arXiv subjects

Valtteri Lahtinen

Publications and source records attributed to Valtteri Lahtinen.

26 records · Page 2Linked to original sources

arXiv2021

Thin-Shell Approach for Modeling Superconducting Tapes in the $H$-$ϕ$ Finite-Element Formulation

This paper presents a novel finite-element approach for the electromagnetic modeling of superconducting coated conductors. We combine a thin-shell (TS) method to the $H$-$ϕ$ formulation to avoid the meshing difficulties related to the high aspect ratio of these conductors and reduce the computational burden in simulations. The interface boundary conditions in the TS method are defined using an auxiliary 1-D finite-element (FE) discretization of $N$ elements along the thinnest dimension of the conductor. This procedure permits the approximation of the superconductor's nonlinearities inside the TS in a time-transient analysis. Four application examples of increasing complexity are discussed: (i) single coated conductor, (ii) two closely packed conductors carrying anti-parallel currents, (iii) a stack of twenty superconducting tapes and a (iv) full representation of a HTS tape comprising a stack of thin films. In all these examples, the profiles of both the tangential and normal components of the magnetic field show good agreement with a reference solution obtained with standard $2$-D $H$-$ϕ$ formulation. Results are also compared with the widely used $T$-$A$ formulation. This formulation is shown to be dual to the TS model with a single FE ($N=1$) in the auxiliary 1-D systems. The increase of $N$ in the TS model is shown to be advantageous at small inter-tape separation and low transport current since it allows the tangential components of the magnetic field to penetrate the thin region. The reduction in computational cost without compromising accuracy makes the proposed model promising for the simulation of large-scale superconducting applications.

physics.comp-ph↗

arXiv2021

Photon-number-dependent effective Lamb shift

The Lamb shift, an energy shift arising from the presence of the electromagnetic vacuum, has been observed in various quantum systems and established as the part of the energy shift independent of the environmental photon number. However, typical studies are based on simplistic bosonic models which may be challenged in practical quantum devices. We demonstrate a hybrid bosonic-fermionic environment for a linear resonator mode and observe that the photon number in the environment can dramatically increase both the dissipation and the effective Lamb shift of the mode. Our observations are quantitatively described by a first-principles model which we develop here also to guide device design for future quantum-technological applications. The device demonstrated here can be utilized as a fully rf-operated quantum-circuit refrigerator to quickly reset superconducting qubits.

quant-ph↗

arXiv2021

A Robust Method for Image Stitching

We propose a novel method for large-scale image stitching that is robust against repetitive patterns and featureless regions in the imagery. In such cases, state-of-the-art image stitching methods easily produce image alignment artifacts, since they may produce false pairwise image registrations that are in conflict within the global connectivity graph. Our method augments the current methods by collecting all the plausible pairwise image registration candidates, among which globally consistent candidates are chosen. This enables the stitching process to determine the correct pairwise registrations by utilizing all the available information from the whole imagery, such as unambiguous registrations outside the repeating pattern and featureless regions. We formalize the method as a weighted multigraph whose nodes represent the individual image transformations from the composite image, and whose sets of multiple edges between two nodes represent all the plausible transformations between the pixel coordinates of the two images. The edge weights represent the plausibility of the transformations. The image transformations and the edge weights are solved from a non-linear minimization problem with linear constraints, for which a projection method is used. As an example, we apply the method in a large-scale scanning application where the transformations are primarily translations with only slight rotation and scaling component. Despite these simplifications, the state-of-the-art methods do not produce adequate results in such applications, since the image overlap is small, which can be featureless or repetitive, and misalignment artifacts and their concealment are unacceptable.

cs.CV↗

arXiv2020

Effects of device geometry and material properties on dielectric losses in superconducting coplanar-waveguide resonators

Superconducting coplanar-waveguide (CPW) resonators are one of the key devices in circuit quantum electrodynamics (cQED). Their performance can be limited by dielectric losses in the substrate and in the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. In a practical scenario, uncertainties in the the dielectric constants and loss tangents of these regions introduce uncertainties in the theoretically predicted participation ratios. We present a method for combining loss simulations with measurements of two-level-system-limited quality factors and resonance frequencies of CPW resonators. Namely, we solve an inverse problem to find model parameters producing the measured values. High quality factors are obtainable by properly designing the cross-sectional geometries of the CPW structures, but more accurate modeling and design methods for low-loss CPW resonators are called for major future improvements. Our nonlinear optimization methodology for solving the aforementioned inverse problem is a step in this direction.

