Quench Current Research Articles

Characteristics of Cu Stabilized ${\rm Nb}_{3}{\rm Al}$ Strands With Low Cu Ratio

Characteristics of recently developed F4-Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand with low Cu ratio are described. The overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand could be easily increased by decreasing of the Cu ratio. Although the quench of a pulse-like voltage generation is usually observed in superconducting unstable conductor, the F4 strand with a low Cu ratio of 0.61 exhibited an ordinary critical transition of gradual voltage generation. The F4 strand does not have magnetic instabilities at 4.2 K because of the tantalum interfilament matrix. The overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the F4 strand achieved was 80-85% of the RRP strand. In the large mechanical stress above 100 MPa, the overall <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the F4 strand might be comparable to that of high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> RRP-Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. The Rutherford cable with a high packing factor of 86.5% has been fabricated using F4 strands. The small racetrack magnet, SR07, was also fabricated by a 14 m F4 cable. The quench current, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> , of SR07 were obtained 22.4 kA at 4.5 K and 25.2 kA at 2.2 K. The tantalum matrix Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strands are promising for the application of super-cooled high-field magnets as well as 4.2 K operation magnets.

Quench Properties of a 7-T Force-Balanced Helical Coil for Large-Scale SMES

Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. Based on the FBC design, a superconducting model coil has been developed. The outer diameter of the model FBC is 0.53 m. The model FBC will have 270 kJ magnetic energy with the critical magnetic field of 7.1 T. The critical coil current and self inductance are 552 A and 1.8 H, respectively. The hand-made winding, using NbTi/Cu composite strands with a diameter of 1.17 mm, was finished with 10584 poloidal turns after four months. The helical windings of the model FBC were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Three test runs were conducted with liquid helium cooling at intervals of several months. The number of quench tests was 81 in total. In the third test run, the quench position of the FBC windings was identified using acoustic emission measurements. The first quench current was 293 A, which was 53% of the critical coil current. The training phenomena could be observed even after the coil was warmed up to room temperature. After successive quenches the quench current was improved to 476 A, corresponding to 86% of the critical coil current, and it was successfully excited up to 6.1 T.

Assessment of Small Bend Diameter Magnesium Diboride Wire for a Superconducting Fault Current Limiter Application

This paper reports results of tests on several MgB2 round wire samples wound with small bend diameters. Wire suitability for use in a resistive type superconducting fault current limiter with a practical aspect ratio is assessed in terms of current limiting performance and tolerance to mechanical process handling. A fully controllable low current supply was used to determine the variation of the wire impedance against frequency and temperature. The frequency response enabled the calculation of coil inductance and superconducting resistance. Critical temperatures measured were in the range of 34 K-39 K. A high current supply was used to test the coil current limiting properties in the temperature range 25-32 K in self field at 50 Hz. Good, repeatable current limiting properties were observed with quench currents in the range 95-390 A. Good tolerance to mechanical process handling was also evident.

Design and Loading Test Results of 21 T@4.2 K Magnet Using RHQT ${\rm Nb}_{3}{\rm Al}$ Insert Coils

In this experiment, we designed, constructed and tested a 21 T superconducting magnet to be operated at 4.2 K, consisting of a Nb3Al inner coil with an exclusive power supply, and a series of Nb3Al middle, Nb3Sn outer-1 and Nb-Ti outer-2 coils with a common power supply. The flux jumps of the Nb3Al middle coil occurred in low magnetic fields and caused frequent shutdowns of an overvoltage-sensitive common power supply; however, we discovered this could be avoided by reducing the current ramp rate. The outer-middle coil unexpectedly quenched at 17.7 T, which was less than the expected 19.3 T. Simultaneous excitation of the inner coil was scarcely effective in increasing the quench current of the outer-middle coil, indicating that the Ic degraded location is relatively far from the inner coil. However, energization of the inner coil to a quench, in a backup field of 17.6 T by the outer-middle coil generated a central field of 20 T in a 25.4 mm clear bore at 4.2 K.

