I. Falorio

Quench Characteristics of bi2212 Solenoid Insert Coils in Background Field up to 20 T

The use of wind and react bi2212 wire for high field ( >;20 T) insert coils has been demonstrated to be a promising technology. While cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets there is little data for the high field application. The present work shows data from systematic quench measurements at 4.2 K on two 6-layer 300 mm high bi2212 solenoid coils at different currents I between 50% and 95% of IC in different background fields of 15, 18 and 20 T from a wide bore compact NbTi-Nb3Sn magnet developed by Oxford Instruments. The quench was induced by localized heat pulses (20-100 ms) and recorded with temporal-spatial resolved voltage taps and thermometers. By precise control of the power delivered in the heat pulse, the minimum quench energy MQE and the time constant for the quasi-stationary minimum propagation zone MPZ was obtained for each test condition. MQE was found to follow a scaling law of IC2/I4 in three different coils. The MPZ exhibited the expected anisotropy, extending predominately in the tangential direction and confined in a single winding layer. The propagation was also predominantly along the winding layer with a relatively slow velocity less than 50 cm/s, with an even lower radial propagation velocity of ~ 3 mm/s. In the present study, the interplay between the HTS coils and with the LTS background field were also measured and the results are highly relevant to the design of 20 T plus high field magnets with integrated LTS and HTS coils. The results were analysed in the context of a wide current temperature range for current sharing in the bi2212 wire and the T3 dependence of heat capacity at low temperatures.

Quench Characteristics of a Cu-Stabilized 2G HTS Conductor

The prospect of medium/high field superconducting magnets using 2G HTS tapes is approaching to reality with continued enhancement in the performance of these conductors. Direct measurements of 1D adiabatic quench initiation and propagation of a Cu-stabilized 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of various local heat pulses to the conductor at different temperatures between 40K and 64K carrying different transport currents. It was found that the stabilizer-free 2G tape maintains the unique characteristics previously measured in non-stabilized tape of increasing minimum propagation zone, (MPZ), with transport current and higher quench energy at lower temperatures. The minimum propagation zone length is investigated at low temperatures and in a background field of ~ 0.1T. The change in MPZ size is discussed in the context of the temperature dependent physical properties including the E-J characteristics. The results add more detail to help understand the unique characteristics of increasing MPZ with transport current and lower temperatures.

Quench Characteristic and Minimum Quench Energy of 2G YBCO Tapes

Due to a much extended current sharing regime and to the power law like E-J characteristics instead of the critical state, the quench characteristics of high temperature superconductors (HTSs) potentially differs from LTS. The understanding of thermal stability is therefore of crucial importance to meet the performance requirements set up by applications. In this work the quench characteristic and MQE measurements were carried out on 2G YBCO tape in self-field and field (0.1 T) scenario, in a wide range of temperature (40-80 K), where the conductors have a modest power exponent n ~ 20-30. The study focuses in particular on high current in the vicinity of Ic (j = I/Ic ~ 1), where power law current sharing dominates. Instead of vanishing with (1 - j), as expected for the critical state, the measured MQE is much higher near Ic. The high MQE and the heat generation of a non-(1 - j) scaling were constantly found in the temperature range 40 K-80 K of measurements. Such an enhanced MQE in the high current regime could be significant for HTS applications and suggests the possibility of sufficient stability very close to Ic. We show that the results can be explained by approximating the non-linear current sharing power-law heat generation with temperature by an equivalent critical state at a higher current sharing temperature. While the power-law and critical state converge at low j regions, the non-linear temperature dependence of thermal properties needs to be considered due to elevated quench/current sharing temperatures.

Quench Property of Twisted-Pair Superconducting Cables in Helium Gas

CERNs twisted-pair superconducting cable is a novel design which offers filament transposition, low cable inductance and is particularly suited for tape conductors such as 2G YBCO coated conductors, Ag-sheathed Bi2223 tapes and Ni/Monel-sheathed MgB2 tapes. A typical design of such twistedpair cables consists of multiple superconducting tapes intercalated with thin copper tapes as additional stabilizers. The copper tapes are typically not soldered to the superconducting tapes so that sufficient flexibility is retained for the twisting of the tape assembly. The electrical and thermal contacts between the copper and superconducting tapes are an important parameter for current sharing, cryogenic stability and quench propagation. Using an MgB2 twisted-pair cable assembly manufactured at CERN, we have carried out minimum quench energy (MQE) and propagation velocity (vp) measurements with point-like heat deposition localized within a tape. Furthermore, different contacts between the copper and superconductor around the hot spot have also been studied, including the co-twisted assembly in Kapton wrapping and locally separated tapes. The measurements have been performed in cooling helium gas at temperatures between 20 K and 35 K, with different current fractions with respect to the thermal runaway current. The results suggest a potential optimization strategy, compromising between: a higher stabilization with better contact between the copper and superconducting layers; and a faster propagation velocity and easier quench detection with a higher contact resistance.

