. Moreover, the current density and the distribution of the electromagnetic fields in the superconducting filament are also studied.