High Current Cables Research Articles

High Current Density Cables for Simpler HTS Magnets in Fusion Energy Systems

High Current Density Cables for Simpler HTS Magnets in Fusion Energy Systems

Numerical Analysis on Performances of Critical Current for Rutherford Cable Made From HTS Quasi-Isotropic Strands

2G high temperature superconducting (HTS) tapes are frequently cabled into strands that can be assembled as conductors in cables with various layouts and thus potentially employed in large-scale superconducting magnets with high magnetic fields and large transport currents. One option meeting the requirements for manufacturing high-current cables is the Rutherford cable configuration. This article proposes a flat Rutherford cable with a copper core made up of 12 quasi-isotropic strands. The critical current characteristics of the Rutherford cable are numerically analyzed at 77, 20, and 4.2 K by using the finite element method and a self-consistent model in self-field and applied magnetic fields with different magnitudes and orientations, respectively. Additionally, the simulated results of the Rutherford cable are compared with those of a directly stacked conductor. The results show that the proposed Rutherford cable can attain critical currents in the hundreds of kA range at 20 and 4.2 K in high fields. This Rutherford cable design is a potential and prospective solution for high-current superconducting cables in power transmissions and large-scale magnets with a high magnetic field.

PROXIMITY EFFECT IN HIGH-CURRENT CONDUCTORS - A CASE STUDY OF BREAKDOWNS IN A HYDROPOWER PLANT

The proximity effect plays an important role when calculating the ampacity of high-current cables, transmission lines and busbars. If ignoring the distance between the conductors, the number of parallel circuits and the conductor phase orders, there will be excessive heat losses in conductors and screens. This was the case in a hydropower plant, where thermal overload was root cause for dielectric breakdowns in insulated busbars. Combined magnetic and thermal calculations provide examples of how pronounced the proximity effect is, by using the hydropower plant as case study. In one location, with three parallel circuits in a 3×3 arrangement, heat losses in the most exposed busbar were reduced by 39 % by simply optimizing phase order of the conductors. At other locations, with conductors in flat formation, the heat losses could be reduced by 11 % by increasing centre-centre distance from 400 to 600 mm. In the 500 m repaired section, heat losses were reduced by 23 % after reorganizing the busbar layout. This corresponds to heat losses of 255 kW and annual energy reduction of 2 GWh at continuous rated current.

Conceptual design of HTS magnets for fusion nuclear science facility

Second-generation high temperature superconductors (HTS) are available for producing >20 T at the magnet bore compared to 13–16 T for lower temperature superconducting (LTS) toroidal field magnets proposed in recent fusion energy systems studies (FESS) of Fusion Nuclear Science Facility (FNSF). HTS may enable higher fusion power density and smaller device size. High current density cables of multi-layered REBCO tapes have achieved >10 kA at 4–20 K operation in short sample tests for fusion. High current density cables are required for engineering design of FNSF to allow space for interior plasma components. High current density HTS magnets are particularly attractive in reducing the size of a fusion device, beneficial for compact tokamaks, due to their space constraints. Successful HTS magnet development may enable the design of smaller and cheaper fusion pilot plants with a mission of demonstrating net electricity. It may also offer significant cost and performance advantages in non-fusion applicants such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). We developed HTS magnet design concepts for a compact FNSF radial build in order to define the coil size, winding pack mechanical loading and engineering requirements. Partnering with vendors in the US, PPPL is also testing high current cable prototypes aiming at enabling low cost cable technology toward 100 A/mm2 engineering current density over the winding pack desired in high field model coil development for compact fusion pilot plants.

