Davide Uglietti

Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

Recent progresses in the second generation REBa2Cu3O7 - x (RE123) coated conductor (CC) have paved a way for the development of superconducting solenoids capable of generating fields well above 23.5 T, i.e. the lim it of NbTi-N b3Sn-based magnets. However, the RE123 magnet still poses several fundamental and engineering challenges. In this work we review the state-of- the-art of conductor and magnet technologies. The goal is to illustrate a close synergetic relationship between evolution of high-field magnets and advancement in superconductor technology. The paper is organized in three parts: (1) the basics of RE123 CC fabrication technique, including latest developments to improve conductor performance and production throughput; (2) critical issues and innovative design concepts for the RE123-based magnet; and (3) an overview of noteworthy ongoing magnet projects.

Effect of YBCO-Coil Shape on the Screening Current-Induced Magnetic Field Intensity

A numerical simulation method which deals with the screening current-induced magnetic field for YBCO coils, including the self field effect induced by the transport current, has been developed. The simulation agrees well with the experimental results for an YBCO solenoid. Based on the numerical simulation, the effect of coil shape on the screening current-induced magnetic field intensity for the YBCO coils has been investigated. The field was demonstrated to reach a maximum if the solenoid corresponds to the minimum-volume design; it amounts to as large as -18% of the central magnetic field. Two major problems must be considered for YBCO coils regarding the screening current: (a) a reduction in the central magnetic field by the screening current and (b) a temporal drift of the apparent magnetic field due to relaxation of the screening current by flux creep. It is suggested that the latter can be suppressed by a current sweep reversal technique.

Magnitude of the Screening Field for YBCO Coils

Screening current induced in a YBCO-coated conductor coil causes two major problems; (i) reduction in the central magnetic field and (ii) temporal magnetic field drift due to flux creep. They constitute disadvantages for YBCO coil applications such as NMR, MRI, accelerator and high field magnets. The second problem is effectively suppressed by current sweep reversal, while the first remains unsolved. The present paper demonstrates that the screening current-induced magnetic field (screening field) is dominated by (a) the YBCO coil shape, (b) the YBCO-coated conductor width, (c) the coil inner diameter and (d) the ratio of operating current to the coil critical current. The dependence on these quantities is systematically investigated by numerical simulations. We conclude that coils with a smaller width of YBCO-coated conductor, a larger inner diameter and a higher ratio of operating current to the coil critical current generate a smaller central screening field ratio.

Design and Strand Tests of a Fusion Cable Composed of Coated Conductor Tapes

The design of a flat cable suitable for future fusion reactors has been carried out. The cable consists of twisted round strands, which are composed of tapes stacked between copper profiles. According to calculations, the strand twist pitch and the cable twist pitch should be at least 1.5 or 2 m long to limit the strain and thus the reduction of the critical current. A 4-m-long strand (Ø6.2 mm) was fabricated on a continuous stacking and soldering line to demonstrate that the scaling up to industrial production is feasible. The strand can carry about 940 A in self-field at 77 K. The critical current evolution under bending strain was measured but it turned out to be rather fragile in “hard” bending direction. Three types of joints between strands were also manufactured and tested at 77 K.

Angular Dependence of Critical Current in Coated Conductors at 4.2 K and Magnet Design

Coated conductor tapes are promising materials for the construction of high field inserts (> 20 T). In a magnet, the magnetic field at the coil ends is not parallel to the wide face of the tape, and the angle can reach values larger than 10 degrees. Coated conductors have very anisotropic transport properties, thus it is very important to measure the angular dependence of the critical current. The critical current of commercial YBCO coated conductors made by SuperPower has been measured at 12 T, 4.2 K as a function of the angle between the magnetic field and the wide face of the tape, with a precision of less than 0.2 degree. The experimental results have been used for designing insert coils for high field magnets: depending on the coil parameters and current, the high sensitivity to the magnetic field angle of YBCO tape can affect the performances of the coil.

