Persistent current joints are crucial components of superconducting magnets—enabling the production of the high and ultra-stable magnetic fields required, for instance, for magnetic resonance measurements. At this critical juncture when persistent mode magnets containing commercial high temperature superconductors may soon become a reality, it is of value to take stock and evaluate current challenges faced in the field of jointing. This paper provides a review of progress made to date on the production and characterization of joints between the five major technological superconductors—NbTi, Nb3Sn, MgB2, BiSCCO and REBCO, including the materials that are used to make these joints.

https://iopscience.iop.org/article/10.1088/0953-2048/28/9/093001/pdf

Persistent current joints between technological superconductors

An efficient cooling system and the superconducting magnet are essential components of magnetic resonance imaging (MRI) technology. Herein, we report a solid nitrogen (SN2) cooling system as a valuable cryogenic feature, which is targeted for easy usability and stable operation under unreliable power source conditions, in conjunction with a magnesium diboride (MgB2) superconducting magnet. The rationally designed MgB2/SN2 cooling system was first considered by conducting a finite element analysis simulation, and then a demonstrator coil was empirically tested under the same conditions. In the SN2 cooling system design, a wide temperature distribution on the SN2 chamber was observed due to the low thermal conductivity of the stainless steel components. To overcome this temperature distribution, a copper flange was introduced to enhance the temperature uniformity of the SN2 chamber. In the coil testing, an operating current as high as 200 A was applied at 28 K (below the critical current) without any operating or thermal issues. This work was performed to further the development of SN2 cooled MgB2 superconducting coils for MRI applications.

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Solid cryogen: a cooling system for future MgB2 MRI magnet.

Magnesium diboride (MgB2) is known to have good potential to be used in a commercial liquid helium-free magnetic resonance imaging (MRI) magnet. The magnet is expected to be fabricated using reacted MgB2 conductors to minimize modifications in the existing magnet production technologies and operated in the persistent mode by forming a closed-loop using superconducting joints. The superconducting joints of the reacted MgB2 conductors are typically formed by placing unreacted magnesium and boron powders in between them followed by sintering. During this process, the MgB2 conductors will inevitably experience dual sintering in the vicinity of the superconducting joints. However, the effects of dual sintering on the performance of MgB2 conductors are still unknown. Therefore, herein, we dual sintered commercially available multifilament in situ and laboratory-made monofilament internal magnesium diffusion (IMD) processed MgB2 conductors (un-doped and C-doped) and evaluated their transport performance, microstructure, lattice parameters, and critical temperature in detail. In the IMD processed wires, the C-doped boron powder was found to promote a better diffusion of magnesium compared with the un-doped boron powder. Under typical dual sintering conditions, surprisingly, the C-doped in situ wire showed a simultaneous improvement in the engineering critical current density and n-value in different magnetic fields at 4.2 K and 20 K. On the other hand, the remaining conductors showed a strong dual sintering induced transport performance variation. Our findings provide insights into MgB2 conductors' performance after dual sintering for their further development towards MRI applications.

MgB2 for MRI applications: dual sintering induced performance variations in in situ and IMD processed MgB2 conductors

The goal of this theoretical study is to illustrate the potential of three different MgB2 tapes, developed by Columbus Superconductors, for application in cylindrical coils. First, the distribution of critical currents and electric fields of individual turns is compared when the winding of the model coil is made with tapes having different Ic(B) and anisotropy values. Second, the influence of the winding geometry on basic parameters of cylindrical coils which consist of a set of pancake coils, such as critical current Icmin, central magnetic field B0 and stored energy E, is analysed. The winding geometry of the coils, i.e. the outer winding radius and the coil length, with the same inner winding radius, was changed from a disc shape to a long thin solenoid in such a way that the overall tape length was held constant, and considered as a parameter. Finally, the winding cross-section of the coil is optimized with respect to the constant tape length in order to reach the maximum central field. The results of calculations show that for a given overall tape length and inner winding radius there exists only one winding geometry which generates the maximum central field. The overall tape length, as a parameter, is changed in a broad range from 500 m to 10 km. All calculations were performed using the experimental data measured at 20 K while the effect of the anisotropy in the Ic(B) characteristic of the short samples is taken into account.

http://iopscience.iop.org/article/10.1088/0953-2048/28/5/055012/pdf

Study of the potential of three different MgB2 tapes for application in cylindrical coils operating at 20 K

The Facility for Antiproton and Ion Research is now fast approaching its realization phase. Roughly 800 superconducting magnets are required for the SIS100 and the Super-FRS machines. Given their long lead time of procurement for this large number of items, the SIS100 dipoles series is already ordered with the quadrupole modules following swiftly. The Super-FRS dipole and multiplets are being tendered. In a second realization phase the SIS300 accelerator will be built and placed in the same common tunnel as the SIS100. We report the status of magnet production of the different projects together with the experiences obtained during the manufacturing process of the first SIS100 series dipole.

