European Dipole Research Articles

Progress in the design and manufacturing of the EDIPO 2 cryostat

A major upgrade of the European DIPOle (EDIPO) test facility, which will allow the testing of superconducting samples both for fusion and for high-energy physics applications, is taking place at the Swiss Plasma Center. With a target background field of 15 T, the new magnet design requires a large engineering current density and thus an accelerator-like magnet cooled by a saturated helium bath is foreseen. Therefore, a helium vessel has been designed and procured.In this work, we present recent advances in the design and manufacturing activities of the EDIPO 2 cryostat. First, we provide an analytical estimation of the thermal budget at the temperature levels of interest. Then, we describe the design and manufacturing of the helium vessel, according to relevant norms. Finite-element numerical simulations are employed both to refine the design and to provide a comparison with the experimental pressure tests.

Preliminary design of the pressure relief system of the EDIPO 2 helium vessel

The European Dipole (EDIPO) test facility is undergoing a major upgrade of its magnet assembly, which will provide ground for testing both fusion and high-energy physics superconducting samples at variable temperature and with a background field of 15 T. In view of a cooling of the superconducting magnet based on a saturated helium bath, a helium vessel has been designed and procured. In this work, we present a preliminary design of the pressure relief system of the helium vessel. After having introduced the main requirements of the cryogenic circuit, we describe a staged pressure protection concept. A bursting disc, chosen as safety pressure relief device, is then sized following the appropriate European norms and according to credible accidental scenarios. Finally, we introduce the need of pressure relief and helium recovery following a quench of the superconducting magnet, after having determined the actual heat load transferred to the helium bath. Numerical simulations are also employed to study the dynamics of the pressure and temperature evolutions, and ideas for future studies are briefly discussed.

Damage Investigations in the HTS Cable Prototype After the Cycling Test in EDIPO

During the test of 60 kA high-temperature superconducting cable prototypes in the European DIPOle (EDIPO), the performance of the prototypes in the cycling test was gradually degrading with the increasing number of electromagnetic and warmup-cool-down cycles. At the end of the test, after about 2000 cycles, the performance drop of the SuperPower and SuperOx conductors reached 10% and 20%, respectively. After the strands were extracted from the SuperOx cable, the critical current of the strand sections that were located in a high-field zone of EDIPO was measured at 77 K /self-field. Reduction of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> . was found not only in the loaded flat side of the cable, but also (and more evident) on the edges of the cable. Several strand sections with the highest degradation of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> . were cut and desoldered. The results of the measurements on the extracted tapes demonstrate highly nonuniform distribution of the retained critical current. Additional investigations have been performed to investigate in details influence of such nonuniformity on the overall I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> performance of the strand. Based on the obtained results, concluding recommendations for the next cable prototypes are discussed.

AC Loss in HTS Fusion Cables: Measurements, Modeling, and Scaling Laws

Currently, the use of high-temperature superconducting (HTS) cables in the high-field sections of central solenoid (CS) of DEMO is one of the most appealing applications of the HTS cables in fusion magnets. Due to the pulsed operation of such coils minimization of the ac loss is necessary for the CS conductors. For the stacked-tape HTS cable layout proposed at the Swiss Plasma Center (SPC), the main contributions in the total ac loss-hysteresis loss in stacks, intra- and inter-strand coupling losses-have been studied. The electrical network model is used for the coupling losses, based on which the scaling laws for the ac energy loss are proposed. On the basis of the developed numerical tools, the assessment of the ac loss for the next HTS cable prototypes at SPC is also reported. More than 50% reduction of the total ac loss is expected compared with the first TF cable prototypes tested in the European DIPOle (EDIPO).

