T. Lecrevisse

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.

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.

Quench Considerations and Protection Scheme of a High Field HTS Dipole Insert Coil

The large scale particle accelerators of the future in the 20 T regime are enabled by high temperature superconducting magnets. The dipole magnets needed in new high-field accelerators can be constructed with an YBCO insert and a Nb3Sn outsert. Such a configuration makes the quench analysis and magnet protection challenging because the quench behavior in both of these coils is different and there is very strong inductive coupling between the coils. The Nb3Sn coil is characterized by high energy and current and relatively fast quench propagation velocity. However, quench propagates slowly in YBCO coils because of typically wide spread large temperature margin. Currently, in the EuCARD project, a European collaboration is targeting to construct a small-scale high field YBCO-Nb3Sn hybrid magnet. In this paper, we scrutinize quench in the YBCO insert. We utilized an approach based on a solution of the heat diffusion equation with the finite element method. Additionally, we present a protection scheme for the coil.

Quench Propagation in YBCO Pancake: Experimental and Computational Results

High-temperature superconductors are promising materials for future applications such as high field magnets thanks to their ability to carry high current densities. Nevertheless, their protection still remains a key issue mainly due to the slow velocity of quench propagation. To understand the quench behavior of a YBCO coil, a simulation code has been developed using the CASTEM-CEA finite element software. Simulations can be performed considering constant current and magnetic field. Results of those simulations will be displayed and a way for improving the protection by adding a stabilizer will be discussed. Two well instrumented YBCO coils were fabricated in order to obtain experimental data on quench propagation in pancake configuration. Their design and some measurements are reported in this paper along with another experiment on a double pancake made by and tested at CNRS Grenoble. Finally, we compare the numerical and experimental results and discuss the accuracy of our simulations.

Performance Tests of Prototype High-Field HTS Coils in Grenoble

Longitudinal and transverse quench propagation tests were performed at the Laboratoire National des Champs Magnetiques Intenses (LNCMI) high-field test facility on instrumented double pancake coils fabricated by CEA-Saclay using co-wound high-temperature superconductor (HTS) tapes. Energy deposited on an embedded heater initiated a quench and its subsequent propagations. Following the resulting thermomechanical analysis, a conductor design with stacked HTS tapes co-wound with stabilizers was conceived. A ten-turn demonstrator racetrack coil has been fabricated from the stacked HTS conductor with an aim to investigate the operation margin of an HTS coil in a background magnetic field misaligned from the coil axis as expected for the operating condition of an accelerator magnet insert. A test setup with a high current capacity up to 3 kA and angular variability that utilizes a room-temperature 376-mm-bore 10-T resistive magnet at LNCMI in Grenoble has been built. The performances and operation margin of the racetrack coil were investigated.

Design Study of a 10-T REBCO Insert Solenoid

As a first step to develop HTS insert technology, we plan to build a 10-T REBCO solenoid that will be tested at the French National High Magnetic Field Laboratory of Grenoble in France (LNCMI) at 4.2 K, in the aperture of a 20-T resistive magnet. Two winding approaches are being considered for the HTS insert, i.e., the no-insulation (NI) and metallic insulation (MI) winding technics. Both methods allow turn-to-turn bypassing current but differ in their turn-to-turn resistance value by three orders of magnitude. In this paper, we provide a comparative study between the two winding options using numerical simulations applied to a preliminary design of the insert, which features a cold bore diameter of 50 mm and is made of 15 double-pancake coils wound from a 6-mm REBCO tape.

Geometry Optimization for SMES Solenoids Using HTS Ribbons

SMES devices are an attractive solution for pulsed current sources. The specific stored energy is moderate therefore it has to be improved. HTS materials offer opportunities in terms of current carrying capabilities as well as mechanical strength. The specific magnetic storage capability is presented as a function of the aspect ratio of solenoid. Shape optimization is discussed using the strongest superconducting tapes available.

Characterization of YBCO Coated Conductors Under High Magnetic Field at LNCMI

The second generation (2G) HTS conductors, the YBaCuO Coated Conductors, show very exciting performances in terms of critical currents under very high fields whereas their mechanical properties, the key issues for very high field magnets, are outstanding for the IBAD (Ion Beam Assisted Deposition) route. Preliminary measurements have shown critical currents Ic above 500 A at 15 T and 4 K on commercial YBCO coated conductors. They are now available in reasonable length and manufacturing focus to provide reliable, consistent performance conductors. With these high critical current and tensile strength above 600 MPa, these conductors are very promising for making HTS superconducting coils able to generate very high field. But very few characterizations exist above 20 T. LNCMI is one of the few facilities in the world where such measurement can be performed.

PROTECTION OF THE 6 T YBCO INSERT IN THE 13 T NB3SN FRESCA II DIPOLE

In the EuCARD project, we aim to construct a dipole magnet in YBCO producing 6 T in the background field of a 13 T Nb3Sn dipole FRESCA II. This paper reviews the quench analysis and protection of the YBCO coil. In addition, a recommendation for the protection system of the YBCO coil is presented. MAGNET SYSTEM