NbTi Cables Research Articles

Preliminary Electromagnetic and Structural Analyses of the Conductors and Clamps of the DTT Current Feeders

The Divertor Tokamak Test facility (DTT) is being constructed in the Research Centre of ENEA Frascati. This nuclear fusion device is devoted to tackle the power exhaust issue in the view of future fusion power plant. The conceptual design and the performances of the DTT superconducting magnet feeders, with NbTi cables, are evaluated based on coupled electromagnetic and structural analyses. We here present a preliminary structural assessment of the feeder clamps. We have considered the self-field condition and the higher force per unit length on feeder paths as evaluated from a 3D EM simulation. The stress state is evaluated by means of a 2D model with plane stress with thickness formulation. Further, the bolt dimension and class, together with the distance between two clamps, is provided in both self-field and maximum force per unit length conditions.

Feasibility Study of GaToroid Gantries for Carbon Ions

The use of carbon ions in particle therapy can enhance the treatment quality, introducing higher biological effectiveness compared to photons and protons. Rotating gantries are nowadays used to deliver particle beams from different directions to obtain precise conformal dose mapping at the tumor location. The mechanical stability requirements during the rotation usually require heavy structures. Superconducting magnets can be used to reduce the size and weight of the machine, but complex cryogenics systems are needed to maintain superconductivity during the gantry rotation. The seek for novel gantry configurations for carbon ions is one of the greatest challenges that need to be tackled to widen the accessibility of hadron therapy to a larger number of patients. The concept of GaToroid, a steady-state toroidal gantry implemented with superconducting magnets, can be used to deliver the beam from different directions without the rotation of neither the structure nor the patient. In this work, we present the magnetic design of two GaToroid solutions for Carbon ions, conceived to operate with Nb-Ti cables in the range of 6-7 Tesla. The first solution was designed with 20 pairs of coils, to maximize the flexibility of the treatment, and with an internal diameter of 3.5 meters, to allow a complete couch rotation on the plane. The second option, based on a more conservative design with an internal diameter of 2.3 meters and eight couples of coils, aims to be a compromise between the complexity and cost of the machine, and the flexibility of the treatments. For both configurations, the coil geometry and current distribution were optimized to maximize the carbon ion momentum acceptance for complete coverage of the treatment range without variation of the toroidal magnetic field. The studies here presented can be considered as the first milestone toward the design of toroidal gantries for heavy ions.

Protection Strategy and Quench Study of MCBXF Magnets

Nested combined orbit correctors are needed for the upgrade of LHC. They are developed at CIEMAT, in collaboration with CERN in the framework of HL-LHC project. There are two types of magnets, so-called MCBXFA and MCBXFB, with different lengths (2.5 and 1.5 m, respectively), but sharing the same cross section. The coils of both magnets are wound with the same Rutherford-type NbTi cable, composed of 18 strands of 0.48 mm diameter, insulated with a braided glass fiber sleeve. Various magnet protection schemes were considered for MCBXF magnets. In this work, self-protection, energy extraction resistor and quench heaters are discussed emphasizing advantages and disadvantages of each strategy. This analysis is supported by quench calculations performed with Squid (CIEMAT in-house developed code) and Roxie. Selected quench simulation results are presented and compared with measurements done on magnet prototypes. Based on the presented study, a protection scheme with energy extraction resistor was selected for the series magnets. The value of the resistor was optimized and related quench calculations are also included.

Calibration of a superconducting transformer by measuring critical current of a NbTi Rutherford cable

Large high field superconducting magnets often require high current superconducting cables. To develop these cables, a facility capable of providing a high magnetic field with large sampling area as well as electrical current of tens of kA is essential. A superconducting transformer (SCT) is an energy-efficient and low-cost way to provide large current to superconducting cables. Previously, we co-developed an SCT and successfully tested it to a maximum output current of 45 kA in zero magnetic field. In this work, this SCT is installed to the 12 T split solenoid magnet at the National High Magnetic Field Laboratory (NHMFL). We calibrated it by using this facility to measure the critical current of an NbTi Rutherford cable as a function of magnetic field up to 10 T, and compared the results with those available in the literature. In addition, a strand extracted from the NbTi cable was tested for critical current. The critical current of the extracted strand is scaled and compared with the critical current of the cable. The accuracy of the critical current measurement using this SCT is discussed in detail. This work concludes the commissioning of this SCT, which combined with the 12 T split magnet will provide a unique cable testing capability for future cable development for the NHMFL and its users.

