Junsheng Cheng

High Magnetic Field Superconducting Magnet System Up to 25 T for ExCES

The ultra-high-field superconducting magnets have been widely applied in scientific instruments for condensed matter physics. A superconducting magnet with the center field of 25 T in a warm bore-size of 32 mm in diameter has been designed for the Extreme Condition Experimental Science Facility (ExCES). The superconducting magnet consists of NbTi, Nb3Sn superconducting coils and YBCO high-temperature superconducting (HTS) insert operated at the 4.2 K. In order to prove the technical feasibility to achieve the target of 25 T, high-temperature superconductor YBCO and Bi2223 inserts have been designed, fabricated and tested in the operating temperature of 4.2 K. Inner diameter, outer diameter, and height for the YBCO insert are 40 mm, 68.9 mm, and 253 mm, respectively. The larger Bi2223 insert has the inner diameter, outer diameter, and height of 120 mm, 212 mm, and 268.8 mm, respectively. Tests at liquid helium temperature show that the YBCO and Bi2223 inserts can generate the center field of 5 T and 5.05 T, respectively. The assembly of the Bi2223 and YBCO insert coils can generate a center magnetic field of 7.6 T when tested at the liquid helium temperature. In this paper, the design, fabrication, and test of the HTS insert and the 25 T magnet are reported.

A Superconducting Magnet System for Whole-Body Metabolism Imaging

A 9.4 Tesla superconducting magnet is designed and fabricated with a warm bore of 800 mm for neuroscience research. The superconducting magnet will be made of a NbTi Wire-in-Channel (WIC) conductor with a higher ratio of copper to non-copper, which thus sustains the high stresses. It is cooled to operate temperature at 4.2 K liquid helium. The cryostat system is cooled through GM cryocoolers, some used to cool the radiation shield, and the others realize the re-condensed liquid helium. The MRI magnet system has a high level of stored energy, about 134 MJ, and a relatively-lower nominal current, about 212.5 A. The magnet will be operated in a persistent current mode with a superconducting switch. The WIC wires are employed to meet the cryostability criteria to avoid any risks from quench. The protection circuit with the subdivision of the coil reduces the terminate voltage and hot-spot temperature. In the paper, the specifications of magnet system will be presented.

High Magnetic Field Superconducting Magnet for 400 MHz Nuclear Magnetic Resonance Spectrometer

A superconducting magnet with the center field of 9.4 T is designed and fabricated for 400 MHz Nuclear Magnetic Resonance. Superconducting coil with NbTi/Cu superconducting wire is employed and cooled by re-condensed liquid helium and the magnet system with the clear-bore of 54 mm. The pulsed tube refrigerator with separated valve is employed to cool the magnet system. The superconducting magnet has an active shield with high pure copper shield to protect during quench of the shielding coil. The paper reports the electromagnetic design, and fabrication is detailed.

Fabrication of NbTi Superconducting Joints for 400-MHz NMR Application

NbTi superconducting joints (SJs) for a 400-MHz nuclear magnetic resonance (NMR) magnet system were fabricated using the superconducting solder matrix replacement in an open-air condition. A detection device for testing the resistance of SJ has been established. The results show that the overall resistance of SJs is 9.58 × 10-12 Ω under the background field of 1 T by summation of individual joint resistance. The resistance of SJs and the capability for current load should meet the demands of the NMR system. The SJs are placed inside the cylindrical vessel above the magnet. The magnetic flux inside the top of the vessel is no more than 0.3 T to assure performance of joints. As results, there is only 0.0001 ppm for homogeneity deviation caused by SJs on this NMR system, and therefore, the negative effect is negligible.

Structural Design of a 9.4 T Whole-Body MRI Superconducting Magnet

A project to develop a 9.4 T magnetic resonance imaging system is proposed for bioscience research applications. A whole body superconducting magnet system will be manufactured and test in the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS). This magnet system features a room temperature bore of 800 mm in diameter, helium bath cooing, 9.4 T center magnetic field and passive iron shielding. The magnet is designed with radial layer-winding method. Five coaxial coils will be wound independently and assembled together as the main magnet. Coil length of the magnet is 3000 mm. In the magnet design, current density grading is performed to optimize the magnetic field distribution and stress level in the coil windings. The maximum magnetic field is 9.505 T, corresponding to an operating current of 224.515 A. The total magnetic energy storage is 138 MJ. Detailed magnetic and mechanic structure design as well as structure stress analysis are presented in this paper.

