NbTi Coils Research Articles

Quench Protection System for a 13-T Magnet With a 700-mm Bore

A 13-T test facility with a 700-mm cold bore is being prepared for the research of high-field and high-current superconductors. Considering reuse of the existing cryostat, we select a pool-cooled and closely wound coil for the background field magnet to attain a large bore with the restricted outer diameter. The magnet is divided into two in the longitudinal direction and six in the radial direction. Rectangular monolithic conductors of Nb 3 Sn and NbTi are selected for the inner six coils and the outer six coils, respectively. To reduce the highest voltage of the Nb 3 Sn coil below 1.5 kV, the protection circuit of the Nb 3 Sn coils is separated by a circuit breaker from that of the NbTi coils. In addition, the half of protection resistors for the Nb 3 Sn coils is connected between the upper half and lower half of the Nb 3 Sn coils. The quench protection system consists of eleven 1-kV dc circuit breakers, ten protection resistors, and two diodes. According to the quench analyses with taking account of heat transfer between layers and the effect of resistance of a propagating normal zone on the current, the highest temperature is considerably lower than the adiabatic hotspot temperatures.

The AC loss evaluation of central solenoid model coil for CFETR

The AC loss of Central Solenoid Model Coil (CSMC) of China Fusion Engineering Test Reactor (CFETR) is calculated in order to be able to determine the allowable excitation current shape in time with respect to the available cooling capacity at liquid helium temperature. Firstly, the structure and operation parameters of CFETR CSMC are summarized including the superconductor, conductor and magnetic field distribution characteristic. Secondly, the main AC losses type and mechanics of CFETR CSMC are introduced, where the hysteresis and coupling loss of Nb3Sn and NbTi coil are estimated using of the model data and a calculation flux density distribution in the cross-section of model coil by ANSYS 2-D model. Thirdly, the low frequency eddy current loss calculation of stainless steel structural components is carried out by ANSYS/EMAG code with 3-D separately for tension rods, plates and beams model. The interaction between several components is not considered. In the case of conductor joints, a 3-D ANSYS code is also used with current force to obtain the eddy current losses for the metal components. Lastly, the individual loss components are summed up. The significant conclusions are made which can provide the valuable guidance for safe operation of CSMC.

The Effect of Temperature Dependence of AC Losses in a Bi-2223/Ag Insert of an 8-T Superconducting Magnet

A conduction-cooled split-gap superconducting magnet system with a center field of 8 T has been designed and fabricated in the Institute of Electrical Engineering, Chinese Academy of Sciences. The system consists of two Bi-2223/Ag coils and six NbTi coils. Due to a large aspect ratio of the high-temperature superconducting tape, there will be large ac losses when the magnet is ramped up and down. An accurate estimation of the total ac losses in the high-temperature superconducting coils is essential for the cryogenic system design. In the Bi-2223/Ag coils, the total ac losses mainly originate from two parts: One is transport loss caused by the variation of transport current with respect to time, and the other is magnetization loss due to alternating external magnetic field. In this paper, the effect of temperature variation on ac losses generated in the Bi-2223/Ag coils is studied. The magnetic field and temperature dependence relations of the critical current density $J_{c}$ are considered. The calculations are carried out in three steps. First, to estimate the magnitude of ac losses rapidly in the Bi-2223/Ag coils, the ac losses are calculated when the Bi-2223/Ag and NbTi coils are simultaneously charged, in which the temperature distribution is not considered. Second, the temperature variation is considered to calculate the ac losses under the same operating conditions with those in the first case. Finally, the NbTi coils are charged first, followed by the Bi-2223/Ag coils; the ac losses calculated are less than those in the second case. Hence, it is a good way to reduce the ac losses by changing the charging sequences of the Bi-2223/Ag and NbTi cols. Afterward, the calculated results are compared with the experimental data, and they show a good agreement.