Upgrade and Commissioning of the SULTAN Facility to Host Quench Experiments on HTS High Current Conductors

High Temperature Superconductors (HTS) are promising materials for future fusion magnets. One of HTS conductors' main issues is the quench protection: the quench develops much slower than in LTS, delaying the quench detection by voltage monitoring and causing a massive temperature increase before detection. The Swiss Plasma Center (SPC), with the support of EUROFusion, has upgraded the SULTAN facility to test large current conductors during the whole quench evolution. The 100 kA superconducting transformer has been replaced by an 18 kA direct power supply, which allows sustaining the operating current in the sample during a quench. Five HTS conductors have been manufactured at SPC: they are sub-size of the HTS high-current cables designed by the group for an HTS high current high field cable proposed for EU-DEMO Central Solenoid (CS) hybrid magnet. Every conductor has a specific feature. The aim is to compare the quench behavior of the five conductors “parametrically”. The commissioning of the upgraded facility and the preliminary results of the measurements are presented in this work.

Investigation of an Opportunity of Using 2G HTS Tapes for High-Current Cables With a Current-Carrying Capacity More Than 10 kA

There had been considered optimized design versions of high-current three-phase HTS cables with 2G HTS tapes as current-carrying elements. Their characteristics (rms phase current 12 kA, line voltage 24 kV) conform to the parameters required for the first stage of the power transmission line from the generator to the first step-up transformer. Three types of cables are under consideration: one with screened phases and triaxial and flat, as well. The phase critical current value is taken 21 kA. For all the cables designs are given their exact geometric parameters together with the estimation of AC losses and HTS tapes consumption. It has been shown, that the optimization performed allows keeping up AC loses at a level acceptable for practical applications. The best design versions proposed have the losses within the range of 0.27–0.5 W/m, which are significantly lower than ones caused by the natural heat transfer into the cryostat – about 1–2 W/m, vs. its dimensions. All the cable types with one and the same design modification have an approximately equal level of losses. But triaxial and flat cables have an advantage in HTS tapes consumption.

Fiber optic quench detection for large-scale HTS magnets demonstrated on VIPER cable during high-fidelity testing at the SULTAN facility

Fiber-optic thermometry has the potential to provide rapid and reliable quench detection for emerging large-scale, high-field superconducting magnets fabricated with high-temperature-superconductor (HTS) cables. Developing non-voltage-based quench detection schemes, such as fiber Bragg grating (FBG) technology, are particularly important for applications such as magnetic fusion devices where a high degree of induced electromagnetic noise impose significant challenges on traditional voltage-based quench detection methods. To this end, two fiber optic quench detection techniques—FBG and ultra-long FBG (ULFBG)—were incorporated into two vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed (VIPER) high-current HTS cables and tested in the SULTAN facility, which provides high-fidelity operating conditions to large-scale superconducting magnets. During surface heater induced quench-like events under a variety of operating conditions, FBG and ULFBG demonstrated strong signal-to-noise ratios (SNRs) ranging from 4 to 32 and measured single-digit temperature excursions; both the SNR and temperature sensitivity increase with temperature. Fiber thermal response times ranged between effectively instantaneous to a few seconds depending on the operating temperature. Strain sensitivity dominates the thermal sensitivity in the conditions achievable at SULTAN; however, measurements at higher quench evolution temperatures, coupled to future work to increase the thermal-to-strain signal, show promise for quench detection capability in full-scale magnets where temperature and strain may occur simultaneously. Overall, FBG and ULFBG were proven capable to quickly and reliably detect small temperature disturbances which induced quench initiation events for high current VIPER HTS conductors in realistic operating conditions, motivating further work to develop FBG and ULFGB quench detection systems for full-scale HTS magnets.

Degradation of REBCO coated conductors due to a combination of epoxy impregnation, thermal cycles, and quench: Characteristics and a method of alleviation

REBa2Cu3Ox (REBCO) coated conductors are an enabling technology for >1 GHz nuclear magnetic resonance spectrometers that need >23.5 T solenoids, compact high-field fusion reactors beyond the International Thermonuclear Experimental Reactor, and 20 T class accelerator magnets. An in-depth understanding of stress and strain management is critically needed for these applications. Here, we report experimental data and an analysis of damages of REBCO-coated conductors in several scenarios relevant for their applications in high-field magnets and unique to their multi-layer thin film structure. The examined scenarios include failure induced by quench, failure induced by epoxy impregnation, and failure induced by a combination of epoxy impregnation and quench. The root cause is film blistering and peeling induced by thermally interfacial stresses characteristic of multilayered heteroepitaxial thin film structures on thick substrates utilized. The quantitative stress analysis is introduced using simple and easy to use formulas and clearly indicates how materials and structural characteristics affect the magnitude and the distribution of stresses and deflections. Finally, a simple method is introduced for removing degradation of REBCO coated conductors due to epoxy impregnation. Our data and analysis help our understanding of the practical limit of REBCO-coated conductors and can be utilized as a guidance for the physical design of REBCO coated conductors and high-current cables assembled from them and for minimizing damages.

