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  • 18101.
    Zwiller, Val
    KTH, School of Engineering Sciences (SCI), Applied Physics. KTH, School of Electrical Engineering (EES).
    Quantum optics with nanowires (Conference Presentation)2017In: QUANTUM DOTS AND NANOSTRUCTURES: GROWTH, CHARACTERIZATION, AND MODELING XIV / [ed] Huffaker, DL Eisele, H, SPIE-INT SOC OPTICAL ENGINEERING , 2017, article id UNSP 101140IConference paper (Refereed)
  • 18102.
    Zwiller, Val
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Nano Photonics.
    Zadeh, I. E.
    Los, J. W. N.
    Gourgues, R. B. M.
    Steinmetz, V.
    Dobrovolskiy, S. M.
    Dorenbos, S. N.
    Single-photon detection with near unity efficiency, ultrahigh detection-rates, and ultra-high time resolution2017In: CLEO: Science and Innovations part of CLEO: 2017 : 4-19 May 2017, San Jose, California, United States, Optical Society of America, 2017Conference paper (Refereed)
    Abstract [en]

    Single-photon detectors with high efficiency, high time resolution, low dark counts and high photon detection-rates are vital for most demanding quantum optics experiments. Combining all performances in a single device has been challenging. Here, we demonstrate a broadband detector with efficiency higher than 92%, over 150 MHz photon detection-rate and dark counts below 130 Hz operated in a conventional Gifford-McMahon cryostat. Furthermore, using our custom made cryogenic amplifiers and optimized detector, we reach a record low jitter of 14.80 ps while maintaining high efficiency.

  • 18103.
    Zytomierski, Patrik
    et al.
    KTH, School of Engineering Sciences (SCI).
    Pettersson, Marcus
    FLYGTEKNIK – DESIGN AV ETT TRANSPORTFLYGPLAN2017Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The purpose of this research is to find and display the methods involved in the conceptual design of a transport aircraft. The goal is to examine this process and utilize the restrictions of a specific mission to find the most important aspects of aircraft design. The primary parameters portrayed are payload and aircraft weight, wing loading, and wing and tail design. These have been studied primarily using a literature study, concluding in overarching use of “Aircraft Design: A Conceptual Approach”, by J. S. Przemieniecki for assumptions and methods. Through this a final conceptual design of a transport aircraft could be made, with the capacity of delivering large payloads over vast distances.

  • 18104.
    Zyuzin, Alexander A.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Zyuzin, A. Yu.
    Flat band in disorder-driven non-Hermitian Weyl semimetals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 4, article id 041203Article in journal (Refereed)
    Abstract [en]

    We study the interplay of disorder and band-structure topology in a Weyl semimetal with a tilted conical spectrum around the Weyl points. The spectrum of particles is given by the eigenvalues of a non-Hermitian matrix, which contains contributions from a Weyl Hamiltonian and complex self-energy due to electron elastic scattering on disorder. We find that the tilt-induced matrix structure of the self-energy gives rise to either a flat band or a nodal line segment at the interface of the electron and hole pockets in the bulk band structure of type-II Weyl semimetals depending on the Weyl cone inclination. For the tilt in a single direction in momentum space, each Weyl point expands into a flat band lying on the plane, which is transverse to the direction of the tilt. The spectrum of the flat band is fully imaginary and is separated from the in-plane dispersive part of the spectrum by the "exceptional nodal ring" where the matrix of the Green's function in momentum-frequency space is defective. The tilt in two directions might shrink a flat band into a nodal line segment with "exceptional edge points." We discuss the connection to the non-Hermitian topological theory.

  • 18105.
    Zyuzin, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Nematic Skyrmions in Odd-Parity Superconductors2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 16, article id 167001Article in journal (Refereed)
    Abstract [en]

    We study topological excitations in two-component nematic superconductors, with a particular focus on CuxBi2Se3 as a candidate material. We find that the lowest-energy topological excitations are coreless vortices: a bound state of two spatially separated half-quantum vortices. These objects are nematic Skyrmions, since they are characterized by an additional topological charge. The inter-Skyrmion forces are dipolar in this model, i.e., attractive for certain relative orientations of the Skyrmions, hence forming multi-Skyrmion bound states.

  • 18106.
    Zyuzin, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Zyuzin, A. Y.
    Chiral anomaly and second-harmonic generation in Weyl semimetals2017In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 95, no 8, article id 085127Article in journal (Refereed)
    Abstract [en]

    We study second-harmonic generation in centrosymmetric Weyl semimetal with broken time reversal symmetry. We calculate electric current density at the double frequency of the propagating electromagnetic field in the presence of an applied constant magnetic field, using the method of kinetic equation for electron distribution function. It is shown that the chiral anomaly contribution to second-harmonic generation in the lowest order is linearly proportional to the applied magnetic field. The limit when the chiral anomaly dominates over the Lorentz-type contribution to second-harmonic generation is discussed.

  • 18107.
    Zäll, E.
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Karoumi, Raid
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Ülker-Kaustell, Mahir
    KTH. Tyréns AB, Stockholm, Sweden.
    Andersson, Andreas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges. Swedish Transport Administration, Solna, Sweden.
    Evaluation of load model for crowd-induced vibrations of footbridges2016In: IABSE Congress Stockholm, 2016: Challenges in Design and Construction of an Innovative and Sustainable Built Environment, International Association for Bridge and Structural Engineering (IABSE) , 2016, p. 65-72Conference paper (Refereed)
    Abstract [en]

    Due to a trend in designing light and slender structures, many modern footbridges are prone to excessive vibrations. Severely vibrating footbridges can give rise to discomfort for the pedestrians. Therefore, during the last decades, pedestrian-induced vibrations of footbridges have become a subject of great interest. In this study, the performance of a coupled crowd-structure model, where the bridge is described using its first two modes of vibrations and each pedestrian is described as a moving mass-spring-damper system, in combination with a walking load, is evaluated. The model is used to estimate vertical deck accelerations of a real footbridge which is known to be susceptible to vibrations, and the results are then compared to measurements. The model performs satisfactory in the time domain, but poorly in the frequency domain, which is concluded to be mainly due to discrepancies in the simulated load compared to the measured load.

  • 18108.
    Zöld, Thomas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics.
    Performance assessment of a hybrid electric-powered long-range commercial airliner 2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Despite the recent increase in the amount of smaller electric general aviation aircrafts, a fully electric airliner is not likely to fly in the near future. Partially inspired by the automotive industry’s success with the hybrid car, this thesis investigated the feasibility of an electric-hybrid propulsion system for an Airbus A340-600 on a long-haul flight and its effect on the aircraft’s performance. First, an analysis was done of the reference aircraft, A340-600, using conventional propulsion. Second, a 5700 nautical miles flight was modelled to determine performance data such as the power and thrust requirements during the different flight phases. Third, the flight phases where electric propulsion would be implemented were identified and an optimum ratio between conventional and electric propulsion was calculated. Finally, a detailed performance analysis of the new hybrid electric aircraft comparing it to a conventional aircraft was conducted.

    The maximum available conventional thrust was reduced to a certain percentage of the maximum thrust. Primarily conventional thrust is used, however when it is no longer sufficient, additional thrust is gained through electric propulsion. Conventional thrust ratio of 69.5%, 63.5% and 59.5% of total thrust was investigated yielding 8680 kg, 10500kg and 8585kg of payload decrease respectively. Net energy of 6.70MWh, 11.71MWh and 31.06MWh is required and the electric engines need to provide 21.3 MW, 25.5 MW and 28.3 MW of net power respectively.

    Partial electric propulsion will result in increased weight; however, it will also give room for further performance optimisation and technical innovations. On the one hand, the conventional engines will run at a constant speed throughout the flight allowing for better optimisation at a specific design point. On the other hand, electric engines are more reliable and require less maintenance than conventional engines. Furthermore, lower fuel consumption means less carbon-dioxide emissions. An exemption from CO2-taxes, similar to measures implemented for hybrid cars in certain countries, could financially justify use of the aircraft by airlines and compensate for the decrease in payload. Since a fully electric propelled airliner is not likely to fly for several decades, a hybrid-airliner would be a suitable alternative for the transition period from fossil fuels to electric energy.

  • 18109. Älfvåg, Hector
    et al.
    Lindbergh, Jakob
    KTH, School of Engineering Sciences (SCI).
    Optimering av kontakt med kontaktledning hos tåg:: Utplacerade massors påverkan på kontaktkraftens kvalitet2019Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This project aims to answer whether or not masses placed along a catenary can improve dynamic performance in the contact force between the train's pantograph and the catenary, and more specifically reduce the fluctuation of the contact force. This would be a way to improve the performance with only minor modifications to the infrastructure, rather than needing to replace entire catenaries, which is both economically and environmentally costly. The idea of masses as stabilizers comes from other applications, such as skyscrapers and power lines, where said masses have a positive effect on vibrations and movements. Simulations in ANSYS and post-processing in MATLAB are used to determine the contact force, which then can be quantified and compared between different scenarios. Eight different catenary positions, four different masses and two train speeds are tested in hope of seeing trends of what improves the performance and what doesn't. A few conclusions about the effect of the masses can be drawn from the results. Bigger masses gave a bigger effect, and a very big mass gave fluctuations where it was tested. Regarding the impact the speed has on the effect of the masses, the case studied here indicates lower speed decreases the local effect but increases the global effect. All mass placements that improved the performance, by reducing the fluctuations of the contact force, were at the end of a catenary span, and all placements that decreased the performance were at the centre and start of a span. Some practical limitations on the application of masses on real catenaries, such as mass placements on the contact wire and increased wear on the components involved, are discussed.

  • 18110.
    Ängeby, William
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Använding av geometrisk akustik för beräkning av efterklangstid i idrottshallar2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    When measures for acoustic improvements in sports halls, and other rooms, are to be implemented it is a great advantage to be able to predict how the action is going to affect for example the reverberation time even before it is done in reality. Since it costs money to install acoustic absorbers you want it to be right the first time around. Through the use of geometrical acoustic calculations digital models of the sports halls can be used to predict the results of a certain action. An often used method is to create a model with the existent absorbing materials and compare the calculated reverberation time to the measured reverberation time from the actual hall to check if the model is valid. Apart from calculations of the reverberation time the influence of the measured reverberation time to STI, a measure of speech intelligibility, is analyzed. A sports hall is often used as work places for sports teachers and it is therefore important to achieve a good acoustic environment in the hall.

    One of the aims of this project is to increase the knowledge of what gives a good model for acoustic calculations and how to choose absorption- and scattering coefficients. The goal is to make the calculated reverberation times to match the measured reverberation times. The other aim is to examine if there is a certain reverberation time curve which helps to achieve as good STI as possible.

    This project is part of the final examination in technical acoustics at KTH and has been carried out at Akustikbyrån T4p in Stockholm.

    The reverberation time for five sports halls has been measured. Models and geometrical acoustic calculations has been done for three of the halls. The absorption- and scattering coefficients used have been chosen from an absorption database as well as from analysis using the Eyring reverberation time model. The calculated reverberation times has been compared to the measured reverberation times. The measured STI for each of the five halls has been compared to the measured reverberation time for an analysis of if there is a certain reverberation curve which provides good speech intelligibility.

    The project resulted in recommendations for how detailed a model should be and also a procedure for estimating absorption- and scattering coefficients. Small details in the model usually don’t have a big impact on the calculated reverberation time at the same time as they do have a huge effect on the calculation times. The results also show that the scattering coefficients can have a huge impact on the calculated reverberation times and that they should not be neglected. It was found that both absorption- and scattering coefficients must often be overestimated for the 125 and 250 Hz octave bands for the calculated reverberation time to match the measured reverberation time in those bands. It turns out to be hard to give a specific reverberation time curve which provides for as good STI as possible since there are other parameters affecting the results. A requirement for reverberation times in sports halls used by school students is 1.2 seconds in all octave bands. The results shows that a reverberation time of 1.2 seconds in all octave bands is not necessary for good speech intelligibility.

    The procedure for estimation of absorption- and scattering coefficients hopefully leads to better understanding of how the coefficients can be chosen and how they affect the calculation results. The results could also give better understanding of the limitations when using software based on geometrical acoustics

  • 18111.
    Åberg, Jonas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Processing.
    Vynnycky, Michael
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory.
    Widell, Björn
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Processing.
    Development of a finite element model for study of the developing stress and strain in a solidifying shell2006Report (Other academic)
  • 18112.
    Åberg, Viktor
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Automatic projector warping using a multiple view camera approach2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The main objective of this master thesis was to construct an automatic method for calibrating a projector to display images on a curved screen without the images looking deformed from a certain intended viewing position. Since the method was thought to be used in a flight simulator, where the intended viewing position has an occluded view of the screen, the method needed to be able to handle these occlusions in some way, and the proposed solution was to use two cameras for the calibration; one in the intended viewing position and one with a more clear sight of the screen.This thesis adds the multi-camera functionality to an existing algorithm for projector calibration using a single camera, which was developed in 2013. This algorithm performs well in calibrating projectors with respect to views that have a clear sight of the screen but lacks the functionality to do a calibration when its single camera cannot capture all parts of the screen from its viewing position.The algorithm developed uses point transfer between camera views to supply the camera in the viewing position with enough information to make a suitable calibration even for the regions of the screen it cannot capture itself.A program has been developed, showing that it is possible to do this projector calibration for situations where up to half of the screen is occluded from the intended viewing position, with a result that is not notably worse than when using the single camera algorithm for similar situations with clear sight of the screen. It might be possible to run the algorithm with less than half the screen visible from the viewing position, but an upper limit of how much of the screen can be occluded with an accepted result has not been found.The algorithm should be usable with any pair of cameras, and any projector, and does not assume that the cameras are stereo calibrated beforehand. However in the testing done in this thesis, camera images with resolution 640x480 have been used, and the displayed projector images have had the resolution 256x192 in the calibration.

  • 18113.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Acoustic multi-ports with application to duct acoustics2010In: 17th International Congress on Sound and Vibration 2010, ICSV 2010: Volume 5, 2010, p. 3930-3942Conference paper (Refereed)
    Abstract [en]

    In this paper the development of multi-port models to describe linear acoustic problems in ducts with flow is presented. From an engineering point of view this field covers many important applications ranging from ventilation ducts in vehicles or buildings to intake/exhaust ducts on ICengines and power plants. Historically the use of multi-port models for ducts started in the 1920's, when the four-pole (2-port) filter models used by electrical engineers were applied to analyse transmission of low frequency 1D (plane) waves with application to automotive mufflers. An important step was then taken in 1971, when Cremer presented the idea that such "black box" models can be applied to describe aerodynamically generated sound in ducts. This implies that any fluid machine or unsteady flow process can be modelled as a "black box", with a "passive part" that describes how incident waves are scattered and an "active part" that describes the sound generation. The active part is normally assumed independent of the acoustic state, which makes the "black box" or acoustic multi-port model consistent with Lighthills acoustic analogy.

  • 18114.
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Method for reducing noise of a high power combustion engine2003Patent (Other (popular science, discussion, etc.))
  • 18115.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Dissipative silencers based on micro-perforated plates2013In: SAE Technical Papers, 2013, Vol. 6Conference paper (Refereed)
    Abstract [en]

    Micro-perforated plates (MPP:s) can be defined as a perforated plate where the hole impedance is dominated by viscous losses. This will always be true for sufficiently low frequencies or small holes. In addition for a standard MPP the perforation ratio is chosen so that the normalized acoustic resistance is between 1-2, which yields optimum damping for incident plane waves. Historically MPP:s have been used as panel absorbers to reduce reflections in rooms and enclosures. More recently the potential for machinery and vehicle applications has come into focus, e.g., dissipative exhaust silencers. Some advantages offered by a MPP solution, when compared to traditional dissipative silencers, are that it can reduce the weight and the problem with fibre breakout. In this paper the work on cylindrical MPP dissipative silencers at KTH is summarized. One important question being how an optimum damping is achieved, for a certain frequency band and for a given volume (length & area ratio) of the silencer.

  • 18116.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Allam, Sabry
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Industrial Education College, Cairo, Egypt .
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Aero-acoustics of flow duct singularities at low mach numbers2006In: Collection of Technical Papers - 12th AIAA/CEAS Aeroacoustics Conference, 2006, p. 3708-3717Conference paper (Refereed)
    Abstract [en]

    This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.

  • 18117.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH-Centre for Sustainable Aviation.
    Bolin, Karl
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH-Centre for Sustainable Aviation.
    Ulfvengren, Pernilla
    KTH, School of Industrial Engineering and Management (ITM), Industrial Economics and Management (Dept.), Industrial Management. KTH-Centre for Sustainable Aviation.
    Air traffic management and noise2018In: INTER-NOISE 2018 - 47th International Congress and Exposition on Noise Control Engineering: Impact of Noise Control Engineering, Institute of Noise Control Engineering , 2018Conference paper (Refereed)
    Abstract [en]

    With the steady increase of air traffic the need for developing sustainable aviation increases as well. To meet this need, the Swedish Transport Administration and KTH Royal Institute of Technology have established a Centre for Sustainable Aviation. In a global perspective, aviation research focuses on meeting future capacity needs for increased travel and at the same time achieve sustainability with reduced environmental impact and sustained or increased safety. This paper describes four on-going projects, as examples of the variety of research that may contribute to a sustainable society in both shorter and longer terms as well as both in a local and in an international perspective. Initially the centre will apply and direct existing knowledge towards noise abatement initiatives in aviation. I the long-term perspective the research will contribute to knowledge on a broader spectrum of sustainability aspects of aviation.

  • 18118.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Kabral, Raimo
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Turbocharger noise - Generation and control2014In: SAE technical paper series, ISSN 0148-7191, Vol. 2014Article in journal (Refereed)
    Abstract [en]

    An important part of modern engine design is the concept of downsizing where a key role is carried by the charging devices. These devices are effective aero-acoustic sources forming a coupled acoustic system with the connected flow-channel components. At KTH a unique test facility for determination of the complete acoustic Two-port for turbochargers has been built. Using this facility both the passive (transmission & reflection) as well as the active (sound generation) data for turbochargers can be measured at a given operating point. One important issue which has been studied in detail using this data is the coupling between the aerodynamic and acoustic fields close to "surge". In addition, the control of compressor noise is an increasing concern. For instance heavy duty diesels and light duty engines with screw (roots) compressors can create strong charging harmonics well below 10 kHz. The standard noise control solution for these cases is to build a series of resonators. As an alternative KTH has developed a novel compact and very efficient silencer in the form of an expansion chamber with locally reacting cavities. The cavities consists of a micro-perforated plate in front of a closed air volume. The micro-perforate and volume are then chosen so that the cavity impedance equals the so called Cremer impedance at a target frequency. This ensures a very high damping at one frequency (hundreds of dB/m) and using this concept compact silencers with a damping higher than 30 dB in octave around the target frequency can be designed.

  • 18119.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Karlsson, Mikael
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Can acoustic multi-port models be used to predict whistling2010In: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference), 2010, p. 2010-4009-Conference paper (Refereed)
    Abstract [en]

    In duct aeroacoustic problems can be described using so called acoustic multi-port models. Such models represent a linear and time-invariant aeroacoustic model, which split the problem in a passive part, a scattering matrix in the frequency domain, describing the reflection and transmission and an active part describing the source strength. In accordance with Lighthill one normally assumes in this kind of model that the source part is uncoupled from the acoustic field. However, this assumption can be relaxed and it is fully possible to assume that the source strength can be affected by incident sound waves via a linear and time-invariant mechanism. The most general frequency domain model for this is a matrix which formally can be added to the scattering matrix describing the passive part. This leads to a model that has the same structure as the traditional multi-port model, but where the scattering matrix also contains information about fluid-acoustic interaction effects which is the origin for creating fluid driven feedback loops or whistles. The implication of these ideas is that multi-port models can be used to analyze amplification of sound and whistling.

  • 18120.
    Åbom, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Karlsson, Mikael
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kierkegaard, Axel
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the use of linear aero-acoustic methods to predict whistling2009In: 16th International Congress on Sound and Vibration 2009, ICSV 2009, 2009, p. 2406-2413Conference paper (Refereed)
    Abstract [en]

    In duct aero-acoustic problems can be described using so called acoustic multi-port models. Such models represent a linear and time-invariant aero-acoustic model, which split the problem in a passive part, a scattering matrix in the frequency domain, describing the reflection and transmission and an active part describing the source strength. In accordance with Lighthill one normally assumes in this type of model that the source part is uncoupled from the acoustic field. However, this assumption can be relaxed and it is fully possible to assume that the source strength can be affected by incident sound waves via a linear and time-invariant mechanism. The most general frequency domain model for this is a matrix which formally can be added to the scattering matrix describing the passive part. This leads to a model that has the same structure as the traditional multi-port model, but where the scattering matrix also contains information about fluid-acoustic interaction effects which is the origin for creating fluid driven feedback loops or whistles.

  • 18121. Ågren, Jan
    et al.
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Railway Technology.
    Flash Temperature in Wheel/Rail contact and Martensite Formation: A Review2006In: Proc. of the 7th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2006), 2006Conference paper (Refereed)
  • 18122. Ågren, O.
    et al.
    Moiseenko, V. E.
    Noack, K.
    Hagnestal, A.
    Kallne, J.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Hybrid Reactor Studies Based On The Straight Field Line Mirror2013In: Fusion science and technology, ISSN 1536-1055, E-ISSN 1943-7641, Vol. 63, no 1T, p. 52-57Article in journal (Refereed)
    Abstract [en]

    The straight field line mirror (SFLM) hybrid reactor studies aim to identify a concept where the safety of fission power production could be enhanced. A fusion neutron source could become a mean to achieve this. The SFLM studies address critical issues such as reactor safety, natural circulation of coolants, steady state operation for a year or more and means to avoid too strong material loads by a proper geometrical arrangement of the reactor components. A key result is that power production may be possible with a fusion Q factor as low as 0.15. This possibility arises from the high power amplification by fission, which within reactor safety margins may exceed a factor of 100. The requirements on electron temperature are dramatically lower for a fusion hybrid compared to a stand-alone fusion reactor. This and several other factors are important for our choice to select a mirror machine for the fusion hybrid reactor studies.

  • 18123. Ågren, O.
    et al.
    Moiseenko, V. E.
    Noack, K.
    Hagnestål, A.
    Källne, J.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    The straight field line mirror concept aiming at a hybrid reactor2012In: 39th EPS Conference on Plasma Physics 2012, EPS 2012 and the 16th International Congress on Plasma Physics: Volume 1, 2012, 2012, p. 5-8Conference paper (Refereed)
  • 18124. Ågren, O.
    et al.
    Noack, K.
    Moiseenko, V. E.
    Hagnestål, A.
    Källne, J.
    Anglart, Henryk
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    The hybrid reactor project based on the straight field line mirror concept2012In: Fusion For Neutrons And Subcritical Nuclear Fission / [ed] Kallne, J; Ryutov, D; Gorini, G; Sozzi, C; Tardocchi, M, American Institute of Physics (AIP), 2012, p. 173-185Conference paper (Refereed)
    Abstract [en]

    The straight field line mirror (SFLM) concept is aiming towards a steady-state compact fusion neutron source. Besides the possibility for steady state operation for a year or more, the geometry is chosen to avoid high loads on materials and plasma facing components. A comparatively small fusion hybrid device with "semi-poor" plasma confinement (with a low fusion Q factor) may be developed for industrial transmutation and energy production from spent nuclear fuel. This opportunity arises from a large fission to fusion energy multiplication ratio, Qr=Pfis/P fus>>1. The upper bound on Qr is primarily determined by geometry and reactor safety. For the SFLM, the upper bound is Qr≈150, corresponding to a neutron multiplicity of k eff=0.97. Power production in a mirror hybrid is predicted for a substantially lower electron temperature than the requirement T e≈10 keV for a fusion reactor. Power production in the SFLM seems possible with Q≈0.15, which is 10 times lower than typically anticipated for hybrids (and 100 times smaller than required for a fusion reactor). This relaxes plasma confinement demands, and broadens the range for use of plasmas with supra-thermal ions in hybrid reactors. The SFLM concept is based on a mirror machine stabilized by qudrupolar magnetic fields and large expander tanks beyond the confinement region. The purpose of the expander tanks is to distribute axial plasma loss flow over a sufficiently large area so that the receiving plates can withstand the heat. Plasma stability is not relying on a plasma flow into the expander regions. With a suppressed plasma flow into the expander tanks, a possibility arise for higher electron temperature. A brief presentation will be given on basic theory for the SFLM with plasma stability and electron temperature issues, RF heating computations with sloshing ion formation, neutron transport computations with reactor safety margins and material load estimates, magnetic coil designs as well as a discussion on the implications of the geometry for possible diagnostics. Reactor safety issues are addressed and a vertical orientation of the device could assist passive coolant circulation. Specific attention is put to a device with a 25 m long confinement region and 40 cm plasma radius in the mid-plane. In an optimal case (keff = 0.97) with a fusion power of only 10 MW, such a device may be capable of producing a power of 1.5 GWth.

  • 18125.
    Ågren, Peter
    et al.
    KTH, Superseded Departments, Physics.
    Walter, Jochen
    KTH, School of Engineering Sciences (SCI), Physics.
    Haviland, David B.
    KTH, Superseded Departments, Physics.
    Switching Current of a Cooper Pair Transistor with Tunable Josephson Junctions2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 1, p. 14510-Article in journal (Refereed)
    Abstract [en]

    We investigate the switching current of a Cooper pair transistor with tunable Josephson energy. The junctions are fabricated in a superconducting quantum interference device (SQUID) geometry which allows for an in situ tunable effective Josephson energy by application of a magnetic field. We find a 2e-periodic switching current versus gate charge. As the magnetic field is increased the switching current stays 2e-periodic but the magnitude is suppressed. At a magnetic field of half a flux quantum through the SQUID's the switching current is minimum. We can theoretically model the experimental data by assuming a switching current which is proportional to the ideal critical current squarred. We show that such a dependence is expected in the limit where the effect of thermal fluctuations on the system is strong.

  • 18126.
    Ågren, Peter
    et al.
    KTH, Superseded Departments, Physics.
    Walter, Jochen
    KTH, School of Engineering Sciences (SCI), Physics.
    Schöllmann, Volker
    KTH, School of Engineering Sciences (SCI), Physics.
    Haviland, David B.
    KTH, Superseded Departments, Physics.
    Switching Currents and Quasi-Particle Poisoning in the Superconducting Single Electron Transistor2002In: INTERNATIONAL WORKSHOP ON SUPERCONDUCTING NANO-ELECTRONICS DEVICES, 2002, p. 25-31Conference paper (Refereed)
  • 18127.
    Åhlfeldt, Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Time-dependent change of the phase-matching wavelength due to refractive-index instabilities in proton-exchanged LiTaO3 waveguides1994Conference paper (Refereed)
  • 18128.
    Åhlgren, Erik
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
    Ett kvantitativt verktyg för utfärdande avsäljoptioner2017Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This study investigates whether differences between options implicit volatility and the underlying assets historical volatility can be used to generate returns to a portfolio. This has been done by posing an investment problem which solution is based on maximizing the investor return given the individual preference and attitude towards risk and belief in the market. The structure of the investment problem is such that the investor can choose to issue put options or not to do so. How many options to be issued are largely due to differences between the implicit volatility and the historical. To illustrate this a fictitious portfolio with data from the market has been established where its returns and transactions have been evaluated and analyzed.

    The result shows that the investment strategy has performed well and that the investors beliefs in the development of the market versus the prices of the options has resulted in returns significantly higher than the underlying asset have. In other words, the solution of the investment problem can serve as an excellent tool for issuing put options.

  • 18129.
    Åkerblom, Karl
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Investigations of Single Longitudinal Mode and Frequency Stabilityin Standing Wave and Unidirectional Cavity Lasers2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 18130.
    Åkerlind, Kristina
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Danielsson, Rebecka
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Assessment of Life Extension for Welded Bridge Details Using HFMI2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 18131.
    Åkermo, Malin
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Thermoforming of closed cell polymer foam and its residual compressive mechanical properties2008In: 8th International Conference on Sandwich Structures, FEUP edições (Faculdade de Engenharia da Universidade do Porto Edicoes), 2008Conference paper (Refereed)
  • 18132.
    Åkermo, Malin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Larberg, Ylva
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallander, Per
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Forming of advanced components out of pre-stacked crossplied UD prepreg2011In: 18th International Conference on Composite Materials, ICCM-18, 2011Conference paper (Refereed)
  • 18133.
    Åkermo, Malin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Larberg, Ylva
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Sjölander, Jens
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallnader, Per
    Saab.
    Influence of interply friction on the forming of stacked prepreg2013In: Proceedings of 19th International Conference on Composite Materials (ICCM19), Curran Associates, Inc., 2013, p. 919-928Conference paper (Refereed)
  • 18134.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Feedback Control of Spatially Evolving Flows2007Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis we apply linear feedback control to spatially evolving flows in order to minimize disturbance growth. The dynamics is assumed to be described by the linearized Navier--Stokes equations. Actuators and sensor are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier--Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier--Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. One possible approach is to perform Fourier decomposition along (almost) homogeneous spatial directions and another is by constructing a reduced order model by Galerkin projection on a suitable set of vectors. The first strategy is used to control the evolution of a range of instabilities in the classical family of Falkner--Skan--Cooke flows whereas the second is applied to a more complex cavity type of geometry.

  • 18135.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Global stability and feedback control of boundary layer flows2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis the stability of generic boundary layer flows is studied from a global viewpoint using optimization methods. Global eigenmodes of the incompressible linearized Navier-Stokes equations are computed using the Krylov subspace Arnoldi method. These modes serve as a tool both to study asymptotic stability and as a reduced basis to study transient growth. Transient growth is also studied using adjoint iterations. The knowledge obtained from the stability analysis is used to device systematic feedback control in the Linear Quadratic Gaussian framework. The dynamics is assumed to be described by the linearized Navier-Stokes equations. Actuators and sensors are designed and a Kalman filtering technique is used to reconstruct the unknown flow state from noisy measurements. This reconstructed flow state is used to determine the control feedback which is applied to the Navier-Stokes equations through properly designed actuators. Since the control and estimation gains are obtained through an optimization process, and the Navier-Stokes equations typically forms a very high-dimensional system when discretized there is an interest in reducing the complexity of the equations. A standard method to construct a reduced order model is to perform a Galerkin projection of the full equations onto the subspace spanned by a suitable set of vectors, such as global eigenmodes and balanced truncation modes.

  • 18136.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Marxen, Olaf
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Steady solutions of the Navier-Stokes equations by selective frequency damping2006In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 18, no 6, p. 068102-Article in journal (Refereed)
    Abstract [en]

    A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient.

  • 18137.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ehrenstein, Uwe
    IRPHÉ, Université de Provence.
    Gallaire, Francois
    Laboratoire J.A. Dieudonné.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Global two-dimensional stability measures of the flat plate boundary-layer flow2008In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 27, no 5, p. 501-513Article in journal (Refereed)
    Abstract [en]

    The stability of the two-dimensional flat plate boundary-layer is studied by means of global eigenmodes. These eigenmodes depend both on the streamwise and wall-normal coordinate, hence there are no assumptions on the streamwise length scales of the disturbances. Expanding the perturbation velocity field in the basis of eigenmodes yields a reduced order model from which the stability characteristics of the flow, i.e. the initial condition and forcing function leading to the largest energy growth, are extracted by means of non-modal analysis. In this paper we show that, even when performing stability analysis using global eigenmodes, it is not sufficient to consider only a few of the least damped seemingly relevant eigenmodes. Instead it is the task of the optimization procedure, inherent in the non-modal analysis, to decide which eigenmodes are relevant. We show that both the optimal initial condition and the optimal forcing structure have the form of upstream tilted structures. Time integration reveals that these structures gain energy through the so called Orr mechanism, where the instabilities extract energy from the mean shear. This provides the optimal way of initiating Tollmien-Schlichting waves in the boundary layer. The optimal initial condition results in a localized Tollmien-Schlichting wavepacket that propagates downstream, whereas the optimal forcing results in a persistent Tollmien-Schlichting wave train.

  • 18138.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Haepffner, Jerome
    Ehrenstein, Uwe
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Model reduction and control of a cavity-driven separated boundary layer2008In: IUTAM Symposium On Flow Control And Mems / [ed] Morrison, JF; Birch, DM; Lavoie, P, 2008, Vol. 7, p. 147-155Conference paper (Refereed)
    Abstract [en]

    The control of a globally unstable boundary-layer flow along a two-dimensional cavity is considered. When perturbed by the worst-case initial condition, the flow exhibits a large transient growth associated with the development of a wave packet along the cavity shear layer followed by a global cycle related to the least stable global eigenmodes. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes. The LQG controller is run in parallel to the Navier-Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 18139.
    Åkervik, Espen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hoepffner, Jérôme
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Uwe, Eherenstein
    IRPH́E, Université de Provence.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Optimal growth, model reduction and control in a separated boundary-layer flow using global eigenmodes2007In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 579, p. 305-314Article in journal (Refereed)
    Abstract [en]

    Two-dimensional global eigenmodes are used as a projection basis both for analysing the dynamics and building a reduced model for control in a prototype separated boundary-layer flow. In the present configuration, a high aspect ratio smooth cavity-like geometry confines the separation bubble. Optimal growth analysis using the reduced basis shows that the sum of the highly non-normal global eigenmodes are able to describe a localized disturbance. Subject to this worst-case initial condition, a large transient growth associated with the development of a wavepacket along the shear layer followed by a global cycle related to the two unstable global eigenmodes is found. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes, and the resulting linear-quadratic-gaussian controller is applied to the Navier--Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

  • 18140.
    Ålander, Alexandra
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Dufek, Jan
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Gudowski, Waclaw
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    From once-through nuclear fuel cycle to accelerator-driven transmutation2006In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 562, no 2, p. 630-633Article in journal (Refereed)
    Abstract [en]

    In this study simulation of different nuclear fuel cycle scenarios are performed. The reference scenario corresponds to a medium size nuclear power country, with 10 light water reactors (LWRs). The study addresses long-term, equilibrium fuel cycle scenarios, with and without plutonium recycling (MOX) in LWRs and transuranics (TRU) burning in accelerator-driven systems (ADS). However, also short-term phase-out scenarios, including TRU burning in ADS, are performed. The equilibrium simulation showed that four ADS units, each of 800 MWt, are sufficient to burn an amount of plutonium and americium corresponding to the build-up of those elements. The phase-out simulation of a country adopting an approach to reduce the spent nuclear fuel inventory, showed that complementary burning of TRU in three to four ADS units appear suitable. The fuel cycle simulations have been performed using the Nuclear Fuel Cycle Simulation (NFCSim) code [C.G. Bathke, E.A. Schneider, NFCSim User's Manual, Los Alamos National Laboratory Report LA-UR-04-8369, 2004.] and the Monteburns code [D.I. Poston, H.R. Trellue, User's Manual, Version 2.0 for Monteburns, Version 1.0, LA-UR-99-4999, 1999.].

  • 18141. Århem, P.
    et al.
    Klement, G.
    Blomberg, Clas
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Channel density regulation of firing patterns in a cortical neuron model2006In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 90, no 12, p. 4392-4404Article in journal (Refereed)
    Abstract [en]

    Modifying the density and distribution of ion channels in a neuron (by natural up- and downregulation or by pharmacological intervention or by spontaneous mutations) changes its activity pattern. In this investigation we analyzed how the impulse patterns are regulated by the density of voltage-gated channels in a neuron model based on voltage-clamp measurements of hippocampal interneurons. At least three distinct oscillatory patterns, associated with three distinct regions in the Na-K channel density plane, were found. A stability analysis showed that the different regions are characterized by saddle-node, double-orbit, and Hopf-bifurcation threshold dynamics, respectively. Single, strongly graded action potentials occur in an area outside the oscillatory regions, but less graded action potentials occur together with repetitive. ring over a considerable range of channel densities. The relationship found here between channel densities and oscillatory behavior may partly explain the difference between the principal spiking patterns previously described for crab axons (class 1 and 2) and cortical neurons ( regular. ring and fast spiking).

  • 18142. Århem, Peter
    et al.
    Blomberg, Clas
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Ion channel density and threshold dynamics of repetitive firing in a cortical neuron model2007In: Biosystems (Amsterdam. Print), ISSN 0303-2647, E-ISSN 1872-8324, Vol. 89, no 1-3, p. 117-125Article in journal (Refereed)
    Abstract [en]

    Modifying the density and distribution of ion channels in a neuron (by natural up- and down-regulation, by pharmacological intervention or by spontaneous mutations) changes its activity pattern. In the present investigation, we analyze how the impulse patterns are regulated by the density of voltage-gated channels in a model neuron, based on voltage clamp measurements of hippocampal interneurons. At least three distinct oscillatory patterns, associated with three distinct regions in the Na-K channel density plane, were found. A stability analysis showed that the different regions are characterized by saddle-node, double-orbit, and Hopf bifurcation threshold dynamics, respectively. Single strongly graded action potentials occur in an area outside the oscillatory regions, but less graded action potentials occur together with repetitive firing over a considerable range of channel densities. The presently found relationship between channel densities and oscillatory behavior may be relevance for understanding principal spiking patterns of cortical neurons (regular firing and fast spiking). It may also be of relevance for understanding the action of pharmacological compounds on brain oscillatory activity.

  • 18143.
    Åslund, Magnus
    KTH, School of Engineering Sciences (SCI), Physics.
    Digital Mammography with a Photon Counting Detector in a Scanned Multislit Geometry2007Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Mammography screening aims to reduce the number of breast cancer deaths by early detection of the disease, which is one of the leading causes of deaths for middle aged women in the western world. The risk from the x-ray radiation in mammography is relatively low but still a factor in the benefit-risk ratio of screening. The characterization and optimization of a digital mammography system is presented in this thesis. The investigated system is shown to be highly dose efficient by employing a photon counting detector in a scanning multislit geometry.

    A novel automatic exposure control (AEC) is proposed and validated in clinical practise. The AEC uses the leading detector edge to measure the transmission of the breast. The exposure is modulated by altering the scan velocity during the scan. A W-Al anode-filter combination is proposed.

    The characterization of the photon counting detector is performed using the detective quantum efficiency. The effect of the photon counting detector and the multislit geometry on the measurement method is studied in detail. It is shown that the detector has a zero-frequency DQE of over 70\% and that it is quantum limited even at very low exposures.

    Efficient rejection of image-degrading secondary radiation is fundamental for a dose efficient system. The efficiency of the scatter rejection techniques currently used are quantified and compared to the multislit geometry.

    A system performance metric with its foundation in statistical decision theory is discussed. It is argued that a photon counting multislit system can operate at approximately half the dose compared to several other digital mammography techniques.