Temperature and Background Field Dependence of a Compact React and Wind

The manufacture of react and wind MgB2 coils with bend diameters suitable for compact applications, such as narrow bore magnets, or demonstrators, such as generators, remains a challenge due to the finite strain tolerance of the MgB2 filaments. One solution is to improve the conductor strain tolerance under bending by reducing the wire diameter and increasing the number of filaments. A 243-m single length of 36-filament, 0.55-mm-diameter conductor with 50-μm-thick s-glass insulation, manufactured by Hyper Tech, is continuously wound onto a 100-mm-inside-diameter, 100-mm-long, 770-turn, 5-layer solenoid coil. As a preliminary verification of short-length conductor properties, transport critical current was measured on samples as reeled. Using both dc and pulsed currents, the solenoid coil is characterized in a background field of up to 5 T, and from 4.2 K to Tc, in both liquid and vapor. Layer-by-layer instrumentation of the coil, together with short-length test data, enables voltage characteristics to be spatially determined. Dissipation in the coil was evenly distributed across all layers with a joint contact resistance of ~20 nΩ. With an inductance of 0.37 H, the coil achieved a magnetic field of 0.82 T at 22.5 K at 100 A, 415 A/mm2, with no background field, and 0.32 T in a background field of 5 T at 4.2 K. The injected current values correlated with short-length critical current to within 10%.

\hbox{MgB}_{2}

Performance of state of the art MgB2 multifilamentary conductor at a required bend radius is essential for many applications including but not limited to magnets and motors. The characterisation is generally done with benchmark transport Ic but further detail can be seen in IV characteristics which are undertaken in this paper. Two conductors with the same architecture but different diameters, 0.89 and 0.45 mm were measured from 32 K to 20 K in self-field in conditions of as received and deformed to a 50 mm bend diameter, corresponding to strains of 1.4 % and 0.7 % respectively. The qualifying 0.45mm conductor was further measured in background fields up to 3 T. The smaller diameter wire was found to have no signs of degradation of critical behaviour in Ic or IV characteristics.

Solenoid Coil

E-J characteristics of SuperPower YBCO 2G tapes have been measured in the temperature range 15K-80K. It was found that the E-J characteristics deviate significantly from the standard power-law behaviour with apparent power exponent decreasing continuously at high voltage with increasing current. The deviation of E-J characteristic from the standard power-law was found to be consistent with a Weibull distribution of critical current. The identical scaling of E-J characteristics above 40K suggests a common critical current distribution. At lower temperature the critical current distribution becomes narrower to give higher apparent power exponent at lower temperature. With the critical current distribution model it is shown that the dissipation can be correlated directly to the flux flow resistance of Bardeen. The distribution of the critical current can be associated with a distribution of pinning potential of collective pinning and the headline pinning range is obtained as function of temperature.

Transport IV characterisation of MgB2 conductor at a bend radius of 50mm

Roebel cable with second-generation YBCO strands is one of the promising high-temperature superconducting solutions of fully transposed high current conductors for high field accelerator magnets. Following the considerable research effort on the manufacturing of Roebel cables in recent years, sample conductors are now available in useful lengths with reproducible performances to allow for detailed characterizations beyond the standard critical current measurements. The ac loss and strands coupling are of significant interest for the field quality of the accelerator magnets. We report a set of systematic ac loss measurements on two different Roebel cable samples prepared for the EuCARD-2 collaboration. The measurements were performed over a wide range of temperature values between 5 and 90 K, and the results were analyzed in the context of strands architecture and coupling. The results show that the transposed bundles are partially decoupled, and the strands in transposition sections behave as an isolated single tape if the strands are insulated.

Flux pinning distribution and E-J characteristics of 2G YBCO Tapes

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 were taken in self-field at 30 K, 30.5 K, 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 nonzero 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 1-D adiabatic, 3-D infinite solenoid, and 3-D with heat transfer at the boundary to explore the effect of dimensionality and conduction cooling on the propagation and MQE behavior.

Magnetization Losses of Roebel Cable Samples With 2G YBCO Coated Conductor Strands

AC loss measurements have been carried out on Bi2212 round wires of different twist pitches. With systematical variation of sample temperature (5 K-70 K) and ac field amplitude (≤ 0.2 T) as well as frequency (20 Hz-2 kHz) , this work shows (1) the full coupling of the non-twisted wire, (2) the uncoupling of sub-wires by twisting at moderate pitch up to 12 mm, (3) a relatively short time constant of milliseconds for the coupling current due to a high transverse resistivity, and (4) an effective (de)coupling diameter of 0.4 mm about 1.6 × of sub-wire bundles. The results suggest a promising potential for improving filament decoupling.