| 1. |
- Loberg, John, 1980-
(författare)
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Novel Diagnostics and Computational Methods of Neutron Fluxes in Boiling Water Reactors
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The focus in this thesis is to improve knowledge of the BWR related uncertainties void, channel bow, and control rods. The presence of void determines the moderation of neutrons in BWRs. A high void fraction is less efficient in moderating neutrons than a low one. As a consequence, the ratio of thermal to fast neutrons is dependent on the surrounding void fraction. In this thesis, calculations with 2D/3D codes corroborate this dependence, the void correlation, to be linear and very robust to changes in different reactor parameters. The void fraction could be predicted from the ratio of simultaneously measured reaction rates from thermal and fast neutron detectors over the whole core with an uncertainty of ±1.5%. The only parameter found disturbing the void correlation significantly is channel bow. However, since channel bow is the only phenomenon found biasing the void correlation, it is found that the void prediction methodology can be used to indicate channel bow with a sensitivity of 4% per mm bow. Consequently, large channel bows could easily be detected. Increased knowledge of void fractions and channel bow could increase both safety and economy of nuclear power production. This thesis also investigates how 2D/3D codes used in production perform in calculating detailed impact of control rods on pin powers and their ability to perform control rod depletion calculations in the reflector region. It is found that the axial resolution used in 3D nodal codes has very large impact on pin power gradients, i.e., using a standard nodal size of ~15 cm can cause underestimations of 50% in pin power gradients, which could lead to fuel damages. In addition, two methods for determining the neutron flux in the control rod when it is withdrawn from the core are presented. Both methods can be used in a 3D nodal code to reproduce the neutron flux in the reflector region with an uncertainty of ±3%.
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| 2. |
- Shukla, Padma Kant
(författare)
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Excitation of zonal flows by kinetic Alfven waves
- 2005
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Ingår i: Physics of Plasmas. - Melville : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear couplings between dispersive kinetic Alfven waves (DKAWs) and electrostatic,convective cells/zonal flows are reexamined. A set of equations that exhibit nonlinear couplings between the scalar and parallel vector potentials of the DKAWs and the scalar potential of zonal flows that are reinforced by the Reynolds stresses of the DKAWs in a magnetized plasma is presented. The equations are then Fourier-analyzed to obtain the nonlinear dispersion relation. The latter exhibits modulational instabilities, which could be responsible for enhanced zonal flows in a uniform magnetized plasma. Zonal flows can regulate the transport of plasma particles in laboratory magnetoplasmas as well as in the Earth's magnetosphere and in the solar corona.
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| 3. |
- Flammini, Francesco, Senior Lecturer, 1978-
(författare)
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Automatic train protection systems
- 2013
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Ingår i: Industrial Engineering & Management. - : OMICS Publishing Group. - 2169-0316. ; 2:5, s. 1-3
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Tidskriftsartikel (refereegranskat)abstract
- Vehicles used in railway applications are requested to be more and more performable, reliable and safe. The use of computer based railway control systems is nowadays widespread, as it has been proven to be the most effective, if not the only practicable way to pursue such hard requirements.
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| 4. |
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| 5. |
- Weurlander, Maria, 1969-
(författare)
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Lärande och studentcentrerad undervisning
- 2023
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Ingår i: Ämnesintegrerad undervisning inom professionsutbildningar. - Malmö : Gleerups Utbildning AB. - 9789151109466 ; , s. 29-48
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 6. |
- Aharonian, Felix, et al.
(författare)
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Hitomi observations of the LMC SNR N 132 D : Highly redshifted X-ray emission from iron ejecta
- 2018
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Ingår i: Publications of the Astronomical Society of Japan. - : Oxford University Press (OUP). - 0004-6264 .- 2053-051X. ; 70:2
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Tidskriftsartikel (refereegranskat)abstract
- We present Hitomi observations of N 132 D, a young, X-ray bright, O-rich core-collapse supernova remnant in the Large Magellanic Cloud (LMC). Despite a very short observation of only 3.7 ks, the Soft X-ray Spectrometer (SXS) easily detects the line complexes of highly ionized S K and Fe K with 16-17 counts in each. The Fe feature is measured for the first time at high spectral resolution. Based on the plausible assumption that the Fe K emission is dominated by He-like ions, we find that the material responsible for this Fe emission is highly redshifted at similar to 800 km s(-1) compared to the local LMC interstellar medium (ISM), with a 90% credible interval of 50-1500 km s(-1) if a weakly informative prior is placed on possible line broadening. This indicates (1) that the Fe emission arises from the supernova ejecta, and (2) that these ejecta are highly asymmetric, since no blueshifted component is found. The S K velocity is consistent with the local LMC ISM, and is likely from swept-up ISM material. These results are consistent with spatial mapping that shows the He-like Fe concentrated in the interior of the remnant and the S tracing the outer shell. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of similar to 1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments. This highlights the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomi and will be greatly widened with future missions such as the X-ray Astronomy Recovery Mission (XARM) and Athena.
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| 7. |
- Ahani, Ghafour, et al.