Improvement of a Large Bore Cryogen-Free Superconducting Magnet for a Hybrid Magnet

A 360-mm room-temperature bore cryogen-free superconducting magnet (CSM), consisting of Nb 3Sn coils and NbTi coils, for a hybrid magnet (HM) has generated the maximum magnetic field of 9.5 T. However, the magnetic field of the CSM has been limited to 8.5 T in the hybrid magnet mode because of a cooling problem. As a result, the hybrid magnet composed of a 19-T water-cooled resistive magnet (WM) had the utmost field generation of 27.5 T. Therefore, we improved the CSM to generate higher magnetic fields. For the improvement of the cooling problem, Nb 3Sn coils were replaced, and thermal conduction was improved between coils and a 4K-GM cryocooler. Furthermore, support structures with a tensile strength over 80 kN and a spring support were adopted against the magnetic force to support the self-weight of coils and to absorb stress caused by thermal contraction difference between each coil. After the improvement, the CSM generated 9.5 T within 1 h and the maximum magnetic field of 9.7 T in a 360-mm room-temperature bore. The HM succeeded in generating 28 T in a 32-mm room-temperature bore with the CSM operated at 9.0 T.

Successful Upgrading of 920-MHz NMR Superconducting Magnet to 1020 MHz Using Bi-2223 Innermost Coil

We succeeded in upgrading the 920-MHz nuclear magnetic resonance (NMR) superconducting magnet (21.6 T) to 1020 MHz (24.0 T) by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10 (Bi-2223) coil. The 920-MHz NMR spectrometer had been installed in the National Institute for Materials Science, Tsukuba, Japan, in 2001. It has been operated in the persistent mode for six years. The upgrading project started in 2006. A Bi-2223 coil was developed as the innermost coil instead of the Nb3Sn one. The newly installed Bi-2223 innermost coil is connected to Nb3Sn and NbTi coils in series. The upgraded NMR magnet was seriously damaged by the Great East Japan Earthquake in March 2011. After more than two years of restoration and additional improvements of current leads and the power supply system, the magnet was cooled down to below 1.8 K in August 2014. The magnet successfully generated 24.0 T, corresponding to 1020 MHz, in October 2014. To achieve the required homogeneity and stability of the magnetic field, not only superconducting and room-temperature shim coils but also ferromagnetic shims were used. The 1020-MHz superconducting NMR magnet has been operated in a power-supply-driven mode for six months.

Effects of different coil combinations on the optimal design of a 25 T superconducting magnet

High field above 20 T is required in diverse physical programs and nuclear magnetic resonance (NMR) systems. For intended science program requirements, as a demonstration of the development in high field superconducting magnet technology, a 25 T (4.2 K) 52 mm cold-bore all-superconducting magnet consisting of a 10 T high-temperature superconducting insert magnet and a 15 T low-temperature superconducting background magnet, is being developed at the Institute of Electrical Engineering, Chinese Academy of Sciences. The development of such a magnet requires its optimization, and the choosing the number and type of coils is crucial to the final optimal design. However there are few researches focusing on the effect of coil combinations. To study the relationship between the number of coils and the magnet parameters, we first discuss the magnet optimization. The objective function of the optimization is defined as the weighted function of coil volume according to the costs of different superconductors, and the following constraint conditions are taken into considerations: center field, YBCO conductor characterization, hoop stress in Nb3Sn coils, and the critical performances of these wires. All those constraint conditions are taken in the analytical form, and the magnetic field, stress results are verified with the finite element method. To guarantee the reliability of the optimal results, in addition to consider the constraint conditions, a method of combining global optimization and local optimization is adopted. 20 different coil combinations are selected according to the investigation of superconducting wires, and their optimal results are calculated. The following conclusions are drawn from the analyses of these results. Firstly, in the design of high field magnet, the number of coils and magnet cost demonstrate a "V"-shaped relationship, that is, there exist an optimal number of coils. Secondly, when the objective function demonstrates good values, Nb3Sn coils generate fields in a range of 6-7 T, whereas NbTi coils generate fields in a range of 8-9 T. Finally, the objective functions under two different situations, i.e., Nb3Sn coils and NbTi coils are powered together and separately, are calculated. From the comparisons we find that the effect of reducing one power supply is acceptable when the number of coils is not too big.

Development status of a next generation ECRIS: MARS-D at LBNL.

To demonstrate a Mixed Axial and Radial field System (MARS) as the best magnet scheme for future ECRISs, MARS-D, a demonstrative ECRIS using a NbTi MARS magnet is progressing at Lawrence Berkeley National Laboratory. An optimized MARS design can use either NbTi or Nb3Sn coils with reduced engineering complexities to construct the needed high-field magnets. The optimized magnet design could enhance MARS-D to a next generation ECRIS by producing minimum-B field maxima of 5.6 T axially and 3.2 T radially for operating frequencies up to 45 GHz. In-progress test winding has achieved a milestone demonstrating the fabrication feasibility of a MARS closed-loop coil.

Protecting a Full-Scale <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex-math></inline-formula> Magnet With CLIQ, the New Coupling-Loss-Induced Quench System

A new protection system for superconducting magnets called coupling-loss induced quench system (CLIQ) has been recently developed at CERN. Recent tests on Nb-Ti coils have shown that CLIQ is a valid, efficient, and promising method for the protection of high-magnetic-field superconducting magnets. However, the protection of new-generation Nb3Sn accelerator magnets is even more challenging due to the much higher stored energy per unit volume and to the significantly larger enthalpy needed to initiate and propagate a normal zone in such coils. Now, the CLIQ system is tested for the first time on a Nb3Sn magnet in the CERN magnet test facility in order to investigate its performance in practice, thereby validating the method for this type of superconducting magnets as well. Furthermore, we successfully reproduced the electrothermal transients during a CLIQ discharge. Finally, the implementation of various CLIQ-based protection schemes for the full-scale Nb3Sn quadrupole magnet for the LHC high luminosity upgrade is discussed. The impact of key system parameters on CLIQ performance and the advantages and drawbacks of using multiple CLIQ units on a single magnet are discussed.

Beam Heat Loads to Superconducting Quadrupoles for BigRIPS In-Flight Separator at RIKEN

The BigRIPS in-flight fragment separator in the RIKEN Radioactive Isotope Beam Factory (RIBF) project was commissioned in 2007 and has been operated for six years. The RIBF accelerator complex has provided a wide range of heavy ion beams from deuteron to uranium with the energy of 250-345 MeV/nucleon. More than 70 experiments of unstable nuclei have been performed using the RI beams produced by the BigRIPS fragment separator. The production target of the RI beams is in close proximity to the first superconducting triplet quadrupoles (STQs) in the BigRIPS fragment separator, so that the STQ is exposed to the high radiation, which causes “beam heat loads” to the cryogenic system. The STQ consists of three air-core NbTi coils with the maximum field gradients of 24 T/m, assembled in the liquid-helium bath cryostat with the warm bore radius of 90 mm. This paper presents measurements of the radiation heat loads to the STQ cryostat for a number of experienced heavy ion beams from oxygen to uranium with the beam power of 1-7 kW. Corresponding radiation transport calculations are presented, and radiation doses and damages to the STQ system are also discussed.

Design Study on a 9.2-T NbTi Superconducting Magnet With Long-Length Uniform Axial Field

A NbTi superconducting magnet with long-length uniform axial field is designed and will be fabricated for high-frequency gyrotron experiments. The magnet has a warm bore of 90 mm in diameter and a uniform axial field length of 200 mm. The magnetic field on the 200-mm axial length is adjustable from 0 to 9.2 T, accommodating a wide range of gyrotron experiment frequencies. The magnet is composed of ten coaxial NbTi coils connected in series. Optimization of the field configuration is carried out to obtain the magnet parameters. Cryogenics of the magnet system is adopted as conduction cooling with a single GM cryocooler. The magnet design and integration are presented in this paper.