Completion of the Commissioning of the EDIPO Test Facility

The main coils of the new European DIPOle (EDIPO) test facility, which is able to provide a background magnetic field up to 12.35 T, were commissioned at the Center for Research in Plasma Physics in July 2013. A field homogeneity of +/-1% was achieved over a 900-mm length, as compared to SULTAN's 10.9 T over 425 mm. To complete the commissioning of the test facility, the new superconducting transformer has been operated in 2015, and a high-current NbTi sample, which was earlier tested in the SULTAN test facility, has been tested in EDIPO under the same conditions as in SULTAN to benchmark the results. During the initial commissioning of the 100-kA transformer in 2013, the primary coil was found to be insufficiently cooled due to the debonding of the primary winding from its former that also served as the liquid helium cryostat. The NbTi primary coil was therefore modified to be in direct contact with liquid helium. At the next attempt of commissioning, in March 2014, a mechanical failure due to the imperfect alignment of the primary and secondary windings caused major damage. A replacement primary coil, which was wound in Summer 2014, was used in the third commissioning campaign in November 2014, but it failed due to the poor impregnation of the NbTi winding. Eventually, the original primary winding was repaired and commissioned in April 2015. The commissioning included a calibration of the current meter for the secondary winding and the high-current test, i.e., up to 95 kA at various background fields, of the “Trasek” conductor sample, which is a rectangular cable-in-conduit conductor made of 324 SnAg-coated NbTi strands. An excellent agreement was found between the results obtained from the two facilities. The sample holder unit is also equipped with a high-temperature superconducting (HTS) adapter and a counterflow heat exchanger to allow testing in EDIPO of HTS high-current samples over a broad range of operating temperatures, from 4.5 up to 50 K.

EDIPO: The Test Facility for High-Current High-Field HTS Superconductors

The commissioning of the European DIPOle (EDIPO) test facility at the Center for Research in Plasma Physics in Villigen was completed in Spring 2015, and the first test of a 60-kA/12-T high-temperature superconductor (HTS) was carried out in June 2015. User specifications are issued to allow external users to prepare their own sample matching the interface of the test facility. EDIPO offers a background magnetic field up to 12.35 T in a rectangular test well with a cross section of 89 mm $\times$ 138 mm (sample dimension). The homogeneous high field length is 910 mm (±1%) or 680 mm (±0.5%) at full current. A set of small copper coils provides a superimposed ac field with amplitude up to ±0.3 T and frequency up to 1 Hz. A superconducting transformer provides steady-state operating current for the sample up to 100 kA. The cooling circuit of the sample supplies supercritical helium at 10-bar inlet pressure. The mass flow rate, which is adjustable by cryogenic valves, is up to 8 g/s in each branch of the sample at 4.5 K. The operating temperature ranges from 4.5 to 60 K (at high temperature, the range of mass flow rate is reduced). To test samples at temperature higher than 10 K, an HTS “adapter” connects the sample to the NbTi transformer plates, which must remain cold. In such case, the length of the sample is reduced from 3600 mm (typical low-temperature superconductor sample) to 2900 mm (HTS sample with adapter).

Test of 60 kA coated conductor cable prototypes for fusion magnets

Coated conductors could be promising materials for the fabrication of the large magnet systems of future fusion devices. Two prototype conductors (flat cables in steel conduits), each about 2 m long, were manufactured using coated conductor tapes (4 mm wide) from Super Power and SuperOx, with a total tape length of 1.6 km. Each flat cable is assembled from 20 strands, each strand consisting of a stack of 16 tapes surrounded by two half circular copper profiles, twisted and soldered. The tapes were measured at 12 T and 4.2 K and the results of the measurements were used for the assessment of the conductor electromagnetic properties at low temperature and high field. The two conductors were assembled together in a sample that was tested in the European Dipole (EDIPO) facility. The current sharing temperatures of the two conductors were measured at background fields from 8 T up to 12 T and for currents from 30 kA up to 70 kA: the measured values are within a few percent of the values expected from the measurements on tapes (short samples). After electromagnetic cycling, Tcs at 12 T and 50 kA decreased from about 12 K to 11 K (about 10%), corresponding to less than 3% of Ic.

