7 research outputs found

    Influence of the internal architecture of MgB2 conductors in the load line of magnet coils

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    MgB2 conductors have a multifilamentary superconducting structure encased in sheaths of different materials. Conductors with ferromagnetic sheaths, such as Ni or Fe, redistribute the magnetic field, which can affect the critical current of the superconducting filaments. The internal magnetic field distribution inside the conductor is not considered in general when modeling magnets, where the winding is considered as a bulk. The previous assumption can lead to discrepancies between the experimental results and numerical Ic predictions. Hence, a correct analysis of the effects of the ferromagnetic matrix in conductors will lead to savings in cost and weight of the coil design. In this work, the magnetic field distribution inside tapes with different architectures based on existing MgB2 tapes has been obtained. The magnetic field in the filaments is modeled focusing on the influence of the ferromagnetic matrix in the field distribution. The study is extended to round pancake coils based on the prototype coils aimed for its application in wind turbines in the SUPRAPOWER project

    Locked rotor and transient tests of a 100 kW HTS machine

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    Locked rotor and “short circuit at the stator terminals” are two standard motor tests that provide operators with useful data about the electro-mechanical performance of rotating machines for installation. A programme of motor tests was performed on a novel machine with an HTS rotor winding and a conventional copper wound stator. The rotor winding was cooled down to 77 K. The differences when driving 0 and 190 A in the stator winding whilst ramping the field current in the locked rotor close to its critical current are discussed by interpreting small changes in the voltages measured at different locations in the winding. The perpendicular field generated by the stator produced the greatest impact on the HTS winding and its starting torque. Finally, a comparison of single-phase short circuits with three-phase short circuits unveiled a doubling in the peak stator current induced and the period for the transient to dissipate, but a moderate reduction in the peak (spiking) current in the HTS winding

    Transient Thermoelectrical Behavior of MgB2 Cables in the Superconducting Links for the High Luminosity Upgrade of the LHC

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    Abstract: High-current superconducting links (SC-Link) consisting of multiple MgB2 cables are being developed for the cold powering of the high luminosity upgrade of the LHC (HL-LHC). The SC-Links are designed to work over a temperature gradient between 4.2 and 20 K cooled by flowing subcritical helium gas to provide up to 20 kA per cable to the new inner-triplets to gain a higher luminosity. The SC-Links have a compact configuration of six 20-kA cables for three magnet circuits and several lower current cables. Upon the quench of one magnet, the neighboring circuits are exposed to a transient field up to 0.3 T, which may induce significant magnetization and coupling current losses with the potential of quenching the cables in these circuits. In this paper, we present a thermoelectrical analysis of the transient electromagnetics of the cable system, the thermal stability of the cables in temperature gradient, and the effect of lateral cooling by the flowing helium gas on the quench behavior. Furthermore, the analysis was compared with a comprehensive experimental study of direct thermometric and electrical measurements of the transient magnetic losses at different temperatures (4-30 K) and field sweep rates (0.3-30 T/s) as well as quench measurements under different cooling configurations. Cables samples of different stabilization matrices and strand configurations were measured to provide insight for the overall optimization by compromising between loss reduction and cryogenic stabilization

    Temperature and background field dependence of thermal stability in a compact MgB2 solenoid coil

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    A conduction cooled compact react and wind MgB2 solenoid magnet was tested for quench propagation and minimum quench energy MQE in a background field which allows to an extent a decoupling of the peak field in the windings from the conductor field performance. MQE and propagation data was taken in self field at 30, 30.5 and 31.5 K and at fields/temperatures of 1 T/25 K, 3 T/18 K and 5 T/10 K where the power law n-value characteristic was similar at n~30. At all temperatures and at low I/Ic quench propagated first through the insulating layers, in a radial direction, and at high I/Ic within the layer. The MQE values as a function of I/Ic are shown to consistently have non zero values as I? Ic which is characteristic of an analytical quench model that includes the n-value power law dissipation. At 30.5 K a sharp drop in MQE occurs along with the change in propagation direction. The analytical model is then exploited as 1D adiabatic, 3D infinite solenoid, and 3D with heat transfer at the boundary to explore the affect of dimensionality and conduction cooling on the propagation and MQE behavior

    Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3_3Sn properties

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    Future applications requiring high magnetic fields, such as the proposed Future Circular Collider, demand a substantially higher critical current density, JcJ_c, at fields ≥16 T than is presently available in any commercial strand, so there is a strong effort to develop new routes to higher JcJ_c Nb3_3Sn. As a consequence, evaluating the irreversibility field (HirrH_{irr}) of any new conductor to ensure reliable performance at these higher magnetic fields becomes essential. To predict the irreversibility field for Nb3_3Sn wires, critical current measurements, IcI_c, are commonly performed in the 12-15 T range and the Kramer extrapolation is used to predict higher field properties. The Kramer extrapolation typically models the contribution only for sparse grain boundary pinning, yet Nb3Sn wires rely on a high density of grain boundaries to provide the flux pinning that enables their high critical current density. However, whole-field range VSM measurements up to 30 T recently showed for Nb3_3Sn RRP® wires that the field dependence of the pinning force curve significantly deviates from the typical grain boundary shape, leading to a 1-2 T overestimation of HirrH_{irr} when extrapolated from the typical mid-field data taken only up to about 15 T. In this work we characterized a variety of both RRP® and PIT Nb3Sn wires by transport measurements up to 29 T at the Laboratoire National des Champs Magnétiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique and whether or not it is a common feature across different designs. Indeed we also found that when measured in transport the 12-15 T Kramer extrapolation overestimates the actual HirrH_{irr} in both types of conductor with an inaccuracy of up to 1.6 T, confirming that high field characterization is a necessary tool to evaluate the actual high field performance of each Nb3Sn wire
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