Fabrication and Test of an 8-T Superconducting Split Magnet System With Large Crossing Warm Bore

A conduction-cooled superconducting split magnet system with large crossing warm bore has been successfully constructed in our laboratory for material processing applications. The magnet system design was described in a previous paper. The magnet is composed of six NbTi low temperature superconducting coils, which generate 5.5-T central magnetic field and two Bi2223/Ag high temperature superconducting (HTS) insert coils, which generate 2.5-T central magnetic field and assembled in the form of split coil groups. The magnet has a 136-mm split gap to accommodate the crossing warm bore of 100 mm in diameter. The magnet system is cooled by two GM cryocoolers. The initial cooldown takes 9.2 days and the final temperature of the magnet is about 4.0 K. The HTS coils and NbTi coils are to be operated in the driven mode with two independent power supplies, under the operating currents of 200 A (HTS) and 139 A (NbTi), respectively. The magnet is successfully powered up to 8 T with a the ramp time of 290 min. In this paper, the fabrication and test of the superconducting split magnet system are presented.

Note: control of liquid helium supply to cryopanels of Kolkata superconducting cyclotron.

The Kolkata superconducting cyclotron utilises liquid helium to cool the main magnet niobium-titanium (NbTi) coil and the cryopanels. Three liquid helium cooled cryopanels, placed inside the dees of the radio-frequency system, maintain the high vacuum in the acceleration region of the superconducting cyclotron. The small cryostat placed inside the cryogenic distribution manifold located at the basement of the superconducting cyclotron building supplies liquid helium in parallel branches to three cold heads, used for cooling their associated cryopanels. The level in the cryostat has to be maintained at an optimum value to ensure uninterrupted flow of liquid helium to these three cold heads. This paper describes the transfer function of the overall system, its tuning parameters, and discusses the actual control of cryostat level by using these parameters.

Superconducting solenoid magnet for the Multi-Purpose Detector at the NICA facility

The Multi-Purpose Detector (MPD) is a 4πspectrometer to be used for studying charged hadrons, electrons and photonsgenerated in heavy ion collisions at energies provided by the NICA colliderof the Joint Institute for Nuclear Research (Dubna). A constituent part ofMPD is a solenoid magnet with superconducting NbTi coil and steel fluxreturn yoke. The magnet is intended for providing a highly homogeneousmagnetic field of 0.5 T in an aperture of 4596 mm in diameter to ensure thetransverse momentum resolution within the range of 0.1–3 GeV/c at NICA.The mass of the magnet is eight hundred tons. Structurally, the MPD magnetyoke is a cylindrical barrel-like structure, which consists of 24 beams thatreturn the magnetic flux of the coil, two support rings, two poles and twosupport cradles that carry the total weight of the detector. The stainlesscryostat with the superconducting coil is rigidly fixed inside the yokebarrel. The structural arrangement of the yoke guarantees high rigidity ofthe whole magnet and will ensure the required field homogeneity and integralof the radial magnetic induction component in the TPC region aftersubsequent withdrawals of the poles and multiple movements of the magnet tothe assembly site for updating or repair. The coil is a one-layer solenoidmade of an aluminum stabilized superconductor. The conductor is wound ontoinside the aluminum support cylinder. The cooling method chosen for the MPDmagnet is based on the forced two-phase helium flow using a heliumrefrigerator. The coil conductor is cooled indirectly via the thermalcontact with the aluminum cylinder and heat removal through the cylinder tothe aluminum tube with circulated liquid helium. In this paper the detailsof the technical design and results of the magnetic and mechanicalcalculations are presented.

