Earticle

To observe the superconducting current and structural properties of high critical temperature (Tc) superconductors (HTS), we suggest the following imaging methods: Room temperature imaging (RTI) through thermal heating, low-temperature bolometric microscopy (LTBM) and Raman scattering imaging. RTI and LTBM images visualize thermal-electric voltages as different thermal gradients at room temperature (RT) and superconducting current dissipation at near-Tc, respectively. Using RTI, we can obtain structural information about the surface uniformity and positions of impurities. LTBM images show the flux flow in two dimensions as a function of the local critical currents. Raman imaging is transformed from Raman survey spectra in particular areas, and the Raman vibration modes can be combined. Raman imaging can quantify the vibration modes of the areas. Therefore, we demonstrate the spatial transport properties of superconducting materials by combining the results. In addition, this enables visualization of the effect of current flow on the distribution of impurities in a uniform superconducting crystalline material. These imaging methods facilitate direct examination of the local properties of superconducting materials and wires.

We present an experimental investigation of the superconducting transition temperatures, Tc, of superconductor/ferromagnet bilayers with varying the thickness of ferromagnetic layer. FeN was used for the ferromagnetic (F) layer, and NbN and Nb were used for the superconducting (S) layer. The results were obtained using three different-thickness series of the S layer of the S/F bilayers: NbN/FeN with NbN thickness, dNbN ≈ 9.3 nm and dNbN ≈ 10 nm, and Nb/FeN with Nb thickness dNb ≈ 15 nm. Tc drops sharply with increasing thickness of the ferromagnetic layer, dFeN, before maximal suppression of superconductivity at dFeN ≈6.3 nm for dNbN ≈10 nm and at dFeN ≈2.5 nm for dNb ≈ 15 nm, respectively. After shallow minimum of Tc, a weak Tc oscillation was observed in NbN/FeN bilayers, but it was hardly observable in Nb/FeN bilayers.

Superconducting properties of thin film MgB2 superconductors irradiated with 45 MeV α-particle beam were studied. After the irradiation, enhancement of the critical current density and pinning force was observed, scaling close to strong pinning formula. Double logarithmic plots of the maximum pinning force density with irreversible magnetic field show a power law behavior close to carbon-doped MgB2 film or polycrystals. Variation of normalized pinning force density in the reduced magnetic field suggests scaling formulas for strong pinning mechanism like planar defects. We also observed a rapid decay of critical current density as the vortex lattice constant decreases, due to the strong interaction between vortices and increasing magnetic field.

Annealing as-grown electron-doped cuprates under a low oxygen-partial-pressure condition is a necessary step to achieve superconductivity. It has been recently found that the so-called protect annealing results in much better superconducting properties in terms of the superconducting transition temperature and volume fraction. In this article, we report on angle-resolved photoemission spectroscopy studies of a protect-annealed electron-doped cuprate Pr0.9La1.0Ce0.1CuO4 on annealing condition dependent superconducting and pseudo-gap properties. Remarkably, we found that the one showing a better superconducting property possesses almost no pseudo-gap while others have strong pseudo-gap feature due to an anti-ferromagnetic order.

The study of superconducting fault current limiter (SFCL) is continuously being studied as a countermeasure for reducing fault-current in the power system. When the fault occurred in the power system, the fault-current was limited by the generated impedance of SFCLs. The operational characteristics of the flux-offset type SFCL according to turn ratios between the primary and the secondary winding of a reactor were compared in this study. We connected the secondary core to a superconductor and a SCR switch in series in the suggested structure. The fault current in the primary and the secondary winding of the reactor and the voltage of the superconductor on the secondary were measured and compared. The results showed that the fault current in the load line was the lowest and the voltage applied at both ends of the superconductor was also low when the secondary winding of the reactor had lower turn ratio than the primary. It was confirmed based on these results that the turn ratio of the secondary winding of the reactor must be designed to be lower than that of the primary winding to reduce the burden of the superconductor and to lower the fault current. Also, the suggested structure could increase the duration of the limited current by limiting the continuous current after the first half cycle from the fault with the fault current limiter.

