6,365 research outputs found
3D Modeling of the Magnetization of Superconducting Rectangular-Based Bulks and Tape Stacks
In recent years, numerical models have become popular and powerful tools to
investigate the electromagnetic behavior of superconductors. One domain where
this advances are most necessary is the 3D modeling of the electromagnetic
behavior of superconductors. For this purpose, a benchmark problem consisting
of superconducting cube subjected to an AC magnetic field perpendicular to one
of its faces has been recently defined and successfully solved. In this work, a
situation more relevant for applications is investigated: a superconducting
parallelepiped bulk with the magnetic field parallel to two of its faces and
making an angle with the other one without and with a further constraint on the
possible directions of the current. The latter constraint can be used to model
the magnetization of a stack of high-temperature superconductor tapes, which
are electrically insulated in one direction. For the present study three
different numerical approaches are used: the Minimum Electro-Magnetic Entropy
Production (MEMEP) method, the -formulation of Maxwell's equations and the
Volume Integral Method (VIM) for 3D eddy currents computation. The results in
terms of current density profiles and energy dissipation are compared, and the
differences in the two situations of unconstrained and constrained current flow
are pointed out. In addition, various technical issues related to the 3D
modeling of superconductors are discussed and information about the
computational effort required by each model is provided. This works constitutes
a concrete result of the collaborative effort taking place within the HTS
numerical modeling community and will hopefully serve as a stepping stone for
future joint investigations
Finite Difference Synthetic Acoustic Logs
Synthetic seismograms of elastic wave propagation in a fluid-filled borehole were generated using both the finite difference technique and the discrete wavenumber summation technique. The latter is known to be accurate for both body and surface (guided) waves. The finite difference grid has absorbing boundaries on two sides and axes of symmetry on the remaining two sides. A grid size no less than 10 points per wavelength was used. The far absorbing boundary was located at a distance of five to 10 radii from the borehole. Two types of solid-liquid interfaces were investigated: 1) a velocity gradient using the heterogeneous formulation, and 2) a sharp boundary using a second order Taylor expansion of the displacements. The results from the finite difference modeling were compared with the synthetic seismograms generated by the
discrete wavenumber summation method. No comparison the heterogeneous formulation and the discrete wavenumber method has been made. The second order approximation to the solid-liquid interface produced seismograms that compared 'well with the discrete wavenumber seismograms. A detailed comparison between the seismograms generated by the two methods showed that the body waves (refracted P and S waves) are identical. while the guided waves showed a slight difference in both phase and amplitude. These
differences are believed to be due to the approximations introduced in the
fluid-solid interface, the absorbing boundary at the edge of the grid, and the
grid and time step sizes involved. Owing. to the fact that they are interface
waves, the guided waves, especially the higher modes, are much more sensitive
to the above mentioned approximations
β-decay half-lives and β-delayed neutron emission probabilities for several isotopes of Au, Hg, Tl, Pb, and Bi, beyond N = 126
Background: There have been measurements on roughly 230 nuclei that are β-delayed neutron emitters. They
range from 8
He up to 150La. Apart from 210Tl, with a branching ratio of only 0.007%, no other neutron emitter
has been measured beyond A = 150. Therefore, new data are needed, particularly in the region of heavy nuclei
around N = 126, in order to guide theoretical models and help understand the formation of the third r-process
peak at A ∼ 195.
Purpose: To measure both β-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl,
Pb, and Bi isotopes beyond N = 126.
Method: Ions of interest were produced by fragmentation of a 238U beam, selected and identified via the GSI-FRS
fragment separator. A stack of segmented silicon detectors (SIMBA) was used to measure ion implants and β
decays. An array of 30 3
He tubes embedded in a polyethylene matrix (BELEN) was used to detect neutrons with
high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time
correlations. The latter were analyzed with an analytical model and results for the half-lives and neutron-branching
ratios were derived by using the binned maximum-likelihood method.
Results: Twenty new β-decay half-lives are reported for 204−206Au, 208–211Hg,
211–216Tl,
215–218Pb, and 218–220Bi,
nine of them for the first time. Neutron emission probabilities are reported for 210,211Hg and 211–216Tl.