Investigation of Mechanical Strains in Thermal Compensation Loop of Superconducting NbTi Cable during Bending and Cyclic Operation.

In the paper, the thermal compensation loops on a composite, superconducting NbTi cable were investigated. This type of cable is used in the superconducting, fast ramping magnets of the SIS100 synchrotron, part of the Facility for Antiproton and Ion Research (FAIR) under construction in Darmstadt, Germany. The influence of space restrictions and electromagnetic cross-talk on the design of the thermal compensation loop was discussed. Plastic deformation of cable components during bending was analyzed by numerical simulations and experiments. A three-dimensional numerical model of the cable was prepared with individual superconducting wires in contact with a central cooling pipe. The bending of a straight cable into a compensation loop shape was simulated, followed by cyclic operation of the cable during thermal cycles. The maximum strains in the superconducting strands and cooling tube were analyzed and discussed.

R&D Activities of Joint Manufacture for CFETR CSMC

The Central Solenoid Model Coil (CSMC) project was launched in the Institute of Plasma Physics Chinese Academy of Sciences for the China Fusion Engineering Test Reactor (CFETR). The CFETR CSMC is designed to generate 12 T peak magnetic field when the running current is 47.65 kA. The CSMC is composed of two Nb3Sn coils in the internal high field and three NbTi coils in the external low field. Electrical joints are one of the key technologies for the CSMC, because the superconducting coils are electrically connected into a single circuit by the joints. Lap type joints have been designed for the CSMC for its proven reliability and easy assembly. It is a great challenge to develop special equipment and technologies for the joint manufacture, including vacuum brazed terminal sleeve assembly, jacket removal, chrome coating removal for Nb3Sn cable, nickel coating removal and silver plating for NbTi cable, compact termination, etc. This paper mainly focuses on the research and development activities of the joint manufacture and the solution of the technical issues. The techniques and equipment can provide a basis for the electrical joint manufacture for the CSMC and other large-scale superconducting magnets.

Note: Electroplating process for connectorizing superconducting NbTi cables.

Experiments done at cryogenic temperatures below the 4.2 K temperature of liquid helium frequently require superconducting coaxial cables to efficiently transmit high-frequency signals while minimizing heat transfer to the experiment's cold stage. These cables are often made of niobium-titanium (NbTi) alloy which is difficult to solder directly. This note describes a new electroplating procedure for plating NbTi coax directly with copper, which enables connector attachment matching the structural and electrical properties of standard coaxial cables. Here, a cable is first electrochemically coated with a thin oxide layer and then electroplated with copper in an acidic copper sulfate solution. The procedure has modest safety requirements and may be carried out in a standard vented laboratory fume hood.

Refinement and Application of a Generic CFD Toolkit Covering the Heat Flows in Combined Solid-Liquid Systems to Investigate Thermal Quench Limits of Superconducting Magnets

The recently developed robust multiregion numerical toolkit for the modeling of heat flows in combined solid-liquid systems is extended to cover larger temperature domains, crossing the superfluid (HeII) to normal helium (HeI) phase transition, and to include NbTi cables that feature open electrical insulation, porous to superfluid helium. The aim is to probe the influence of thermal design details on the temperature margin of superconducting magnets. The model enhancements are discussed and the model is applied to analyze the results of a particle beam loss test in an LHC main dipole. We show and quantify the degree to which the build characteristics of the superconducting magnet, such as electrical insulation and interlayer fishbones, influence the local temperature margin and, hence, the quench level seen by the magnet.

