The influence of Lorentz force on the ac loss in sub-size cable-in-conduit conductors for ITER

The cable-in-conduit superconductors for the ITER coils have operating current in excess of 40 kA and function under last ramp conditions and fields up to 13 T. The transverse Lorentz force acting on strands may reduce the effective contact resistance between strands in the cable and as a consequence, the coupling loss will increase. This influence is investigated with a sub-size jacketed cable having 81 Cr-coated Nb3Sn strands. The AC loss is measured with a sinusoidal and trapezoidal magnetic field superimposed to a stationary background field of 1 or 2 T while the cable carries a constant transport current up to about 30 kA. The AC loss is determined by a pick-up coil system and partly with a calorimeter for calibration purposes. The nτ at 0 current declines after cyclic loading, from 9 ms in the virgin state to 2 ms after several loads. The increase of the interstrand coupling loss due to Lorentz effects, accompanied by resistance-hysteresis and relaxation effects as observed in the loss are discussed. The total loss increases considerably due to interference of transport current and induced coupling currents with rising transport current and DC field

The influence of the diffusion barrier on the AC loss of Nb3Sn superconductors

Nb3Sn superconductors as they are applied in the ITER fusion programme, are equipped with diffusion barriers made of V, Ta, and V-Nb, all having superconducting properties. If an external ac magnetic field is applied, superconducting shielding currents are induced in the barrier which enclose a bundle of Nb3Sn filaments. As a consequence they are shielded and no ac loss will occur in the filaments. When the penetration field of the barrier material is exceeded, additionally the loss of the barrier and for higher fields also the loss of the Nb3Sn multifilamentary zone is generated. As long as the barrier is superconducting it will cause a substantial increase of ac loss. As a consequence the ac loss of the conductor in terms hysteresis loss per cycle and coupling time constants are strongly influenced. This aspect has to be considered carefully when properties of Nb3Sn conductors are determined at low magnetic fields

Simulation of the ITER Poloidal Field Coil Insert DC Performance with a New Model

The Poloidal Field (PF) Coil Insert is made from a NbTi cable in conduit conductor and has been subjected to tests in the Central Solenoid Model Coil facility at JAEA in Japan. For the interpretation of the voltage tap signals from these tests, we adapted the JackPot model – which was used previously to analyse short sample tests – to simulate also the model coil experiments. A key ingredient of JackPot is that the local magnetic field on the superconducting strands and the inter-strand contact resistances all depend on the “trajectories” of the strands within the cable. These trajectories areprecisely calculated, ensuring a realistic distribution of magnetic field- and contact resistance values. The results of the model calculations show that the applied joints are most likely responsible for the poor performance of short samples of similar PF conductors in earlier experimental tests. The model predicts that the influence of the joints is significantly less pronounced for the Poloidal Field Coil Insert

A novel numerical mechanical model for the stress–strain distribution in superconducting cable-in-conduit conductors \ud

Besides the temperature and magnetic field, the strain and stress state of the superconducting Nb3Sn wires in multi-stage twisted cable-in-conduit conductors (CICCs), as applied in ITER or high field magnets, strongly influence their transport properties. For an accurate quantitative prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the strain distribution along all individual wires is required. For this, the thermal contraction of the different components and the huge electromagnetic forces imposing bending and contact deformation must be taken into account, following the complex strand pattern and mutual interaction by contacts from surrounding strands. In this paper, we describe a numerical model for a superconducting cable, which can simulate the strain and stress states of all single wires including interstrand contact force and associated deformation. The strands in the cable can be all similar (Nb3Sn/Cu) or with the inclusion of different strand materials for protection (Cu, Glidcop).\ud \ud The simulation results are essential for the analysis and conductor design optimization from cabling to final magnet operation conditions. Comparisons are presented concerning the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties and thus on the eventual strain distribution in the Nb3Sn filaments when subjected to electromagnetic forces, axial force and twist moment. Recommendations are given for conductor design improvements. \ud \ud \u

Electromagnetic and mechanical AC loss of an ITER TF model coil conductor (DP4) under transverse cyclic loading

