High temperature superconductivity: Magnets and applications of naval interest

The Navy has had a long standing interest in power applications of superconductivity. Recent advances in high temperature superconducting (HTS) materials have caused the Navy science and technology community to assess the potential of HTS magnets for Naval applications. A program has begun to test HTS conductors and magnets in order to determine realistic performance expectations from which to determine proper systems integration. This paper presents some early results of industrially produced HTS magnets and discusses possible Naval applications. Speculative application toward magneto-hydrodynamic (MHD) propulsion is also discussed-specifically with respect to HTS materials.     

Correlations between field quality and geometry of components in the collared coils of the LHC main dipoles

The Large Hadron Collider (LHC) (1995), a proton-proton superconducting accelerator, will consist of about 8400 superconducting magnet units, all operating in superfluid helium at a temperature of 1.9 K. The design of the superconducting main dipole magnets for the LHC is guided by the requirement of an extremely high field quality in the magnet aperture which is mainly defined by the layout of the superconducting coil and the position of the conductors. In order to avoid conductor movements within the magnet cross-section, the superconducting coils are held in place by surrounding stainless steel collars. In this paper, we review the dependence of field harmonics in the LHC main dipoles on dimensions of the hardware components of the collared coils. An analysis of the dimensional measurements of these components which are used in the collared coils produced so far is given. Sensitivity tables which are worked out through a coupled magneto-static model give the variation of the multipoles on collars, copper wedge dimensions and cable geometry. A Monte Carlo method is used to simulate the effects of possible errors on the multipoles.     

SuperPower's YBCO Coated High-Temperature Superconducting (HTS) Wire and Magnet Applications

Recent developments in second-generation (2G) HTS wire and coil technology are presented highlighting the ability of 2G HTS wire to function under demanding operating conditions associated with many applications including linear motors for transportation. The challenges of use in various coil constructions and applications are discussed. The 2G wire architecture of a structural substrate, buffer stack, HTS layer, and stabilization enables the 2G wire to tolerate high stress levels while providing the high current density required for lightweight, compact magnets. The high winding current density that is available with SuperPower's thin (0.1 mm) 2G HTS wire has been utilized in several coil demonstrations, including one generating central fields in excess of 26.8 T. The ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed.      

High-field superconductor technology

The past two years have seen the acceptance of superconducting magnets for engineering installations, the development of more advanced and robust engineering materials than the earlier conductors, and the wider application of high-field superconducting magnets in development programs for high-energy physics and thermonuclear fusion research. Their principal advantage is the great saving in operating cost when used in large-volume magnets up to 3 teslas. At higher fields a further advantage is their lower capital cost as compared with that of conventional electromagnets. They also have the potential, not as yet realized, of being more compact than a conventional electromagnet of identical performance.     

Design of a linear bulk superconductor magnet synchronous motor for electromagnetic aircraft launch systems

High-temperature superconducting (HTS) material in bulk form is used to design a linear synchronous motor for an electromagnetic aircraft launch system. The motor is designed without an iron core. Stator coils are placed in the air while the permanent magnets used in conventional design of linear permanent magnet synchronous motors are replaced by the HTS bulk magnets. The physical, operational, and equivalent circuit parameters of the linear motor with HTS bulk magnets are compared with those of a linear permanent magnet synchronous motor and linear induction motor designed for the same application. Results show that utilizing superconducting magnets is only superior at temperatures below 40 K.     

Superconducting Magnets and Cooling System in BEPCII

A pair of dual-purpose superconducting quadrupole magnets and a superconducting detector solenoid were fabricated and installed in Beijing Electron-Positron Collider Upgrade (BEPCII). The magnets are symmetrically inserted into the BESIII detector with respect to the interaction point. They are identical, iron-free, non-collared, multi-layered and active shielded superconducting magnets for the micro-beta focusing at the interaction region of the collider rings. Each quadrupole magnet is composed of seven coils at different operating currents wound layer by layer on a common cylindrical support. The magnet has an overall effective length of 0.96 m and provides a good field aperture of 65 mm in diameter. They are cooled by supercritical helium in order to eliminate the flow instabilities in constrained cooling channels. The BESIII superconducting solenoid magnet was designed to provide an axial magnetic field of about 1.0 T over the tracking volume and to meet the requirement of particle momentum resolution to particle detectors. A single layer of coil, in-direct cooling by forced two-phase helium, high purity aluminum based stabilizer and NbTi/Cu superconductor is adopted for the solenoid. The solenoid is 3.4 m in diameter and 3.89 m in length. This paper presents the design of the superconducting magnets in the BEPCII as well as their cryomodules. The cooling system for the magnets is also discussed.     