Development of Large Scale Superconducting Magnet With Very Small Stray Magnetic Field for 2 MJ SMES

A superconducting magnet for the superconducting magnetic energy storage system (SMES) fabricated by NbTi monolithic conductor is cooled down and operated at the temperature of liquid helium. The large-scale superconducting magnet with four parallel solenoids was designed, fabricated and tested for the high storage energy density SMES. The superconducting magnet stores 2 MJ of energy with a current of 490 A and a peak magnetic field of 5.4 T. Two GM cryo-coolers cool the whole system to realize zero evaporation of liquid helium. The high temperature superconducting current leads of Bi2223 are used and cooled through one GM cryocooler. The ZnO resistor is used to protect the superconducting magnet. In the paper, the system of superconducting magnet is introduced in detail for the superconducting magnetic energy storage system.

Preliminary Mechanical Analysis of a 9.4-T Whole-Body MRI Magnet

A 9.4-T/800-mm superconducting magnet for whole-body magnetic resonance imaging system has been designed and will be constructed from NbTi conductors. Main coils wound on five aluminum alloy formers provide the center field strength of about 9.4 T. In addition, compensation coils wound on another aluminum alloy former are used to improve the field uniformity of the imaging region. The operating point of the inner main coil is very close to the critical properties of its wire, and its temperature margin is about 0.4 K. It is necessary to know the mechanical stress in the main coils during all operating conditions. The mechanical behavior of the main coils during winding, cooldown, and energizing is analyzed. The effects on mechanical disturbances are predicted, and one available method is proposed to reduce premature quench. In addition, the dimension variations and position changes due to pretension, thermal contraction, and magnetic compression are present.

Conduction-Cooled Superconducting Magnet With Persistent Current Switch for Gyrotron Application

A superconducting magnet with a center field of 4.5 T cooled by GM cryocooler and operated in the persistent current mode has been designed, fabricated and tested for gyrotron. The superconducting magnet has a warm bore with diameter of 90 mm, the homogenous region with the diameter of 40 mm and length of 230 mm. The ratio of the axial field to the center field located at 180 mm is lower than 88%. In the other special points, the ratios of the radial field to the axial field should be less than from 3% to 11%. The thermally-controlled NbTi/CuNi switch with superconducting joint is connected to the conduction cooled magnet. In this paper, the detailed design, fabrication and test are reported.

Evaluation of NbTi Superconducting Joints for 400 MHz NMR Magnet

Persistent current NbTi superconducting joint were fabricated based on the cold-pressing welding method for 400 MHz nuclear magnetic resonance (NMR) magnet. The electrical properties of the joints were tested using the current decay measurement method. To simulate the actual coil assembly, a nine-joint series loop was made and tested. Test results show that the total resistance of the nine persistent joints made based on cold-pressing welding method is 3×10-14 Ω at 120 A under 1 T background magnetic field, which meets the requirements of the 400 MHz NMR magnet. We have found that the induced current in the joint loop decays obeying three decay patterns, which is relevant to the flux creep and can be explained with n-value losses in the joint loop. Relaxation of magnetization in persistent current joint loops was also observed under 1 T background magnetic field.

Effect of Pretension, Support Condition, and Cool Down on Mechanical Disturbance of Superconducting Coils

The applied pretension on the conductor and support condition during coil fabrication has a great effect on the stress state of the coil. Otherwise, the thermal contraction properties of mandrels and coils would cause residual thermal stress during cool down. A combined homogeneous cylinder method was used to analyze the mechanical performance of winding process for the condition of no mandrel support. A finite-element model of solenoid was created to calculate the stress of winding process and cool down. The mandrel support conditions of fixed support at ends of former and radial support along the whole inner surface of former were researched. Mechanical behaviors of one coil used for a 9.4-T magnetic resonance imaging magnet during winding process and cool down were studied based on the aforementioned approach.