Optimisation of ITER Nb3Sn CICCs for coupling loss, transverse electromagnetic load and axial thermal contraction

The ITER cable-in-conduit conductors (CICCs) are built up from sub-cable bundles, wound in different stages, which are twisted to counter coupling loss caused by time-changing external magnet fields. The selection of the twist pitch lengths has major implications for the performance of the cable in the case of strain sensitive superconductors, i.e. Nb3Sn, as the electromagnetic and thermal contraction loads are large but also for the heat load from the AC coupling loss. Reduction of the transverse load and warm-up cool-down degradation can be reached by applying longer twist pitches in a particular sequence for the sub-stages, offering a large cable transverse stiffness, adequate axial flexibility and maximum allowed lateral strand support. Analysis of short sample (TF conductor) data reveals that increasing the twist pitch can lead to a gain of the effective axial compressive strain of more than 0.3 % with practically no degradation from bending. For reduction of the coupling loss, specific choices of the cabling twist sequence are needed with the aim to minimize the area of linked strands and bundles that are coupled and form loops with the applied changing magnetic field, instead of simply avoiding longer pitches. In addition we recommend increasing the wrap coverage of the CS conductor from 50 % to at least 70 %. The models predict significant improvement against strain sensitivity and substantial decrease of the AC coupling loss in Nb3Sn CICCs, but also for NbTi CICCs minimization of the coupling loss can be achieved. Although the success of long pitches to transverse load degradation was already demonstrated, the prediction of the combination with low coupling loss needs to be validated by a short sample test.Comment: to be published in Supercond Sci Techno

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 x 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)

Performance evolution of 60kA HTS cable prototypes in the EDIPO test facility

During the first test campaign of the 60 kA HTS cable prototypes in the EDIPO test facility, the feasibility of a novel HTS fusion cable concept proposed at the EPFL Swiss Plasma Center (SPC) was successfully demonstrated. While the measured DC performance of the prototypes at magnetic fields from 8 T to 12 T and for currents from 30 kA to 70 kA was close to the expected one, an initial electromagnetic cycling test (1000 cycles) revealed progressive degradation of the performance in both the SuperPower and SuperOx conductors. Aiming to understand the reasons for the degradation, additional cycling (1000 cycles) and warm up-cool down tests were performed during the second test campaign. I-c performance degradation of the SuperOx conductor reached similar to 20% after about 2000 cycles, which was reason to continue with a visual inspection of the conductor and further tests at 77 K. AC tests were carried out at 0 and 2 T background fields without transport current and at 10 T/50 kA operating conditions. Results obtained in DC and AC tests of the second test campaign are presented and compared with appropriate data published recently. Concluding the first iteration of the HTS cable development program at SPC, a summary and recommendations for the next activity within the HTS fusion cable project are also reported

Central solenoid winding pack design for DEMO

The present study aims to reduce the outer radius of the central solenoid (CS) with respect to its nominal size specified by EUROfusion for a maintained CS magnetic flux. A reduced outer CS radius would allow the reduction of the overall size and cost of the DEMO magnet system. The proposed outline design of the winding pack for the CS1 module is based on layer winding. To achieve the same magnetic flux in a CS coil of significantly reduced outer radius the peak magnetic field at the CS conductors needs to be substantially increased. The use of high-temperature superconductors is therefore envisaged in the highest field sections of the CS coil. It is planned to use react & wind Nb3Sn conductors for intermediate field sections and NbTi at the lowest fields. In order to make a most economic use of the superconductors the proposed winding pack design considers a superconductor grading

Commissioning of the Main Coil of the EDIPO Test Facility

The EDIPO test facility is erected at CRPP Villigen with the aim of providing a flexible, high field test bed for high current force flow superconductors. The EDIPO main coil is a tilted-head race-track pair wound by a graded Nb3Sn cable-in-conduit conductor. The whole project, partly funded by the European Commission, started in 2004 and entered the commissioning phase in 2013. The final steps of instrumentation and installation of the main coil, delivered by industry in May 2011, lasted about 18 months. The first cool-down of the facility started in November 2012. The commissioning of the main coil, including the precise measurement of the generated magnetic field, was carried out in March 2013. At an operating current of 17.2 kA, a +/- 1% homogenous field of 12.35 T was generated over a length of 900 mm in the center of the test well, 140 mm x 91 mm in cross section. Details about cool-down, flux jumps, forces and displacements, field map, and charging rate are presented in this paper