Upgrade of SULTAN/EDIPO for HTS Cable Test

CRPP hosts two unique conductor test facilities SULTAN (SUpraLeiter TestANlage) and EDIPO (European DIPOle). They allow the test of high current superconductors in high magnetic fields (SULTAN 11 T, EDIPO 12.5 T). In both facilities sample currents up to 100 kA can be supplied by means of a NbTi transformer. Presently the facilities are upgraded for the test of high current high-temperature superconductor (HTS) samples. For HTS conductor testing at temperatures between 20 and 50K, the heat flux between the HTS sample under test and the NbTi transformer needs to be limited to around 10W per conductor leg by means of an HTS adapter connecting them. The second required upgrade is the supply of intermediate temperature helium (20-50 K) to the HTS test conductor. It is mandatory that the helium gas coming from the HTS conductor under test can be returned to the cryoplant as cold gas (T < 20K). To reach this goal a tube-in-tube heat exchanger has been manufactured in which 4.5K helium coming from the cryoplant is in counter flow with the warm gas leaving the HTS test conductor.

The EDIPO Project: Status and Outlook

The aim of this paper is to present an up to date review of the status of the European DIPOle (EDIPO) project - whose objective is to build a new facility to perform both DC and AC tests of large force flow superconductor samples in high magnetic background field (up to ~12.5 T) - as well as an outlook on the final commissioning of the magnet in CRPP-PSI foreseen for 2012. Concerning the actual status, the results presented focus on the recent steps carried out in the manufacturing of the magnet assembly, in particular the cold mass assembly, its impregnation and the final acceptance tests. Moreover, results are reported regarding the identification and repair of an inter-turn short carried out on EDIPO Pole 1 in the second half of 2010. As far as the commissioning of EDIPO is concerned, an up-to-date report is provided on the preparation of the facility at CRPP (e.g. mechanical structures, high temperature current leads, quench detection, control system, etc.) as well as its acceptance and final commissioning (e.g. magnet reception tests, commissioning program, etc.).

Thermo-hydraulic analysis of the cool-down of the EDIPO test facility

The first cool-down of the EDIPO (European DIPOle) test facility is foreseen to take place in 2011 by means of the existing 1.2 kW cryoplant at EPFL-CRPP Villigen. In this work, the thermo-hydraulic analysis of the EDIPO cool-down is performed in order both to assess the its duration and to optimize the procedure. The cool-down is driven by the helium flowing in both the outer cooling channel and in the windings connected hydraulically in parallel. We take into account limitations due to the pressure drop in the cooling circuit and the refrigerator capacity as well as heat conduction in the iron yoke. Two schemes of the hydraulic cooling circuit in the EDIPO windings are studied (coils connected in series and coils connected in parallel). The analysis is performed by means of an analytical model complemented by and numerical model. The results indicate that the cool-down to 5 K can be achieved in about 12 days.

Status Report of the EDIPO Project

The aim of this paper is to present an up to date review of the status of the European DIPOle (EDIPO) project whose objective is to build a new facility to perform both DC and AC tests of large superconductor samples in high magnetic background field (up to ~12.5 T). EDIPO was designed by EFDA, is being built by BNG (Babcock-Noell) and, once completed, it shall be installed near the existing SULTAN facility at the Paul-Scherer-Institute, (CRPP, Villigen, Switzerland). The results presented focus on the manufacture of the dummy coil (which is a 1:1 model of one of the EDIPO poles) that was completed in spring 2009. Details are given about the dummy coil winding results, coil reaction heat treatment, impregnation and acceptance tests. Moreover, a progress status is provided about the manufacturing of the other parts of the EDIPO magnet assembly, in particular with respect to the manufacturing of the test-well, AC coils, yoke sheets cutting and assembly, the outer cylinder production and welding (316 LN steel, 35 mm thick) and the coils ends preparation. Some results regarding the R&D studies are also reported about the winding trials carried out to improve the winding accuracy and the numerical simulations made to support coil winding (e.g. conductor spring back).

Test results of the 18 kA EDIPO HTS current leads

For the new test facility EDIPO (European DIPOle), to be hosted by CRPP, two 18 kA HTS current leads were manufactured and successfully tested. The HTS module, made of AgMgAu/Bi-2223 tapes, is cooled only by heat conduction to the cold end, while the copper part is cooled by forced flow helium gas. The current leads were tested at low voltage up to the maximum current of 18 kA. The helium mass flow rates required for stable operation at various currents were determined. In addition to the steady state operation, the transient behavior in the case of a loss of flow was studied experimentally. The test results provide an estimate of the operational limits of the EDIPO HTS current leads.

