High temperature superconductors (HTSs), like for instance REBCO (RE = rare earth) coated conductors, are of high potential for building large superconducting magnets. Some magnets, such as ...accelerator magnets, require the use of superconducting cables to allow fast ramping, and low magnetization loss to mitigate field quality issues. One of the methods to lower ac loss is to divide the superconducting layer in the tape into filaments. In this paper, conductors with copper stabilization for practical applications are laser scribed into narrow filaments. Striated tapes are then wound into conductor on round core (CORC ) cables. The critical current and magnetization ac loss of single tapes were measured. We found that the stabilizing copper layer causes difficulties for laser scribing. The degradation of the critical current is more pronounced than in the case of non-stabilized tapes. The selection of the number of filaments is therefore a compromise between critical current degradation and reduction of ac loss. Based on the results obtained from single tape experiments, the optimum number of filaments in 4 mm wide tapes was chosen, and CORC cables with 2, 3 and 4 layers of tapes with and without filaments were manufactured. Magnetization ac loss measurements at 77 K showed a reduction of ac loss in the cables with filaments. This reduction corresponds almost to the number of filaments. Measurement at different frequencies also showed that the coupling loss in CORC cables with a short twist-pitch is relatively small in comparison to hysteretic loss.
EuCARD-2 is a project supported by FP7-European Commission that includes, inter alia, a work-package (WP10) called "Future Magnets." This project is part of the long term development that CERN is ...launching to explore magnet technology at 16 T to 20 T dipole operating field, within the scope of a study on Future Circular Colliders. The EuCARD2 collaboration is closely liaising with similar programs for high field accelerator magnets in the USA and Japan. The main focus of EuCARD2 WP10 is the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, The cable will be used to wind a stand-alone magnet 500 mm long and with an aperture of 40 mm. This magnet should yield 5 T, when stand-alone, and will enable to reach a 15 to 18 T dipole field by placing it in a large bore background dipole of 12-15 T. REBCO based Roebel cables is the baseline. Various magnet configurations with HTS tapes are under investigation and also use of Bi-2212 round wire based cables is considered. The paper presents the structure of the collaboration and describes the main choices made in the first year of the program, which has a breadth of five to six years of which four are covered by the FP7 frame.
The European project ECCOFLOW aims at a versatile resistive-type superconducting fault current limiter. For the first time, such a device will be tested at two different sites. The project partners ...have developed a superconducting fault current limiter design based on REBCO tapes with respect to the specifications provided by two hosting utilities. The limiter will operate at 1 kA at a rated voltage of 24 kV, and will be tested in both a busbar and a transformer feeder application. The design started with extensive investigations on the superconductor tape properties, especially with respect to the limitation behavior in all possible scenarios, and subsequent wide-range modeling of its in-grid behavior. The general integrated layout provides a limitation time of up to 1 s. The limiter is cooled using Gifford-McMahon cryocoolers to recondense the evaporated nitrogen. The present status of production and testing will be presented as well.
In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is ...exposed to AC magnetic field. The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches. In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis. The overall geometry of the models was identical: each consisted of three tapes 4 mm wide that were placed with a pitch of 50 mm in a single layer on the 8 mm diameter round core. The cable length was designed to reach two complete tape pitches. In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing. The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding. Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments. In particular, at AC fields of 0.1 T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable. Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields.
In the frame of the EuCARD2 collaboration, aimed at developing the technology for 20 T class accelerator magnets, several demonstrator dipole magnets are being built using high critical current ...density and fully transposed ReBCO tape-based Roebel-type cables. In accelerator magnets the dynamic magnetic field quality is one of the key parameters, which is affected by the effective inter-strand resistances in the cables. For this reason, measurements of the inter-strand resistances on ReBCO Roebel cables were carried out at 4.2 and 77 K. Acquiring these data is also essential for input of cable simulation models. The cable samples are impregnated with epoxy resin to reduce the effect of transverse stress degradation due to Lorentz forces acting on the strands in the Roebel cables. The measured inter-strand resistance is used to estimate the AC coupling loss in different magnetic field orientations. Moreover, the contributions of diverse interface contact resistances to overall inter-strand resistance of Roebel cables were determined using a novel theoretical model. For validation, the AC loss of cables were examined in various orientations of applied field at 4.2 K. With three analytical models the hysteresis loss was calculated and compared to the measured data. The average inter-strand resistance of the cable samples impregnated with the unfilled epoxy CTD-101K range from 3 to 16 at 77 K and 1.5 to 9 at 4.2 K. Between the tapes the copper to copper interface resistance dominates the inter-strand resistance of impregnated Roebel cables. The calculated and measured AC loss for the CTD-101K impregnated Roebel cable lead to equivalent conclusions that the coupling loss is lower than the hysteresis loss within the range of the experiment. These observations substantially differ from earlier results extracted from a similar cable but impregnated with the alumina-filled epoxy resin CTD-101G, which showed considerable coupling loss when exposed to magnetic field parallel the wide face of the cable.