physics.app-ph↗

arXiv2019

Semantics of HTS AC loss modelling: Theories, models and experiments

Computer-assisted modelling is an essential approach to design new devices. It speeds up the process from the initial idea to an actual device and saves resources by reducing the number of built prototypes. This is also a significant practical motivator behind scientific research in contemporary high-temperature superconductor (HTS) AC loss modelling. However, in the scientific literature in this field, consistent practices about modelling terminology have not been established. Then, it is up to the reader to decide, what is the true intent and meaning of the authors. Consequently, the interpretation of such literature might be very much reader-dependent. Moreover, an inseparable part of the whole modelling process is the development of modelling approaches and numerical methods and comparing the predictions obtained via modelling to experimentally achieved results: It is commonplace to discuss the accuracy of modelling results or the validation of a model. In this paper, we discuss the terminology related to theories, models and experiments in the context of HTS AC loss modelling. We discuss the recursive nature of theories and models in this context, discuss the compatibility of discrete formulations of physics utilized in our field with the corresponding continuum descriptions, as well as with intuition, and interpret the perceived meaning of validation of a self-consistent model, shedding light on the relationships between theories, models and measurements. We present our view on understanding these relations in the familiar context of AC losses in HTS. As a result, we end this paper with four conjectures describing our views.

physics.gen-ph↗

arXiv2019

Cuts for 3-D magnetic scalar potentials: visualizing unintuitive surfaces arising from trivial knots

A wealth of literature exists on computing and visualizing cuts for the magnetic scalar potential of a current carrying conductor via Finite Element Methods (FEM) and harmonic maps to the circle. By a cut we refer to an orientable surface bounded by a given current carrying path (such that the flux through it may be computed) that restricts contour integrals on a curl-zero vector field to those that do not link the current-carrying path, analogous to branch cuts of complex analysis. This work is concerned with a study of a peculiar contour that illustrates topologically unintuitive aspects of cuts obtained from a trivial loop and raises questions about the notion of an optimal cut. Specifically, an unknotted curve that bounds only high genus surfaces in its convex hull is analyzed. The current work considers the geometric realization as a current-carrying wire in order to construct a magnetic scalar potential. Moreover, we consider the problem of choosing an energy functional on the space of maps, suggesting an algorithm for computing cuts via minimizing a conformally invariant functional utilizing Newton iteration.

physics.comp-ph↗

arXiv2018

Open Material Property Library With Native Simulation Tool Integrations -- MASTO

Reliable material property data is crucial for trustworthy simulations throughout different areas of engineering. Special care must be taken when materials at extreme conditions are under study. Superconductors and devices assembled from superconductors and other materials, like superconducting magnets, are often operated at such extreme conditions: at low temperatures under high magnetic fields and stresses. Typically, some library or database is used for getting the data. We have started to develop a database for storing all kind of material property data online called Open Material Property Library With Native Simulation Tool Integrations -- MASTO. The data that can be imported includes, but is not limited to, anisotropic critical current surfaces for high temperature superconducting materials, electrical resistivities as a function of temperature, RRR and magnetic field, general fits for describing material behaviour etc. Data can also depend on other data and it can be versioned to guarantee permanent access. The guiding idea in MASTO is to build easy-to-use integration for various programming languages, modelling frameworks and simulation software. Currently, a full-fledged integration is built for MATLAB to allow users to fetch and use data with one-liners. In this paper we briefly review some of the material property databases commonly used in superconductor modelling, present a case study showing how selection of the material property data can influence the simulation results, and introduce the principal ideas behind MASTO. This work serves as the reference document for citing MASTO when it is used in simulations.

cond-mat.mtrl-sci↗

arXiv2016

Towards a unified framework for decomposability of processes

The concept of process is ubiquitous in science, engineering and everyday life. Category theory, and monoidal categories in particular, provide an abstract framework for modelling processes of many kinds. In this paper, we concentrate on sequential and parallel decomposability of processes in the framework of monoidal categories: We will give a precise definition, what it means for processes to be decomposable. Moreover, through examples, we argue that viewing parallel processes as coupled in this framework can be seen as a category mistake or a misinterpretation. We highlight the suitability of category theory for a structuralistic interpretation of mathematical modelling and argue that for appliers of mathematics, such as engineers, there is a pragmatic advantage from this.

math.CT↗