Non-twisted stacks of coated conductors for DC magnets: analysis of inductance and AC losses

In the last 10-15 years, the most common strategy in the development of High Temperature Superconducting (HTS) cable for magnets has been to imitate Low Temperature Superconducting (LTS) cable designs. However, requirements for LTS materials are not valid for HTS materials, which are extremely stable. For example, non-twisted multifilamentary Bi-2223 tapes have been successfully used in several magnets. This paper review stability and analyse inductance and AC losses in non-twisted stack of HTS tapes. Numerical calculations show that twisting has negligible effect on inductance variations in a stack of tapes. Regarding AC losses, any magnet built with coated conductors have larger losses than LTS ones, because of the aspect ratio and large width of the tape. If a wide tape is replaced by a non-twisted stack of narrow tapes, losses and residual magnetisation could be reduced. In contrast with multifilamentary wires, twisting a stack of tapes reduces losses only marginally. Therefore, cables composed of non-twisted stack could be designed to have losses comparable to the one of twisted stack concepts. Few examples of large cables for fusion applications are discussed. Designs based on non-twisted stacks can be simpler, more robust and cost effective than twisted ones.Comment: 20 pages, 10 figures, submitted to Cryogenic

Design of the HTS Fusion Conductors for TF and CS Coils

The main electrical and mechanical requirements for the LTS fusion conductors of DEMO are retained as a starting point for the development of HTS fusion cables. Based on the HTS coated conductor technology, a flat cable design was proposed by CRPP Swiss Plasma Center (SPC) using the strands made of twisted stack of tapes soldered into copper profiles. Analytical modeling of the cable geometry is developed and presented in this work. The model was used to estimate various properties of cable. Addressing the issue of bending strain and related performance degradation, an optimization model of the cable properties was built, which allows to best fulfill the cable requirements. Design options are developed for both toroidal field (TF) coils operating at 63 kA and central solenoid (CS) coils operating at 50 kA. Paying attention to the DC and pulsed operation of the TF and CS coils, proposals for the design of the forced-flow HTS conductors are reported and discussed for each type of the coils

Design Optimization of Round Strands Made by Twisted Stacks of HTS Tapes

Atwisted stack of high-temperature superconducting (HTS) tapes soldered into copper profiles was used as a strand in the fabrication of the 60-kA/ 12-T HTS cable prototype at the Center for Research in Plasma Physics (CRPP). Considering the strand in general as a modular element for the high-current cables, design parameters of the strand need to be optimized in order to best fulfill requirements of appropriate application field. During the development program of the HTS fusion cable at CRPP, influence of the design parameters on the strand's performance was obtained as input data for the optimization process. In this paper, we summarize the results of the twisting, bending, and transverse pressure tests on strands of various tapes and profile's geometries. Finite-element model for the transverse pressure test was developed, validated with the test results, and used for the design proposals. Effect of the stack aspect ratio, tape properties, and anisotropy, as well as selection of HTS material for the strand, will be discussed

A review of commercial high temperature superconducting materials for large magnets: from wires and tapes to cables and conductors

High temperature superconducting (HTS) materials have the potential to generate a magnetic field beyond the level obtainable with low temperature superconducting (LTS) materials. This review reports on past and present R&D on HTS cables and conductors for high field tokamaks, accelerator dipoles, and large solenoids. Among the HTS wires and tapes available commercially, coated conductor tapes are the most appealing because of their outstanding critical strength and future improvement margin. Limitations are the weakness against peeling and delamination and the short piece length. The prices of technical superconductors are reviewed because they play an important role in large projects; moreover HTS wires and tapes are discussed from the perspective of industrial production considering the historical development of the LTS wire market. Various designs have been proposed for HTS cables and conductors: some are better suited for soft materials, while others can exploit the anisotropy of coated conductors (by aligning the tape with the field), thus providing the highest current density. Recently, there has been an increase in the size and complexity of the prototypes; however some peculiar features of HTS, such as high stability margins and high mechanical limits, have not yet been fully incorporated into the designs: for example the transposition requirements for HTS have not yet been studied in detail. There are elements indicating that rectangular wires and tapes (even if anisotropic) can be used for manufacturing cables and magnets of any size and have advantages with respect to round wires

Design Study of a 50-kA React-and-Wind Bi-2212 Cable for Fusion Magnet

In the last few years, the current density of Bi-2212 wires has been greatly improved by filament densify technologies. One further step toward applications in fusion magnets is to design a high current cable, which must overcome the drawbacks of Bi-2212 wires: brittleness and complex heat treatment process. In this article, react-and-wind flat cables are proposed. With react-and-wind technique, a larger number of choices of jacket and quench protection materials are available including stainless steel and copper, both of which are widely used for Nb3Sn and NbTi superconducting cables but are not directly usable for wind-and-react Bi-2212 cables due to their reaction during heat treatment. Two wires, representing, respectively, modest and top performance of the state-of-the-art wires, are considered. Several cable layouts are studied in terms of dimension, compaction rate, and transverse mechanical pressure. Materials for quench protection and jacket materials are also discussed

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

Impact of Hysteresis Losses in Hybrid (HTS-LTS) Coils for Fusion Applications

Several conductor designs featuring High Temperature Superconducting (HTS) stacked tapes for fusion coils are being proposed. These conductors are planned to operate in time-varying magnetic field and current; thus, the estimation of AC losses is fundamental for the conductor design and the accurate analysis of its performance in operation. The case study of an HTS conductor proposed for the hybrid (HTS-LTS) Central Solenoid coil for the EU DEMO tokamak is considered in this work. Here, a numerical model based on the finite element method (FEM) and the H-formulation is used, in order to estimate the hysteresis losses. The FEM model is first benchmarked against available analytical formulae as well as available literature data. Then it is applied to the real case operational scenario. It is shown that for the conductor design analyzed, the coupling losses are orders of magnitude lower than the hysteresis ones. The impact of the hysteresis+coupling losses on the temperature margin of the coil is assessed with a thermal-hydraulic model. It is shown that the heat generated in the HTS layers is partially transferred to the LTS layers, leading these layers to quench. An alternative conductor concept is also analyzed, showing that, however, in the top and bottom modules of the CS coil, due to the bending of the magnetic field, a too large heat deposition is present

Modelling of hysteresis losses in HTS Cable-in-Conduit Conductors for large scale applications

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