Low-temperature Superconducting Magnet Research Articles

A High-Efficiency Protecting Scheme for HTS Inserts in Case of Background Magnet Quenches

For extremely high field magnets, the LTS/HTS structure is an efficient and economical solution. In an LTS/HTS magnet system, the LTS (Low-temperature superconducting) background field magnet is deeply mutual inductance coupled with the HTS (High-temperature superconducting) insert magnet. A sudden background LTS magnet quench may potentially destroy the double pancakes (DPs) of the insert HTS magnet because of the induced current and the resulting additional electromagnetic stress. To improve the safety and stability of the LTS/HTS magnet system, this article proposed a new passive protecting scheme of the LTS/HTS system. In this scheme, a magnetic dam is arranged between the insert magnet and the background field magnet. When the background field magnet quenches, the magnetic dam can help to slow down the magnetic flux variation in the insert magnet. According to the simulation results, a magnetic dam can significantly reduce the instantaneous stress rise of the insert magnet by 65.3protective effect of the magnetic dam was also verified by the experiments with a Bi-2223 insulating coil magnet. The structural parameters affecting the isolating effect of the magnetic dam were also discussed. Based on these work, researchers can further improve the safety and stability of the LTS/HTS magnet system.

A dynamic strain-based quench-detection method in an LTS sextupole magnet during excitation and quench

The dynamic strain characteristics and responses of a low-temperature superconducting (LTS) magnet during excitation and a quench are investigated in the present work. For the strain measurements, strain gauges in the form of a half-bridge circuit comprising cryogenic strain gauges and their dummy resistances are embedded directly within the superconducting magnet structure. A wireless high-speed data acquisition system with a resolution of 1 ms is also used to obtain the strain history of the LTS magnet during operation. The dynamic strain induced by thermal or mechanical disturbances is detected promptly and compared with the transport current and temperature signals recorded during a quench. This indicates that the dynamic strain measured in the LTS magnet can capture a quench feature in a timely manner. For a better understanding of the dynamic strain histories in the magnet, the dynamic strain signals are subjected to spectral analysis during the excitation and pre- and post-quench processes. It is shown originally that several spectral peaks on strain measured are always observed at the onset of a quench. Thus, the dynamic strain characteristics and responses provide a evaluation means of superconducting magnet.

Performance Evaluation of Current Leads for a 5 Tesla Electromagnetic Properties Measurement System

The development of the Electromagnetic Property Measurement System is underway at the Korea Basic Science Institute. The Electromagnetic Property Measurement System is used for electrical, thermal, and magnetic specimen property measurements in variable magnetic fields and temperatures with a low-temperature superconducting magnet. To activate low-temperature superconducting magnet that operates with liquid helium, we propose the use of vapor-cooled current leads. The low-temperature superconducting magnet is connected to the power supply at room temperature through a pair of current leads. Accordingly, heat inflows through the current leads and represents one of the major contributory factors of the entire heat inflow. Therefore, design optimization and evaluation of the current leads is necessary to minimize heat and liquid helium consumption. Vapor-cooled, and hybrid current leads were designed and fabricated, and testing in liquid helium cryostat was conducted. The low-temperature superconducting magnet was charged with electrical current up to the operating level, and the liquid helium boil-off rate was measured with respect to the supplied electrical current level. The performances of the two current leads were accessed based on the thermal load and boil-off rate of the liquid helium. The experimental and analyzed liquid helium boil-off rates associated with the current leads were in close agreement.

Quench Protection Optimization of Multi-Coil LTS Magnets

Quench protection is critical for superconducting systems, especially those containing significant energy to damage the system or its components during quench. We propose a new quench protection approach for multi-coil low temperature superconducting (LTS) magnet systems. The approach minimizes the number of protection components that must be activated during quench. In this approach, inductively-coupled coils are electrically connected in several possible ways: all in parallel; several parallel-connected groups, with each group consisting of several series connected coils; several series-connected groups, with each group consisting of several coils in parallel. If one of the parallel-connected coils or groups quenches, current in this coil or group decreases while current in the other parallel-connected coils or groups tries to increase to keep the total magnetic flux in the circuit the same, and this tendency will be affected by the nature of the inductive coupling among the coils. If during this event the non-quenching parallel coils remain superconducting, then the quench is limited to only the quenched coil or group. In some cases, the equipment may continue operation after quench, although at a somewhat reduced capacity. The proposed approach is potentially more advantageous for superconducting systems that consist of many weakly-coupled coils of similar size.