Progress in Wind-and-React Bi-2212 Accelerator Magnet Technology

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report on our progress in the development of the technology for the application of <formula formulatype="inline"><tex Notation="TeX">$\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{8+{\rm x}}$</tex></formula> (Bi-2212) in Wind-and-React accelerator magnets. A series of superconducting subscale coils has been manufactured at LBNL and reacted at the wire manufacturer SWCC. Selected coils are impregnated and tested in self-field, even though the coils exhibited leakage during the partial melt heat treatment. Other coils have been disassembled after reaction and submitted to critical current <formula formulatype="inline"><tex Notation="TeX">$(I_{c})$</tex> </formula> tests on individual cable sections. We report on the results of the current carrying capacity of the coils. Voltage-current <formula formulatype="inline"> <tex Notation="TeX">$(V\ I)$</tex></formula> transitions were reproducibly measured up to a quench currents around 1400 A, which is 25% of the expected performance. The results indicate that the coils are limited by the inner windings. We further compare possibilities to use Bi-2212 and <formula formulatype="inline"> <tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> tilted solenoid, and <formula formulatype="inline"><tex Notation="TeX">$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$</tex> </formula> (YBCO) racetrack inserts to increase the magnetic field in HD2, a 36 mm bore <formula formulatype="inline"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex> </formula> dipole magnet which recently achieved a bore magnetic field of 13.8 T. The application of Bi-2212 and/or YBCO in accelerator type magnets, if successful, will open the road to higher magnetic fields, far surpassing the limitations of <formula formulatype="inline"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex> </formula> magnet technology. </para>

超電導磁気エネルギー貯蔵用電磁力平衡コイルの実証モデル開発

In large-scale SMES, the superconducting coils require special considerations for induced electromagnetic forces to limit allowable tensile stress. Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. In order to demonstrate the feasibility of the FBC concept for SMES, the authors have developed a superconducting model coil. The outer diameter of the model FBC is 0.53m. The hand-made winding, using NbTi/Cu composite strands with a diameter of 1.17mm, was finished with 10584 poloidal turns after four months. The helical windings of the model FBC were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Three test runs were conducted with liquid helium cooling at intervals of several months. The number of quench tests was 81 in total. The first quench current was 293A, which was 53% of the critical coil current. The training phenomena could be observed even after the coil was warmed up to room temperature. After successive quenches the quench current was improved to 476A, corresponding to 86% of the critical coil current, and it was successfully excited up to 6.1T.

Experimental Results of a 7-T Force-Balanced Helical Coil for Large-Scale SMES

A superconducting force-balanced coil (FBC), which is geometrically one tenth the size of a 100 MJ class SMES coil, has been developed. The FBC is a helically wound hybrid coil of toroidal field coils and a solenoid. This coil configuration can minimize the required mass of the structure for induced electromagnetic forces. The model FBC with an outer diameter of 0.53 m will have 270 kJ magnetic energy at the critical magnetic field of 7.1 T using NbTi strands. The critical coil current and self inductance are 552 A and 1.8 H, respectively. The diameter of the NbTi strand is 1.17 mm. The hand-made winding of the model FBC was carried out without reinforcing materials such as stainless steel wires. The first experiment was conducted with liquid helium cooling. The quench properties of the model FBC were mainly investigated. The first quench current is 293 A, which is 53% of the critical coil current. Training phenomena were observed in the model FBC without any sudden decreases in the quench current. After 33 successive excitations the quench current improved to 419 A, and it was successfully excited up to 5.4 T without reinforcing materials for the NbTi strand. This fact demonstrates a primary capability of the FBC as a SMES coil.

Self-Field Effects in Magneto-Thermal Instabilities for Nb-Sn Strands

Recent advancements in the critical current density of conductors, coupled with a large effective filament size, have drawn attention to the problem of magneto-thermal instabilities. At low magnetic fields, the quench current of such high strands is significantly lower than their critical current because of the above-mentioned instabilities. An adiabatic model to calculate the minimum current at which a strand can quench due to magneto-thermal instabilities is developed. The model is based on an dasiaintegralpsila approach already used elsewhere . The main difference with respect to the previous model is the addition of the self-field effect that allows to describe premature quenches of non-magnetized strands and to better calculate the quench current of strongly magnetized strands. The model is in good agreement with experimental results at 4.2 K obtained at Fermilab using virgin Modified Jelly Roll (MJR) strands with a low residual resistivity ratio (RRR) of the stabilizing copper. The prediction of the model at 1.9 K and the results of the tests carried out at CERN, at 4.2 K and 1.9 K, on a 0.8 mm Rod Re-Stack Process (RRP) strand with a low RRR value are discussed. At 1.9 K the test revealed an unexpected strand performance at low fields that might be a sign of a new stability regime.