Things to Think About When Estimating the Cost of Magnets Made With Conductors Other Than Nb-Ti

There have been a number of papers written about estimating the budgetary cost of superconducting magnets based on the magnet stored energy, magnetic induction times field volume, or the magnet coil and cryostat mass. The cost equations were based on using Nb-Ti as the superconductor. In all cases, a portion of the cold mass was devoted to carrying the magnetic forces. The cost equations could be applied to Nb-Ti cable in conduit magnets, because conduit is a part to the force carrying structure. This paper explains the factors that are involved that influence a cost estimate for magnets fabricated with other conductors such as Nb 3 Sn, MgB 2 , BSCCO, and second-generation ReBCO tapes. All of these conductors will add to the cost of the magnet because the conductor cost per ampere-meter is higher. In a number of cases there is a reaction and special insulation step added to the process of magnet fabrication. All of these conductors are much more strain sensitive than Nb-Ti, which can also add to the magnet cost. The effects of field orientation on conductor critical current and quenching will likely increase the magnet cost as well. The unknown factor is the cost of uncertainty and project delay.

The numerical boundary conditions of the wrapping pattern of thin insulation

The wrapping pattern of thin insulation is an important issue in cryogenic installations because it can significantly affect heat transport. The major objective of this study is to develop the numerical boundary conditions (BC) needed to model the wrapping pattern of thin insulation. An example of the practical application of the proposed BC includes the heat transfer of Rutherford NbTi cables immersed in superfluid helium (He II) across thin layers of electrical insulation. The proposed BC and a mathematical model of heat transfer in He II are implemented in the open source CFD toolbox OpenFOAM. The implemented mathematical model and the BC are compared with experiments. The study confirms that the thermal resistance of electrical insulation can be lowered by implementing the proper wrapping pattern. The proposed BC can be useful in the study of new patterns for wrapping schemes. Further possible improvements of the tool are indicated in the paper.

Field Quality and Mechanical Analysis of the Beam Separation Dipole for HL-LHC Upgrade

High luminosity upgrade of the Large Hadron Collider (HL-LHC) project has been launched to attain a ten times higher integrated luminosity than the current LHC that has been in operation for over ten years. For this goal, the quadruple and dipole magnets around two interaction points, the ATLAS and the CMS, will be upgraded. High Energy Accelerator Research Organization (KEK) is in charge of developing the new superconducting beam separation dipole magnet (D1). The main dipole field of 5.6 T in a large aperture of 150 mm is generated using a cos-theta coil wound with Nb-Ti cables at nominal operating current of 12.0 kA at 1.9 K corresponding to 75% of the load line ratio. The main challenges for the D1 are larger aperture, a high level of iron saturation, radiation resistance, and tight constraints on field quality. This article summarizes the results of a detailed analysis on field error. Electromagnetic simulation with ROXIE was carried out for the 2-D model of the new D1. As possible design changes, a diameter and position of heat exchanger holes in the iron yoke, and shape of the cryostat were considered. To reveal the influence on systematic error, packing factor of iron yoke, relative permeability of the stainless steel collar, and coil deformation during yoking were reflected to the baseline design. In addition, random geometric error was also estimated. Based on these results, contribution of each factor to field quality is studied.

The Influence of the Al Stabilizer Layer Thickness on the Normal Zone Propagation Velocity in High Current Superconductors

We present a two dimensional numerical model for predicting the normal zone propagation velocity in Aluminum stabilized Rutherford NbTi cables. By solving two coupled differential equations under adiabatic conditions, the model takes into account the thermal diffusion and the current redistribution process following a quench. Both the temperature and magnetic field dependencies of the materials properties are included. We study the influence of the thickness of the cladding on the propagation velocity for varying operating current and magnetic field. To assist in the comprehension of the numerical results, we introduce an analytical formula for the longitudinal normal zone propagation.

