Pusheng Yuan

Enhancement of transport critical current density of SmFeAsO1−xFx tapes fabricated by an ex-situ powder-in-tube method with a Sn-presintering process

SmFeAsO1−xFx (Sm1111) tapes were prepared by an ex-situ powder-in-tube method with a Sn-presintering process. Scanning electron microscopy revealed apparent difference in microstructure between Sn-presintered tapes and the previously reported polycrystalline Sm1111 bulk, since Sn has reduced FeAs wetting phase and filled the voids between Sm1111 grains. The Sn-presintered tapes showed significant enhanced field dependences of transport Jc compared with Sn-added tapes. A highest transport critical current density (Jc) of 3.45 × 104 A cm−2 at 4.2 K and self-field is achieved. Magneto-optical imaging further confirmed large and well-distributed global and intergranular Jc in Sn-presintered Sm1111 tapes.

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

Design of a 7 T Superconducting Magnet for Lanzhou Penning Trap

A penning trap system called LPT (LANZHOU PENNING TRAP) is now being developed for precise mass measurements in IMP (Institute of Modern Physics). The most key component of LPT is a superconducting magnet. A 156 mm warm bore and two cylinder good field regions with a distance of 220 mm are required for trapping ions and measurements. As the required homogeneity is better than 0.5 ppm, several complicated coaxial coils are used to produce such a magnetic field. The size and position of these coils are optimized by using a method combining linear program with multiobjective optimization. Superconducting shim coils and passive shim pieces are used to eliminate inevitable winding tolerances and environmental influence. The fringe field is decreased to 5 Gs at 2 m line from the center of the magnet by active shielding coils. The designs of the mechanical structure, the quench protection system are also introduced in this paper.

Microstructure and superconducting properties of nanocarbon-doped internal Mg diffusion-processed MgB2 wires fabricated using different boron powders

MgB2/Nb/Monel monofilament wires were fabricated using four different boron powders by an internal Mg diffusion (IMD) process. The microstructure, morphology and the critical current density (J c) of the used boron powders and the formative MgB2 layers were analyzed and compared. It was found that the purity and particle size of the boron powder influence the superconducting properties of MgB2 wires; further that the optimized heat-treatment condition also depends on the quality of the boron powder. The highest J c was obtained in the MgB2 layer made using amorphous boron (AB) powder, although a certain amount of voids existed in the superconducting layer. The IMD-processed MgB2 layer fabricated using high-purity boron (HB) powder had also a high J c compared with the powder-in-tube (PIT) process and a few unreacted boron particles remained in it. MgB2 wire fabricated using low-purity boron (LB) powder had a high cost-performance ratio compared with the others, which is expected to allow the fabrication of large-scale and low-cost superconducting wires for practical application. However, the enhancement of the J c was not found in the MgB2 layer manufactured using the ball-milled LB (MLB) powder as expected due to the increased percentage of impurity.

High performance FeSe0.5Te0.5 thin films grown at low temperature by pulsed laser deposition

We report on fully epitaxial FeSe0.5Te0.5 (FST) thin films with high-quality grown on CaF2 (00l) substrate at a low temperature of 300{\deg}C by pulsed laser deposition. The transport Jc of thin films is up to 1.36 MA/cm2 in self-field and 0.97 MA/cm2 in 9 T at 4.2 K, indicating very weak field dependence. A nearly isotropy of Jc ({\gamma}= Jc(H//ab)/Jc(H//c) as low as 1.09 at 9 T is achieved in the FST thin films. Moreover, no clear amorphous interfacial layer presents between the film and the substrate probably ascribing to low temperature and low laser repetition rate grown, while the thickness of reaction layer is approximate 5 nm in many other works. The evidences of transmission electron microscopy show some lattices with lateral size of < 5 nm x 20 nm seem to be disturbed. Those location defects are thought to be responsible for nearly isotropic behavior of superconductivity.