Development of a pre-bent react-and-windCuNb/Nb3Sn multilayer superconducting coil

A superconducting multilayer coil usingCuNb/Nb3Sn wire was fabricated by the pre-bent react-and-wind (R&W) method. The pre-bent R&Wmethod is a combination of the pre-bending treatment and the conventional R&W process. A0.8% pre-bending strain was applied by ten fixed pulleys. The coil was tested inmagnetic fields of 14, 12, 11 and 10 T at 4.2 K. The coil quench current achieved100% load ratio of the short sample critical current in the 14 T back-up field. Thequench current of the outer section coil achieved 100% of the short sample criticalcurrent in the 12 T back-up field. The coil quench currents in various magneticfields are discussed from a hoop stress and mechanical disturbance point of view.

Quench characteristics of Ag/AuBi2223 HTS-stainless steel stack used for the hybrid current leads of the large hadron collider

The quench characteristics of Ag/Au sheathed Bi2223 tapes have been investigated in an adiabatic condition and in a configuration similar to that used in hybrid high temperature superconducting current leads, namely the 13000A leads used for the Large Hadron Collider at CERN. A specialised rig was designed and constructed to provide a carefully controlled environment. The samples were prepared from HTS tape soldered onto a stainless steel substrate with a number of temperature sensors at various positions along the length of tape. One end of the lead (cold end) was maintained at 6K using G-M cryo-cooler whereas the temperature of the other end (warm end) can be varied and maintained at temperatures up to 100K. The thermal runaway currents (quench currents) at various warm end temperatures (in the range of 40-100K) were determined. The temperature evolutions at various locations along the tape were recorded at different top end temperatures and currents. The effect of the stainless steel mechanical reinforcement on the thermal properties of the stack was also investigated and presented.

Study on the Quench Current of YBCO Thin Film FCL

We present the result of a theoretical analysis on the quench current of YBCO thin films for the superconducting resistive fault current limiter. Generally, a transport current exceeds a critical current without quenching and the quench current is usually more than 1.5 times of the critical current. The quench current is calculated theoretically by considering the effects of a small temperature rise DeltaT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> . The temperature rise produces the Joule heat increase because the critical current density decreases by the temperature rise. If this Joule heat can not be removed completely by the heat transmission (hDeltaT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) from the superconductor surface to liquid nitrogen, the superconductor quenches. It is shown that the theoretically calculated quench currents accord with the experimental results.

Second Generation High-Temperature Superconducting Wires for Fault Current Limiter Applications

In this paper, we report the results from evaluation on the use of 2nd generation high-temperature superconducting wires, or 2G HTS conductors, as elements for superconducting fault current limiter (SFCL). The unique features of 2G HTS conductors such as high N-values, superior electromechanical performance, large surface area available for cooling, and their availability in long-lengths that could be produced by high-throughput and low-cost manufacturing, may provide advantages for SFCL applications. We tested SuperPower's standard ion-beam-assisted-deposition based 2G HTS conductors under various conditions. First, individual conductors 10-20 cm long with dc critical current (Ic) ranging from 180 amps to 277 amps were tested at prospective fault current up to 3 kA (peak). 2G HTS conductors demonstrated good fault current limiting performance, including first peak limitation. Quench current was in the range of 1.8 to 3 times Ic, and the response time was within 1 ms. Secondly, 3-5 conductors in parallel connections demonstrated uniform current sharing and fast recovery under no-load condition. Finally, an assembly consisted of 12 elements in series connection was tested at high-power condition at KEMA PowerTest. Each element had four 40 cm long conductors with Ic 120 A in parallel connection. With 1080 V supply voltage and 90 kA prospective fault peak current, the fault current was limited to 32 kA at the 1st peak with 3.2 kA in the HTS elements. The response time was less than 1 ms. All these testing results indicate that our 2G HTS conductors are promising for practical SFCL applications.