Status of the Procurement of the European Superconductors for the ITER Magnets

The European share of the ITER magnet superconductors includes the supply of around 20 km of Toroidal Field and 7 km of Poloidal Field conductors. This represents 20% and 11% of the total conductor amounts respectively needed for all ITER Toroidal Field (TF) and Poloidal Field (PF) coils. For TF conductor, around 97 tons of superconducting niobium tin strand and 60 tons of copper wire are needed to be purchased, cabled and inserted within a stainless steel jacket tube to form the Cable-In-Conduit-Conductor (CICC). For PF conductor, about 45 tons of superconducting multifilamentary niobium titanium wire need to be cabled and jacketed. Altogether, Fusion for Energy (F4E) placed 4 supply contracts for the European TF and PF coil conductors (one for copper strand, two for niobium tin strand and a single contract for cabling and jacketing). In addition, the PF conductor procurement is based on a bi-lateral agreement with the Russian Federation Domestic Agency (RF DA) where NbTi cables are supplied by RF DA and their jacketing is provided by F4E. In this article, the procurement strategy and the current procurement status are reported for the European contribution of TF and PF conductors.

Design study of 4.5 MJ sector-toroidal SMES coil

In continuation of SMES technology development in our centre, design study of a 4. 5 MJ (1.25 kW h) sector-toroidal SMES coil using custom make Rutherford type NbTi cable is reported. The sector- toroid is optimized with six modular type solenoid coils connected in series. The basic module is considered to be solenoid type because of its easier winding technique though it is not the best choice from stress considerations. The coil operating current as well as maximum magnetic field at the inner layer has been determined in order to limit voltage across the coil during discharging mode of its operation and also to minimize the overall dynamic and static load in the cryostat. Due to asymmetric field distribution around the coil, each coil experiences a huge centered force that needs to be arrested with proper support structures. Therefore, extensive magneto-structural stress analysis has been carried out using commercial finite element code ANSYS. The paper describes the magnetic design, three dimensional stress analyses in coil and its support structure during cool-down and energisation, design scheme of sector- toroidal SMES cryostat, etc.

Nanostructure of superconducting Nb-Ti cable

Scanning-probe, electron, and optical microscopes are used to study the structural evolution in the intermediate stage of drawing (to diameter 1.3–1.2 mm) for superconducting cable based on Nb-Ti alloy (47.5 wt % Ti or, equivalently, 63.7 at % Ti). Such cable is used for current transmission in the magnetic system of the International Thermonuclear Experimental Reactor (ITER). The microstructure and phase composition of the superconducting alloy are investigated, as well as their influence on its properties after cold drawing and intermediate annealing. Zones where plastic deformation is localized are found at breaks in the super-conductor. The variation in shape and chemical composition of Nb-Ti fibers is studied in a defect-free region and in the fracture zone of the cable. A diffusional niobium barrier is identified around the Nb-Ti fibers in the copper matrix.

Characterization of a Large Size Co-Extruded Al-Ni Stabilized Nb-Ti Superconducting Cable for Future Detector Magnets

Future detector magnets call for the development of next-generation large-sized Al stabilized Nb-Ti superconducting cables exhibiting high yield strength for coping with the large stress in wide bore magnets with peak magnetic fields up to 6 T, while avoiding significant degradation in residual resistivity ratio. Precipitation type alloys obtained by dilute-alloying of high purity Al with suitable additives like Ni, subjected to partial annealing following cold drawing, can feature a yield strength up to 110 MPa at 4.2 K. For the ATLAS central solenoid, a Nb-Ti cable has been plated with a precipitation-type Al-0.1wt%Ni alloy. However, this conductor with a critical current of 20 kA at 5 T, features a cross-section of only 130 mm2. Here, a first step in process scale-up to a 60 kA at 5 T class conductor is described. For the first time, a continuous co-extruded Al-0.1wt %Ni stabilized conductor has been produced with a cross-section as large as 700 mm2. Sections of the conductor are work hardened in order to increase the mechanical properties of the as-extruded temper. The mechanical and transport characteristics as a function of the amount of work hardening have been assessed by removing samples after every subsequent step.

Microstructure of the elements of a superconducting Alloy Nb-Ti cable

The formation of a micro- and macrostructure in the Nb-Ti-based technical superconductors that are intended for the current-carrying elements in the magnetic system of the International Thermonu-clear Experimental Reactor and are subjected to deformation by drawing is considered. The deformation structure and microstructure of cable samples are studied by optical microscopy, atomic force microscopy, and scanning electron microscopy. The specific features of plastic flow localization at the sites of fracture of a multiconductor cable are revealed.

