Tim Mulder

Introduction of CORC ® wires: highly flexible, round high-temperature superconducting wires for magnet and power transmission applications

Conductor on Round Core (CORC®) technology has achieved a long sought-after benchmark by enabling the production of round, multifilament, (RE)Ba2Ca3O7−x coated conductors with practical current densities for use in magnets and power applications. Recent progress, including the demonstration of engineering current density beyond 300 Amm−2 at 4.2 K and 20 T, indicates that CORC® cables are a viable conductor for next generation high field magnets. Tapes with 30 μm substrate thickness and tape widths down to 2 mm have improved the capabilities of CORC® technology by allowing the production of CORC® wires as thin as 3 mm in diameter with the potential to enhance the engineering current density further. An important benefit of the thin CORC® wires is their improved flexibility compared to thicker (7–8 mm diameter) CORC® cables. Critical current measurements were carried out on tapes extracted from CORC® wires made using 2 and 3 mm wide tape after bending the wires to various diameters from 10 to 3.5 cm. These thin wires are highly flexible and retain close to 90% of their original critical current even after bending to a diameter of 3.5 cm. A small 5-turn solenoid was constructed and measured as a function of applied magnetic field, exhibiting an engineering current density of 233 Amm−2 at 4.2 K and 10 T. CORC® wires thus form an attractive solution for applications between 4.2 and 77 K, including high-field magnets that require high current densities with small bending diameters, benefiting from a ready-to-use form (similar to NbTi and contrary to Nb3Sn wires) that does not require additional processing following coil construction.

Design and Manufacturing of a 45 kA at 10 T REBCO-CORC Cable-in-Conduit Conductor for Large-Scale Magnets

The European Organization for Nuclear Research (CERN) is developing high-current ReBCO-CORC strand-based cables for use in future large-scale detector magnets. A six-around-one, forced flow gas-cooled ReBCO-CORC cable-in-conduit conductor (CICC) is envisioned for application in magnets operating in the 20-40 K temperature range. A CICC, rated for 45 kA at 4.2 K and 10 T, is designed and in production. The CICC comprises a cable of six CORC strands helically wound around a tube. The cable has an expected current density of 105 A/mm2 at 10 T/4.2 K, which corresponds to an overall current density of 53 A/mm2. A cable current density of 110 A/mm2 can be reached when increasing the temperature to 20 K and operating in a magnetic field of 5 T.

Quench Protection of Very Large, 50-GJ-Class, and High-Temperature-Superconductor-Based Detector Magnets

An investigation is performed on the quench behavior of a conceptual 50-GJ 8-T high-temperature-superconductor-based solenoid. In this design, a 50-kA conductor-on-round-core cable-in-conduit conductor utilizing ReBCO technology is envisioned, operating at 40 K. Various properties such as resistivity, thermal conductivity, and heat capacity are very different at this temperature, which affects the quench behavior. It is found that the envisioned conductor is very stable with a minimum quench energy of about 2 kJ. However, the quench propagation velocity is typically about 20 mm/s, so that creating a wide-spread normal zone throughout the coil is very challenging. Moreover, an extraction voltage exceeding 20 kV would be required to ensure a hot-spot temperature below 100 K once a thermal runaway occurs. A novel concept dubbed “rapid quench transformation” is proposed whereby the superconducting conductor is co-wound with a normal conductor to achieve a high degree of inductive coupling. This geometry allows for a significant electric noise reduction, thus enabling low-threshold quench detection. The secondary circuit is connected in series with a stack of diodes, not allowing current transfer during regular operation, but very fast current transfer once a quench is detected. With this approach, the hot-spot temperature can be kept within 20 K of the cold mass temperature at all times, the hot-spot temperature is well below 100 K, and just under 80% of the stored energy can be extracted during a quench.

Optimized and practical electrical joints for CORC type HTS cables

Within CERN the development of REBCO-CORC (Conductor On Round Core) type cables is pursued in view of possible application in future detector and accelerator magnets. An important issue is the design and qualification of terminations for connecting CORC cables mutually or to bus-bars. A termination design is envisaged that combines a simple manufacturing process with a lowest possible joint terminal resistance in the few nΩ range at 4.2 K, first for a single CORC cable and subsequently for CORC based Cable-in-Conduit Conductors. The investigation concerns the effect of tapering the CORC cable within the joint to form a staircase like geometry, which allows current to pass more directly from the copper joint casing to the inner REBCO layers of the CORC cable. Simulations have shown a substantial decrease in joint resistance at operating current in the case both CORC cable and joint casing are tapered. The CORC cable and new joint were tested at CERN. In this paper, some details of the new joint design, fabrication process, and model are presented and the results are summarized.

Development of Joint Terminals for a New Six-Around-One ReBCO-CORC Cable-in-Conduit Conductor Rated 45 kA at 10 T/4 K

The European Organization for Nuclear Research (CERN) is developing a six-around-one conductor on round core (CORC)-strand-based cable-in-conduit conductor (CICC) for use in detector and other large magnet systems. The CICC comprises six ReBCO-CORC strands helically wound around a central tube or rod and inserted in a square aluminum jacket. A major design challenge is finding a simple yet low-resistive method of injecting current homogeneously into the CORC strands of the CICC. In the production of joints for single-CORC cables, we are currently pursuing a method in which the different ReBCO layers at both ends of the CORC cable are trimmed into a staircase-like geometry. A similar trimming method is developed for joint terminals for the ReBCO-CORC-based CICC. A demonstration joint terminal is made to test the various steps of the trimming and manufacturing process before fabricating a joint terminal with real CORC strands. This paper presents an overview of CIC-joint terminal design, simulation results, and the different steps in the manufacturing process.

