Arend Nijhuis

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system R&D activities were conducted in parallel, together with broad investigations on high temperature superconductor (HTS) technologies. In this paper, we present the outcomes of the work conducted in two areas in the 2014 magnet work program: 1) the EU inductive reactor (called DEMO1) 2014 configuration (power plant operating under inductive regime) was the basis of conceptual design activities, including further optimizations; and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

Experiments and FE modeling of stress-strain state in ReBCO tape under tensile, torsional and transverse load

For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress–strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress–strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts.

Electromechanical modeling of Nb3Sn superconducting wires subjected to periodic bending strain

The transport performance of Nb3Sn cable-inconduit conductors (CICCs) depends on the strain distribution along the superconducting filaments determined by the combination of electromagnetic and mechanical forces applied to the strands. Experimental studies on the effect of bending strain were performed at the University of Twente by means of the Test Arrangement for Strain Influence on Strands (TARSIS) facility. The aim of this paper is to verify the agreement between a detailed electromechanical model of the wire and the experimental results obtained in TARSIS. A numerical model of two Nb3Sn internal tin strands was developed, which describes the wire through a distributed parameter nonlinear electrical circuit. The model requires a single strand to be discretized into a number of elements, connected by transverse conductances and subjected to a given strain distribution. The strain distribution maps were computed at the École Centrale Paris by means of the MULTIFIL code at different experimental conditions in the TARSIS facility. The simulation results show good agreement with the experimental ones in terms of both critical current and n-value degradation.

Local Strain Exerted on Nb3Sn Filaments in an ITER Strand

As part of an international project to benchmark facilities for measuring the strain dependence of critical current in International Thermonuclear Experimental Reactor (ITER) Nb3Sn strands, direct measurement of local strain exerted on Nb3Sn filaments was attempted at cryogenic temperature by means of a pulsed neutron technique. The lattice axial strain increased linearly with a slope close to unity against applied strain, while the thermal axial strain was −0.22% at 8.5 K. As a result, the force-free strain was evaluated to be 0.22–0.23%. This key parameter should provide an accurate estimate of the peak location of critical current versus applied strain. The lattice transverse strain decreased linearly as a function of applied strain with a slope of 0.33–0.34. The lattice strains of the Nb and Cu components were also measured and their behavior was analyzed by computing diffraction elastic moduli based on micromechanics theories. The stress–strain curve calculated according to the rule of mixtures described quite well the macroscopic curve measured for the present ITER Nb3Sn strand.

Cable Rotation and Twist Pitch Variation for ITER TF Conductor in China

Toroidal field (TF) coil is made of cable-in-conduit conductor (CICC) with an operating current of 68 kA. In total, 900 Nb3Sn strands and 522 copper strands are cabled around the central spiral and then wrapped with stainless-steel tape of 0.1-mm thickness. The cable is then inserted into a jacket under tensile force that is increasing with length of insertion. Insertion and compaction are key technologies injacketing. The tensile force can lead to rotation of a cable, resulting into an increase in a twist pitch. Understanding the twist pitch variation is very important; in particular, the twist pitch of a cable inside a CICC strongly affects properties such as ac loss. In this paper, we present the results of a Chinese TF conductor during manufacture. A rotation sensor and a straight line on the cable surface were used to measure the rotation during insertion and compaction. X-ray was used to check the changed twist pitch during compaction. Short samples were subjected to destructive examination to assess the exact twist pitch. The test results show that the Chinese TF conductor has a slight rotation and very small twist pitch variation during conductor manufacture.

Analysis of ITER PF Coil Joint Design Under Reference Operating Scenario

One of the critical components of the ITER poloidal field (PF) coils is the electrical joint connecting two conductor lengths. The lap “shaking hands” joints will operate under variable field, causing parasitic-induced currents in superconducting strands and temperature rise of the strands. Previously, some design changes for decreasing the induced currents in the joints were proposed and assessed with the JackPot-ACDC model. In this paper, we use the same model to compare the behavior of two designs under the reference operation cycle of the PF coils. It is concluded that the joints with the proposed design changes will have sufficient stability margin against thermal and electromagnetic disturbances.

