Trapping a magnetic field of 17.89 T in stacked coated conductors by suppression of flux jumps

We have successfully trapped a field of 17.89 T at 6.5 K at the center of a compact coated-conductor (CC) stacks (13×12×11.7 mm3) within 75 min by suppressing flux jumps. The CC stack consists of 200 sheets of EuBa2Cu3O7 CCs with BaHfO3 nanorods to increase the critical current density at high fields and low temperatures. To enhance thermomagnetic stability, the central 50 CCs are coated with 1 µm thick Pb with large specific heat at low temperatures. Numerical calculations based on the actual J c-H characteristics reproduces the trapped field quantitatively. New directions for achieving even higher trapped field at higher temperatures and making use of the trapped field are discussed.

Parameter Space Mapping for Blood Oxygenation Measurement with Low Field NMR

<p><b>Blood oxygenation is a critical physiological parameter for patient health. The clinical importance of this parameter means that measurement of blood oxygenation is a routine part of care. Magnetic resonance provides a way to measure blood oxygenation through the paramagnetic effect of deoxy-haemoglobin, which decreases the T2 relaxation time of blood. This effect has been well characterised at high fields (>1:5 T) for use in Magnetic Resonance Imaging, and it is a contributing factor to the Blood Oxygenation Level Dependent contrast used in functional MRI. However there are relatively few studies of this effect at low magnetic fields, and these have only looked at extreme levels of oxygenation/deoxygenation. To study this effect for potential application in a low-field device, we measured this effect to determine how factors such as oxygenation, field strength and CPMG echo time affect the T2 of blood.</b></p> <p>A continuous flow circuit, similar to a cardiopulmonary bypass circuit, was used to control parameters such as oxygen saturation and temperature, before the blood sample flowed into a variable field magnet (set at fields between 5-40 MHz), where a series of CPMG experiments with echo times ranging from 1 ms to 20 ms were performed to measure the T2. Additionally, the oxygen saturation was continually monitored by an optical sensor, for comparison with the T2 changes. This allowed us to test the sensitivity of this effect at low fields.</p> <p>These results show that at low fields, the T2 relaxation change still follows the trends shown in the literature, with a dependence on B0 squared, and on the fraction of deoxyhaemoglobin squared. Additionally, these results were also compared with two theoretical models for the dependence on echo time, which have previously been tested at high fields: the Luz-Meiboom equation, and the Jensen and Chandra model. Both models gave good agreement with the data measured at low fields. These experiments show that the T2 changes in blood due to oxygenation are still visible at low field, and that this technique should be feasible in a low field device.</p>

Parameter Space Mapping for Blood Oxygenation Measurement with Low Field NMR

<p><b>Blood oxygenation is a critical physiological parameter for patient health. The clinical importance of this parameter means that measurement of blood oxygenation is a routine part of care. Magnetic resonance provides a way to measure blood oxygenation through the paramagnetic effect of deoxy-haemoglobin, which decreases the T2 relaxation time of blood. This effect has been well characterised at high fields (>1:5 T) for use in Magnetic Resonance Imaging, and it is a contributing factor to the Blood Oxygenation Level Dependent contrast used in functional MRI. However there are relatively few studies of this effect at low magnetic fields, and these have only looked at extreme levels of oxygenation/deoxygenation. To study this effect for potential application in a low-field device, we measured this effect to determine how factors such as oxygenation, field strength and CPMG echo time affect the T2 of blood.</b></p> <p>A continuous flow circuit, similar to a cardiopulmonary bypass circuit, was used to control parameters such as oxygen saturation and temperature, before the blood sample flowed into a variable field magnet (set at fields between 5-40 MHz), where a series of CPMG experiments with echo times ranging from 1 ms to 20 ms were performed to measure the T2. Additionally, the oxygen saturation was continually monitored by an optical sensor, for comparison with the T2 changes. This allowed us to test the sensitivity of this effect at low fields.</p> <p>These results show that at low fields, the T2 relaxation change still follows the trends shown in the literature, with a dependence on B0 squared, and on the fraction of deoxyhaemoglobin squared. Additionally, these results were also compared with two theoretical models for the dependence on echo time, which have previously been tested at high fields: the Luz-Meiboom equation, and the Jensen and Chandra model. Both models gave good agreement with the data measured at low fields. These experiments show that the T2 changes in blood due to oxygenation are still visible at low field, and that this technique should be feasible in a low field device.</p>