Aluminum Stabilized NbTi Conductor Test Coil Design, Fabrication, and Test Results

A new generation of precision muon conversion experiments is planned at both Fermilab and KEK. These experiments will depend upon a complex set of solenoid magnets for the production, momentum selection and transport of a muon beam to a stopping target, and for tracking detector momentum analysis of candidate conversion electrons from the target. Baseline designs for the production and detector solenoids use NbTi cable that is heavily stabilized by an extruded high RRR aluminum jacket. A U.S.-Japan research collaboration has begun whose goal is to advance the development of optimized Al-NbTi conductors, gain experience with the technology of winding coils from this material, and test the conductor performance as modest length samples become available. For this purpose, a “conductor test” solenoid with three coils was designed and built at Fermilab. A sample of the RIKEN Al-NbTi conductor from KEK was wound into a “test” coil; this was sandwiched between two “field” coils wound from doubled SSC cable, to increase the peak field on the RIKEN test coil. All three solenoid coils were epoxy impregnated, and utilized aluminum outer bandage rings to apply preload to the coils when cold. The design and fabrication details, and results of the magnet quench performance tests are presented and discussed.

Electro-thermal FEM simulations of the 13 kA LHC joints

The interconnections between the superconducting main dipole and main quadrupole magnets are made of soldered joints of two superconducting Nb–Ti cables embedded in a copper busbar stabilizer. The primary cause of the September 2008 incident in the LHC was a defect in an interconnection between two dipole magnets. Analyses of the incident show that possibly more defects might be present in the 13kA circuits, which can lead to unprotected resistive transitions. To avoid the reoccurrence of such an event, thorough experimental and numerical investigations have taken place to determine the safe operating conditions of the LHC. However to show measured curves is beyond the scope of this article. Furthermore, improvements in the design have been proposed in the form of additional parallel copper pieces, or shunts, which bridge the possible voids in the soldering and offer a bypass for the current in case of a quench.The purpose of this work is to support the design choices and to indicate the sensitivity to some of the free parameters in the design. Electro-thermal Finite Element Method (FEM) simulations are performed, making use of COMSOL Multiphysics. The use of FEM allows for a profound three-dimensional analysis and some interesting features of the shunted busbar can only be revealed this way. Especially current redistribution in the shunted area of the interconnect gives important insights in the problem. The results obtained using the model are very sensitive to the exact geometrical properties as well as to the material properties, which drive the Joule heating inside the interconnection. Differences as compared to a one-dimensional model, QP3, are presented. QP3 is also used for simulations of non-shunted busbar joints as well as shunted busbars. Furthermore, margins are given for the soldering process and the quality control of the shunted interconnections, since the contact area between the stabilizer pieces and the shunt is an important quality aspect during the manufacturing of a safe interconnection.

Stability Margin of NbTi CIC Conductor for JT-60SA Equilibrium Field Coil

The equilibrium field (EF) coil conductors are designed with the NbTi cable in conduit (CIC) conductor because the maximum magnetic field of each conductor is up to 4.8 T for the EF-L conductor and 6.2 T for the EF-H conductor. The first EF conductor was manufactured and processed into the performance verification test sample. Then the performance verification tests had been operated before beginning the mass production. The current sharing temperature (Tcs) measurement tests and the quench tests were operated as the performance verification test to evaluate the Tcs and the minimum quench energy (MQE). The results of the Tcs measurement test indicate that both the Tcs of the EF-H conductor and the EF-L conductor are almost same as the expected value estimated by the strand characteristics. Additionally, the Tcs results satisfied the requirements of the EF coils, therefore the conductor design and the fabrication process are going well. The results of the quench test indicate that the EF conductors have enough stability marginuch as more than several hundred mJ/cc at the temperature margin of more than 0.5 K and the current of 20 kA. In addition, the supercritical helium (SHe) mass flow seldom influenced on the MQE. Thus the EF coil can be operated in stable condition even if the SHe mass flow will be decreased.