Fabrication Trials of Round Strands Composed of Coated Conductor Tapes

Various techniques have been proposed in order to assemble coated conductor tapes into high current strands. In this work, the preparation of high current capacity round strands starting from industrial coated conductors has been investigated. Round, twisted strands (about 50 cm long) consisting of stacked tapes sandwiched between two semicircular copper profiles have been assembled following two manufacturing routes: in one case the stack was first soldered to the copper profiles and then twisted; in the second method a twisted strand (stacked tapes between copper profiles) was soldered. The round conductors carry over 400 A at 77 K in self field. Three types of solder alloy were tested: BiSn, InSn, and PbSn. The critical current variation under bending strain was also measured: critical current retention of 99% was observed up to 0.3 % peak bending strain. At 0.6 % bending strain the strands prepared with PbSn solder exhibit a reversible reduction of the critical current of less than 2%, irrespective of the manufacturing route. Such round strands could be used to manufacture flat cables with conventional cabling methods.

Magnetic Flux Concentrator Using Gd-Ba-Cu-O Bulk Superconductors

Magnetic flux concentration is a very important technique for the effective generation of high magnetic fields. We propose the use of the diamagnetism of a high T c bulk superconductor (HTS bulk) instead of ferromagnetic materials such as Ho and Dy. We fabricated a magnetic field concentrator using Gd-Ba-Cu-O bulk. The essential point was slits to suppress the current along the circumference. The concentrator was cooled at the center of a superconducting magnet with liquid helium. By increasing the external field to 1.00 T, a magnetic flux density of 3.20 T was obtained at the center of the concentrator. At an external field of 2.00 T, a field of 5.65 T was also obtained. A magnetic lens using HTS bulk was successfully demonstrated. The experimental results were well explained by numerical analyses assuming HTS bulk as a perfect conductor.

Development of HTS Conductors for Fusion Magnets

In view of the development of cables for next-generation fusion reactors, research activities are carried out on all HTS materials available from industrial production. Preliminary design of a react and wind cable using Bi-2212 wires is carried out, inspired by the Nb3Sn cable for European DEMO. The design and construction of a 60-kA prototype cable made of coated conductors have been carried out. The strands in the cable are composed of a stack of coated conductor tapes (4 mm wide) embedded in a copper profile of 6.3 mm in diameter. Tapes and copper profiles are soldered together in order to obtain a mechanically solid strand and to keep the inter tape resistance at a minimum, so that current can be easily redistributed among the tapes in a strand. The strands are fabricated in pieces 2 m long, but scaling up to industrial production should not present any major problem. A flat cable is manufactured by winding 20 strands around a central copper former; the cable will be inserted in a steel jacket for force flow cooling. The critical current of each strand was measured at liquid nitrogen in self-field just after the manufacturing process (twisted and straight) and after cabling: No reduction of the critical current was observed. Two pieces of cable, each 2 m long, are going to be prepared and assembled to form a sample that can be tested in the EDIPO facility.

Specific Heat, A Method to Determine the

A new calorimeter has been built with the special purpose to determine the distribution of Tc in industrial superconducting wires. Specific heat measurements have been carried out on a series of multifilamentary Nb3Sn wires, using a long relaxation technique. The advantage of this technique consists in the fact that the measurement is performed in presence of the Cu-Sn matrix, i.e. the filaments are measured under the same stress conditions as under operation, i.e. under the same state of mechanical precompression. In addition, the Tc distribution is obtained for the whole sample volume, ruling out shielding effects. The deconvolution of the data in the region of the superconducting transition was used for getting the precise distribution of Tc, which in turn allows a determination of the Sn distribution across the filaments. These data confirm previous TEM measurements showing a Sn gradient inside of the filaments of bronze route processed Nb3Sn wires. The Tc distribution has been determined in Nb3Sn wires processed by bronze route, internal Sn and powder-in-tube technique. Based on this information, the various processing parameters can be varied to get narrower Tc distributions at transition temperatures closer to 18 K.

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The best performing HTS conductors are available in the form of thin tapes with operating current in the range of 200 A at the operating conditions. To realize conductors with high current carrying capability, in the range of several tens of kA, a large number of tapes must be assembled in a cable. The conventional stranding methods for round conductors are not applicable to thin tapes. CRPP has developed an assembly concept based on twisted stacks of tapes soldered into a copper profile. The ability to withstand bending and torsion strain is the main issue for the design of a high current flat cable based on twisted stacks of HTS tapes. An analytical mechanical model was developed and validated with the results of measurements. Parametric analyses are used to select the optimum layout for the first full size prototype of the coming HTS conductor for fusion. Various configurations of the cable are presented and compared, paying attention in particular to the strain acting in the tape. The chosen set of cable parameters satisfies electromechanical limitations and provides feasibility of strand winding.