Strain Responses of Superconducting Magnets Based on Embedded Polymer-FBG and Cryogenic Resistance Strain Gauge Measurements

Large Lorentz forces commonly arise from the intense field and high current in superconducting coils, which can induce maximum stress up to several hundred megapascal (MPa) in magnets. It is a big challenge in compact high-field magnets with small mechanical margin during energization. In this paper, the mechanical characteristics and strain responses of two superconducting magnets with different configurations and low-temperature superconducting (LTS) materials are investigated experimentally during their excitation process and quench events. To explore the strain behavior in the magnets, several cryogenic resistance strain gauges (CRSGs) and homemade polymer-fiber Bragg grating (FBG) sensors are used for measurements. The strain responses during excitation and quench training tests of the two LTS magnets are recorded to show that both measurement techniques have good precision and repeatability. Compared to the CRSG with complex compensation bridges, the polymer-FBG sensors exhibit more advantages like smaller size, noncircuit compensation, and insensitivity to electromagnetic interferences, and most important of all is that the polymer-FBG can be embedded in coils with good survival to measure the internal strain in the magnets. Based on FEM, we further calculated the electromagnetic field and deformation of the two magnets, and the predicted results show good agreement with the experimental data.

Conceptual Design of a Bitter-Like Superconducting Magnet Stacked by REBCO Annular Plates and Magnetized by Flux Pump

The superconducting magnet wound by high-temperature superconducting (HTS) wires attracts extensive interest in high-magnetic field and at low temperature. However, different from the conventional low-temperature superconducting magnet, the persistent operation of the HTS magnet with persistent current switch is not commercial at present because of the immature jointing technique between HTS wires. This paper presents a conceptual design of a bitter-like superconducting magnet by stacking RE-Ba-Cu-O (RE = rare earth) annular plates magnetized by flux pump, which can operate in persistent mode. The induced field characteristics and temporal response of the superconducting magnet were first analyzed by theoretical simulation using the finite element method. The design method of REBCO annular plates and construction details of the bitter-like superconducting magnet were then detailed herein. A model bitter-like superconducting magnet was fabricated by stacking REBCO annular plates without joint resistance and its feasibility with persistent mode was experimentally validated at 77 K. The technology has potential application in nuclear magnetic resonance magnet and high field magnets.

Design and Test Results of Superconducting Magnet for Heavy-Ion Rotating Gantry

Heavy-ion radiotherapy has a high curative effect in cancer treatment and also can reduce the burden on patients. These advantages have been generally recognized. Furthermore, a rotating gantry can irradiate a tumor with ions from any direction without changing the position of the patient. This can reduce the physical dose on normal cells, and is thus commonly used in proton radiotherapy. However, because of the high magnetic rigidity of carbon ions, the weight of the rotating gantry for heavy-ion therapy is about three-times heavier than those used for proton cancer therapy, according to our estimation. To overcome this issue, we developed a small and lightweight rotating gantry in collaboration with the National Institute of Radiological Sciences (NIRS). The compact rotating gantry was composed of ten low-temperature superconducting (LTS) magnets that were designed from the viewpoint of beam optics. These LTS magnets have a surface-winding coil-structure and provide both dipole and quadrupole fields. The maximum dipole and quadrupole magnetic field of the magnets were 2.88 T and 9.3 T/m, respectively. The rotating gantry was installed at NIRS, and beam commissioning is in progress to achieve the required beam quality. In the three years since 2013, in a project supported by the Ministry of Economy, Trade and Industry (METI) and the Japan Agency for Medical Research and Development (AMED), we have been developing high-temperature superconducting (HTS) magnets with the aim of a further size reduction of the rotating gantry. To develop fundamental technologies for designing and fabricating HTS magnets, a model magnet was manufactured. The model magnet was composed of 24 saddle-shaped HTS coils and generated a magnetic field of 1.2 T. In the presentation, recent progress in this research will be reported.

Development of Conduction-cooled Cryostat for Superconducting Probe Station

A conduction-cooled cryostat has been developed for NbTi superconducting magnet used in a cryogenic probe station. In a cryogenic probe station, a sample stage for transient measurement is located in the bore of a low-temperature superconducting (LTS) magnet and thermally connected to the second-stage cold head of a cryocooler to cool the sample down to the liquid helium temperature. Two separate structures exist in order to maintain isolation between the LTS magnet and sample stage. The system is cooled by a two-stage cryocooler, and the LTS magnet is charged and generates the designed magnetic field. When the magnet is charged and maintains a certain magnetic field, the temperature of sample stage is varied depending on the amount of heating. The thermal linkage between the LTS magnet and second-stage cold head as well as the sample stage and second-stage cold head is investigated in terms of the stability of LTS magnet, magnetic field, and temperature of sample stage. The mechanical module including the thermal link to control the sample stage is integrated into the system and the results of the performance test are reported.

Performance of a 14-T CuNb/Nb3Sn Rutherford coil with a 300 mm wide cold bore

A large-bore 14-T CuNb/Nb3Sn Rutherford coil was developed for a 25 T cryogen-free superconducting magnet. The magnet consisted of a low-temperature superconducting (LTS) magnet of NbTi and Nb3Sn Rutherford coils, and a high-temperature superconducting magnet. The Nb3Sn Rutherford coil was fabricated by the react-and-wind method for the first time. The LTS magnet reached the designed operation current of 854 A without a training quench at a 1 h ramp rate. The central magnetic field generated by the LTS magnet was measured by a Hall sensor to be 14.0 T at 854 A in a 300 mm cold bore.

Modeling of Minimum Energy Required to Quench an HTS Magnet With a Strip Heater

Due to the wide spectrum of current sharing temperatures in a high-temperature superconducting (HTS) magnet, estimating the energy required to quench the magnet is a complicated task. On the other hand, quenching a low-temperature superconducting (LTS) magnet for quench characterization purposes with a heater is straightforward due to the small temperature margin and correspondingly low minimum quench energy (MQE). To estimate the required energy for the LTS magnet, the analytical concept of MQE can be utilized. In this paper, we propose that only numerical simulations can give adequate estimates to the MQE of an HTS magnet, for measurement purposes. Furthermore, due to the high enthalpy margin, the utilization of spot heaters with short energy pulses becomes questionable. We present in detail the effect of heater's pulselength to the MQE, when a strip heater is utilized for quenching. In addition, the effect of the heater area on MQE is studied. We consider the model of a REBCO coil to be constructed and tested in the Enhanced European Coordination for Accelerator Research and Development (EuCARD-2) Project. According to the results: 1) MQE increases almost linearly for pulselengths between 100 and 500 ms; 2) when the heater area is enlarged, the required energy per area saturates to a certain value related to the coil's enthalpy margin; 3) MQE obtained with a traditional analytic approach based on a minimum propagating zone considerably underestimates the numerically obtained MQE.

Low Temperature Superconducting Magnet Connected to a Cryocooler by Conductive Link

A low-temperature superconducting (LTS) magnet cooled by a cryocooler has been designed, fabricated, and tested for lab-scale material control device. The superconducting magnet incorporates a solenoidal configuration with a copper former which has a 52-mm room-temperature bore. The superconducting coil is installed in a cryostat maintaining high vacuum and is cooled by a two-stage cryocooler. In order to maintain the operating temperature of magnet at the designed level, the cold head temperature of a cryocooler must be lower so that heat can be removed from the superconducting coil. Also, a temperature difference occurs between the magnet and cryocooler and its magnitude is dependent upon the contact resistance at the interface between the metals in the conductive link. The performance of the LTS magnet is investigated with respect to the conductive link between the magnet former and the cold head of a cryocooler. In addition, the effects of the contact pressure and interfacial materials on the temperature distribution along the conductive link are discussed.