Steady and Unsteady Current Modes and Thermal Runaway Conditions of High-${\rm T}_{\rm c}$ Composite Superconductors

Transport current properties of the steady and unsteady operating modes and thermal runaway conditions of the Ag-sheathed Bi2Sr2CaCu2O8 composite superconductors are theoretically studied in DC magnetic fields. It is shown that the temperature of a composite is not equal to the coolant temperature before thermal runaway. As a result, the shape of the voltage-current characteristics of high-Tc superconducting composites has only a positive slope during continuous current charging both before and after thermal runaway. That is why the voltage-current characteristics of high-Tc superconductors do not define the boundary of the thermal runaway. This peculiarity has to be considered during experiments at which the critical or quench currents are defined.

Design of a High Temperature Superconducting Coil for a 8.3 MVA Fault Current Limiter

In this study, a high temperature superconducting (HTS) coil is designed for a single phase 8.3 MVA fault current limiter (FCL) whose ratings are 13.2 kV in voltage and 630 A in current. The coil is wound with the bifilar winding method to limit the fault current only by resistance. The wire used in the HTS coil is YBCO coated conductor (CC) with metal stabilizer such as copper or stainless steel. In order to design the HTS FCL, the thermal and electrical analyses are performed considering the transient thermal conduction and the resistance development according to the temperature. The HTS coil is designed with a temperature limit of 300 K. The stainless steel stabilized wire is desirable than the copper stabilized wire because of the high resistance and the low temperature rise. As the design result, the total length of the wire is calculated as 2.56 km with 25 mum stainless steel stabilizer. The parallel number of wires is 8 and the quench current of the coil is approximately 1200 A at 65 K. The height and the outer diameter of the coil except the terminals and the connection parts are 0.5 m and 0.9 m, respectively. The HTS coil is composed of 7 concentric solenoid coils and each coil has 19 turns of clockwise winding and same turns of counter-clockwise winding for non-inductive property.

Experimental studies of the quench behaviour ofMgB2 superconducting wires for fault current limiter applications**This work is supported by Rolls-Royce plc and the UK Department of Trade and Industry(DTI).

Various MgB2 wires with different sheath materials provided by Hyper Tech Research Inc., havebeen tested in the superconducting fault current limiter (SFCL) desktop tester at24–26 K in a self-field. Samples 1 and 2 are similarly fabricated monofilamentaryMgB2 wires with a sheath of CuNi, except that sample 2 is doped with SiC and Mg addition. Sample3 is a CuNi sheathed multifilamentary wire with Cu stabilization and Mg addition. All thesamples with Nb barriers have the same diameter of 0.83 mm and superconducting fractionsranging from 15% to 27% of the total cross section. They were heat-treated at temperatures of700 °C for a hold time of 20–40 min. Current limiting properties ofMgB2 wires subjected to pulse overcurrents have been experimentally investigated inan AC environment in the self-field at 50 Hz. The quench currents extractedfrom the pulse measurements were in a range of 200–328 A for differentsamples, corresponding to an average engineering critical current density(Je) of around4.8 × 104 A cm−2 at 25 K in theself-field, based on the 1 µV cm−1 criterion. This work is intended to compare the quench behaviourin the Nb-barrier monofilamentary and multifilamentaryMgB2 wires with CuNi and Cu/CuNi sheaths. The experimental results canbe applied to the design of fault current limiter applications based onMgB2 wires.