Conclusions: The new β-decay half-lives are in good agreement with previous measurements on nuclei in this
region. The measured neutron emission probabilities are comparable to or smaller than values predicted by global
models such as relativistic Hartree Bogoliubov plus the relativistic quasi-particle random phase approximation
(RHB + RQRPA).Spanish Ministerio de EconomÃa y Competitividad-FPA2011- 28770-C03-03, FPA2008-04972-C03-3, AIC-D2011-0705, FPA2011-24553, FPA2008-6419, FPA2010-17142, FPA2014-52823-C2-1-P, FPA2014- 52823-C2-2-P, and CPAN CSD-2007-00042 (Ingenio2010)Program Severo Ochoa-SEV-2014-0398German Helmholtz Association (Young Investigators)-VH-NG 627 (LISA-Lifetime Spectroscopy for Astrophysics)Nuclear Astrophysics Virtual Institute-VH-VI-417German Bundesministerium für Bildung und Forschung-06MT7178 / 05P12WOFNFSpanish Nuclear Security Council (CSN)-Catedra ArgosUK Science & Technology Facilities Council (STFC)-ST/F012012/
ac Losses in a Finite Z Stack Using an Anisotropic Homogeneous-Medium Approximation
A finite stack of thin superconducting tapes, all carrying a fixed current I,
can be approximated by an anisotropic superconducting bar with critical current
density Jc=Ic/2aD, where Ic is the critical current of each tape, 2a is the
tape width, and D is the tape-to-tape periodicity. The current density J must
obey the constraint \int J dx = I/D, where the tapes lie parallel to the x axis
and are stacked along the z axis. We suppose that Jc is independent of field
(Bean approximation) and look for a solution to the critical state for
arbitrary height 2b of the stack. For c<|x|<a we have J=Jc, and for |x|<c the
critical state requires that Bz=0. We show that this implies \partial
J/\partial x=0 in the central region. Setting c as a constant (independent of
z) results in field profiles remarkably close to the desired one (Bz=0 for
|x|<c) as long as the aspect ratio b/a is not too small. We evaluate various
criteria for choosing c, and we show that the calculated hysteretic losses
depend only weakly on how c is chosen. We argue that for small D/a the
anisotropic homogeneous-medium approximation gives a reasonably accurate
estimate of the ac losses in a finite Z stack. The results for a Z stack can be
used to calculate the transport losses in a pancake coil wound with
superconducting tape.Comment: 21 pages, 17 figures, accepted by Supercond. Sci. Techno
Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance
A liquid isooctane (CH) filled ionization linear array for
radiotherapy quality assurance has been designed, built and tested. The
detector consists of 128 pixels, each of them with an area of 1.7 mm
1.7 mm and a gap of 0.5 mm. The small pixel size makes the detector ideal for
high gradient beam profiles like those present in Intensity Modulated Radiation
Therapy (IMRT) and radiosurgery. As read-out electronics we use the X-Ray Data
Acquisition System (XDAS) with the Xchip developed by the CCLRC.
Studies concerning the collection efficiency dependence on the polarization
voltage and on the dose rate have been made in order to optimize the device
operation.
In the first tests we have studied dose rate and energy dependences, and
signal reproducibility. Dose rate dependence was found lower than 2.5 % up to 5
Gy min, and energy dependence lower than 2.1 % up to 20 cm depth in
solid water. Output factors and penumbras for several rectangular fields have
been measured with the linear array and were compared with the results obtained
with a 0.125 cm air ionization chamber and radiographic film,
respectively. Finally, we have acquired profiles for an IMRT field and for a
virtual wedge. These profiles have also been compared with radiographic film
measurements. All the comparisons show a good correspondence. Signal
reproducibility was within a 2% during the test period (around three months).
The device has proved its capability to verify on-line therapy beams with
good spatial resolution and signal to noise ratio.Comment: 16 pages, 12 figures Submitted to Phys. Med. Bio
Current profiles and AC losses of a superconducting strip with elliptic cross-section in perpendicular magnetic field
The case of a hard type II superconductor in the form of strip with elliptic
cross-section when placed in transverse magnetic field is studied. We approach
the problem in two steps, both based on the critical-state model. First we
calculate numerically the penetrated current profiles that ensure complete
shielding in the interior, without assuming an a priori form for the profiles.
In the second step we introduce an analytical approximation that asumes that
the current profiles are ellipses. Expressions linking the sample magnetization
to the applied field are derived covering the whole range of applied fields.
The theoretical predictions are tested by the comparison with experimental data
for the imaginary part of AC susceptibility.Comment: 12 pages; 3 figure
Personal data broker instead of blockchain for students’ data privacy assurance
Data logs about learning activities are being recorded at a growing pace due to the adoption and evolution of educational technologies (Edtech). Data analytics has entered the field of education under the name of learning analytics. Data analytics can provide insights that can be used to enhance learning activities for educational stakeholders, as well as helping online learning applications providers to enhance their services. However, despite the goodwill in the use of Edtech, some service providers use it as a means to collect private data about the students for their own interests and benefits. This is
showcased in recent cases seen in media of bad use of students’ personal information. This growth in cases is due to the recent tightening in data privacy regulations, especially in the EU. The students or their parents should be the owners of the information about them and their learning activities online. Thus they should have the right tools to control how their information is accessed and for what purposes. Currently, there is no technological solution to prevent leaks or the misuse of data about the students or their activity. It seems appropriate to try to solve it from an automation technology perspective. In this paper, we consider the use of Blockchain technologies as a possible basis for a solution to this problem. Our analysis indicates that the Blockchain is not a suitable solution. Finally, we propose a cloud-based solution with a central personal point of management that we have called Personal Data Broker.Peer ReviewedPostprint (author's final draft
Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys
X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)2. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formedThanks to the MCYT for the financial support given to this work (Project MAT2006-13179-C02-01-02)Peer reviewe
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