Vanadium oxide coatings to self-regulate current sharing in high-temperature superconducting cables and magnets

High-temperature superconductors such as REBa2Cu3O7 − δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-to-normal transition (quench), i.e., when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a much-prolonged magnet charging time. To address this issue, we investigate vanadium oxide (VOx) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VOx coatings (with 1.70 ≤ × ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are x-ray amorphous but with a short-range crystalline ordering according to Raman spectrometry. The resistivity of VOx decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change in the critical current caused by the coating process. The results from current sharing experiments and circuit analysis suggest that the VOx coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents, and enable self-protection during quenches.

Development and test of a 35 kA - HTS CroCo cable demonstrator

The answer to energy-efficient electric power transfer of high currents in the range of several tens of kA can be given by high temperature superconducting (HTS) cables. BSCCO and MgB2 have been used widely for such cables, reaching maximum currents of about 20 kA. REBCO coated conductors are promising for future HTS cables beyond 20 kA and allow the operation based on subcooled liquid nitrogen. Several cabling concepts based on REBCO tapes were developed world-wide to realize such cables. Using the stacked-tape concept, a scalable semi-industrial process was developed by KIT, called HTS CrossConductor (HTS CroCo). Key aspects of the conceptual design of high-current HTS cables are discussed and the design of a 35 kA DC cable demonstrator made from HTS CroCo strands is presented. Aspects regarding joints, current redistribution between individual strands and electrical stabilization are highlighted. The performance of this demonstrator cable was tested, reaching the envisaged current.

Measurement of splice resistance and normal zone propagation velocity in REBCO tapes for the Superconducting Link of HL-LHC

The Superconducting Link (SC-Link) being developed at CERN in the context of the LHC High-Luminosity upgrade (HL-LHC) will supply the current to superconducting magnets of the HL-LHC Inner Triplets and Matching Sections. The SC-Link consists of MgB2 highcurrent cables, and it includes high temperature superconducting REBCO cables making the electrical transition between the MgB2 and the current leads. Electrical protection of superconducting devices made of REBCO against unexpected quenches is challenging especially when operation is at high current density. Adequate electrical protection of REBCO cables is crucial to insure good performance of the circuit over the lifetime of the machine; protection relies on the detailed knowledge of the quench propagation velocity. In this paper we reviewed the electromagnetic properties of REBCO tapes procured for prototype work. We measured the splice resistance (Rs ) of the REBCO conductors and found a significant variation among the different batches of conductor. The normal zone propagation velocity (NZPV) was reported to be affected by the interfacial resistance which is governing the splice resistance: increasing the interfacial resistance enhances the NZPV. In order to determinate the influence of the internal resistance on the quench behaviour of the tapes, we investigated the normal zone propagation velocity of the epoxy-impregnated tapes at 77 K in self-field and 4.2 K in field up to 7 T in the FRESCA test station. We found that despite the Rs variation observed (up to factor 4), the tapes show similar NZPV at both temperatures. In addition we observed a significant field dependence of the NZPV of the tapes at 4.2 K. The results are compared to numerical simulation and discussed.