(författare)
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Optimal Scheduling of Content Caching with recommendation and Freshness of Information
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Tidskriftsartikel (refereegranskat)abstract
- Content caching at the network edge has been considered an effective way of mitigating backhaul load and improving user experience. Caching efficiency can be enhanced by content recommendation and by keeping the information as fresh as possible. To the best of our knowledge, there is no work that jointly takes into account both these aspects in content caching. By content recommendation, a requested content that is not in the cache can be alternatively satisfied by a related cached content recommended by the system. Informationfreshness, arising naturally in content caching can be quantified by age of information (AoI). We address, optimal scheduling of cache updates for a time-slotted system accounting for both content recommendation and AoI, subject to cache capacity as well as backhaul capacity for updating the cache. For each content, there are requests that need to be satisfied and there is a cost function capturing the freshness of information which is monotonically increasing in the AoI. We present the following contributions. First, we prove that the problem is NP-hard even with contents of uniform size. Second, we derive an integer linear formulation for the problem, by which the optimal solution can be obtained for small-scale scenarios. Third, we develop an algorithm based on Lagrangian decomposition. Fourth, we develop efficient algorithms for solving the resulting subproblems. In addition, our algorithm computes a bound that can be used to evaluate the performance of any suboptimal solution. Finally, we conduct simulations to show the effectiveness of our algorithm in comparison to a greedy schedule.
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| 8. |
- Ahani, Ghafour, et al.
(författare)
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Routing and scheduling of network flows with deadlines and discrete capacity allocation
- 2020
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Ingår i: Networks. - : WILEY. - 0028-3045 .- 1097-0037. ; 76:1, s. 54-74
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Tidskriftsartikel (refereegranskat)abstract
- Joint scheduling and routing of data flows with deadline constraints in communication networks has been attracting research interest. This type of problem distinguishes from conventional multicommodity flows due to the presence of the time dimension. In this paper, we address a flow routing and scheduling problem with delivery deadline, where the assignment of link capacity occurs in discrete units. Discrete capacity allocation is motivated by applications in communication systems, where it is common to have a base unit of capacity (e.g., wavelength channel in optical communications). We present and prove complexity results of the problem. Next, we give an optimization formulation based on a time slicing approach, which amounts to a discretization of the time into time slices to enable to formulate the deadline constraints. We then derive an effective reformulation of the problem, via which a column generation algorithm is developed. In addition, we propose a simple and fast max-flow-based algorithm. We use a number of networks and traffic scenarios to study various performance aspects of the algorithms.
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| 9. |
- Ahari, Mostafa Tanhayi, et al.
(författare)
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Partons as unique ground states of quantum Hall parent Hamiltonians : The case of Fibonacci anyons
- 2023
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Ingår i: SciPost Physics. - : Stichting SciPost. - 2542-4653. ; 15:2
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Tidskriftsartikel (refereegranskat)abstract
- We present microscopic, multiple Landau level, (frustration-free and positive semi-definite) parent Hamiltonians whose ground states, realizing different quantum Hall fluids, are parton-like and whose excitations display either Abelian or non-Abelian braiding statistics. We prove ground state energy monotonicity theorems for systems with different particle numbers in multiple Landau levels, demonstrate S-duality in the case of toroidal geometry, and establish complete sets of zero modes of special Hamiltonians stabilizing parton-like states, specifically at filling factor ν = 2/3. The emergent Entangled Pauli Principle (EPP), introduced in Phys. Rev. B 98, 161118(R) (2018) and which defines the “DNA” of the quantum Hall fluid, is behind the exact determination of the topological characteristics of the fluid, including charge and braiding statistics of excitations, and effective edge theory descriptions. When the closed-shell condition is satisfied, the densest (i.e., the highest density and lowest total angular momentum) zero-energy mode is a unique parton state. We conjecture that parton-like states generally span the subspace of many-body wave functions with the two-body M-clustering property within any given number of Landau levels, that is, wave functions with Mth-order coincidence plane zeroes and both holomorphic and anti-holomorphic dependence on variables. General arguments are supplemented by rigorous considerations for the M = 3 case of fermions in four Landau levels. For this case, we establish that the zero mode counting can be done by enumerating certain patterns consistent with an underlying EPP. We apply the coherent state approach of Phys. Rev. X 1, 021015 (2011) to show that the elementary (localized) bulk excitations are Fibonacci anyons. This demonstrates that the DNA associated with fractional quantum Hall states encodes all universal properties. Specifically, for parton-like states, we establish a link with tensor network structures of finite bond dimension that emerge via root level entanglement.
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| 10. |
- Aharonian, F., et al.
(författare)
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Constraints on the Intergalactic Magnetic Field Using Fermi-LAT and HESS Blazar Observations
- 2023
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 950:2
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades. The gamma-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of B > 7.1 x 10(-16) G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of 10(4) (10(7)) yr, IGMF strengths below 1.8 x 10(-14) G (3.9 x 10(-14) G) are excluded, which rules out specific models for IGMF generation in the early universe.
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