Energising a coil results in a transverse force on the strands pushing the cable towards one side of the jacket. This load causes a transverse compressive strain in strands and in particular in strand crossover points. Besides this, contact surfaces interfere by micro-sliding resulting in friction and anomalous contact resistance behaviour versus force. Two Central Solenoid Model Coil conductors have been tested previously in a cryogenic press and now the experimental results are presented for the Toroidal Field Model Coil (TFMC) conductor (DP4). The press can transmit a variable (cyclic) force of at least 650 kN/m directly to a cable section of 400 mm at 4.2 K. The magnetisation of the conductor and the interstrand resistance (Rc) between various strands inside the cable can be measured by varying pressure. The force on the cable and the displacement are monitored simultaneously in order to determine the effective cable Young's modulus and the mechanical heat generation due to friction and deformation. The mechanical heat generation, the coupling loss time constant n¿ and the Rc of the full-size ITER TFMC conductor have been studied under load up to 40 full loading cycles. The evolution of Rc is comparable to the behaviour found for the CS Model Coil type of conductors. A significant decrease of the cable coupling current time constant, n¿ and mechanical heat generation after cyclic loading is foun

Electromagnetic and mechanical characterisation of ITER CS-MC conductors affected by transverse cyclic loading, part 1: coupling current loss

The magnetic field generated by a coil acts on the cable which results in a transverse force on the strands. This affects the interstrand contact resistances (Rc), the coupling current loss and current redistribution during field changes. A special cryogenic press has been built to study the mechanical and electrical properties of full-size ITER conductor samples under transverse, mechanical loading. The cryogenic press can transmit a variable (cyclic) force up to 650 kN/m to a conductor section of 400 mm length at 4.2 K. The jacket is partly opened in order to transmit the force directly onto the cable. In addition a superconducting dipole coil provides the magnetic field required to perform magnetisation measurements using pick-up coils. The various Rc's between strands selected from different positions inside the cable have been studied. The coupling loss time constants (nτ) during and after loading are verified for the Nb3Sn, 45 kA, 10 and 13 T, ITER Model Coil conductors. A summary of the results obtained with up to several tens of full loading cycles is presented. A significant decrease of the cable nτ after several cycles is observed. The values of the nτ's are discussed with respect to the Rc measurements and a multiple time constant model (MTC)

Electromagnetic and mechanical characteristaion of ITER CS-MC conductors affected by transverse cyclic loading, part 3: mechanical properties

The magnetic field and current of a coil wound with a cable-in-conduit conductor causes a transverse force pushing the cable to one side of the conduit. This load causes elastic and plastic deformation with friction as well as heating due to friction. A special cryogenic press has been built to study the mechanical and electrical properties of full-size ITER conductors under transverse mechanical loading. The cryogenic press can transmit at 4.2 K cyclic forces of 650 kN/m to conductor sections of 400 mm length representative of the peak load on a 50 kA conductor at 13 T. In order to transmit the force directly onto the cable, the conduit is opened partly to allow the cable deformation. The force acting on the cable as well as the displacement are monitored simultaneously in order to determine the mechanical heat generation due to friction. The mechanical loss under load is investigated for the Nb3Sn, 45 kA, 10 and 13 T, central solenoid model cell conductors (CSMC). The mechanical heat generation is determined from the hysteresis in the measured curves of displacement versus applied force. The first results of the effect of some 40 loading cycles are presented and the two conductors are compared. A significant decrease of the cable mechanical heat generation after loading cycles is observed

CORD, a novel numerical mechanical model for Nb3Sn CICCs

The strain state of the superconducting Nb3Sn strands in multi-stage twisted ITER Cable-In-Conduit Conductors (CICCs) strongly determines the transport properties. For an accurate prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the stress and strain distribution along all individual strands is imperative. Also during the cabling process, the axial stress of the individual strands must be well controlled to avoid kinks, in particular when mixing different strands, e.g., Nb3Sn and copper strands. A mechanical model for a superconducting cable (CORD) was developed, which can predict the strain and stress states of all single strands including interstrand contact force and the associated deformation. The simulation results are not only important for analysis but can be used for optimization of cable manufacturing and conductor design optimization. We discuss the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties