45-T Hybrid Magnet Research Articles

Analysis and redesign of the thermal shield for the 40-T hybrid magnet at the CHMFL

A 40 T hybrid magnet has been constructed and commissioned at the High Magnetic Field Laboratory of the Chinese Academy of Sciences (CHMFL). It consists of a 10 T superconducting outsert magnet and a 30 T resistive insert magnet. As an important component of the hybrid magnet, the 80 K thermal shield is located inside of the cryostat protecting the superconducting magnet from thermal radiation. During the commissioning phase, a trip of the water-cooled resistive magnet triggered the quench protection system of the superconducting magnet, huge induced currents resulting in expansion and rupture of the oxygen-free copper thermal shield. In this paper, the cause of thermal shield rupture was investigated via finite element simulations. And the structure design and mechanical assessment of a new alternative thermal shield adopted for the 40 T hybrid magnet is presented.

30 T scanning tunnelling microscope in a hybrid magnet with essentially non-metallic design

We report on the construction and performance of the first hybrid resistive-superconducting magnet (HM) based scanning tunnelling microscope (STM) above 30 T. This custom-design HM-STM features a novel design of the STM head unit, whose tip-sample approach is implemented using a slender piezoelectric tube (PZT). The scanner shares part of PZT by fixing a sapphire frame onto the front quarter of PZT to construct a compact tip-sample loop, realising an outer diameter of 8.8 mm, which makes it compatible with a narrow sample space. Its main components are made of non-metallic materials of sapphire, which allows it to be immune from eddy currents and to operate under the condition of strong magnetic field fluctuation from a hybrid magnet, as well as cryogen-free cryocooler magnet systems. To analyse the stiffness of the STM head unit, the eigenfrequencies with 11 kHz and 12 kHz in bending modes, 25 kHz in a torsional mode, and 67 kHz in a longitudinal mode were simulated by finite element analysis; also, the drifting rates of the STM in ambient conditions in the X-Y plane and Z direction were measured at 25.5 and 38.2 pm/min, respectively. We present the first atomic images in magnetic fields up to 30.1 T in an HM. The raw data show the stable and distinguished performance while ramping up to maximum fields, indicating the new device's potential capability of operating in the future 45T-hybrid magnet and hundred-field pulsed magnet. Meanwhile, our compact and concentric cylindrical STM insert can operate in the low-temperature tubular sample space housed by the HM bore to develop low-temperature and extreme high-magnetic field STM.

Progress in the Development of the HFML 45 T Hybrid Magnet

To extend its user's facilities, the High Field Magnet Laboratory (HFML-EMFL) at the Radboud University is in the process of building a 45-T hybrid magnet. The magnet system will consist of a 22 MW 32.7 T resistive insert and a 600-mm-bore 12.3 T superconducting outsert magnet, and the design was significantly adjusted after a thorough design revision in 2011. The HFML hybrid magnet will be operated with separate current sources for the superconducting and resistive coils (20 kA at 10 V and 40 kA at up to 550 V, respectively). The outsert coil is a solenoid layer wound with all-Nb 3 Sn/Cu cable-in-conduit conductor (CICC), cooled by a forced flow of supercritical helium and operated at 20 kA. Similar to the series-connected hybrids for the HZB (Berlin, Germany) and the NHMFL (Tallahassee, FL, USA), the HFML outsert coil contains three grades of conductor. All CICC grades are based on high-current density Nb3Sn strand produced by Oxford Superconducting Technology. The CICC production and qualification program has been completed successfully. The coil will be wound at the NHMFL and, after heat treatment and impregnation, sent to Nijmegen for integration into the cryostat. In this paper, the design choices and current status of the program are presented.

Test of the MF-CICC Conductor Designed for the 12-T Outsert Coil of the HFML 45-T Hybrid Magnet

A 45-T hybrid magnet is being built at the High Field Magnet Laboratory of the Radboud University in Nijmegen, The Netherlands. The hybrid magnet consists of a 12-T cable-in- conduit-conductor (CICC) Nb 3Sn superconducting outsert and a 33-T resistive insert magnet. To verify the CICC design, a thorough testing has been completed in the SULTAN facility at Swiss Plasma Center, EPFL in Villigen (Switzerland) for the medium- grade conductor of the outsert. In two test campaigns, the dc cable performance (current-sharing temperature, critical current), the ac loss, and the conductor's performance stability during cyclic loading and after one warmup and cooldown cycle have been investigated. Two different cable layouts were tested—one with a very short twist pitch (STP) and the second one with a long twist pitch (LTP) cabling pattern. As both conductors were made of the same Nb 3Sn strand and underwent the same heat treatment and sample preparation procedure, the effect of the twist pitch on the ac loss and on the dc performance with respect to cyclic loading could be reliably evaluated. The test results show that both cable layouts are actually very robust. The cable could withstand 2000 load cycles and the warmup and cooldown cycle without any significant degradation of the dc performance, and even the overloading at BI product (field multiplied by current) approximately two times larger than those foreseen during magnet operation did not lead to a big performance change. Small differences between the STP and LTP options have been observed, indicating that the STP conductor withstands high electromagnetic loads better than the LTP one.

Upgrade of the NHMFL Cryogenic System and the Index Heating Test Results on the 45-T Hybrid

The 45-T Hybrid at the National High Magnetic Field Laboratory (NHMFL) has been the only facility in the world capable of providing a dc field of above 40 T. The 45-T hybrid magnet, consisting of a superconducting outsert and resistive insert, was commissioned in 2000 reaching a field of 45 tesla. The cryogenic system for the NHMFL has been in service since 1994 and has provided cryogenic service for the 45-T hybrid superconducting outsert since testing in 1998. It was recently upgraded, replacing a majority of the original system. The 45-T Hybrid was cooled down successfully with the new cryogenic system in February 2014. The new cryogenic system has been proved more efficient and reliable in the past years, providing increased availability to the users. After the upgrade of the cryogenic system, a standard index heating test was performed on the outsert of the hybrid. This test is mainly for evaluating the performance of the outsert, particularly the degradation of the innermost Nb3Sn coil of the outsert that suffered an unprotected quench in 2000 and degradation since then. The results of index heating tests in the past are compared and presented.

Study of Fault Conditions During Heat Treatment of Nb3Sn Strand

Heat treatment of large-scale superconducting coils is limited by the power of the furnace and the weight of the superconducting coils. For the superconducting coils to be used in a 45-T hybrid magnet, the practical temperature curve will lag behind the setting temperature curve, particularly for the lowest temperature stage (210 °C). There is also a risk that the heat treatment might be interrupted due to a general power failure or equipment failure. The highest temperature stage (640 °C) is the key stage to form Nb3Sn crystals; thus, interruption of the heat treatment during the 640 °C stage is expected to cause a great impact on the performance of a Nb3Sn superconducting magnet. This paper compares the performance of Nb3Sn strand samples given the recommended heat treatment with samples subjected to simulated temperature lag or simulated power failure. All of the samples had Residual Resistance Ratios, critical currents, and calculated heat losses that were satisfactory for use in the superconducting part of the 45-T hybrid magnet.

Water Cooled Resistive Magnets at CHMFL

The steady high magnetic field facility was founded by the National Development and Reform Commission of China (NDRC) in 2008 and is undertaken by the High Magnetic Field Laboratory of the Chinese Academy of Sciences. The project includes the construction of a series of water-cooled resistive magnets, superconducting magnets, and a hybrid magnet. Up-to-date, five water-cooled resistive magnets have been successfully installed. In addition, the six coils of the resistive magnet insert for the 45-T hybrid magnet have also been assembled and will be tested early next year. Two new world records of water-cooled resistive magnets have been achieved, i.e., 38.5 T/O32 and 35 T/O50 mm. Design, optimization, and assembly of the magnets, as well as their characterization are described in detail in this presentation.

Analysis of the Experimental Results for the Nb3Sn Model Coil Being Built in China

A model coil for the 40-T hybrid magnet superconducting outsert was designed and manufactured at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The performance test of the model coil was successfully performed in 2011 and has confirmed that the manufacturing principles are applied for the 40-T hybrid magnet superconducting outsert. The model coil was charged successfully with an operating current of 16,000 A to produce 12.14 T magnetic field under the 7.57 T background magnetic field generated by two NbTi superconducting coils. The test results fulfilled the goals of the model coil. This paper presents the main test results of the model coil including the relevant thermal and electromagnetic parameters obtained.

Cable-in-Conduit Conductor Fabrication for the Hybrid Magnet of CHMFL

The superconducting magnet for a 40-T hybrid magnet is being developed at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The hybrid magnet consists of a resistive insert and a superconducting outsert providing 29 and 11 T, respectively. The superconducting magnet was designed to be wound with four grades of cable-in-conduit conductors (CICCs). The superconducting coils contain over 5000 kg of $\hbox{Nb}_{3}\hbox{Sn/Cu}$ CICC in seven piece lengths. Three dummy conductors and seven prototype conductors have been successfully fabricated without incident. The conductor design, strand cabling, and conductor fabrication details will be introduced in this paper.

DC Resistance Performance of the Low-Resistance <inline-formula> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex-math></inline-formula> Joints for the CHMFL Hybrid Magnet

The superconducting coils of a 40-T hybrid magnet were designed with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductors wound in layers or double pancakes, which are connected in series by joints. Two kinds of electrical joints have been designed for the different terminals of the superconducting coils. The “terminal joint” is for the joints between the terminals of the superconducting coils and the ends of NbTi buslines. The “interlayer joint” is for the joints between the layers of the coils. Two prototype joint samples have been manufactured, and their dc resistance tested. The joints' manufacturing process and test results are presented in this paper.

Cold Mass Design of the Superconducting Outsert for the 40-T Hybrid Magnet

A 40-T hybrid magnet is under construction at the High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences (CHMFL), Hefei, China. It includes a superconducting outsert magnet that can provide a central field of 11 T and a warm bore of Ø 800 mm for the insertion of a water-cooled magnet. This superconducting magnet has three superconducting coils, all of which are wound with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductors (CICCs). The structural design of the superconducting outsert's cold mass has been worked out starting from the design of the conductors and the configuration of the coils. In this paper, we present the considerations and designs for the major components of the cold mass, i.e., the winding pack, the preload structure, the CICC lead support structures, and the cooling circuit. In addition, the design of the support structure of the coils is introduced and discussed. This is a key element and a special component with the function to support the cold mass and to transfer the fault loads from the coils to the base plate of the cryostat while its heat load to the cryogenic side remains as low as possible.

Quench Analysis of the Hybrid-Magnet Superconducting Outsert

Quench propagation analysis of the superconducting outsert for the 45-T hybrid magnet being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences was performed. The superconducting outsert was wound with a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductor cabled in a 316LN jacket cooled with supercritical helium at 4.5 K. The superconducting outsert can produce 11.0-T central magnetic field at an operating current of 13410 A. An active protection circuit with a dump resistor of 0.272 Ω was adopted to transfer the stored magnetic energy during a dump operation. In this paper, the magnetic field and strain of the superconducting outsert were analyzed. During a quench, the hot spot temperature of the cable and the helium pressure inside a jacket for different quench delay times were introduced.

Thermal-Electromagnetic Analysis for 14 kA Fast Discharge of a Model Coil

A model coil for 40-T hybrid magnet superconducting outsert magnet has been constructed and tested at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The model coil was wound with Nb3Sn cable-in-conduit conductor (CICC) cabled in a 316LN jacket cooled with supercritical helium. The model coil alone can produce about 4 T maximum magnetic field with an operating current of 14 kA. The model coil, in combination with 7.57-T NbTi background coil, can produce 11.5 T central field at 14 kA. During the test campaigns, a fast discharge was triggered by a dump resistor of 3.6 mΩ to evaluate the thermal-electromagnetic behavior of the model coil. In order to avoid a quench of the background coil, no current was exerted on the background coil through a power supply during the fast discharge of the model coil. The test results show that the central magnetic field is not scaled proportionally to the current decay of the model coil. The circuit model gives excellent results compared with the measured ones for the central magnetic field evolution as a function of time in this paper. For the thermal-hydraulic behavior during the fast discharge, the maximum temperature at the inlet simulated by the 1-D Gandalf code gives excellent agreement results compared with the measured ones with the conductor coupling time constant of 63 ms.

Thermal–Hydraulic Analysis of a Model Coil for 40-T Hybrid Magnet Superconducting Outsert

A thermal–hydraulic performance analysis of a model coil for a 40-T hybrid magnet superconducting outsert being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences, was performed. The model coil was wound with a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> cable-in-conduit conductor cabled in a 316LN jacket cooled with supercritical helium. The model coil, in combination with 7.5-T NbTi solenoid coils, will be capable of generating a 12-T central field. Only one cooling channel was used to cool the model coil due to its short length. Both the temperature margin associated with a given scenario and the quench propagation following an artificial disturbance are discussed. The thermal–hydraulic analysis of the temperature margin showed that there is a sufficient minimum temperature margin for 100-A/s current ramp rate under a 7.5-T background magnetic field. The quench analysis showed that the hot-spot temperature of the cable is about 60 K, which is less than the maximum allowable value of 150 K for 0.7-s delay time. In addition, the convergence study was carried out for different time steps and space sizes.

Fabrication Technique of the Model Coil for the 40-T Hybrid Magnet Superconducting Outsert

In the High Magnetic Field Laboratory, Chinese Academy of Sciences, a model coil has been fabricated and tested with the intention of developing the manufacturing technique and confirming the research and development routine of the superconducting outsert for a 40-T hybrid magnet. The model coil was layer wound using a cable-in-conduit conductor with small radius due to the restriction of the inner diameter of a NbTi solenoid coil, which will provide a central 7.5-T background magnetic field for the model coil tests. In normal operation, the highest field on the model coil conductor would reach about 12 T. The manufacturing process of the model coil included conductor forming, winding, heat treatment, vacuum impregnation, and assembly. For the purpose of summarizing the whole manufacturing process, in this paper, the important fabrication techniques are introduced and discussed.

Thermal Performance Simulation of a Model Coil for a 40-T Hybrid Magnet

A model coil for the 40-T hybrid magnet is under construction at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The model coil is layer wound with a rectangular <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{Nb}_{3}\hbox{Sn}$</tex></formula> cable-in-conduit conductor and adopts “wind-and-react” manufacturing technology. The model coil will simulate same electromagnetic loads of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$B \times I$</tex></formula> as the superconducting outsert of the 40-T hybrid magnet in the performance tests. The thermohydraulic behavior of the model coil during various operating scenarios was simulated by the numerical model Gandalf. The charging process and the cyclic operation of the model coil are analyzed in this study. The temperature rise and margin of the conductor due to the ac losses are computed and presented.

Characterization of $\hbox{Nb}_{3}\hbox{Sn}$ Strands for a 40-T Hybrid Magnetic Model Coil

A 40-T hybrid magnet under construction at the High Magnetic Field Laboratory of the Chinese Academy of Sciences will use cable-in-conduit conductors for its superconducting outsert. The design necessitates the use of high current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ≥ 2100 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 T/4.2 K). The high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> restacked-rod-processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand from Oxford Instruments Superconductor Technology was selected as one of the candidate strands. A series of tests was carried out to qualify the strand performance. Tests mainly included magnetization and critical current measurements as a function of axial strain, magnetic fields, and temperature. The test results are presented and discussed.

Heat Treatment and Characterization of $\hbox{Nb}_{3} \hbox{Sn}$ Strands for the Model Coil of the 40-T Hybrid Magnet

A model coil for the 40-T hybrid magnet is being built at the Chinese High Magnetic Field Laboratory, Chinese Academy of Sciences. /restack-rod-process (RRP) strands adopted for the model coil were heat-treated according to the heat treatment (HT) schedule recommended by the manufacturer, which is Oxford Superconducting Technology. The microstructure of the reacted strand was analyzed. Measurements of the critical current, the residual resistivity ratio and the hysteresis losses were also carried out. The results of critical-characteristic measurements confirmed that the performance of the heat-treated wire was good. All characteristics indicated that the HT of the /RRP strands was successfully completed.

Mechanical Behavior Analysis of Model Coil for the 40-T Hybrid Magnet Superconducting Outsert

A 40 T hybrid magnet system will be designed and constructed at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The superconducting outsert consists of two concentric superconducting coils. After the conceptual design of the superconducting magnet, the RD the first is a traditional method called global-local finite element technique, the second is a detailed 2-D FEA method.

A Conceptual Design of Model Coil for the 40-T Hybrid Magnet Superconducting Outsert

The High Magnetic Field Laboratory of the Chinese Academy of Sciences will develop a 40-T hybrid magnet. The superconducting outsert, which consists of a NbTi coil and a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil, will provide more than 11 T of central field in a 580-mm bore. The superconducting coils will be wound of cable-in-conduit conductor (CICC) and cooled by flowing 4.5-K supercritical helium. A model coil of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil has been designed and will be manufactured to develop the technology of cabling, jacketing, winding, heat treatment, etc., and the model coil can also simulate the performance of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting coil under its operating conditions. This paper gives an overview of the conceptual design for the model coil and predicts its current-sharing temperature (Tcs).