Stable, predictable and training-free operation of superconducting Bi-2212 Rutherford cable racetrack coils at the wire current density of 1000\u2009A/mm2

High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb3Sn, but two challenges have so far been the low engineering critical current density JE, especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record JE up to 950 A/mm2 at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a JE of 1020 A/mm2. Different from the unpredictable training performance of Nb-Ti and Nb3Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb3Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

A 1.2 T canted cosθ dipole magnet using high-temperature superconducting CORC® wires

REBa2Cu3Ox (REBCO, RE = rare earth elements) coated conductors can carry high current in high background fields, in principle enabling dipole magnetic fields beyond 20 T. Although model accelerator magnets wound with single REBCO tapes have been successfully demonstrated, magnet technology based on high-current REBCO cables for high-field accelerator magnet applications has yet to be established. We developed a two-layer canted cosθ dipole magnet with an aperture of 70 mm using 30 m long commercial Conductor on Round Core (CORC®) wires. The 3.1 mm diameter CORC® wire contained 16 commercial REBCO tapes with a 30 μm thick substrate. The magnet was tested at 77 and 4.2 K. It generated a peak dipole field of 1.2 T with 4.8 kA at 4.2 K with neither thermal runaway nor training. Reasonable geometric field quality and strong magnetization-current effects with multipole decay were observed. Our work demonstrated a feasible high-temperature superconducting magnet technology as a first step toward a new accelerator magnet paradigm that will enable high-field inserts for next-generation circular colliders and stand-alone magnets that can operate over a wide temperature range for a broad range of applications.

Measurements of the Strain Dependence of Critical Current of Commercial REBCO Tapes at 15 T Between 4.2 and 40 K for High Field Magnets

Author(s): Pierro, F; Delgado, M; Chiesa, L; Wang, X; Prestemon, SO | Abstract: Interest for high magnetic fields (g16 T) for applications in high energy physics (HEP) and fusion machines, requires the development of high current cables capable to withstand the large forces, mechanical and electromagnetic, experienced during manufacturing and operations. The critical current (Ic) of REBCO tapes depends on strain, magnetic fields, and operational temperatures. Understanding how these parameters affect the Ic of the conductor will be critical to develop robust high-current REBCO cables. However, there are limited reports on the strain dependence of Ic, in particular at high fields and elevated temperatures relevant for future high-field compact fusion reactor magnets. We present Ic of commercial REBCO tapes measured as a function of compressive and tensile strain (between-0.6% and +0.65%) at high magnetic fields (12 and 15 T) and different temperatures (within 4.2-40 K). Results at 4.2 and 20 K showed less than 5% reduction in the normalized Ic at high strain, while a stronger strain dependence was observed at 40 K. Samples tested at 12 T and 4.2 K showed similar strain dependence as 15 T and 4.2 K. In all tested conditions, the tape experienced reversible Ic reduction in both tension and compression. Finite element analysis was used to predict the residual thermal strain accumulated in the REBCO layer prior of testing to account for the effect of the cooldown. A method was also developed to account for the current sharing observed between the sample and the sample holder during the ramp of the current. Our results provide useful input for the development of high-field fusion and HEP magnets using REBCO conductors.

Finite-Element Analysis of the Strain Distribution Due to Bending in a REBCO Coated Conductor for Canted Cosine Theta Dipole Magnet Applications

High-current cables using REBCO tapes can be used to develop high-field dipole magnets. However, the strain accumulated during cable fabrication and coil winding may reduce the critical current of the conductor. Therefore, it is important to properly consider the strain when designing high-field magnets. In this paper, we used structural finite-element analysis (FEA) to predict the strain experienced by a REBCO tape during bending in configurations relevant to the fabrication of high-field accelerator magnets, in particular, the mechanical strain generated during cable fabrication and winding in a canted-cos θ dipole configuration. We considered two different cable options: (A) Flat tape that lay in the mandrel channel and (B) a REBCO tape helically wound around a circular copper core, the typical configuration of the conductor in round core cable (CORC). Strain accumulated during tape winding is studied for different core diameters and winding tilt angles. FEA longitudinal strain results were compared with the simulations for configuration A, where higher strain was observed experimentally. Configuration B was verified indirectly by comparing experimentally measured Ic with the one predicted (based on the longitudinal strain) as a function of the bending diameter. Good agreement was found up to a bending diameter of 30 mm. The presented results will help to understand the impact of bending on REBCO tapes and CORC wires to develop high-field magnets.