High capacity cable's role in once and future grids

High temperature superconducting (HTS) cables cooled to 77 K are starting to be tested and could soon be carrying a lot more current through the same old underground city pipes. If superconducting transmission cables can be made to work compatibly with other emerging high-temperature superconducting technologies then it may be possible to layout grids in innovative ways and to position generators closer to customers without having to step voltage up and down. Conventional underground cables normally incorporate fluid, such that the use of liquid-nitrogen coolant in superconducting cables is not such a departure as it might at first seem. The main immediate challenge in developing superconducting cables is to acquire operational experience. Only by working with live-networks and with the users of network equipment will it be possible to evaluate compatibility with existing components, system reliability, maintenance and total system costs. Two basic types of superconducting cable designs are emerging. In one the HTS conductor is enclosed in a cryogenic environment, which in turn is covered by conventional room temperature dielectric. In the other, a cryogenic-dielectric design, two concentric HTS conductors are used transmit electricity. These designs are discussed as are superconducting tapes for 77 K operation, cooling and insulation, joints and terminations, and testing parameters     

Interaction region magnets for VLHC

The interaction region (IR) magnets for the proposed Very Large Hadron Collider (VLHC) require high gradient quadrupoles and high field dipoles for high luminosity performance. Moreover, the IR magnets for high energy colliders and storage rings must operate in an environment where the amount of energy deposited on superconducting coils is rather large. In the case of doublet IR optics with flat beams, the design of the first 2-in-1 quadrupole defines the geometry and pole tip field in this and other IR magnets. This paper will present a novel design of this magnet that allows a very small separation between the two apertures. A brief discussion of the conceptual magnetic design of this and other magnets for interaction regions is given. The influence of critical current density in superconductor (a higher value of which is most beneficial to high performance IR magnet design) is also discussed. Since high temperature superconductors (HTS) retain most of their critical current density at high fields and at elevated temperatures, they offer an attractive possibility for the IR magnet designs of future colliders or upgrades of present colliders.     

Recent issues in fabrication of Ag-clad BSCCO superconductors

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (J_c) up to 12,000 A/cm² has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.     

Thermomechanical properties of the coil of the superconducting magnets for the Large Hadron Collider

The correct definition and measurement of the thermomechanical properties of the superconducting cable used in high-field magnets is crucial to study and model the behavior of the magnet coil from assembly to the operational conditions. In this paper, the authors analyze the superconducting coil of the main dipoles for the Large Hadron Collider. They describe an experimental setup for measuring the elastic modulus at room and at liquid nitrogen temperature and for evaluating the thermal contraction coefficient. The coils exhibit strong nonlinear stress-strain behavior characterized by hysteresis phenomena, which decreases from warm to cold temperature, and a thermal contraction coefficient, which depends on the stress applied to the cable during cooldown.     

Fabrication of HTS dc Bias Coil for 35kV/90MVA SFCL

For a saturated iron core fault current limiter, superconductor is the only suitable material to make the dc bias coil, especially when the device is used in a high voltage power grid. Commonly, superconducting wires are used to wind the dc bias coil. Since the performance of the wires changes greatly under magnetic fields, the calculation of the field spatial distraction is essential to the optimization of the superconducting magnet. A superconducting coil with 141000 ampere-turns magnetizing capacity made of 17600 meters of BSCCO 2223 HTS tapes was fabricated. This coil was built for a 35kV/90MVA saturated iron-core fault current limiter. Computer simulations on magnetic field distribution were carried out to optimize the structural design, and experiments were done to verify the performance of the coil. The configuration and the key parameters of the coil will be reported in this paper.     

