147 research outputs found
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.Comment: Proceedings of EUCAS 09 conferenc
Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
We present a method for characterizing the propagation of the magnetic flux
in an artificially drilled bulk high-temperature superconductor (HTS) during a
pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical
sample, the magnetic flux density is measured simultaneously in 16 holes by
means of microcoils that are placed across the median plane, i.e. at an equal
distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes
during a pulse and measure the time taken by the external magnetic flux to
reach each hole. Our data show that the flux front moves faster in the median
plane than on the surface when penetrating the sample edge; it then proceeds
faster along the surface than in the bulk as it penetrates the sample further.
Once the pulse is over, the trapped flux density inside the central hole is
found to be about twice as large in the median plane than on the surface. This
ratio is confirmed by modelling
Recent Decisions
Comments on recent decisions by J. Patrick O\u27Malley, Peter H. Lousberg, David J. Eardley, James M. Corcoran, Jr., Joseph B. Joyce, James E. Murray, Edmund L. White, Berry L. Reece, Jr., A. J. Deutsch, and George N. Tompkins, Jr
Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux ?
Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing
and the heat exchange with the cooling liquid. However, drilling holes also
reduces the amount of magnetic flux that can be trapped in the sample. In this
paper, we use the Bean model to study the magnetization and the current line
distribution in drilled samples, as a function of the hole positions. A single
hole perturbs the critical current flow over an extended region that is bounded
by a discontinuity line, where the direction of the current density changes
abruptly. We demonstrate that the trapped magnetic flux is maximized if the
center of each hole is positioned on one of the discontinuity lines produced by
the neighbouring holes. For a cylindrical sample, we construct a polar
triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ~20% higher than in a squared lattice, for which the holes do
not lie on discontinuity lines. This result indicates that one can
simultaneously enhance the oxygen annealing, the heat transfer, and maximize
the trapped field
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
JPCam: A 1.2Gpixel camera for the J-PAS survey
JPCam is a 14-CCD mosaic camera, using the new e2v 9k-by-9k 10microm-pixel
16-channel detectors, to be deployed on a dedicated 2.55m wide-field telescope
at the OAJ (Observatorio Astrofisico de Javalambre) in Aragon, Spain. The
camera is designed to perform a Baryon Acoustic Oscillations (BAO) survey of
the northern sky. The J-PAS survey strategy will use 54 relatively narrow-band
(~13.8nm) filters equi-spaced between 370 and 920nm plus 3 broad-band filters
to achieve unprecedented photometric red-shift accuracies for faint galaxies
over ~8000 square degrees of sky. The cryostat, detector mosaic and read
electronics is being supplied by e2v under contract to J-PAS while the
mechanical structure, housing the shutter and filter assembly, is being
designed and constructed by a Brazilian consortium led by INPE (Instituto
Nacional de Pesquisas Espaciais). Four sets of 14 filters are placed in the
ambient environment, just above the dewar window but directly in line with the
detectors, leading to a mosaic having ~10mm gaps between each CCD. The massive
500mm aperture shutter is expected to be supplied by the Argelander-Institut
fur Astronomie, Bonn. We will present an overview of JPCam, from the filter
configuration through to the CCD mosaic camera. A brief outline of the main
J-PAS science projects will be included.Comment: 11 pages and 9 figure
Introducing the DizzyQuest: an app-based diary for vestibular disorders
BACKGROUND Most questionnaires currently used for assessing symptomatology of vestibular disorders are retrospective, inducing recall bias and lowering ecological validity. An app-based diary, administered multiple times in daily life, could increase the accuracy and ecological validity of symptom measurement. The objective of this study was to introduce a new experience sampling method (ESM) based vestibular diary app (DizzyQuest), evaluate response rates, and to provide examples of DizzyQuest outcome measures which can be used in future research. METHODS Sixty-three patients diagnosed with a vestibular disorder were included. The DizzyQuest consisted of four questionnaires. The morning- and evening-questionnaires were administered once each day, the within-day-questionnaire 10 times a day using a semi-random time schedule, and the attack questionnaire could be completed after the occurrence of a vertigo or dizziness attack. Data were collected for 4~weeks. Response rates and loss-to-follow-up were determined. Reported symptoms in the within-day-questionnaire were compared within and between patients and subgroups of patients with different vestibular disorders. RESULTS Fifty-one patients completed the study period. Average response rates were significantly higher than the desired response rate of \textgreater 50% (p \textless 0.001). The attack-questionnaire was used 159 times. A variety of neuro-otological symptoms and different disease profiles were demonstrated between patients and subgroups of patients with different vestibular disorders. CONCLUSION The DizzyQuest is able to capture vestibular symptoms within their psychosocial context in daily life, with little recall bias and high ecological validity. The DizzyQuest reached the desired response rates and showed different disease profiles between subgroups of patients with different vestibular disorders. This is the first time ESM was used to assess daily symptoms and quality of life in vestibular disorders, showing that it might be a useful tool in this population
Randomised controlled trial of cervical radiofrequency lesions as a treatment for cervicogenic headache [ISRCTN07444684]
BACKGROUND: Cervicogenic headache (CEH) is a unilateral headache localised in the neck or occipital region, projecting to the frontal and temporal regions. Since the pathogenesis of this syndrome appears to have an anatomical basis in the cervical region, several surgical procedures aimed at reducing the nociceptive input on the cervical level, have been tested. We developed a sequence of various cervical radiofrequency neurotomies (facet joint denervations eventually followed by upper dorsal root ganglion neurotomies) that proved successful in a prospective pilot trial with 15 CEH patients. To further evaluate this sequential treatment program we conducted a randomised controlled trial METHODS: 30 patients with cervicogenic headache according to the Sjaastad diagnostic criteria, were randomised. 15 patients received a sequence of radiofrequency treatments (cervical facet joint denervation, followed by cervical dorsal root ganglion lesions when necessary), and the other 15 patients underwent local injections with steroid and anaesthetic at the greater occipital nerve, followed by transcutaneous electrical nerve stimulation (TENS) when necessary. Visual analogue scores for pain, global perceived effects scores, quality of life scores were assessed at 8, 16, 24 and 48 weeks. Patients also kept a headache diary. RESULTS: There were no statistically significant differences between the two treatment groups at any time point in the trial. CONCLUSION: We did not find evidence that radiofrequency treatment of cervical facet joints and upper dorsal root ganglions is a better treatment than the infiltration of the greater occipital nerve, followed by TENS for patients fulfilling the clinical criteria of cervicogenic headache
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