Superconducting Solenoids for RF Accelerators and Electron Guns

Compact solenoids are applied at an increasing rate for beam focusing in superconducting RF accelerating structures, for instance in secondary ion beam linacs and for free electron lasers. High field solenoids with a center field of 7.9 T for the HIE-ISOLDE high-beta cryomodules at CERN are currently being designed and manufactured at Danfysik. NbTi coils and a soft iron yoke are carefully designed for operation in a LHe bath at a temperature of 4.5 K at field integral requirements of ≤ 13.5 T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> m. The quenching characteristics of this solenoid have been studied and simulated, and a passive quench protection circuit is part of the solenoid design. The yoke outer diameter is 200 mm; the total length 312 mm. An HTS solenoid based on second-generation high-temperature superconducting technology was designed, built, tested, and delivered by Danfysik to the free electron laser project of the University of Wisconsin. The compact 136-mm-long HTS solenoid with a center field of 0.2 T operates in the temperature range between 5 and 70 K. The field integral is 3.1 T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> mm with very small integrated field errors. Conduction cooling of the compact solenoid makes it easy to assemble and operate very close to the RF cavity.

A New Test Facility to Characterize React and Wind &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$ \hbox{MgB}_{2}$&lt;/tex&gt;&lt;/formula&gt; Conductor

A new test facility was developed at CEA/Saclay in collaboration with the SigmaPhi Company in order to perform studies of a dry MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnet operating at 10 K and medium range field (1 T up to 4 T). For a rapid use of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors in an industrial product, notably in the medical sector, the “react and wind” technique must be used to wind magnets. However, the reacted MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors have a minimum winding radius, classically around 100 mm, which is incompatible with the existing Saclay test facility. A new test facility has been constructed with a 600 A dc current power, a 300 mm diameter sample holder, and conduction cooling by a two-stage GM cryocooler. The background field for this test facility is provided by a NbTi coil, which has a warm bore of 350 mm and a maximum field of 3 T. This paper presents the design of this test facility, especially the design of the current leads, the insulated thermal contacts with the development of a new type of insulation based on aluminum nitride and the dry sample holder. Results of critical current tests in self-field are presented and compared with manufacturer data for a reacted MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductor, a Ni/MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tape soldered on an OFHC copper stabilizer tape, supplied by Columbus Superconductors SpA.

Partial-Insulation Winding Technique for NbTi Coils

In 2010, we reported, for the first time, the no-insulation (NI) winding technique for high-temperature superconducting (HTS) pancake coils. Based on our test results of small NbTi NI coils, reported in 2011, the key benefits, i.e., enhanced mechanical integrity, compactness, and thermal stability, of the NI winding technique, appear intact for low-temperature superconducting (LTS) windings. However, the intrinsic charging delay observed in NI coils, caused by the lack of insulation, is more pronounced in LTS coils of a bare round wire than in HTS pancakes of thin, wide tape. Thus, to significantly reduce the charging delay in LTS coils of a bare round wire, we proposed a partial-insulation (PI) winding technique, a variation of the NI technique. In the PI winding of a bare round wire, a thin insulation sheet is introduced every few layers-note that in the PI winding, there are no turn-to-turn insulations. This paper reports results, experimental and analytical, of the PI winding technique in which bare-round-wire NI and PI coils were prepared to quantify the effects of PI winding technique. Three LTS coils of the identical dimension and magnet constant were wound with 0.4 mm diameter NbTi mono-filament wire and tested in a bath of liquid helium at 4.2 K, respectively, with three winding techniques: insulated (INS); NI; and PI. We analyzed the experimental results by applying an equivalent circuit model that had earlier been successfully applied to another set of experimental results. A graph model of resistance matrix was applied to estimate characteristic resistance of both NI and PI coils.