Analysis of the EDIPO Temperature Margin During Current Ramp-Up

The European dipole (EDIPO), currently under construction, will provide background magnetic fields of up to 12.5 T for tests of ITER high-current superconducting cables. The EDIPO winding consists of 7 × 2 double layers of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductors with forced flow cooling of supercritical helium. The performance limits of EDIPO during current ramp-up are analyzed analysed with the CryoSoft suite of codes, recently integrated into a customizable and flexible environment for the analysis of thermal hydraulic and electrical transients in superconducting magnetic systems. The simultaneous analysis of the cryogenic system and all 14 double layers shows that under all charging conditions the EDIPO temperature margin remains sufficiently high.

Status of the EDIPO Project

The aim of this paper is to present an up to date review of the status of the European DIPOle (EDIPO) project whose objective is to build a new facility to perform both DC and AC tests of large superconductor samples in high magnetic background field (up to 12.5 T). EDIPO was designed by EFDA, is being built by BNG (Babcock-Noell) and, once completed, it shall be installed near the existing SULTAN facility at the Paul-Scherer-Institute, (CRPP, Villigen, Switzerland). The results presented focus on the manufacture of the dummy coil (which is a 1:1 model of one of the EDIPO poles) that was completed in spring 2009. Details are given about the dummy coil winding results, coil reaction heat treatment, impregnation and acceptance tests. Moreover, a progress status is provided about the manufacturing of the other parts of the EDIPO magnet assembly, in particular with respect to the manufacturing of the test-well, AC coils, yoke sheets cutting and assembly, the outer cylinder production and welding (316 LN steel, 35 mm thick) and the coils ends preparation. Some results regarding the R&D studies are also reported about the winding trials carried out to improve the winding accuracy and the numerical simulations made to support coil winding (e.g. conductor spring back).

Magnetic Design and Code Benchmarking of the SMC (Short Model Coil) Dipole Magnet

The Short Model Coil (SMC) working group was set in February 2007 within the Next European Dipole (NED) program, in order to develop a short-scale model of a dipole magnet. The SMC group comprises four laboratories: CERN/TE-MSC group (CH), CEA/IRFU (FR), RAL (UK) and LBNL (US). The SMC magnet is designed to reach a peak field of about 13 Tesla (T) on conductor, using a 2500 Powder-In-Tube (PIT) strand. The aim of this magnet device is to study the degradation of the magnetic properties of the cable, by applying different levels of pre-stress. To fully satisfy this purpose, a versatile and easy-to-assemble structure has been realized. The design of the SMC magnet has been developed from an existing dipole magnet, the SD01, designed, built and tested at LBNL with support from CEA. The goal of the magnetic design presented in this paper is to match the high field region with the high stress region, located along the dipole straight section. For this purpose, three-dimensional nonlinear parametric models have been implemented using three codes (CAST3M, ANSYS, and OPERA). This optimization process has been an opportunity to cross-check the codes. The results of this benchmarking are presented here, along with the final design which incorporates the use of end spacers and a surrounding iron structure to deliver a nominal field of 13 T uniformly distributed along the cable straight section.

Numerical Simulations of Superconducting Cables Bending and Winding

The assessment of the cold working, deformation, stress and strain distributions, and elastic spring back of superconducting cables conductors used for nuclear fusion experiments is a crucial issue for their manufacture process. The winding process of the ITER Toroidal Field (TF) conductors foresees their unbending from the drum they are delivered with and re-bending to their final shape. Nowadays the winding of the European Dipole (EDIPO) dummy coil is in progress and the final one will be challenging due to the conductors stiffness and the related spring back effect. In the light of this F4E ongoing activity and in view of the final design of the automatic winding process of the ITER TF coils a numerical tool capable to predict the plastic and elastic behavior of these conductors would be useful. To this end explicit Finite Element (FE) models have been implemented with the commercial code ANSYS/LS DYNA and simplified non linear calculations with CAST3M. Several numerical analyses have been performed by using different material properties, plasticity behaviors and loading conditions. The results of the analyses are presented and compared with experimental results and correlations with analytical solutions are investigated.