Performance Test of an 8 kA @ 10-T 4.2-K ReBCO-CORC Cable

CERN is developing high-current ReBCO conductor on round core (CORC) cables for application in future detector and accelerator magnets. A characterization test on a ReBCO-CORC cable sample and its joints is performed in the 10-T FRESCA cable test facility at CERN. The sample is taken from the first 12-m-long CORC production. Key is the characterization of the field- and temperature-dependent critical currents of the CORC cable at 1.9 K and 4.2 K. Secondary objectives include evaluating the response of the CORC cable to quenches and the performance of cylindrical low resistive cable terminals especially designed and manufactured for use on CORC cables. The 7.6-mm CORC cable features 8 kA at 4.2 K and 10 T, and the joint terminals show a 25 ± 5 - nΩ resistance for 20-cm length.

New Fast Response Thin Film-Based Superconducting Quench Detectors

Quench detection on superconducting bus bars and other devices with a low normal zone propagation velocity and low voltage build-up is quite difficult with conventional quench detection techniques. Currently, on ATLAS superconducting bus bar sections, superconducting quench detectors (SQD) are mounted to detect quench events. A first version of the SQD essentially consists of an insulated superconducting wire glued to a superconducting bus line or windings, which in the case of a quench rapidly builds up a relatively high resistance that can be easily and quietly detected. We now introduce a new generation of drastically improved SQDs. The new version makes the detection of quenches simpler, more reliable, and much faster. Instead of a superconducting wire, now a superconducting thin film is used. The layout of the sensor shows a meander like pattern that is etched out of a copper coated 25 μm thick film of Nb-Ti glued in between layers of Kapton. Since the sensor is now much smaller and thinner, it is easier to install and build up a high resistance with a much shorter response time. The design of the sensors is explained. The test results of the new sensors in a few variants in a calibration setup as well as when mounted on the windings surface of a magnet are reported.

A 2.5-T, 1.25-m Free Bore Superconducting Magnet for the Magnum-PSI Linear Plasma Generator

DIFFERs main experiment, Magnum-PSI, is the only laboratory setup in the world capable of exposing materials to plasma conditions similar to those of future fusion reactors. The success of the Magnum-PSI experiment depends on the generation of a 2.5-T magnetic field without restricting the diagnostic access and operational aspects of the experiment. This has been achieved with a magnet consisting of five superconducting solenoids wound on a 2.5-m-long stainless steel coil former positioned in a cryostat offering a 1.25-m warm bore. A copper stabilized multifilamentary NbTi conductor with a 3.48-mm2 cross section has been used; thus the magnet exhibits a total inductance of 500 H and a stored energy of 16 MJ. This presents quite a challenge for the protection scheme that has been implemented using a mix of back-to-back cold diodes and external dump resistors. The coils generate a plateau-shaped magnetic field adjustable up to 2.5 T while the distance between the coils allows for 16 room temperature view-ports. The coils are cooled with liquid helium using a recondensing system operated with cryocoolers, while the magnet system cycles between zero and full field up to once per day. The magnetic stray field is shielded down to 1 mT outside the experimental area by iron walls that flank the magnet.

New Bridge Temperature Sensor for Superconducting Magnets and Other Cryogenic Applications

A few hundred temperature sensors are used to monitor the temperature behavior of the gigantic ATLAS toroid superconducting magnet system during cool down and normal operation. In order to guarantee good sensitivity of temperature measurements in the range from liquid helium to room temperature, two types of sensors are positioned at the same location: platinum resistance thermometers for the range 30–300 K and carbon composition resistors (Allen-Bradley) for the 4–30 K range. Both types are very well known for use in cryogenics and they have performed satisfactorily during 10 year of ATLAS operation. The sensors themselves are easily available and inexpensive and the main cost is for the many kilometers of cold and warm instrumentation cables, connectors, conditioners, and installation work. A reduction of the amount of measurement channels is an important issue and this motivated us to develop a new compact and robust sensor module covering the entire temperature range that would combine advantages of both platinum and carbon resistors. The solution is trivial, elegant, and simple. Two resistors with positive temperature derivative and two resistors with negative temperature derivative are connected in a full-bridge connection. We used two platinum and two carbon resistors. The output signal is a result of the subtraction of voltages across positive and negative temperature derivative resistors that makes this temperature bridge sensor very sensitive for the entire temperature range. Variable temperature characterization tests were performed in the helium gas environment in the CERN Cryogenic Laboratory. Our measurements have demonstrated that the bridge sensors have a full range sensitivity better than 0.1 mV/K at a supply current of 100 μ A. In the meantime, a few other superconducting detector magnets in experiments operating at CERN are equipped with these new, simple, and robust temperature sensors.

Design and Preparation of Two ReBCO-CORC® Cable-In-Conduit Conductors for Fusion and Detector Magnets

Two new ReBCO-CORC® based cable-in-conduit conductors (CICC) are developed by CERN in collaboration with ACT-Boulder. Both conductors feature a critical current of about 80 kA at 4.5 K and 12 T. One conductor is designed for operation in large detector magnets, while the other is aimed for application in fusion type magnets. The conductors use a six-around-one cable geometry with six flexible ReBCO CORC® strands twisted around a central tube. The fusion CICC is designed to be cooled by the internal forced flow of either helium gas or supercritical helium to cope with high heat loads in superconducting magnets in large fusion experimental reactors. In addition, the cable is enclosed by a stainless steel jacket to accommodate with the high level of Lorentz forces present in such magnets. Detector type magnets require stable, high-current conductors. Therefore, the detector CORC® CICC comprises an OFHC copper jacket with external conduction cooling, which is advantageous due to its simplicity. A 2.8 m long sample of each conductor is manufactured and prepared for testing in the Sultan facility at PSI Villigen. In the paper, the conductor design and assembly steps for both CORC® CICCs are highlighted.