Performance Evaluation and Quality Assurance Management During the Series Power Tests of LHC Main Lattice Magnets

Within the LHC project, a series production of superconducting dipoles and quadrupoles has recently been completed in industry and all magnets were cold tested at CERN. The main features of these magnets are: two-in-one structure, 56 mm aperture, two layer coils wound from 15.1 mm wide Nb-Ti cables, and all-polyimide insulation. This paper reviews the process of the power test quality assurance and performance evaluation, which was applied during the LHC magnet series tests. The main test results of magnets tested in both supercritical and superfluid helium, including the quench training, the conductor performance, the magnet protection efficiency and the electrical integrity are presented and discussed in terms of the design parameters and the requirements of the LHC machine.     

Fabrication of HTS dc Bias Coil for 35 kV/90 MVA SFCL

Abstract---For a saturated iron core fault current limiter, superconductor is the only suitable material to make the dc bias coil, especially when the device is used in a high voltage power grid. Commonly, superconducting wires are used to wind the dc bias coil Since the performance of the wires changes greatly under magnetic fields, the calculation of the field spatial distraction is essential to the optimization of the superconducting magnet. A superconducting coil with 141000 ampere-turns magnetizing capacity made of 17600 meters of BSCCO 2223 HTS tapes was fabricated. This coil was built for a 35 kV/90 MVA saturated iron-core fault current limiter. Computer simulations on magnetic field distribution were carried out to optimize the structural design, and experiments were done to verify the performance of the coil The configuration and the key parameters of the coil will be reported in this paper.     

Modeling of Quench Limit for Steady State Heat Deposits in LHC Magnets

A quench, the transition of a conductor from the superconducting to the normal conducting state, occurs irreversibly in the accelerator magnets if one of the three parameters: temperature, magnetic field or current density exceeds a critical value. Energy deposited in the superconductor by the particle beams provokes quenches detrimental for the accelerator operation. In particular if particles impacting on the vacuum chamber and their secondary showers depose energy in the magnet coils. The large hadron collider (LHC) nominal beam intensity is 3.2 ldr 10¹⁴ protons. A quench occurs if a fraction of the order of 10⁷ protons per second is lost locally. A network model is used to simulate the thermodynamic behavior of the magnets. The heat flow in the network model was validated with measurements performed in the CERN magnet test facility. A steady state heat flow was introduced in the coil by using the quench heaters implemented in the LHC magnets. The value of the heat source current is determined by the network model and the magnet coil current which is required to quench the coil is predicted accordantly. The measured and predicted value comparison is regarded as a sensitive test of the method.     

Demagnetization of Trapped Field in High TC Superconductors Using Pulsed Field Methods

This paper examines the demagnetization of a trapped magnetic field in a high T_C superconducting (HTS) monolith using a single, short, high-amplitude field pulse. With a single optimized pulse, the peak trapped field can be reduced to one-third of the maximum saturated value. Alternatively, the spatial average of the trapped field could be fixed very close to zero. After demagnetization, the trapped field has both positive and negative values at various locations in the HTS monolith. Transient responses of the HTS to pulsed fields with different amplitudes are also reported and discussed. It does not appear that the residual trapped field can be reduced to zero throughout the monolith using pulsed field demagnetization methods in a fixed sample/coil geometry. Bean's critical state model is used to explain the measured equilibrium field distributions. Transient response of the HTS to an externally applied field is qualitatively explained by magnetic diffusion. Drive circuit characteristics and design feasibility issues are also addressed     

Speckle interferometry measurements of the conductor displacementsin the CERN-LHC main dipole

Magnetic field quality in superconducting magnets strongly depends on conductor position in operational conditions. Simulations based on finite-element models (FEM) provide the field of stresses and displacements inside the magnet. Due to the complex mechanical behavior of the coil and to the different materials composing the magnet, it is not trivial to build a reliable mechanical model. We present an experimental method based on optical measurements to validate the accuracy of the FEM in evaluating differential displacements at room temperature. A short sample of dipole coils is loaded through a small press and speckle interferometry measurements detect differential displacements with an accuracy of 1.5 μm. Comparison with the numerical results allows the testing of the most critical features of the model, i.e., a pressure-dependent coil elasticity and the interfaces between different materials. A good agreement between measurements and deformed geometry foreseen by the FEM is found     