Mechanical Design of the SMC (Short Model Coil) Dipole Magnet

The Short Model Coil (SMC) working group was set in February 2007 within the Next European Dipole (NED) program, in order to develop a short-scale model of a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipole magnet. The SMC group comprises four laboratories: CERN/TE-MSC group (CH), CEA/IRFU (FR), RAL (UK) and LBNL (US). The SMC magnet was originally conceived to reach a peak field of about 13 T on conductor, using a 2500 A/mm Powder-In-Tube (PIT) strand. The aim of this magnet device is to study the degradation of the magnetic properties of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable, by applying different level of pre-stress. To fully satisfy this purpose, a versatile and easy-to-assemble structure has to be realized. The design of the SMC magnet has been developed from an existing dipole magnet, the SD01, designed, built and tested at LBNL with support from CEA. In this paper we will describe the mechanical optimization of the dipole, starting from a conceptual configuration based on a former magnetic analysis. Two and three-dimensional Finite Element Method (FEM) models have been implemented in ANSYS and in CAST3M, aiming at setting the mechanical parameters of the dipole magnet structure, thus fulfilling the design constraints imposed by the materials.

Heat Treatment Optimization Studies on PIT ${\rm Nb}_{3}{\rm Sn}$ Strand for the NED Project

For the Next European Dipole (NED) program, a Powder-In-Tube (PIT) strand was successfully developed by SMI. This high-performance Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand presents a non-copper critical current density of ~ 2500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 T applied field and 4.2 K and a filament diameter around 50 mum. Extensive heat treatment optimization studies were performed in order to maximize both critical current and RRR, with a plateau temperature down to 625degC and duration up to 400 hours. It appears that a critical current enhancement of ~ 10% can be achieved for a reaction schedule of 320 hours at 625degC with non-copper critical current density respectively exceeding 2700 and 1500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 and 15 T (4.2 K). Thanks to this modified heat treatment, this strand completely fulfills the NED stringent specification.

Thermal conductivity and Kapitza resistance of epoxy resin fiberglass tape at superfluid helium temperature

The system of materials composed of fiberglass epoxy resin impregnated tape constitutes in many cases the electrical insulation for “dry”-type superconducting accelerator magnet such as Nb 3Sn magnets. Nb 3Sn magnet technology is still under development in a few programs to reach higher magnetic fields than what NbTi magnets can produce. The European program, Next European Dipole (NED), is one of such programs and it aims to develop and construct a 15 T class Nb 3Sn magnet mainly for upgrading the Large Hardron Collider. Superfluid helium is considered as one possible coolant and since the magnet has been designed with a “dry” insulation, the thermal conductivity and the Kapitza resistance of the electrical insulation are the key properties that must be know for the thermal design of such a magnet. Accordingly, property measurements of the epoxy resin fiberglass tape insulation system developed for the NED project was carried out in superfluid helium. Four sheets with thicknesses varying from 40 to 300 μm have been tested in a steady-state condition. The determined thermal conductivity, k, is [(25.8 ± 2.8) · T − (12.2 ± 4.9)] × 10 −3 W m −1 K −1 and the Kapitza resistance is given by R K = (1462 ± 345) · T (−1.86 ± 0.41) × 10 −6 Km 2 W −1 in the temperature range of 1.55–2.05 K.

Mechanical Design of the Next European Dipole

The next european dipole (NED) Consortium is working to develop the technology of high field, dipole magnets for a future luminosity upgrade of the LHC at CERN. The proposed magnet will be a large aperture (88 mm), high field (15 T) superconducting dipole magnet using Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn Rutherford cable and a wind-and-react manufacturing scheme. The magnet geometry (cosine-theta), scale and forces present real challenges for the design of the mechanical structure and the magnet assembly methods. This paper reports on the mechanical design studies carried out during the first phase of the NED project (2004 to 2007). This paper describes the development of a full 2-D mechanical Finite Element model of the dipole magnet including the superconducting coil, the collars, the iron yoke structure and the outer support cylinder. The model has been qualified and used to study the stress levels in the coil at assembly, cooldown and powering. Due to the strain sensitivity of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors, a primary objective has been to limit the compressive stress in the superconducting cable to 150 MPa at full field.