Heat transfer through the flat surface of Rutherford superconducting cable samples with novel pattern of electrical insulation immersed in He II

Future accelerator magnets will be exposed to heat loads that exceed even by an order of magnitude presently observed heat fluxes transferred to superconducting magnet coils. To avoid the resistive transition of the superconducting cables, the efficiency of heat transfer between the magnet structure and the helium must be significantly increased. This can be achieved through the use of novel concepts of the cable’s electrical insulation wrapping, characterized by an enhanced permeability to helium while retaining sufficient electrical resistivity.This paper presents measurement results of the heat transfer through Rutherford NbTi cable samples immersed in a He II bath and subjected to the pressure loads simulating the counteracting of the Lorentz forces observed in powered magnets. The Rutherford cable samples that were tested used different electrical insulation wrapping schemes, including the scheme that is presently used and the proposed scheme for future LHC magnets. A new porous polyimide cable insulation with enhanced helium permeability was proposed in order to improve the evacuation of heat form the NbTi coil to He II bath. These tests were performed in a dedicated Claudet-type cryostat in pressurized He II at 1.9K and 1bar.

Development of a compact superconducting magnet with a GdBCO magnetic lens

Concentration of a magnetic field has been achieved using a Gd–Ba–Cu–O (GdBCO) magnetic lens. A conduction-cooled compact high-field superconducting magnet with a GdBCO magnetic lens was developed. The magnet possessed a 10-mm room-temperature bore and consisted of two Nb–Ti solenoid coils and a GdBCO magnetic lens, which was installed at the center of the Nb–Ti coils in order to concentrate the background field generated by the Nb–Ti coils. The Nb–Ti coils and the GdBCO magnetic lens were cooled using a two-stage pulse-tube cryocooler. A concentrated magnetic field of 10.3 T was obtained at a background field of 5.6 T provided by the Nb–Ti coils. No degradation was found in the magnet during repeat excitation. The large field gradient generated by the GdBCO magnetic lens is expected to be used for the levitation of diamagnetic materials.

Design of a Large Single-Aperture Dipole Magnet for HL-LHC Upgrade

An upgrade of the low-beta insertion system for the ATLAS and Compact Muon Solenoid experiments is proposed in the high luminosity Large Hadron Collider upgrade project. It includes final beam focusing quadrupoles, beam separation and recombination dipoles, and larger aperture matching section quadrupoles. KEK is in charge of the conceptual design of the large aperture separation dipole D1. The latest design parameters are a main field of ~ 5 T at 1.9 K with Nb-Ti superconducting technology, a coil aperture of 160 mm, and a cos-theta one-layer coil with Large Hadron Collider dipole cable. Because the new D1 is expected to be operated in a very high radiation environment, radiation resistance and a cooling scheme are being carefully considered. The collaring-yoke structure is adopted to provide the mechanical support for the single-layer Nb-Ti coil. We summarize the design study of this magnet, including i) the very large iron saturation effect on field quality due to the large aperture and limited size of the iron yoke, ii) the stray field at the outer surface of the iron cryostat, and iii) the stress management from room temperature assembly to final operation.

Cryocooled Facilities for Superconducting Coils Testing in Gaseous Helium

Abstract Two superconducting coil test facilities equipped by Sumitomo SRDK-415D cryocoolers were developed, manufactured and tested. The motivation for their constructing was to make cheaper the testing (and especially training of LTS magnets) by liquid helium (LHe) saving. It is well known that the helium price increases rapidly and this tendency most probably will continue for a long time, as the demand of helium grows faster than its production. The utilization of heat-exchange gas considerably reduces many problems, that arise in the design of completely dry LTS magnets. The goal was to decrease or even completely avoid the consumption of rather expensive liquid helium for testing the laboratory size Nb-Ti and Nb3Sn coils including their training process. Several superconducting magnets were tested by using these facilities. For example, the first facility was successfully used for testing of 13 T, 60 kg coil cooled by cryocooler in helium gas (several torr pressure) heat exchange atmosphere. The precooling time was about 45 hours. The quench current (240 A at 4.2 K) was equal to that reached in the pool boiling LHe cryostat. The second facility with 420 mm wide access bore can be used for testing of corresponding size superconducting coils with very modest consumption of liquid helium with its level well below the lower flange of the coil. Each test facility is equipped by 2 pairs of HTS current leads. Design and operational experience of one of them is described.