Coated conductor applications such as fusion magnets, particle accelerator magnets and generator windings require high current-carrying capabilities. This requirement can be fulfilled by various ...cable concepts using commercial long length REBCO coated conductors with high current-carrying performance. In the past few years, our group has successfully developed the Roebel cable concept for coated conductors. The design advantages of such a cable are high current-carrying capability and low alternating current (AC) losses. Unfortunately, for large-scale applications, the possibilities of a simple scale-up of the Roebel geometry are limited and additional design ideas are needed. One way to reach the required high currents is the Rutherford cable concept. In this concept a conductor is wound with transposition on a flat metal former. In order to design the former, the bending properties of the Roebel assembled coated conductor cables (RACC) must be measured and characterized. This allows the identification of a destruction-free interval for the Roebel cable, in terms of bending angle and transposition length. In this work we designed and assembled a demonstrator of a coated conductor Rutherford cable (CCRC) with three RACC cables. We measured the critical current and the AC losses of the cable demonstrator. Our results show that, despite still needing efforts in terms of reproducibility of the assembly process and of AC loss reduction, this design is a promising and viable solution for high current-capacity cables made of coated conductors.
Roebel cables made from REBCO coated conductors meet the requirements of accelerator magnets for high-current carrying capability, compactness and transposition. Previous work evidenced that the ...transverse stress distribution and the electrical protection of the first generation Roebel cables measured at CERN had to be improved. New Roebel cables incorporating copper stabilizer have been designed, produced at KIT and extensively characterized at CERN at 4.2 K and in external fields of up to 9.6 T. In this paper, we report on the optimisation of the cable design and on the results of the electrical and mechanical measurements performed at CERN on a new generation of cables.
REBCO Roebel cables are considered for application in high-temperature superconducting inserts for accelerator magnets because of their fully transposed geometry, high-engineering current density, ...and adequate bending tolerance. In these magnets the cables experience Lorentz forces leading to transverse stresses up to 100-150 MPa. Previous reports have shown bare Roebel cables to degrade under such high stresses so that additional reinforcement is required. In this work, two identical Roebel cables are vacuum impregnated with a mixture of epoxy and fused silica in order to improve their tolerance to transverse stress. After impregnation, the critical current of the cables is measured under transverse mechanical loading at T = 4.2 K, T. A reference cable without impregnation is tested as well. Pressures up to 350 MPa are applied to a short (30 mm) section of each cable. No degradation was observed for pressures up to 250 MPa and 170 MPa in the two impregnated cables. The critical current of the non-impregnated cable, in contrast, started to decrease at stresses as low as 40 MPa.
Within a German government funded project the partners Siemens AG and KIT have developed an iron-free inductive current limiter. The project aimed at a device for the medium voltage range, i.e. U = ...10 kV with a nominal power of 10 MVA. One phase of this limiter was designed, manufactured and successfully tested. The limiter is based on a stack of closed superconducting ReBCO rings inserted into a conventional reactor coil. Under normal operation conditions, the rings shield the inner part of this coil. With the occurrence of a short circuit the superconducting rings quench and the impedance of the reactor coil becomes effective. The paper reports the principles of the design and the development of the superconducting ReBCO rings. It describes in detail the test of such rings, in particular the contactless determination of contact resistances and critical currents. Of particular importance was the development of a cryostat which minimizes the distance between the primary reactor coil and the superconducting insert. Finally, the device was tested in the Siemens test field in Berlin under the most relevant operation conditions. The limiting behavior complied with the predicted properties.
This paper describes the standalone magnet cold testing of the high temperature superconducting (HTS) magnet Feather-M2.1-2. This magnet was constructed within the European funded FP7-EUCARD2 ...collaboration to test a Roebel type HTS cable, and is one of the first high temperature superconducting dipole magnets in the world. The magnet was operated in forced flow helium gas with temperatures ranging between 5 and 85 K. During the tests a magnetic dipole field of 3.1 T was reached inside the aperture at a current of 6.5 kA and a temperature of 5.7 K. These values are in agreement with the self-field critical current of the used SuperOx cable assembled with Sunam tapes (low-performance batch), thereby confirming that no degradation occurred during winding, impregnation, assembly and cool-down of the magnet. The magnet was quenched many tens of times by ramping over the critical current and no degradation nor training was evident. During the tests the voltage over the coil was monitored in the microvolt range. An inductive cancellation wire was used to remove the inductive component, thereby significantly reducing noise levels. Close to the quench current, drift was detected both in temperature and voltage over the coil. This drifting happens in a time scale of minutes and is a clear indication that the magnet has reached its limit. All quenches happened approximately at the same average electric field and thus none of the quenches occurred unexpectedly.
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