3D modeling of high temperature superconducting staggered array undulator

A 3-D model of a superconducting staggered array undulator has been built, which could serve as a powerful tool to solve electromagnetic problems and to realize field optimization of such design. Given the limitation of 2-D simulation for irregular shapes and complex geometries, 3-D models are more desirable for a comprehensive investigation. An optimization method for the undulator peak field is proposed; up to 32% enhancement can be achieved by introducing major segment bulks. Some improvements of the undulator design are obtained by careful analyzing of the simulation results. © 2002-2011 IEEE.

Modification of the trapped field in bulk high-temperature superconductors as a result of the drilling of a pattern of artificial columnar holes

The trapped magnetic field is examined in bulk high-temperature superconductors that are artificially drilled along their c-axis. The influence of the hole pattern on the magnetization is studied and compared by means of numerical models and Hall probe mapping techniques. To this aim, we consider two bulk YBCO samples with a rectangular cross-section that are drilled each by six holes arranged either on a rectangular lattice (sample I) or on a centered rectangular lattice (sample II). For the numerical analysis, three different models are considered for calculating the trapped flux: (i), a two-dimensional (2D) Bean model neglecting demagnetizing effects and flux creep, (ii), a 2D finite-element model neglecting demagnetizing effects but incorporating magnetic relaxation in the form of an E-J power law, and, (iii), a 3D finite element analysis that takes into account both the finite height of the sample and flux creep effects. For the experimental analysis, the trapped magnetic flux density is measured above the sample surface by Hall probe mapping performed before and after the drilling process. The maximum trapped flux density in the drilled samples is found to be smaller than that in the plain samples. The smallest magnetization drop is found for sample II, with the centered rectangular lattice. This result is confirmed by the numerical models. In each sample, the relative drops that are calculated independently with the three different models are in good agreement. As observed experimentally, the magnetization drop calculated in the sample II is the smallest one and its relative value is comparable to the measured one. By contrast, the measured magnetization drop in sample (1) is much larger than that predicted by the simulations, most likely because of a change of the microstructure during the drilling process.

Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30mT to 130mT and the flux in the superconductor is probed by miniature coils inserted in the holes. In a given hole, three different penetration regimes can be observed: (i) the shielded regime, where no magnetic flux threads the hole; (ii) the gradual penetration regime, where the waveform of the magnetic field has a clipped sine shape whose fundamental component scales with the applied field; and (iii) the flux concentration regime, where the waveform of the magnetic field is nearly a sine wave, with an amplitude exceeding that of the applied field by up to a factor of two. The distribution of the penetration regimes in the holes is compared with that of the magnetic flux density at the top and bottom surfaces of the sample, and is interpreted with the help of optical polarized light micrographs of these surfaces. We show that the measurement of the magnetic field inside the holes can be used as a local characterization of the bulk magnetic properties of the sample.

Thermal material with low curie temperature in a thermally actuated superconducting flux pump system

A thermally actuated flux pump is an efficient method to magnetize the high-temperature superconductor (HTS) bulk without applying a strong magnetic field. A thermal material is employed as a magnetic switch, which decides the efficiency of the system. To measure the Curie temperatures of those samples without destroying them, the nondestructive Curie temperature (NDT) measurement was developed. The Curie temperature of gadolinium (Gd) was measured by the NDT method and compared to the results from superconducting quantum interference device (SQUID). Because the SQUID tests require the sample to be cut into small piece, a constant shape of the testing sample could not be guaranteed. The demagnetizing effect was considered to remove the shape effect. The intrinsic permeability was modified from the apparent susceptibility by considering demagnetization. A thermal material with low Curie temperature, Mg 0.15Cu0.15Zn0.7Ti0.04Fe 1.96O4, was synthesized and its performance was tested and compared with previous thermal materials. Comparisons of three thermal materials, including the Curie temperature and the permeability, will be detailed in the paper. © 2002-2011 IEEE.

Numerical study of the shielding properties of macroscopic hybrid ferromagnetic/superconductor hollow cylinders

We study the magnetic shielding properties of hybrid ferromagnetic/ superconductor (F/S) structures consisting of two coaxial cylinders, with one of each material. We use an axisymmetric finite-element model in which the electrical properties of the superconducting tube are modeled by a nonlinear E-J power law with a magnetic-field-dependent critical current density whereas the magnetic properties of the ferromagnetic material take saturation into account. We study and compare the penetration of a uniform axial magnetic field in two cases: 1) a ferromagnetic tube placed inside a larger superconducting tube (Ferro-In configuration) and 2) a ferromagnetic tube placed outside the superconducting one (Ferro-Out configuration). In both cases, we assess how the ferromagnetic tube improves the shielding properties of the sole superconducting tube. The influence of the geometrical parameters of the ferromagnetic tube is also studied: It is shown that, upon an optimal choice of the geometrical parameters, the range of magnetic fields that are efficiently shielded by the high-temperature superconductor tube alone can be increased by a factor of up to 7 (2) in a Ferro-Out (Ferro-In) configuration. The optimal configuration uses a 1020 carbon steel with a thickness of 2 mm and a height that is half that of the superconducting cylinder (80 mm). © 2009 IEEE.

