J. I. Vestgården

University of Oslo

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T. H. Johansen

University of Oslo

Y. M. Galperin

Ioffe Institute

A. J. Qviller

Institute for Energy Technology

V. V. Yurchenko

University of Oslo

P. Mikheenko

University of Oslo

D. V. Shantsev

University of Oslo

W.A. Ortiz

Universidade Federal de São Carlos

I. J. Maasilta

University of Jyväskylä

Peter B. Mozhaev

Moscow Institute of Physics and Technology

F. Colauto

Universidade Federal de São Carlos

Cooperative Institutions

Norwegian Defence Research Establishment

University of Oslo

Physico-Technical Institute

University of Wollongong

Russian Academy of Sciences

KU Leuven

Norwegian Academy of Science and Letters

Centre for Advanced Study

University of Jyväskylä

Universidade Federal de São Carlos

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Mechanism for flux guidance by micrometric antidot arrays in superconducting films

Physical Review B (2012)

J. I. Vestgården V. V. Yurchenko R. Wördenweber T. H. Johansen

A study of magnetic flux penetration in a superconducting film patterned with arrays of micron-sized antidots (microholes) is reported. Magneto-optical imaging (MOI) of a ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ film shaped as a long strip with perpendicular antidot arrays revealed both strong guidance of flux and, at the same time, large perturbations of the overall flux penetration and flow of current. These results are compared with a numerical flux creep simulation of a thin superconductor with the same antidot pattern. To perform calculations on such a complex geometry, an efficient numerical scheme for handling the boundary conditions of the antidots and the nonlocal electrodynamics was developed. The simulations reproduce essentially all features of the MOI results. In addition, the numerical results give insight into all other key quantities (e.g., the electrical field), which become extremely large in the narrow channels connecting the antidots.

Penetration (warfare)

10.1103/physrevb.85.014516

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Citations (22)

Quasi-1D intermittent flux dynamics in superconducting films

APS (2012)

T. H. Johansen A. J. Qviller V. V. Yurchenko J. I. Vestgården Peter B. Mozhaev

Dynamics

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Bayesian occupancy grid for active sonar detection and localization of moving targets

Global Oceans 2020: Singapore – U.S. Gulf Coast (2020)

Karl Thomas Hjelmervik Kristoffer Engedal Andreassen Elin Margrethe Bøhler J. I. Vestgården Henrik Berg

Active sonar performance is highly dependent on the surrounding environment. Conventional detection algorithms apply thresholds to acoustic data passed through a normalizer in order to detect targets. These methods fail to fully exploit available knowledge of the environment, which leads to higher false alarm rates than necessary, particularly in littoral environments. Furthermore, they do not exploit negative information, e.g. the significance of the repeated lack of threshold crossings in a given area. Sonar performance models may estimate both the probability of detection and false alarm in a known environment. Proper exploitation of this data allows for reduced false alarm rates in cluttering and reverberating environments. Furthermore, areas lacking threshold crossings and with high probability of detection, may be classified as target free. Bayesian occupancy grids is a probabilistic approach that takes into account both sensor information and prior information of surrounding walls or topography for localization of robots. Here we apply the method on synthetic sonar data combined with a sonar performance model. The synthetic sonar data contains moving and stationary targets in a littoral environment. The method's performance is compared to conventional algorithms, and its robustness to errors in the sound speed profile used in the modelling is assessed.

Occupancy grid mapping

Robustness

False alarm

Synthetic aperture sonar

Occupancy

10.1109/ieeeconf38699.2020.9389044

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Citations (3)

Flux guidance by antidot arrays in superconducting films

arXiv (Cornell University) (2011)

J. I. Vestgården V. V. Yurchenko R. Wördenweber T. H. Johansen

J. I. Vestgarden, V. V. Yurchenko, R. Wordenweber, and T. H. Johansen 3, 4 Department of Physics, University of Oslo, P. O. Box 1048 Blindern, 0316 Oslo, Norway Peter Grunberg Institute and JARA-Fundamentals of Future Information Technology, Forschungszentrum Julich, 52425 Julich, Germany Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia Centre for Advanced Study at The Norwegian Academy of Science and Letters, NO-0271, Oslo, Norway

Norwegian

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Dynamics and morphology of dendritic flux avalanches in superconducting films

Physical Review B (2011)

J. I. Vestgården D. V. Shantsev Y. M. Galperin T. H. Johansen

We develop a fast numerical procedure for analysis of nonlinear and nonlocal electrodynamics of type-II superconducting films in transverse magnetic fields taking into account realistic boundary conditions. Using this procedure we explore stability of such films with respect to dendritic flux avalanches. The calculated flux patterns are very similar to experimental magneto-optical images of MgB2 and other superconductors, where the avalanche size and morphology change dramatically with temperature. We also find a threshold magnetic field, which agrees with both experiments and linear stability analysis. The simulations predict the temperature rise during an avalanche, where for a sub-microsecond time T ~ 1.5 Tc, and a precursor stage with large thermal fluctuations.