Performance of Filters in Medical Protein Separation System Using Superconducting Magnet

Medical proteins such as monoclonal antibodies or immunoglobulin are important as medicine for cancer and other uses. Today, we can easily sort and analyze medical proteins using various types of commercially available affinity magnetic beads. However, separation systems for these medical proteins have a very low separation rate, and the cost of the product is extremely high. We successfully developed a high gradient magnetic separation system using a cryocooler-cooled low-temperature superconducting magnet and conducted experiments on separating affinity magnetic nanobeads. Our system demonstrated very high separation efficiency and can achieve low costs with a large production rate compared to systems now used in this field. The design of a filter to trap and recover the nanobeads is important to this application. In order to achieve a filter with a high trapping ratio of magnetic beads, the parameters need to be optimized because the ratio depends on the dimensions and arrangement of the filter made of fine magnetic metal fibers. In this study, we investigated the performance of filters in the high gradient magnetic separation system. The test results show 97.8% of the magnetic nanobeads in pure water were captured, and 94.1% of the total beads were collected.

Conductive link between cryocooler and magnet in cryogen-free LTS magnet system

The conductive link is used as a cooling medium between a cryocooler and magnet in a cryogen-free superconducting magnet system. The low temperature superconducting (LTS) magnet has one solenoidal configuration with a metal former which has a 52 mm room temperature bore. The superconducting coil is installed in the cryostat maintaining high vacuum and cooled by a two-stage cryocooler. In order to maintain the operating temperature of magnet at the designed level, the cold head temperature of the cryocooler must be lower so that heat can be removed from the superconducting coil. Also, temperature difference is occurred between the magnet and cryocooler and its magnitude is dependent upon the contact resistance at the interfacial surface between metals in the conductive link. In the paper, the performance of the LTS magnet is investigated with respect to the conductive link between the magnet former and the cold head of the cryocooler. The effects of the contact pressure and interfacial materials on the temperature distribution along the conductive link are also presented.

Electromagnetic Characteristics of Bi-2212 Test Coils at High Field During Quench

We present further progress on the development of insert coils using round wires of the high-temperature superconductor Bi-2212 material. Several different-sized configurations of coils have been manufactured with variation of coil length as well as number of layers. The coils have been tested at 4.2 K in various background field values up to 20 T using a wide bore low-temperature superconducting magnet. The electromagnetic stress characteristics of the insert coils and performance under quench will be discussed.

Fiber Bragg Grating Sensor for Strain Sensing in Low Temperature Superconducting Magnet

Fiber Bragg grating (FBG) sensor for monitoring the electromagnetic strain in a low temperature superconducting (LTS) magnet was studied. Before used to LTS magnet strain sensing, the strain response of the sensor with 1.54- wavelength at liquid helium was experimentally studied. It was found that the wavelength shift showed good linearity with longitudinal applied loads and the strain sensitivity is constant at 4.2 K. And then, the hoop strain measurement of a LTS magnet was carried out on the basis of measured results. Furthermore, the finite element method (FEM) was used to simulate the magnet strain. The difference between the experimental and numerical analysis results is very small.

HTS magnets: stability; protection; cryogenics; economics; current stability/protection activities at FBML

The unique features that distinguish a high-temperature superconducting (HTS) magnet from a low-temperature superconducting (LTS) magnet are: (1) operating temperature and (2) temperature span over which the magnet remains superconducting. Thus, for two magnets, one HTS and the other LTS, both satisfying the same set of field specifications, the operating temperature and the operating range of temperature of the HTS magnet are generally an order of magnitude greater than that of the LTS magnet. In this CHATS 2002 Workshop, four issues, i.e., stability, protection, cryogenics, and economic issues will be examined as they are impacted by the two unique features, first as a general introduction to this presentation, between HTS and LTS magnets. This is followed by discussion of economics of large-scale HTS devices, specifically of fusion; electric power devices; high-energy physics; research-purpose NMR; and high- B DC magnets. The paper concludes with a presentation of preliminary results of stability/protection research presently being performed at the FBML.