Use of a high-temperature superconducting coil for magnetic energy storage

A high temperature superconducting magnetic energy storage device (SMES) has been realised using a 350 m-long BSCCO tape wound as a pancake coil. The coil is mounted on a cryocooler allowing temperatures down to 17.2 K to be achieved. The temperature dependence of coil electrical resistance R(T) shows a superconducting transition at T 102.5 K. Measurements of the V(I) characteristics were performed at several temperatures between 17.2 K and 101.5 K to obtain the temperature dependence of the critical current (using a 1 νV/cm criterion). Critical currents were found to exceed 100 A for T < 30 K. An electronic DC-DC converter was built in order to control the energy flow in and out of the superconducting coil. The converter consists of a MOS transistor bridge switching at a 80 kHz frequency and controlled with standard Pulse Width Modulation (PWM) techniques. The system was tested using a 30 V squared wave power supply as bridge input voltage. The coil current, the bridge input and output voltages were recorded simultaneously. Using a 10 A setpoint current in the superconducting coil, the whole system (coil + DC-DC converter) can provide a stable output voltage showing uninterruptible power supply (UPS) capabilities over 1 s. © 2006 IOP Publishing Ltd.

The mechanism of high temperature aqueous corrosion of steel

A new model is presented which describes the growth of the duplex layers of Fe3O4 on mild steel in high temperature, deoxygenated, neutral or alkaline aqueous solutions. It is shown that the layers grow by the ingress of water along oxide micropores to the metal-oxide interface and by the rate-limiting outward diffusion of Fe ions along oxide grain boundaries. The new model accounts for the observed temperature-dependence and pH-dependence of the corrosion, the morphology of inner and outer layer crystallites, the segregation of alloying elements, and the location of hydrogen evolution. The model can also be generalized to other steels and alloys. © 1989.

Changes in the Mg profile and in dislocations induced by high temperature annealing of blue LEDs

The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: (i) a significant increase in the photoluminescence efficiency of the devices; (ii) a decrease in the parasitic emission bands located between 380 nm and 400 nm; (iii) an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples. © 2013 SPIE.

Effect of high temperature post-annealing on sidewalls of GaAs NWs grown by MOCVD

The sidewall facets of GaAs nanowires (NWs) were studied. It has been found that the sidewalls of GaAs NWs grown at 450 °C are {112} facets. However, the sidewalls of GaAs NWs start to become {110} during the postannealing at 650 °C for 30 min. © 2010 IEEE.

A high temperature and low power SOI CMOS MEMS based thermal conductivity gas sensor

The design, 3D FEM modelling and measurement results of a novel high temperature, low power SOI CMOS MEMS thermal conductivity gas sensor are presented here. The sensor consists of a circular membrane with an embedded tungsten micro-heater. The high sensing capability is based on the temperature sensitivity of the resistive heating element. The sensor was fabricated at a commercial foundry using a 1 μm process and measures only 1×1 mm 2. The circular membrane has a 600 μm diameter while the heating element has a 320 μm diameter. Measurement results show that for a constant power consumption of 75 mW the heater temperature was 562.4°C in air, 565.9°C in N2, 592.5°C for 1 % H2 in Ar and 599.5°C in Ar. © 2013 IEEE.

First demonstration of a full C-band tunable WDM-PON system with novel high-temperature DS-DBR lasers

We demonstrate automatic operation of a cooler-less tunable-laser based WDM-PON system. Using a pilot-tone based overhead channel and centralized wavelength locking scheme, 1 Gb/s and 10 Gb/s data transmission is demonstrated in a multi-user set-up. © 2013 Optical Society of America.

The influence of a grain boundary on the thermal transport properties of bulk, melt-processed Y-Ba-Cu-O

We report the dependence of thermal conductivity, thermoelectric power and electrical resistivity on temperature for a bulk, large grain melt-processed Y-Ba-Cu-O (YBCO) high temperature superconductor (HTS) containing two grains separated by a well-defined grain boundary. Transport measurements at temperatures between 10 and 300 K were carried out both within one single grain (intra-granular properties) and across the grain boundary (inter-granular properties). The influence of an applied external magnetic field of up to 8 T on the measured sample properties was also investigated. The presence of the grain boundary is found to affect strongly the electrical resistivity of the melt-processed bulk sample, but has almost no effect on its thermoelectric power and thermal conductivity, within experimental error. The results of this study provide direct evidence that the heat flow in multi-granular melt-processed YBCO bulk samples should be virtually unaffected by the presence of grain boundaries in the material. © 2013 IOP Publishing Ltd.