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

Reversal of childhood idiopathic scoliosis in an adult, without surgery: a case report and literature review

<p>Abstract</p> <p>Background</p> <p>Some patients with mild or moderate thoracic scoliosis (Cobb angle <50-60 degrees) suffer disproportionate impairment of pulmonary function associated with deformities in the sagittal plane and reduced flexibility of the spine and chest cage. Long-term improvement in the clinical signs and symptoms of childhood onset scoliosis in an adult, without surgical intervention, has not been documented previously.</p> <p>Case presentation</p> <p>A diagnosis of thoracic scoliosis (Cobb angle 45 degrees) with pectus excavatum and thoracic hypokyphosis in a female patient (DOB 9/17/52) was made in June 1964. Immediate spinal fusion was strongly recommended, but the patient elected a daily home exercise program taught during a 6-week period of training by a physical therapist. This regime was carried out through 1992, with daily aerobic exercise added in 1974. The Cobb angle of the primary thoracic curvature remained unchanged. Ongoing clinical symptoms included dyspnea at rest and recurrent respiratory infections. A period of multimodal treatment with clinical monitoring and treatment by an osteopathic physician was initiated when the patient was 40 years old. This included deep tissue massage (1992-1996); outpatient psychological therapy (1992-1993); a daily home exercise program focused on mobilization of the chest wall (1992-2005); and manipulative medicine (1994-1995, 1999-2000). Progressive improvement in chest wall excursion, increased thoracic kyphosis, and resolution of long-standing respiratory symptoms occurred concomitant with a >10 degree decrease in Cobb angle magnitude of the primary thoracic curvature.</p> <p>Conclusion</p> <p>This report documents improved chest wall function and resolution of respiratory symptoms in response to nonsurgical approaches in an adult female, diagnosed at age eleven years with idiopathic scoliosis.</p

MAGNETIC INSTABILITIES IN MULTIFILAMENT SUPERCONDUCTING COMPOSITES IN A FAST TIME VARYING MAGNETIC FIELD

L'analyse du comportement d'un composite supraconducteur multifilamentaire, soumis à un champ magnétique variable, permet de définir simplement un critère d'instabilité pour un composite se trouvant dans des conditions adiabatiques. Les résultats donnés par ce critère sont en bon accord avec ceux obtenus lors de deux expériences différentes réalisées sur des composites de NbTi.From the study of the behaviour of a superconducting multifilament composite in a time-varying magnetic field, we have derived a simple instability criteria for a composite under adiabatic conditions. The results given by this criteria are in good agreement with those obtained during two different experiments on NbTi composites

Mechanical and Electrical Modeling of Strands in Two ITER CS Cable Designs

Following the test of the first Central Solenoid (CS) conductor short samples for the International Thermonuclear Experimental Reactor (ITER) in the SULTAN facility, Iter Organization (IO) decided to manufacture and test two alternate samples using four different cable designs. These samples, while using the same Nb$_{3}$Sn strand, were meant to assess the influence of various cable design parameters on the conductor performance and behavior under mechanical cycling. In particular, the second of these samples, CSIO2, aimed at comparing designs with modified cabling twist pitches sequences. This sample has been tested, and the two legs exhibited very different behaviors. To help understand what could lead to such a difference, these two cables were mechanically modeled using the MULTIFIL code, and the resulting strain map was used as an input into the CEA electrical code CARMEN. This article presents the main data extracted from the mechanical simulation and its use into the electrical modeling of individual strands inside the CICC

Coupled mechanical-electromagnetic-thermal-hydraulic effects in Nb3Sncable-in-conduit conductors for ITER

The crucial multi-physics problem of how to extrapolate from the performance of an isolated Nb3Sn strand measured in the laboratory to the performance of a superconducting coil using multi-strand twisted cables is addressed here. We consider the particular case of the path going from the LMI strand to the international thermonuclear experimental reactor (ITER) toroidal field model coil (TFMC), through its associated Full Size Joint Sample, the TFMC-FSJS. Mechanical, electromagnetic and thermal-hydraulic conditions are simulated using the ANSYS, ENSIC and Mithrandir/M&M codes, respectively. At least in this case, the DC performance of the short sample turns out to be relatively close to (considering error bars) but not fully representative of that of the coil, showing higher (less compressive) effective thermal strain but also higher sensitivity to the electromechanical load

Approach to Heterogeneous Strain Distribution in Cable-In-Conduit Conductors Through Finite Element Simulation

International audienceThe ITER Cable-In-Conduit Conductors are submitted to high thermal and electromagnetic cyclic loadings responsible for conductivity loss in the strain-sensitive Nb3Sn strands. The complex mechanical phenomena occurring at the local scale of the strands make the final performances of the CICC difficult to predict from single-strand properties. In order to assess the amplitudes of the local strains that drive the conductor electrical behavior, a nonlinear finite element simulation code is used. The successive stages of the conductors' service life, from the forming of the cable to its thermal cool down and Lorentz force loading, are simulated. Each strand is individually modeled along with the great number of contacts-friction interactions between the strands. This paper presents the simulation results obtained for 144 strand cables of two different designs. It is shown that the various loadings result in a heterogeneous distribution of strains along and across the strands with occurrence of extreme tensions and compressions. The use of simulation would eventually help to better characterize the influence of conductor design parameters

Numerical Simulation of ITER Cable-in-conduit Conductors Mechanical Behavior

The ITER Cable-In-Conduit Conductors (CICC) are composed of an assembly of pure copper wires and composite superconducting strands (with embedded brittle Nb3Sn microfilaments) cabled together and inserted in a stainless steal jacket. If the current carrying capacities of individual ITER strands are clearly identified, by a dependence of the critical current on the applied strain and by a statistical quantification of possible microfilaments breakage, the characterization of cable-in-conduit is not yet fully achieved. What are the local strain values of strands inside CICCs under operating conditions is still an open question. A deeper understanding of how local strains develop and where critical strains appear in complex cabled structures could help to optimize CICCs designs in term of losses of conductivity. The present work aims at providing for a finite element model of CIC conductors, able to predict local strains, especially bending, at the scale of individual strands. The finite element software, MULTIFIL, initially developed to model various kinds of entangled media, has been adapted to consider the specific issues related to CIC conductors. The MULTIFIL's main feature is basically to handle the evolution of contact-friction interactions between wires. In this study, the initial conductors' geometry (trajectories of all individual wires), a priori unknown, is determined by a simulation of the shaping process by means of moving rigid tools. Starting from formed cables, both the thermal restraint and the transverse Lorentz loads are simulated through successive applications of proper loading. An important issue concerns proper boundary conditions to be applied at each strands ends. In that sense, the so-called pseudoperiodic boundary conditions, relevant to cable modelling, will be introduced. By the way, experimental and numerical “Force/Displacements” curves, obtained on cables under standard axial and transverse loading, show good agreement. A quantitative approach based on metallographic analysis of sub-cables and comparison of conductor's sections with numerical results will be presented as well. Results of full simulations (from initial shaping to magnetic loading) will be exposed for different conductors. Relevant information at the scale of strands (curvatures and strains distributions) can be retrieved from these simulations. The influence of changes in global design parameters (pitches lengths, void fraction and strands material properties) on local strains will be shown and discussed. This work is supported by CEA and ITER Organization allowing a helpful collaboration with ECP by contributing to the interpretation and providing for experimental database