In recent years, the iron-dominated high-temperature superconductor (HTS) quadrupole magnets are being developed for heavy ion accelerators. Field analyses for iron-dominated quadrupole magnets were based on the normal-conducting (NC) quadrupole magnet early in the development for accelerators. Some conclusions are still in use today. However, the magnetic field of iron-dominated HTS quadrupole magnets cannot fully follow these conclusions. This study established an HTS quadrupole magnet model and an NC quadrupole magnet model, respectively. The harmonic characteristics of two magnets were analyzed and compared. According to the comparison, the conventional iron-dominated quadrupole magnets can be designed for maximum field gradient; the HTS quadrupole magnet, however, should be considered with varying field gradient. Finally, the HTS quadrupole magnet was designed for the changing field gradient. The field quality of the design was improved comparing with the result of the previous study. The new design for the HTS quadrupole magnet has been suggested.

High charge state electron cyclotron resonance (ECR) ion source is important on the performance of heavy ion accelerators. In this paper, a low temperature superconductor (LTS) was used to make a hexapole coil for an 18-GHz ECR ion source. Several hexapole structures, including racetrack, graded racetrack, and saddle were implemented and analyzed for the hexapole-in-solenoid ECR ion source system. Under the appropriate radial confinement field, the smaller outer radius of hexapole can be better for the solenoid design. Saddle hexapole was selected by comparing the wire length, maximum outer radius of the hexapole, the Lorentz force at the end part of the hexapole and the maximum magnetic field in the coil. Based on saddle hexapole, a new design for hexapoles, the snake hexapole, was developed in this paper. By comparative analysis of the Lorentz force at the end part of the saddle and snake hexapoles, the snake hexapole is much better in the ECR ion source system. The suggested design for the ECR ion source with the snake hexapole is presented in this paper.

For the magnetic field analysis or design, it is important to know the behavior of the magnetic field in an interesting space. Magnetic field mapping becomes a useful tool for the study of magnetic field. In this paper, a numerical way for mapping the magnetic field within the cylindrical center volume of magnet is presented, based on the solution of the Laplace’s equation in the cylindrical coordinate system. The expression of the magnetic field can be obtained by the magnetic flux density, which measured in the mapped volume. According to the form of the expression, the measurement points are arranged with the parallel cylindrical line (PCL) method. As example, the magnetic flux density generated by an electron cyclotron resonance ion source (ECRIS) magnet and a quadrupole magnet were mapped using the PCL method, respectively. The mapping results show the PCL arrangement method is feasible and convenience to map the magnetic field within a cylindrical center volume generated by the general magnet.

This paper presents the developed quench analysis code and protection circuit design for a superconducting magnet system of 28GHz electron cyclotron resonance (ECR) ion source. The superconducting magnet is composed of a hexapole magnet and four solenoid magnets located outside of the hexapole one. All magnets are wound with NbTi composite wire and impregnated by epoxy. By using the developed characteristic analysis code, the normal zone resistance, decaying current and temperature rising can be estimated during quench. Also, the stored magnetic energy is successfully consumed from the series resistor of the designed protection circuit. The analytical results are compared with the experimental results to verify the developed quench analysis code and protection circuit.

A free-piston reciprocating expander is a device which operates without any mechanical linkage to a stationary part. Since the motion of the floating piston is only controlled by the pressure difference at two ends of the piston, this kind of expander may indispensably require a sophisticated active control system equipped with multiple valves and reservoirs. In this paper, we have suggested a novel design that can further reduce complexity of the previously developed cryogenic free-piston expander configuration. It is a simple replacement of both multiple valves and reservoirs by a combination of an orifice valve and a reservoir. The functional characteristic of the integrated orifice-reservoir configuration is similar to that of a phase controller applied in a pulse tube refrigerator so that we designate the one as a phase controller. Depending on the orifice valve size in the phase controller, the different PV work which affects the expander performance is generated. The numerical model of this unique free-piston reciprocating expander utilizing a phase controller is established to understand and analyze quantitatively the performance variation of the expander under different valve timing and orifice valve size. The room temperature experiments are carried out to examine the performance of this newly developed cryogenic expander.