AC losses and stability on large cable-in-conduit superconductors

The cable-in-conduit superconductors are preferred for applications where the AC losses and stability are a major concern, e.g., fusion magnets and SMES. A review of coupling currents loss results for both NbTi and Nb₃Sn cable-in-conduit conductors (CICC) is presented and the AC loss relevant features are listed, with special emphasis for the role of the interstrand resistance and strand coating. The transient stability approach for CICCs is discussed and the analytical models are quoted as well as the relevant experimental database. The likely spectrum of transient disturbance in CICC is reviewed and the need to account for interstrand current sharing in the design is outlined. Eventually a practical criterion for the interstrand resistance is proposed to link the stability and AC loss design.     

Coupling current control in Rutherford cables wound with NbTi, Nb3Sn, and Bi:2212/Ag

Calorimetric measurements of AC loss have been performed on Rutherford cables wound with NbTi, Nb₃Sn, and Bi:2212/Ag strands, respectively. For the NbTi cables Ni and stabrite coatings had been applied, while for the Nb₃Sn and Bi:2212/Ag cables the strand surfaces were just bare Cu or Ag, respectively. Most of the cables contained resistive cores: ribbons of kapton, or titanium (NbTi), stainless steel (NbTi and Nb₃Sn), and nichrome-80 (Bi:2212/Ag). In all cases the cores were found to lead to a strong suppression of the face-on (field normal to the broad cable face) coupling current loss; to such an extent that even the Bi:2212/Ag cables, which would otherwise be severely cross-sintered, show the possibility of acceptable coupling loss. Effective crossover interstrand contact resistances are calculated and normalized to a standard cable specification (‘LHC inner') for the purpose of intercomparison.     

Reduction of AC losses applying novel resistive interfilamentary carbonate barriers in multifilamentary Bi(2223) tapes

Hysteresis losses and coupling losses, a main component of the AC losses in Bi(2223) tapes, can effectively be reduced by enhancing the resistivity of the matrix material between the filaments and applying a filament twist. Since through alloying the sheath, as using AgAu(8 wt.%), the resistivity can only be raised by a factor <10 (77 K), a new conductor configuration with a quite novel composite matrix having resistive SrCO₃ barriers inside the Ag matrix between the filaments was developed. These new barriers, a cheap and commercial material, withstand the tape annealing, do not react with the superconductor, sinter dense and have a good bonding to Ag. Applying two different preparation techniques for 19 filament prototype tapes, critical current densities up to 20.7 kA cm⁻² were achieved. We report on tape preparation, the effect on the phase texture and the superconducting properties of such barrier tapes.     

Magnetic and mechanical AC loss of the ITER CS1 model coil conductor under transverse cyclic loading

The magnetic field in a coil results in a transverse force on the strands pushing the cable towards one side of the jacket. A special cryogenic press has been built to study in a unique way the mechanical and electrical properties of full-size ITER Cable-in-Conduit (CIC) samples under a transverse, mechanical load. The press can transmit a variable (cyclic) force of at least 650 kN/m to a cable section of 400 mm at 4.2 K. The jacket around the cable is partly opened in order to transmit the transverse force directly onto the cable. A superconducting dipole coil provides the AC magnetic field required to perform magnetisation measurements with pick-up coils. In addition the interstrand resistance (R_c) between various strands selected from topologically different positions inside the cable is measured. The force on the cable as well as 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 as the force is cycled. The mechanical heat generation, the coupling loss time constant nτ and R_c of a full-size ITER conductor have been studied under load for the first time. An important result is the significant decrease of nτ, after cyclic loading. It is also observed that the mechanical heat generation decreases with cycling.     

AC coupling losses in superconducting multistage cables with and without additional co-twisted copper strands

A simple technique is proposed to evaluate interstrand AC coupling losses deposed in superconducting multistage cables under low excitation of the transverse homogeneous time-varying magnetic field. The technique uses the superposition of the solutions for the induced coupling currents and interstrand or intersubcable AC losses in pairs of strands or subcables, constituting a multistage cable. The technique is valid under assumption of no resistance offered by strands and subcables for the longitudinal currents. The method allows one also to take into account the effect of additional co-twisted pure copper strands or subcables.     

AC losses in multifilamentary Bi(2223) tapes with an interfilamentary resistive carbonate barrier

For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO₃ barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.     

Calculation of AC losses in HTSC wires with arbitrary current voltage characteristics

A method for the calculation of magnetic field dynamics and AC losses in superconductors with smooth current–voltage characteristics is described. It is based on an integral equation for the current density, recently used by Brandt for magnetic relaxation. Brandt's equation is generalized to include arbitrary external magnetic fields and transport currents. One of the benefits of the integral equation formulation is that no boundary conditions ‘at infinity' are required, thus restricting the calculation region to the conductor cross section. The method is applied to superconducting tapes in oblique external fields. A further extension of the theory is shown to be applicable to the calculation of coupling losses in twisted multifilamentary superconductors.     

Experimental investigation of AC losses and effective time constant in the high current cable

The report is dedicated to the results of AC-loss measurements in the ITER (International Thermonuclear Experimental Reactor) Nb₃Sn strand and ITER design subsize cable samples. The test facility described below includes the set of superconducting magnet systems furnished with boil-off calorimetric inserts and calorimetric schemes including gas-flow meters. The calorimetric method and the magnetization loop recording method were used simultaneously for the AC-loss measurements. The coincidence between the results of both measuring techniques was confirmed. The original calorimetric scheme makes it possible to measure the released energy during one pulse of the magnet field. The results of AC loss measurements in ITER subsize conductor samples manufactured in Russia are given as an example.     

