arXiv subjects

Linus Hägg

Publications and source records attributed to Linus Hägg.

4 recordsLinked to original sources

arXiv2026

NESSA: a compact 14 MeV D-T neutron source facility at Uppsala University

The NESSA (Neutron Source in Uppsala) facility hosts a compact 14 MeV deuterium-tritium sealed tube neutron generator at the Ångström Laboratory, Uppsala University. The generator, housed in a bunker inside the FREIA hall, reaches a maximum yield of $4.7\times10^{8}$ n/s. This paper describes the facility: the generator, the bunker and its shielding, the detector systems, and the Monte Carlo models used to characterize the neutron field. We also report the first commissioning measurements: yield calibration with $^{93}$Nb activation foils, fission chamber response at two positions, and simulated air and structural activation. Initial indium foil activations and single event effect (SEE) tests on silicon devices are also presented. The facility will be used for nuclear data measurements, neutron detector response studies, moderation and thermalization experiments, irradiation testing of electronics as well as for training and education.

physics.ins-det↗

arXiv2024

Investigations of an effective time-domain boundary condition for quiscent viscothermal acoustics

Accurate simulations of sound propagation in narrow geometries need to account for viscous and thermal losses. In this respect, effective boundary conditions that model viscothermal losses in frequency-domain acoustics have recently gained in popularity. Here, we investigate the time-domain analogue of one such boundary condition. We find that the thermal part of the boundary condition is passive in time domain as expected, while the viscous part is not. More precisely, we demonstrate that the viscous part is responsible for exponentially growing normal modes with unbounded temporal growth rates, which indicates ill-posedness of the considered model. A finite-difference-time-domain scheme is developed for simulations of lossy sound propagation in a duct. If viscous losses are neglected the obtained transmission characteristics are found to be in excellent agreement with frequency-domain simulations. In the general case, the simulations experience an instability much in line with the theoretical findings.

physics.comp-ph↗

arXiv2021

Well-posed variational formulations of Friedrichs-type systems

All finite element methods, as well as much of the Hilbert-space theory for partial differential equations, rely on variational formulations, that is, problems of the type: find $u\in V$ such that $a(v,u) = l(v)$ for each $v\in L$, where $V, L$ are Sobolev spaces. However, for systems of Friedrichs type, there is a sharp disparity between established well-posedness theories, which are not variational, and the very successful discontinuous Galerkin methods that have been developed for such systems, which are variational. In an attempt to override this dichotomy, we present, through three specific examples of increasing complexity, well-posed variational formulations of boundary and initial--boundary-value problems of Friedrichs type. The variational forms we introduce are generalizations of those used for discontinuous Galerkin methods, in the sense that inhomogeneous boundary and initial conditions are enforced weakly through integrals in the variational forms. In the variational forms we introduce, the solution space is defined as a subspace $V$ of the graph space associated with the differential operator in question, whereas the test function space $L$ is a tuple of $L^2$ spaces that separately enforce the equation, boundary conditions of characteristic type, and initial conditions.

math.AP↗

arXiv2020

On the well-posedness of Galbrun's equation

Galbrun's equation, which is a second order partial differential equation describing the evolution of a so-called Lagrangian displacement vector field, can be used to study acoustics in background flows as well as perturbations of astrophysical flows. Our starting point for deriving Galbrun's equation is linearized Euler's equations, which is a first order system of partial differential equations that describe the evolution of the so-called Eulerian flow perturbations. Given a solution to linearized Euler's equations, we introduce the Lagrangian displacement as the solution to a linear first order partial differential equation, driven by the Eulerian perturbation of the fluid velocity. Our Lagrangian displacement solves Galbrun's equation, provided it is regular enough and that the so-called "no resonance" assumption holds. In the case that the background flow is steady and tangential to the domain boundary, we prove existence, uniqueness, and continuous dependence on data of solutions to an initial--boundary value problem for linearized Euler's equations. For such background flows, we demonstrate that the Lagrangian displacement is well-defined, that the initial datum of the Lagrangian displacement can be chosen in order to fulfill the "no resonance" assumption, and derive a classical energy estimate for (sufficiently regular solutions to) Galbrun's equation. Due to the presence of zeroth order terms of indefinite signs in the equations, the energy estimate allows solutions that grow exponentially with time.

math.AP↗