B. Hervieu

Study and Development of the Superconducting Conductor for the Grenoble Hybrid Magnet

To produce a continuous magnetic field of at least 8.5 T in a 1.1 m cold bore diameter, the superconducting outsert of the Grenoble Hybrid magnet is based on the novel development of a Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled to 1.8 K by a bath of superfluid helium pressurized at atmospheric pressure. The main results of the conductor studies and development are presented after a brief introduction to the specificity of hybrid magnets, namely the electromagnetic couplings between resistive and superconducting coils. Results obtained with short samples of conductor are reviewed including the measurements of the elastic limit, AC losses, stability and critical current. The final specification of the RCOCC is presented highlighting the proposed method for the industrialization of the insertion process of the Rutherford cable on the hollow Cu-Ag stabilizer as well as its validation phase on short samples.

Design Status of the R3B-GLAD Magnet: Large Acceptance Superconducting Dipole With Active Shielding, Graded Coils, Large Forces and Indirect Cooling by Thermosiphon

The R3B-Glad superconducting Magnet provides the field required for a large acceptance spectrometer, dedicated to the analysis of Reactions with Relativistic Radioactive ions Beams. In the framework of the FAIR Project to GSI and within NUSTAR physics program, the technical study started in 2006, and the engineering design is undertaken. One main feature of this butterfly-like magnet with graded, tilted and trapezoidal racetrack coils is the active shielding. It makes it possible to decreasing the field by two orders of magnitude within a 1.2 m length, despite the large opening on the outlet side of the magnet (around 0.8 square meters). The fringe field is lower than 20 mT in the target area beside the entry, while the main field is larger than 2 teslas, out of 2 m length. The other principal characteristics are as follows: first, a high level of magnetic forces (300 to 400 tons per meter), with little place to block the coils, requiring a very specific mechanical structure; then, the magnet protection system that is based on an external dump resistor, coupled to a strong quenchback effect, to prevent any damage of the coils which could be caused by the 24 MJ of stored energy; lastly, the indirect cooling of the cold mass with a two-phase helium thermosiphon. The overall size of the conical cryostat will be around 3.5 m long, 3.8 m high and 7 m broad.

Progress Report on the 43 T Hybrid Magnet of the LNCMI-Grenoble

A CEA-CNRS French collaboration is currently developing a new hybrid magnet to produce in a first step a continuous magnetic field of 43 T in a 34-mm warm bore aperture. This magnet combines a resistive insert, composed of Bitter and polyhelix coils, and a large bore superconducting “outsert.” The superconducting coil is based on the novel development of a Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled down to 1.8 K by a bath of superfluid helium at atmospheric pressure. It aims at producing a nominal magnetic field of 8.5 T in a 1.1-m cold bore diameter. The specifications of the RCOCC will be presented together with the design and parameters of the cryogenic system. The solution to reduce the coupling between resistive and superconducting coils will be recalled as well as the constraints for designing the mechanical structure. The design study phase is coming to an end. The status of the conductor production and the next steps of the project are presented.

Final Design of the New Grenoble Hybrid Magnet

A CEA-CNRS French collaboration is currently developing a new hybrid magnet; this magnet combines a resistive insert composed of Bitter and polyhelix coils and a new large bore superconductor outsert to create an overall continuous magnetic field of 42+ T in a 34 mm warm aperture. The design of the superconducting coil outsert has been completed after thorough studies and successful experimental validation phases. Based on the novel development of a Nb-Ti/Cu Rutherford Cable On Conduit Conductor (RCOCC) cooled down to 1.8 K by the mean of a bath of superfluid helium at atmospheric pressure, the superconducting coil aims to produce a continuous magnetic field of 8.5 T in a 1.1 m cold bore diameter. The main results of the final design studies of the superconducting coil are presented including the 2D and 3D mechanical stress analysis, the conductor and coil specifications, the coil protection system as well as the required cryogenics infrastructure. The final design of the resistive insert coils is also described.