Comparison study of cable geometries and superconducting tape layouts for high-temperature superconductor cables

High-temperature superconductor (HTS) rare-earth-barium-copper-oxide (REBCO) tapes are very promising for use in high-current cables. The cable geometry and the layout of the superconducting tapes are directly related to the performance of the HTS cable. In this paper, we use numerical methods to perform a comparison study of multiple-stage twisted stacked-tape cable (TSTC) conductors to find better cable structures that can both improve the critical current and minimize the alternating current (AC) losses of the cable. The sub-cable geometry is designed to have a stair-step shape. Three superconducting tape layouts are chosen and their transport performance and AC losses are evaluated. The magnetic field and current density profiles of the cables are obtained. The results show that arrangement of the superconducting tapes from the interior towards the exterior of the cable based on their critical current values in descending order can enhance the cable’s transport capacity while significantly reducing the AC losses. These results imply that cable transport capacity improvements can be achieved by arranging the superconducting tapes in a manner consistent with the electromagnetic field distribution. Through comparison of the critical currents and AC losses of four types of HTS cables, we determine the best structural choice among these cables.

AC loss characteristics of CORC® cable with a Cu former

High-temperature superconductors from the REBCO (RE = rare earth) family have attained industrial production and their performance is continuously being enhanced. However, cabling technology for high-current (kA range) cables for magnet technology is still challenging and there are only few cable concepts available (CORC®, Roebel cable, twisted stack cable). Each of them exhibits different characteristics. In this paper we experimentally investigate CORC® cable produced in-house utilizing a copper tube former. Such a former offers a central cooling channel for partial or complete cable cooling by forced flow of coolant. We focus mainly on AC loss due to transporting AC current, an external applied AC magnetic field and their simultaneous action. In the case of transporting AC current we found indications that a large part of the total loss has its origin in eddy currents due to an axial magnetic field. For the investigation of magnetization AC loss, we prepared several samples with different configurations. In this case we found direct evidence for increasing AC loss due to losses in the metallic former. However, we also found that at low field amplitudes the magnetization AC loss of the complete cable is lower than the loss in the bare former. This is caused by shielding of the magnetic field by a superconductor, which was also confirmed by numerical simulations.

Out-of-plane bending characteristics of second generation high temperature superconducting tapes

REBCO tapes, known as second generation (2G) high temperature superconducting (HTS) tapes, are promising for their application in high field magnets and high current cables. However, in practical applications 2G HTS tapes always need to be bent, which causes the degradation of critical current. Therefore, it is necessary to study the bending characteristics of 2G HTS tapes. Studies have been carried out regarding easy bending and hard bending characteristics, but there has been no research into the characteristics of out-of-plane bending with different bending angles. This paper theoretically analyzes the out-of-plane bending with different bending angle characteristics of 2G HTS tapes and deduces the strain formula of different bending modes. Experiments on the bending characteristics of 2G HTS tapes were performed in liquid nitrogen. The experiments verified the theoretical calculation, and the results have significance for the applications of 2G HTS tapes.

Investigation of Splice Resistances of High-Current MgB2 Cables Operated in Liquid and Helium Gas

Investigation of Splice Resistances of High-Current MgB2 Cables Operated in Liquid and Helium Gas

Cyclic load effect on round strands made by twisted stacks of HTS tapes

Various DC tests performed recently with full-size 60kA HTS cable prototypes in the EDIPO test facility demonstrated that proposed design of HTS strand at Swiss Plasma Center (SPC) – stack of HTS tapes twisted and soldered between two copper profiles – is applicable for high-current fusion cables, but improvement of the strand mechanical properties against the cyclic loading is still needed. Based on experimentally obtained correlation between the performances of cable prototypes at different operating conditions, further investigation of cyclic transverse load on the strand performance was performed at 77K. Aiming to obtain a strand design able to withstand a continuous cyclic load operation of some thousand cycles, influence of the strand geometry and tape's manufacturer has been studied. Cyclic load has been applied up to 1000 cycles for straight and bent samples at the load amplitudes up to 4MPa. Based on the obtained data, next design of HTS cable prototype will be discussed.