Comparison of the DC Performance of Full- and Sub-Size&lt;tex&gt;$rm NbTi$&lt;/tex&gt;Cable-in-Conduit Conductors

The DC performances of three full- and five sub-size NbTi cable-in-conduit conductors, measured in the SULTAN test facility, are compared. In general, the self-field effect was found to cause sudden take-offs at high currents. In most of the investigated conductors the transition to sudden take-offs occurs for self-field contributions between 0.5 and 0.8 T. In the region of the sudden take-offs, the quench currents of the full-size conductors were found to be far below the critical current expected for the peak magnetic field. This behavior can be attributed to a current unbalance among the strands, which seems to be more pronounced at higher currents

Effect of Flux Jumps in Superconductor on&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Accelerator Magnet Performance

The performance of some superconducting accelerator magnets based on high-J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands suffered from flux jumps in the coils which significantly limited their maximum quench current. Detailed investigation into the possible cause of these performance-limiting quenches showed a correlation between the magnet test results and the properties of the superconducting strands used in the magnet and the magnet design parameters. This paper discusses the effect of flux jumps in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands and cables on quench performance and on field quality of accelerator magnets

Voltage Spikes in&lt;tex&gt;$hboxNb_3hboxSn$&lt;/tex&gt;and NbTi Strands

As part of the High Field Magnet program at Fermilab several NbTi and Nb3Sn strands were tested with particular emphasis on the study of voltage spikes and their relationship to superconductor instabilities. The voltage spikes were detected under various experimental conditions using voltage-current (V-I) and voltage-field (V-H) methods. Two types of spikes, designated 'magnetization' and 'transport current' spikes, have been identified. Their origin is most likely related to magnetization flux jump and transport current redistribution, respectively. Many of the signals observed appear to be a combination of these two types of spikes; the combination of these two instability mechanisms should play a dominant role in determining the minimum quench current

Excitation of a Superconducting Coil Made of Ta-Reinforced&lt;tex&gt;$rm Nb_3rm Sn$&lt;/tex&gt;Conductor in 14 T

We have been developing Ta reinforced Nb3Sn superconductors. To demonstrate the conductor's performance, we fabricated an epoxy-impregnated magnet and excited in the maximum background field of 14 T. The magnet showed training behavior. To estimate the relation between the premature quenches and epoxy resin's cracks, we analyzed the mechanical stress during excitation of the magnet. Assuming the conductor in the innermost layer was not fixed to the bobbin due to the many cracks, the analytical stress data agreed well with the experimental results. Even if the conductor was not mechanically supported by the bobbin, it was showed that the quench currents of the magnet reached the critical current. We think that the Ta-reinforced conductor is useful in the case of large electromagnetic stress

Operating modes of high-Tc composite superconductors and thermal runaway conditions under current charging

The operating thermal and electric modes of ahigh-Tc superconducting composite in partially and fully penetrated states induced bythe charging current are investigated. They were studied under conditions inwhich the current charging rate, the volume fraction of the superconductor in acomposite or the temperature of the cooling bath were changed. The transientbehaviour of the voltage–current dependence, which is characteristic during stable andunstable increases in electric field inside the composite under a continuous currentcharging, is discussed. Simulations were done using zero- and one-dimensionalsteady and unsteady thermoelectric models with a power equation describing thevirgin voltage–current characteristic of a superconductor. It is found that somethermoelectric trends underlie the shape of the voltage–current characteristic of thehigh-Tc superconducting composite. These have to be considered during experiments in whichthe critical or quench currents are defined. First, in the initial stage of the fullypenetrated regime (in the low voltage range), the electric field distribution does nothave a uniform character. These states depend on the volume fraction of thesuperconductor and the current charging rate: the higher these quantities, thehigher the heterogeneity of the electric field. Second, during the stable over-criticalregime (in the high voltage range) occurring in complete penetration modes, theevolution of the electric field may depend on the relevant temperature increase of acomposite according to the corresponding increase in its temperature-dependent heatcapacity. Consequently, the shape of the voltage–current characteristic of a compositehigh-Tc superconductor during continuous current charging, both before and after thermal runaway,has only a positive slope. Moreover, it is proved that the growth of the fully penetrated part ofthe voltage–current characteristic becomes less intensive when the current charging rate orthe coolant temperature increase. That is why the voltage–current characteristic of ahigh-Tc superconductor cannot determine the boundary of onset of thermal runaway. It is alsoconfirmed that there is a thermal degradation mechanism of the current-carrying capacityof the composite. In particular, according to this mechanism, the quench currents do notincrease proportionally to an increase in the amount of superconductor in a composite.