A New Design for the Superconducting Outsert of the GHMFL 42+ T Hybrid Magnet Project

A new superconducting coil outsert has been designed to be integrated within the existing infrastructure of the GHMFL hybrid project. Based on the novel development of a Nb-Ti Rutherford Cable On Conduit Conductor (RCOCC) cooled at 1.8 K by a bath of superfluid helium at atmospheric pressure, the superconducting coil aims to produce a continuous magnetic field of 8.5 T in a 1.1 m bore diameter. Combined with resistive insert coils, an overall continuous magnetic field of 42+ T will be produced in a 34 mm warm aperture. The main results of the conceptual study are reported including the conductor and coil specifications, the mechanical stress analysis, the coil protection scheme as well as the required cryogenics infrastructure. First developments and tests regarding the RCOCC are also presented.

Dynamical Response of Hybrid Magnet Structure Featuring Eddy-Current Shield During Transient Failure Mode

Hybrid magnets enable to achieve very high magnetic fields by combining resistive insert magnets with a large bore superconducting outer coil. In order to reduce the electromagnetic coupling between the coils, we introduce an eddy-current shield between the resistive and superconducting magnet. This additional shield is responsible for heat loads. To limit the degradation of the thermal behavior of the cold mass, an innovative support ferrule has been designed. It allows rigid connection of the eddy-current shield to the magnet structure. In this paper, the worst magnetic failure scenario is identified and modeled. It results in large transient forces applying to the structure. Transient mechanical analysis of the hybrid magnet structure featuring innovative support ferrule is presented. Simulations show that the proposed configuration leads to lower mechanical stresses than a configuration featuring a suspended shield.

Role and Impact of the Eddy Current Shield in the LNCMI-G Hybrid Magnet

In the new 43-T LNCMI hybrid magnet, a good conducting shield is inserted between the resistive insert magnet and the superconducting outsert magnet. Its goal is to decrease the ac losses induced in the superconducting conductor during a fault of the insert, disruption of the resistive coils (i.e., fast discharge) by smoothing the flux variations, and avoidance of a quench of the superconducting coil. The induced currents within the shield are very large due to the presence of copper material, which is the drawback of a good screening effect. Such currents result in large Lorentz forces, particularly during the most severe fault scenario of the resistive insert with the burnout of half of the Bitter coils. Finally, the detection system must be specially designed to handle the effects of the shield on the induced voltages.

Two-Dimensional and Three-Dimensional Mechanical Analyses of the Superconducting Outsert of the LNCMI Hybrid Magnet

Current technical superconductors, such as Nb3Sn, are limited to fields around 20 T by their intrinsic material properties. Generating significantly stronger magnetic fields can only be done by using resistive magnets, or by combining superconducting and resistive magnets (hybrid magnets), at the expense of large power consumption and operating cost. A CEA-CNRS French collaboration is currently developing a new hybrid magnet, which combines a resistive insert composed of Bitter and polyhelix coils and a new large-bore superconductor outsert to produce an overall continuous magnetic field of more than 42 T in a 34-mm warm aperture. Based on the novel development of a NbTi/Rutherford cable on conduit (RCOCC) cooled down to 1.8 K by means of the bath of superfluid helium at atmospheric pressure, the superconducting coil aims to produce a continuous magnetic field of 8.5 T in a 1.1-m cold bore diameter. This paper summarizes the results of the mechanical behavior study of the cold mass cooled from room to operating temperature and Lorentz forces at operating temperature. Computations have been performed in 2-D axisymmetry and 3-D, to take into account the axisymmetric discontinuous distribution of the tie rods.

Status of the 43-T Hybrid Magnet of LNCMI-Grenoble

Based on a close collaboration between CEA and CNRS, a new hybrid magnet is being built at LNCMI-Grenoble. By combining a resistive insert, which is made of Bitter and polyhelix coils, with a large bore superconducting outsert, an overall continuous magnetic field of at least 43 T will be produced in a 34-mm warm bore aperture. The superconducting coil relies on the novel development of a Nb-Ti/Cu Rutherford cable-on-conduit conductor cooled down to 1.8 K by a bath of superfluid helium at atmospheric pressure and will produce a nominal magnetic field of 8.5 T in a 1.1-m cold bore diameter. After thorough reviews of the hybrid magnet design, which have anticipated possible upgrades of the maximum magnetic field produced, the project has entered in its production phase. The status and the next steps of the project will be reviewed highlighting the remaining technical challenges.