H.G. Knoopers

Effect of self-field and current non-uniformity on the voltage–temperature characteristic of the ITER central solenoid insert coil by numerical calculations

For the optimisation of a magnet design with cable-in-conduit conductor (CICC) technology it is essential to comprehend the scaling of the critical current from the separate strand characteristics to the finally assembled cable performance in a coil. Several model coils have been tested in the framework of research for the International Thermonuclear Experimental Reactor (ITER). At present, the scaling of the critical current from the strand to the full cable performance and the apparent decrease of the n-index from strand to cable in the voltage–current curves is not understood. It is important to recognize the mechanisms behind this phenomenon in relation to the cost of the superconducting strand, which is significant in the manufacture of the magnets. Therefore, basic phenomena like the cable conductor self-field, the current unbalance introduced by the non-uniformity of the joints and a possible reversible or irreversible degradation of the voltage current characteristic of a strand during cable manufacture or electromagnetic loading of the magnet have to be considered. The voltage–current characteristic of the strand is extensively explored for the relevant range of magnetic field, temperature and axial strain space. Accordingly a numerical six-element network model is developed to simulate the conditions and behaviour of the last stage cable elements of a full-size ITER conductor. The experimental data, mainly in terms of voltage–current (VI) or -temperature (VT) characteristics, are obtained on the central solenoid insert coil (CSIC) experiment performed in Naka (Japan) in the framework of the research for ITER. The numerical model, which is briefly introduced, is used to study the cable performance by using experimentally obtained cable parameters like inter-strand (and bundle) contact resistance, strand critical current data as a function of magnetic field, temperature and applied axial strain, and external cable self-field measurements by Hall sensors for reconstruction of the current non-uniformity. The effect of a current redistribution due to the cable self-field on the voltage–temperature curve is calculated in correlation with the transverse resistance between the strands and last cabling stage bundles (petals). A realistic unbalanced current distribution is established by introducing non-uniform joints at the extremities of the CS-insert cable. It appears that the cable self-field effect hardly gives any change in the shape of the VT curve but merely a shift towards lower temperature giving a reduction of the current sharing temperature Tcs (10 μV/m) of <0.1 K. For typical CICCs with Cr-coated Nb3Sn strands, there is practically no current redistribution due to the cable self-field, because of the high inter-strand contact resistance. An unbalanced current distribution also gives an earlier voltage rise in the VT curve, mainly at low levels of the electric field. At a 10 μV/m criterion practically no reduction of the Tcs (<0.1 K) is found by the numerical simulation. However, in the CSIC the experimentally obtained overall reduction in Tcs from strand to cable is 0.7 K for an operating current of 40 kA at 12.5 T background field. According to the results of the numerical simulation, the cable self-field effect and the non-uniform current distribution, which is unavoidably caused by the joints, cannot explain the early voltage rise and low n-index in the VT curve of the CS-insert coil. It is very likely that electromagnetic forces play a role in causing reversible degradation in critical current or even irreversible due to strand (filament) damage. Neither can it be excluded that strand deformation during cabling has an impact on the final conductor performance as well. Therefore additional effort is required in detailed 3D modeling of the possible strand deformations inside a cable and the impact it has on the strand performance by experimental verification on strand level.

Experimental verification of the temperature and strain dependence of the critical properties in Nb/sub 3/Sn wires

The critical current density in Nb/sub 3/Sn conductors is described with an improved scaling formula for the temperature, magnetic field and strain dependence. In an earlier study, it is concluded that the largest uncertainties in this description arise from the temperature dependence that is described with various slightly different empirical relations. For the optimization of the numerical codes, used to predict the stability of large magnet systems, a more accurate description is required. Therefore, two different bronze processed conductors for the ITER CS model coil are analyzed in detail. The critical current is measured at temperatures from 4.2 K up to the critical temperature, in magnetic fields from 1 T to 13 T and with an applied axial strain from -0.6% to +0.4%. The axial strain is applied by a U-shaped bending spring and a comparison is made between brass and Ti-6Al-4V, as substrate material.

Magnetisation loss in twisted multifilament Bi-2223 tape conductors

The magnetisation loss in multifilament HTS tape superconductors can be reduced by twisting the filaments and increasing the matrix resistivity. In this paper the influence of filament twist on the coupling current loss of the tape is studied. Magnetisation measurements on multifilament Bi2223 tape conductors with different twist pitches are performed at 77 K in magnetic field applied parallel and perpendicular to the wide face of the conductor. Both frequency and magnetic field amplitude are varied. In perpendicular magnetic field the filaments are fully coupled. In parallel magnetic field the twisted filaments are decoupled, even for twist pitches around 10 mm and a pure silver core. However, for increasing magnetic field >0.04 T, the influence of the coupling current loss increases. The transition to full coupling and the coupling current loss are analysed with an existing analytical model for a tape superconductor.