Minimum Stored Energy High-Field MRI Superconducting Magnets

A globally optimum minimum stored energy optimization strategy is implemented to design actively shielded superconducting magnet configurations used in high-field applications. The current density map is first obtained and used as a foundation for the magnet configurations by placing coils at current density local extremities. Optimized current density maps based on the stored energy formulation along with final magnet arrangements are provided to illustrate the findings. In this work, the focus was on compact superconducting magnets as measured by physical size and system footprint for given magnetic field properties inside the imaging region. The process of obtaining the current density maps proposed here over the given magnet domain, where superconducting coils are laid out, suggests that peak current densities occur around the perimeter of the domain, where in the most compact designs, with the domain length less than 1 m, the current direction alternates amongst adjacent coils. To reduce the peak magnetic field to acceptable levels on the superconductors in high-field designs, the size of the magnet domain is made larger, to the extent that the current densities no longer alternate between coils.      

Thermal and electrical stabilization of high-temperature superconductor powder-in-tube conductor

High-temperature superconductor (HTS) powder-in-tube conductors often require thermal and/or electrical stabilization to ensure their long-term integrity. An example of such a stabilization need is the safety bypass lead of an HTS current lead intended for application to superconducting magnetic energy, storage (SMES) devices. The bypass lead functions include current sharing during upset conditions and conductor element temperature control. This paper presents a bypass lead design for an HTS current lead in an 0.5 MW h SMES system. Included are predictions of thermal and electrical stabilization and current lead heat leaks. The results of a supporting development program are presented, as are details of a method to connect conductor element/bypass leads, an assembly procedure, a cleaning method, and thermal-stress tolerance measurements.     

Superconducting magnet system containing Bi-2212/Ag Coil Generates 21.8 T at 1.8K

We fabricated a Bi-2212/Ag double stacked pancake coil of 13 mmØ) in inner bore and of 46.5 mmØ in outer diameter, by using Bi-2212/Ag tapes prepared by the combination of continuous dip-coating process and melt-solidification technique. This small superconducting magnet was used as an insert magnet of a conventional superconducting magnet system and tested at saturated superfluid helium temperature (～ 1.8K) in various bias fields. The generated field of Bi2212/Ag coil was 0.9 T, with I_c of 310 A(criterion 10^-13Ω·m), in the bias field of 20.9 T. Thus, this superconducting magnet system achieved generation of magnetic field of 21.8 T in the full superconducting state.     

A new support structure for high field magnets

Pre-stress of superconducting magnets can be applied directly through the magnet yoke structure. We have replaced the collar functionality in our 14 Tesla R&D Nb/sub 3/Sn dipole magnets with an assembly procedure based on an aluminum shell and bladders. Bladders, placed between the coil pack and surrounding yoke inside the shell, are pressurized up to 10 ksi [70 MPa] to create an interference gap. Keys placed into the interference gap replace the bladder functionality. Following the assembly, the bladders are deflated and removed. Strain gauges mounted directly on the shell are used to monitor the stress of the entire magnet structure, thereby providing a high degree of pre-stress control without the need for high tolerances. During assembly, a force of 8.2 /spl times/ 10/sup 5/ lbs/ft [12 MN/m] is generated by the bladders and the stress in the 1.57" [40 mm] aluminum shell reaches 20.3 ksi [140 MPa]. During cool-down the thermal expansion difference between shell and yoke generates an additional compressive force of 6.85 /spl times/ 10/sup 5/ lbs/ft [10 MN/m], corresponding to a final stress in the shell of 39.2 ksi [270 MPa]. Pre-stress conditions are sufficient for 16 T before the coils separate at the bore. Bladders have now been used in the assembly and disassembly of two 14 T magnets. This paper describes the magnet structure, assembly procedure and test results.