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Status of the Superconducting Magnets for FAIR

The Super-FRS (Super FRagment Separator) is a part of FAIR (Facility for Antiproton and Ion Research), which will be constructed at GSI, Germany by 17 countries. The Super-FRS comprises 24 superferric dipole magnets. The 2D and 3D magnetic field simulations of the prototype magnet are described in this paper. A passive trim slot and four chamfered removable poles are used to satisfy the required field homogeneity which is better than at 1.6 T, 0.8 T and 0.16 T in a wide elliptical useable aperture of 380 mm 140 mm. Measurement results at various field levels are shown in this paper as well. It can be seen from the comparison of calculation and measurement results that the magnetic designs of the magnet fulfils the requirements.

Magnetic Field Design of the Dipole for Super-FRS at FAIR

The fragment separator of the HIAF (High Intensity Heavy Ion Accelerator Facility) project called HFRS requires quadrupoles with high gradients (11.4 T/m) and large bores (320 mm in diameter). The iron dominated magnets with superconducting coils have been widely used in the similar facilities such as A1900, BigRIPS, Super!FRS and RISP with the advantages of low request for coils installation precision, simple fabrication and low cost, but they have large cold mass and helium containment, which result in long time cooling down and high pressure rise during a quench. In addition, due to iron saturation, it is hard to guarantee on the field quality in the operated field range. A new coil dominated design based on the Canted!Cosine!Theta geometry is presented for HFRS, which is expected to overcome these problems. The design superimposes several layers of oppositely wound helical windings to generate high quality quadrupole. Sextupole, octupole and steering dipole can also be easily integrated to reduce the length of cryostat. This paper reports the detailed design of HFRS multiplets based on the CCT concept and the construction of a subscale prototype.

Multipole magnets for the HIAF fragment separator using the Canted-Cosine-Theta (CCT) geometry

As promising magnet types, discrete-cosine-theta and canted-cosine-theta (DCT&CCT) superconducting magnets are designed for the High Energy Fragment Separator (HFRS), a fragment separator of the High Intensity Heavy Ion Accelerator Facility (HIAF). Quench protection is one of the key issues for DCT&CCT superconducting magnets due to low thermal conductivity between each turns and slow transverse quench propagation. Thermal quench-back induced by the center cooper wire as a heater automatically is a feasible quench protection option for the magnet wound with 6+1 type superconducting cables (six superconducting wires and one center cooper wire). In the present paper, an electro-thermal coupling model is developed and validated to deal with the dynamic coupling behaviors of electrical and temperature fields inside the superconducting magnets during quench protection process. The transient evolutions of current and voltage-to-ground are numerically obtained by finite element method, which shows obvious quench-back behaviors and agree well with the experimental measurements of the prototype magnet L800 in first round test. The hot-spot temperature can be reduced significantly considering quench-back effect. Furthermore, the quench protection baselines of temperature and quench integral are analyzed and predicted for L800, indicating that the magnet can be protected properly by the protection method with a quench-back at ultimate current 466A.

Electro-Thermal Coupling Model of Quench Protection With a Quench-Back for DCT&CCT Superconducting Magnets

HIAF (High Intensity heavy ion Accelerator Facility) is the new generation heavy ion accelerator under construction in China. The HFRS (FRagment Separator of HIAF) is a fragment separator and also a transfer-line between the Booster Ring and the Spectrometer ring, which has magnetic rigidity up to 25 Tm. HFRS magnet system is composed of 11 superconducting dipoles and 13 sets of triplets. Several multipole magnets (an octupole coil, a quadrupole coil, and a sextupole coil) are nested together to compact the length of the HFRS-line. All of them feature high field gradients (13 T/m, 25 T/m2, 105 T/m3) and large bores (320 mm). To reduce the cold mass and ensure the field quality, the whole multipole magnets are all designed with coil-dominated magnets of CCT (Canted Cosine Theta) and DCT (Discrete Cosine Theta). This paper will introduce the general design process of an HFRS multipole module. The development process and test results of the prototype will be presented.

DCT&CCT Superconducting Multiplets for HIAF-HFRS

A compact solenoid for the China Accelerator Driven Subcritical System (China-ADS) injector II has been developed. It includes a focusing solenoid and two steering dipoles. The magnet is compact with an entire length of 160 mm, including the helium vessel and the side flanges. Nonlinear optimization method was used to meet the requirement of focusing strength and stray field with minimized solenoid volume. The steering dipoles are placed inside the solenoid to reduce the magnet size. The structural analysis, quench protection design, and fabrication and test results are presented in this paper.

Dongsheng Ni

Papers

Magnetic Field Design of the Dipole for Super-FRS at FAIR

Multipole magnets for the HIAF fragment separator using the Canted-Cosine-Theta (CCT) geometry

Electro-Thermal Coupling Model of Quench Protection With a Quench-Back for DCT&CCT Superconducting Magnets

DCT&CCT Superconducting Multiplets for HIAF-HFRS

A Compact Superconducting Solenoid for China-ADS Injector II