Critical Current of Various REBCO Tapes Under Uniaxial Strain

For high-current REBCO superconductors in high magnet fields with currents on the order of 50 kA, single coated conductors must be assembled in a cable. Under operating conditions, such cables are subjected to combined torsion, axial, and transverse loads in the tapes and tape joints. As such, the single tape performance under applied axial strain is crucial to understand cable limitations, and important for choosing an appropriate tape geometry and tape supplier. The critical current (Ic) versus applied uniaxial strain and the relative strain irreversibility limit were measured on HTS tapes produced by five manufacturers. The Ic measurements were performed at 77 K in self-field with the sample tapes soldered to the U-shaped bending spring (U-spring). It has been observed that most of the tapes perform well under applied compressive strain. The dependence of Ic on applied strain and the irreversibility limits were found to be unique for each tape. The n-values of the tapes, unlike the Ic, are less sensitive to applied strain in the reversible regime. The results of different tapes are presented and compared.

Uniaxial Strain Induced Critical Current Degradation of Ag-Sheathed Bi-2212 Round Wire

The critical current degradation of Bi-2212 Ag-sheathed round wire subjected to uniaxial strain was studied at 4.2xa0K in 14xa0T background field. The strains applied on the sample are both tension and compression. The additional tensile strain caused by the difference in thermal expansion between the Bi-2212 round wire and Ti–6Al–4V alloy spring was calculated. The results showed that a drastic degradation of the critical current occurred when the intrinsic strain exceeded around 0.5% in tensile direction. For the compressive strain, the degradation of critical current was almost linear but more gradual than tensile strain. The intention is to use these results as a basis for Bi-2212 conductor and superconducting coil design.

Persistent-current magnetizations of Nb3Sn Rutherford cables and extracted strands

The magnetizations of eight high-gradient quadrupole cables designated HQ and QXF and a pair of strands, identical in architecture but with different effective strand diameters extracted from an HQ and a related QXF cable, were measured. In the service of field quality assessment, the cable magnetizations and losses were measured by pickup coil magnetometry at 4.2 K in face-on fields, B m , of ± 400 mT at frequencies, f, of up to 60 mHz. Based on the coupling component of loss, Q coup , the coupling magnetization M coup = Q coup /4B m was derived for a ramp rate of 7.5 mT/s. Persistent current (shielding) magnetization and loss (M sh and Q h,strand ) were measured on short pieces of extracted strand by vibrating sample magnetometry at 4.2 K. Unpenetrated M-B loops to ± 400 mT and fully penetrated loops to ± 14 T were obtained. M coup can be easily controlled and reduced to relatively small values by introducing cores and adjusting the preparation conditions. But in low fields near injection Nb3Sns high J c and correspondingly high M sh,cable may call for magnetic compensation to preserve field quality. The suitably adjusted cable and strand fully penetrated M-B loops were in reasonable accord leading to the conclusion that strand magnetization is a useful measure of cable magnetization, and that when suitably manipulated can provide input to magnet field error calculations.

Interstrand Coupling Properties of LARP High Gradient Quadrupole Cables in Response to Variations in Cable Design and Heat Treatment Condition

Calorimetric measurement of coupling loss versus frequency has been measured on two sets of cored and uncored large Hadron Collider Accelerator Research Program high gradient quadrupole Nb3Sn Rutherford cables. Studied are the responses of the resulting interstrand contact resistances (ICR) to variation of stainless-steel (SS) core width and position and to variation of reaction-heat-treatment (RHT) condition. One pair of cables (an early HQ-series type) with and without core had received RHT under 20-MPa uniaxial face-on pressure. Another set of cables (recent QXF type) furnished with SS cores of various widths had received RHT under ambient pressure. The results were displayed as cable-cross-sectional micrographs and plots of ICR versus percent core coverage (W). The HQ cables were tightly compacted and produced results consistent with a previously expected continuous ICR versus W variation. On the other hand, the QXF cables were uncompacted such that their upper and lower layers were separated by what is referred to as a full-width “pseudocore;” as a result, their ICRs were independent of the widths of the SS cores. Compaction versus noncompaction is discussed and future research directions are suggested.