Angular-dependent vortex pinning mechanism and magneto-optical characterizations of FeSe0.5Te0.5 thin films grown on CaF2 substrates

Magneto-optical (MO) characterizations and the angular-dependent critical current density (J c(Θ)) of epitaxial FeSe0.5Te0.5 (FST) thin films grown on CaF2 single-crystalline substrates were performed. The MO images show typical rooftop patterns in the remanent state from which a large, homogeneous, and almost isotropic self-field J c over 2 × 106 A cm−2 at 8 K was obtained. The vortex pinning mechanism is investigated measuring the magnetic field and angular-dependent critical current density J c. The FST films exhibit small anisotropy of J c in the whole applied magnetic field range below 15 K. The Dew-Hughes model and angular scaling analyses prove that pointlike normal cores, which are distributed randomly in the FST film, dominate the pinning in the FST films on CaF2 substrates.

The Cold Test of Super-FRS Superconducting Dipole Prototype for FAIR Project

Superconducting dipole prototype of the Super-FRS (Super FRagment Separator) for FAIR Project (Facility for antiproton and Ion Research) which is jointly developed by the Institute of Modern Physics (Lanzhou), the Institute of Plasma Physics (Hefei) and the Institute of Electrical Engineering (Beijing), have passed the final magnetic field measurement and low temperature test. The magnetic flux density is up to 1.6 T when the operating current is 232 A, and magnetic field homogeneity meets the calculation result which is ±1 × 10-4. The single ramping tests up to 232 A and 278 A at the rate of 3 A/s, 3 triangular cycles ramping up to 232 A within 120 seconds, and the quench tests at 232 A and 278 A by using the spot heater were done. Some other tests were finished at the same time. All the test results indicate that the superconducting dipole prototype could meet the requirements of FAIR.

Low-temperature synthesis to achieve high critical current density and avoid a reaction layer in SmFeAsO1−x F x superconducting tapes

A low-temperature (300–500 °C) heat treatment process under ambient pressure or uniaxial pressure was performed on Sn-added SmFeAsO1−x F x superconducting tapes fabricated by the ex situ powder-in-tube method. A highest transport critical current density (J c) of 3.95 × 104 A cm−2 (at 4.2 K and self-field) was achieved by this process. The low-temperature process allows tapes to endure much longer heat treatment without J c degradation than the high-temperature method. Microscopic analysis also revealed that this method could obtain a clear boundary without a reaction layer or interdiffusion between a superconducting core and sheath metal.

Active Shim Design of a 7 T Highly Homogeneous Superconducting Magnet for Lanzhou Penning Trap

Lanzhou Penning Trap (LPT) is now being developed for precise mass measurements in Institute of Modern Physics (IMP). Homogeneity of magnetic field in LPT was about 3 ppm when without active shimming, while the required value is less than 0.5 ppm. Linear programming algorithm was employed to design a 9-coil cylindrical shim set for main magnet in LPT. First, mesh on the cylinder and current paths are expressed by connectings between the grid nodes. Then current paths are calculated to compensate the high order components of magnet field through linear programming algorithm optimization. The resulting designs are relatively sparse and can be readily constructed. The method has good flexibility, which can uses current paths on any given surface to generate any order shim field and set allowable errors separately over different arbitrary regions of interest (ROI). Software simulation is used to evaluate the influence on field due to bend corners and limited current density for actual windings.

High-performance FeSe0.5Te0.5 thin films fabricated on less-well-textured flexible coated conductor templates

We report on the transport properties of FeSe0.5Te0.5 (FST) thin films fabricated on less-well-textured flexible coated conductor templates with LaMnO3 (LMO) as buffer layers using pulsed laser deposition. The LMO buffer layers exhibit large in-plane misalignment of ~7.72°, which is unfavorable for cuprate-coated conductors due to the high grain boundaries. The FST thin films show a superconducting transition temperature of 16.8 K, higher than that of bulk materials due to the compressive strain between LMO and FST. Atomic force microscopy observations reveal that island-like features appear at the surfaces of both LMO and FST, confirming the island growth mode. A self-field transport critical-current density of up to 0.43 MA cm−2 at 4.2 K has been observed in FST thin films, which is much higher than that in powder-in-tube processed FST tapes. The films are capable of carrying current densities of over 105 A cm−2 in the whole applied magnetic field up to 9 T, showing great potential for high-field applications. The results indicate that, for FST, highly textured metal tapes are not needed to produce coated conductors with high performance, which is of great advantage over cuprate-coated conductors.