Angular dependence of the dynamic resistance and its relation to the AC transport current loss in Bi-2223/Ag tape superconductors

The dynamic resistance loss, DC transport current combined with external AC magnetic field, in a Bi2Sr2Ca2Cu3O10/Ag superconducting tape is measured as a function of the orientation of the external AC magnetic field. We investigate the possibility of describing the AC transport current loss in an externally applied AC magnetic field from the much simpler dynamic resistance measurement. The orientation angles of the magnetic field with respect to the plane of the tape are changed from 0° (perpendicular configuration) to 90° (parallel configuration). The angular dependence of the dynamic resistance energy dissipation is described with an empirical formula. It is very similar to the angular dependence observed for the hysteresis loss. The dynamic resistance in parallel and perpendicular applied magnetic field is fairly well described with theoretical predictions based on the critical state model. The increase of the AC transport current loss due to a simultaneously applied AC magnetic field is described quite well with the dynamic resistance in the fully saturated state.

Compressive pre-strain in Nb/sub 3/Sn strand by steel tube and effect on the critical current measured on standard ITER barrel

The large Cable-In-Conduit Conductors (CICC) designed for the magnet coils in the International Thermonuclear Experimental Reactor (ITER), are composed of Nb/sub 3/Sn strand bundles in a Stainless Steel (SS) or Incoloy conduit. Both, the thermal contraction of the strand composite and the conduit material, define the final pre-strain after cooling down and thus affect the strand critical current (I/sub c/). The transverse forces, introduced when charging a coil, in addition affect the overall strain state due to bending and pinching of strands. Recently, periodic bending tests were applied on strand samples without additional axial compressive pre-strain. Here we explore the method of swaging a SS-tube around a strand to imitate the cool-down strain effect of the conduit. The experimental results of the I/sub c/ measurements at 12 T and 4.2 K are presented for a Nb/sub 3/Sn PIT strand with and without swaged SS tube on both Ti-6Al-4V and SS standard ITER sample holder (barrels). The effect of gluing the sample to the barrel is also investigated. The intrinsic strain state of the samples is verified by measurement of the I/sub c/ versus applied strain with the Pacman spring.

PTF, a new facility for pulse field testing of large scale superconducting cables and joints

A magnetic Pulse Test Facility (PTF), in which samples of CICC electrical joints from each ITER home team will be tested, has been fabricated at the MIT Plasma Fusion Center under an ITER task agreement. Construction of this facility has recently been completed, and an initial test phase on the first CICC joint sample has begun. PTF includes capabilities for sample currents up to 50 kA from a superconducting transformer developed by the University of Twente, magnetic fields up to 6.6 T with ramp rates to +1.5 T/s and -20 T/s, and a cryogenic interface, supplying supercritical helium with flow rates to 20 g/s through each CICC leg at controlled temperatures to 10 K and pressures to 10 atmospheres. A sophisticated, multiple-channel data acquisition system is provided to processed, digitally recorded sensor signals from both the sample and the facility. The facility is totally remote-controlled from a control room through a fiber optic link, and qualified users worldwide are afforded secured access to test data on a 24-hour basis via the Internet. The facility has successfully exercised the first joint sample over the ITER test spectrum with positive results.

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.

dc and ac behaviour of a normal joint in a superconducting cable

The application of joints which can be disassembled is studied within the framework of the development of high current (25 kA), high power (0.5 kW) cryogenic current supplies (flux pumps).1,2 The behaviour of a scale model with currents up to 5 kA will be presented here. Several means of diminishing the joint resistance have been researched with special attention being paid to detection methods to measure the resistance, and their accuracies. The role of the solder has been investigated. A simple model gives good qualitative understanding of the matter. The presence of ac currents in a cryogenic current supply causes considerably higher losses in the joint because of the non-homogeneous distribution of the current. A good impression of this distribution can be obtained by measuring the field along the joint. The measured results are in good agreement with calculated ones. A useful length of the joint under ac conditions can be defined and is helpful for design purposes.

Superconducting Magnets for Disk-Shaped MHD Generators Review and Predesign

In this paper possible magnet systems that can be used in combination with disk-shaped MHD generators are reviewed. The main coil configurations are the single solenoidal magnet and the split-pair magnet. Advantages and disadvantages of these magnet systems are discussed. After an enumeration of a few basic design considerations, a predesign of a split-pair magnet for a compact open disk demonstration unit with a thermal input power of 10 MW is presented. Furthermore, the use of Nb3Sn technology is envisaged to obtain a compact magnet with 9 T in the centre of the MHD channel. The preliminary design configuration consists of a NbTi and a Nb3Sn section. The distribution of winding volume among both sections is based upon minimum conductor costs.

Characterisation of superconducting components using PSPICE

A customer library of superconducting (SC) components for the PSPICE is under development in our group. The new library includes models of both low and high T/sub c/ superconductor based components. For large scale applications typical examples of simulated components are: a SC filament, composite wire and tape, a cable, a magnet, a switch, a transformer, a rectifier. Each model can he easily customised for a particular application. When the scheme is ready, it can he analysed using the powerful tools offered by PSPICE. The current status of the theory and model development and a few typical examples together with brief review of the practical conductors are presented and discussed in the paper.