Electro-optical measurement of intense electric field on a high energy pulsed power accelerator

We describe a direct electro-optical approach to measuring a strong 118 MV/m narrow pulse width (~ 33 ns) electric field in the magnetically insulated transmission line (MITL) of a pulsed power accelerator. To date, this is the highest direct external electric field measured electro-optically in a pulsed power accelerator, and it is between two to three orders of magnitude higher than values reported in comparable high energy scientific experiments. The MITL electric field is one of the most important operating parameters in an accelerator and is critical to understanding the properties of the radiation output. However, accurately measuring these high fields using conventional pulsed power diagnostics is difficult due to the strength of interfering particles and fields. Our approach uses a free-space laser beam with a dielectric crystal sensor that is highly immune to electromagnetic interference and does not require an external calibration. Here we focus on device theory, operating parameters, laboratory and pulsed power accelerator experiments as well as challenges that were overcome in the measurement environment.

A precursor mechanism triggering the second magnetization peak phenomenon in superconducting materials

The correlation in type-II superconductors between the creep rate S and the Second Magnetization Peak (SMP) phenomenon which produces an increase in Jc, as a function of the field (H), has been investigated at different temperatures by starting from the minimum in S(H) and the onset of the SMP phenomenon detected on a FeSe0.5Te0.5 sample. Then the analysis has been extended by considering the entire S(H) curves and comparing our results with those of many other superconducting materials reported in literature. In this way, we find evidence that the flux dynamic mechanisms behind the appearance of the SMP phenomenon in Jc(H) are activated at fields well below those where the critical current starts effectively to increase. Moreover, the found universal relation between the minimum in the S(H) and the SMP phenomenon in Jc(H) shows that both can be attributed to a sequential crossover between a less effective pinning (losing its effectiveness at low fields) to a more effective pinning (still acting at high fields), regardless of the type-II superconductor taken into consideration.

Electro-Optical Measurement of Intense Electric Field on a High Energy Pulsed Power Accelerator

We describe a direct electro-optical approach to measuring a strong 118 MV/m narrow pulse width (~33 ns) electric field in the magnetically insulated transmission line (MITL) of a pulsed power accelerator. To date, this is the highest direct external electric field measured electro-optically in a pulsed power accelerator, and it is between two to three orders of magnitude higher than values reported in comparable high energy scientific experiments. The MITL electric field is one of the most important operating parameters in an accelerator and is critical to understanding the properties of the radiation output. However, accurately measuring these high fields using conventional pulsed power diagnostics is difficult due to the strength of interfering particles and fields. Our approach uses a free-space laser beam with a dielectric crystal sensor that is highly immune to electromagnetic interference and does not require an external calibration. Here we focus on device theory, operating parameters, laboratory and pulsed power accelerator experiments as well as challenges that were overcome in the measurement environment.

Optimizing the relaxivity at high fields: systematic variation of the rotational dynamics in polynuclear Gd-complexes based on the AAZTA ligand

A homogeneous series of polynuclear structures containing from 2 to 6 GdAAZTA complexes (AAZTA=6-amino-6-methylperhydro-1, 4-diazepine tetraacetic acid) were synthesized covering a broad range of molecular weights, ca. 1200-6000 Da. A...

Goethite Nanorods: Synthesis and Investigation of the Size Effect on Their Orientation within a Magnetic Field by SAXS

Goethite is a naturally anisotropic, antiferromagnetic iron oxide. Following its atomic structure, crystals grow into a fine needle shape that has interesting properties in a magnetic field. The needles align parallel to weak magnetic fields and perpendicular when subjected to high fields. We synthesized goethite nanorods with lengths between 200 nm and 650 nm in a two-step process. In a first step we synthesized precursor particles made of akaganeite (β-FeOOH) rods from iron(III)chloride. The precursors were then treated in a hydrothermal reactor under alkaline conditions with NaOH and polyvinylpyrrolidone (PVP) to form goethite needles. The aspect ratio was tunable between 8 and 15, based on the conditions during hydrothermal treatment. The orientation of these particles in a magnetic field was investigated by small angle X-ray scattering (SAXS). We observed that the field strength required to trigger a reorientation is dependent on the length and aspect ratio of the particles and could be shifted from 85 mT for the small particles to about 147 mT for the large particles. These particles could provide highly interesting magnetic properties to nanocomposites, that could then be used for sensing applications or membranes.

Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V−1 s−1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.