Microsecond

10.1103/physrevb.84.054537

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Citations (71)

Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots

Physica C Superconductivity (2006)

V. V. Yurchenko R. Wördenweber Yu. M. Galperin D. V. Shantsev J. I. Vestgården

10.1016/j.physc.2005.12.075

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Citations (22)

Quasi-One-Dimensional Intermittent Flux Behavior in Superconducting Films

Physical Review X (2012)

A. J. Qviller V. V. Yurchenko Y. M. Galperin J. I. Vestgården Peter B. Mozhaev

Vortices in type-II superconductors are tiny tornados of circulating electric current of a radius of tens of nanometers surrounding a normal-state core. They move, repel each other, and may get pinned by defects. One-dimensional ``traffic jams'' and intermittent jam-relieving ``avalanches'' of such vortices in a thin superconducting film with a multiterraced surface are revealed vividly by spatially resolved, real-time magneto-optical images, pointing to interesting new physics.

Nanometre

Type-II superconductor

Electric current

10.1103/physrevx.2.011007

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Citations (8)

Sonar scattering from the sea bottom near the Norwegian coast

OCEANS 2017 – Anchorage (2017)

J. I. Vestgården Karl Thomas Hjelmervik Dan Henrik Sekse Stender Henrik Berg

High false alarm rates on active sonar systems in shallow waters is a well known problem which may be a limiting factor for the sonar performance. One way to reduce the false alarm rate is through supervised learning and machine learning algorithms applying relevant classification features. Here we propose backscatter intensity as a classification feature. It is calculated from a topographical map under the assumption of omnidirectional scattering for sound, an unrefracted path and iso-velocity sound speed profile. Analysis of historic data for a towed array sonar near the Norwegian coast showed that clutter echoes to a large extent was located at cliffs, escarpments and ridges where the calculated backscatter intensity was high. With a simple thresholding of the estimated backscatter intensity one can correctly classify 60% of terrain echoes at the expense of 10% potential target echo misclassification rate. The classification rate rises to 75% for the echoes of highest signal-to-noise ratio. The proposed classification feature is computationally inexpensive and does not depend on free parameters.

Backscatter (email)

Feature (linguistics)

Sonar signal processing

False alarm

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Dendritic flux avalanches in superconducting films of different thickness

arXiv (Cornell University) (2013)

J. I. Vestgården Y. M. Galperin T. H. Johansen

At low temperatures the critical state in superconducting films can be unstable with respect to thermomagnetic dendritic avalanches. By numerical simulations of disk-shaped superconductors, we consider how the dynamics and morphology of the avalanches depend on the disk thickness. We find that as the disks get thicker, the jumps in magnetic moment caused by the avalanches get larger and the threshold magnetic field for the appearance of the first avalanche increases. At the same time, the branches are straighter and the number of branches decreases. Comparison with theory suggests that strong spatial disorder to some extent cancels the stabilizing effects of the substrate kept at constant temperature.

Thermomagnetic convection

Constant (computer programming)

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Nonlocal electrodynamics of normal and superconducting films

New Journal of Physics (2013)

J. I. Vestgården P. Mikheenko Y. M. Galperin T. H. Johansen

Electrically conducting films in a time-varying transverse applied magnetic field are considered. Their behavior is strongly influenced by the self-field of the induced currents, making the electrodynamics nonlocal, and consequently difficult to analyze both numerically and analytically. We present a formalism which allows many phenomena related to superconducting and Ohmic films to be modelled and analyzed. The formalism is based on the Maxwell equations, and a material current-voltage characteristics, linear for normal metals, and nonlinear for superconductors, plus a careful account of the boundary conditions. For Ohmic films, we consider the response to a delta function source-field turned on instantly. As one of few problems in nonlocal electrodynamics, this has an analytical solution, which we obtain, in both Fourier and real space. Next, the dynamical behaviour of a square superconductor film during ramping up of the field, and subsequently returning to zero, is treated numerically. Then, this remanent state is used as initial condition for triggering thermomagnetic avalanches. The avalanches tend to invade the central part where the density of trapped flux is largest, forming dendritic patterns in excellent agreement with magneto-optical images. Detailed profiles of current and flux density are presented and discussed. Finally, the formalism is extended to multiply connected samples, and numerical results for a patterned superconducting film, a ring with a square lattice of antidots, are presented and discussed.

Formalism (music)

Ohmic contact

10.1088/1367-2630/15/9/093001

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Citations (50)