Decrease of magnetic AC loss in twisted-filament Bi-2223 tapes

In AC power-engineering applications, a large part of the AC loss in the superconductor is due to magnetization by the external field. This magnetic AC loss has been well described for the low-T_c conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity and flux creep. Magnetic AC loss in various twisted and non-twisted Bi-2223 tapes has been measured at power frequencies by a pickup method. The results are compared to theoretical models of magnetization loss. When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is pure silver. The extra effect of higher-resistance matrix materials is studied. In perpendicular field it is more difficult to decouple the filaments, due to the particular tape geometry. Contrary to a wire, there are essential differences between the AC loss mechanisms in a long twisted tape and those in a short piece of non-twisted tape. Finally, the dynamic resistance caused by the AC magnetic field is examined.     

AC losses in multifilamentary HTS-composite tapes based on BiPbSrCaCuO

Some results of AC loss measurements are presented for 19, 61, 127-filamentary Bi-2223/Ag tapes prepared by the ‘powder-in-tube' method. All measurements have been made at T=77 K under sinusoidal transport current with frequency in the range of 30–600 Hz and the current amplitude up to 30 A. The measurements have been carried out both in self field conditions and at the external magnetic field applied to the tape at the different angles. The dependencies of the AC losses on current amplitude and frequency have been obtained. It is found that for all tapes the current amplitude dependencies of the AC losses show good agreement with the Norris prediction for an elliptical or strip geometry. The AC loss dependencies on frequency were linear. The measurements of AC losses in external magnetic field show that the change of AC losses is only through the change of the critical current. So the transport AC losses in the tapes are the ‘saturation losses' that is they are different from classic hysteresis losses.     

Reduced filament coupling in Bi(2223)/BaZrO3/Ag composite tapes

Multifilamentary Bi(2223)/Ag tapes often exhibit AC loss levels comparable to those measured in monofilamentary samples, which is partly due to the large coupling currents induced in the low resistive sheath material. Surrounding the individual filaments by electrically insulating barrier layers suppresses these currents and strongly reduces the coupling. We demonstrate this effect with various types of magnetic and self-field AC loss measurements on a series of Bi(2223)/BaZrO₃/Ag tapes. We also discuss the influence of barrier thickness, twist pitch length and filament arrangement on the measured losses in these composite conductors.     

Experimental study on AC losses in Ag sheathed PbBi2223 tapes with twist filaments

Experimental measurements of AC losses were carried out on Ag sheathed PbBi2223 tapes with twisted and untwisted filaments. Losses were measured at 77 K as function of frequency and magnetic field parallel and perpendicular to the tape surface, using appropriate pick-up loops. Both the first and third harmonics of the signal were measured, in order to distinguish between the hysteresis loss and other types of loss. The effect of filaments uncoupling by twisting was clearly identified. For a tape with a twist pitch of 10 mm and I_c=40 A (20 kA cm⁻²) operating at 43 Hz, the filaments are uncoupled in fields less than 40 mT, which is greater than the full penetration field for both the filaments and the tape. Hence, a reduction in the hysteretic loss of the superconducting core is realised at power frequency between 10 and 40 mT. Results form the self-field loss measurement implies the uncoupling of twisted filaments at relative low transport current (I<0.5I_c)     

AC losses in a flexible 10 m long conductor model for a HTS power transmission cable

To demonstrate the possibility of manufacturing a HTS power transmission cable with low AC losses, we fabricated a 10 m long cable conductor and a cryosystem. The conductor was wound in a four-layer design out of 2 km Ag/Bi-2223 tapes. We determined a critical current of 5000 A. Loss measurements were performed with an electric method which detects the voltage drop along the conductor with a lock-in technique and a calorimetric method which measures the temperature rise along the conductor. Both methods yield the same low loss values of only 0.8 W/m at 77 K and 2000 A_rms/50 Hz. This is due to the low loss winding scheme we used which assures an equal current distribution in all four layers through transformatoric coupling. We applied the uniform current distribution model and added the nonlinear V–I curve to describe quantitatively the obtained results.     

Estimation of Jc(B) dependence from self-field alternating current (AC) losses measured on Bi-2223/Ag tapes

Transport AC losses measured in self-field conditions on multifilamentary Bi-2223 tapes are often found to be lower than those calculated within the framework of the critical state model for a bulk wire with elliptical cross section, though generally higher than predicted for a strip. This effect is sometimes ascribed to the non-ideal geometry of the tapes, which does not exactly reproduce either shape. Here we propose an alternative explanation assuming that the critical current density of superconducting material depends on magnetic field. In practice, we analyzed the AC loss curve and deduced different I_c values for the individual data points, using the standard Norris equation for elliptical conductor. This gives the relation between ‘calculated' I_c and the self-field associated to AC transport current, which can be regarded as an alternative way to qualify the dependence of J_c on magnetic field. Important is that this procedure covers the range of fields below the self-field at I_c where the measurement in background DC field can not be used to determine J_c(B).     

Mismatch considerations in laser diode to monomode fiber excitation via a hemispherical lens on the elliptical core fiber tip

We present what, as per our knowledge, is the first theoretical investigation of the coupling efficiency of a laser diode to a single-mode fiber via a hemispherical microlens on the elliptical core fiber tip in presence of possible transverse and angular misalignments. Using the ABCD matrix for refraction by a hemispherical lens on the tip of an elliptical core single-mode fiber, we present analytical expressions of such coupling efficiencies and investigate the corresponding losses due to possible transverse and angular misalignments. The concerned calculations involve little computations. Our analysis takes into consideration the limited aperture provided by the hemispherical lens. The results should be useful in optimum launch optics for designing suitable hemispherical microlenses on the tip of elliptical core fibers or circular core fibers which suffer from noncircularity.