  • 18144.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, M
    Danielsson, M
    Optimized AEC for scanning digital mammography based on local variation of scan velocity2005In: Medical Imaging 2005: Physics of Medical Imaging, Pts 1 and 2 / [ed] Flynn, MJ, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2005, Vol. 5745, p. 468-477Conference paper (Refereed)
    Abstract [en]

    In mammography, there is an optimal photon energy and current time product that produce the required image quality at the minimal dose. The task of an automatic exposure control (AEC), in full field digital mammography (FFDM) is to minimize the dose by using optimized exposure settings. Each point in a mammogram has different radiological thickness. A conventional AEC samples the thickness in some regions to set the current time product and possibly also the beam quality. We define an ideal AEC as one that optimizes the beam quality and exposure in each point to produce a constant contrast-to-noise ratio (CNR) of structures of interest throughout the image. This paper presents the results from a theoretical evaluation of an AEC proposed for a scanning photon-counting FFDM system. The geometry enables the AEC to use information from the leading detector line to adjust the scan velocity during the scan. Thus, the irradiation can be better optimized in the scanning-direction as compared to a conventional AEC. The scan time is further reduced by increased velocity over sections that contain no tissue. The results are quantified in terms of reduction of entrance dose and scan time. The presented AEC is compared to an ideal AEC, a conventional AEC and is also benchmarked against an ideal regulator. The effect of the detector width is evaluated. Compared to a conventional AEC, both evaluated on a set of 266 mammograms, the ideal AEC would reduce the entrance dose by 39% on average while the proposed AEC for scanning systems reduces the entrance dose by 10-20% and scan-time by 25-32% on average, depending on detector width.

  • 18145.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    AEC for scanning digital mammography based on variation of scan velocity2005In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 32, no 11, p. 3367-3374Article in journal (Refereed)
    Abstract [en]

    A theoretical evaluation of nonuniform x-ray field distributions in mammography was conducted. An automatic exposure control (AEC) is proposed for a scanning full field digital mammography system. It uses information from the leading part of the detector to vary the scan velocity dynamically, thus creating a nonuniform x-ray field in the scan direction. Nonuniform radiation fields were also created by numerically optimizing the scan velocity profile to each breast's transmission distribution, with constraints on velocity and acceleration. The goal of the proposed AEC is to produce constant pixel signal-to-noise ratio throughout the image. The target pixel SNR for each image could be set based on the breast thickness, breast composition, and the beam quality as to achieve the same contrast-to-noise ratio between images for structures of interest. The results are quantified in terms of reduction in entrance surface air kerma (ESAK) and scan time relative to a uniform x-ray field. The theoretical evaluation was performed on a set of 266 mammograms. The performance of the different methods to create nonuniform fields decreased with increased detector width, from 18% to 11% in terms of ESAK reduction and from 30% to 25% in terms of scan time reduction for the proposed AEC and detector widths from 10 to 60 mm. Some correlation was found between compressed breast thickness and the projected breast area onto the image field. This translated into an increase of the ESAK and decrease of the scan time reduction with breast thickness. Ideally a nonuniform field in two dimensions could reduce the entrance dose by 39% on average, whereas a field nonuniform in only the scanning dimension ideally yields a 20% reduction. A benefit with the proposed AEC is that the risk of underexposing the densest region of the breast can be virtually eliminated.

  • 18146.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Evaluation of an AEC system for scanning photon counting mammography based on variation of scan velocityIn: Medical physics (Lancaster), ISSN 0094-2405Article in journal (Other academic)
  • 18147.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Optimization of operating conditions in photon counting multi-slit mammography based on Si-strip detectors - art. no. 61420A2006In: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3 / [ed] Flynn, MJ; Hsieh, J, 2006, Vol. 6142, p. A1420-A1420Conference paper (Refereed)
    Abstract [en]

    Measurements and simulations of the signal-difference-to-noise ratio (SDNR) and average glandular dose (AGD) have been performed on a photon counting full-field digital mammography system to determine the optimal operating conditions. Several beam qualities were experimentally evaluated by using different combinations of tube voltage, added filters and thickness of BR12 with a tungsten target x-ray tube. The SDNR and AGD were also calculated theoretically for an extended number of operating conditions and a more accurate breast model. As figure of merit for each operating condition, a spectral quantum efficiency (SQE) was calculated as the polychromatic SDNR squared over the optimal monochromatic SDNR squared at the same AGD. The theoretical model agreed within 4% relative the measured SDNR throughout the evaluated breast thickness (30-70 mm) and tube voltage range (26-38 kV). The optimization was performed with a constant SDNR-rate as compared to using a fixed filter thickness. The optimal combinations of tube voltage-filter material were: 32 kV-Ag, 34 kV-Cd, 36 kV-Sn for a breast thickness of 30, 50 and 70 mm respectively. These K-edge filter materials increased the SQE by less than 4% compared to the optimal Al filtration.

  • 18148.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Physical characterization of a scanning photon counting digital mammography system based on Si-strip detectors2007In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 34, no 6, p. 1918-1925Article in journal (Refereed)
    Abstract [en]

    The physical performance of a scanning multislit full field digital mammography system was determined using basic image quality parameters. The system employs a direct detection detector comprised of linear silicon strip sensors in an edge-on geometry connected to photon counting electronics. The pixel size is 50 mu m and the field of view 24 x 26 cm(2). The performance was quantified using the presampled modulation transfer function, the normalized noise power spectrum and the detective quantum efficiency (DQE). Compared to conventional DQE methods, the scanning geometry with its intrinsic scatter rejection poses additional requirements on the measurement setup, which are investigated in this work. The DQE of the photon counting system was found to be independent of the dose level to the detector in the 7.6-206 mu Gy range. The peak DQE was 72% and 73% in the scan and slit direction, respectively, measured with a 28 kV W-0.5 mm Al anodefilter combination with an added 2 mm Al filtration.

  • 18149.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Scatter rejection in multislit digital mammography2006In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 4, p. 933-940Article in journal (Refereed)
    Abstract [en]

    The scatter to primary ratio (SPR) was measured on a scanning multislit full-field digital mammography system for different thickness of breast equivalent material and different tube voltages. Scatter within the detector was measured separately and was found to be the major source of scatter in the assembly. Measured total SPRs below 6% are reported for breast range 3-7 cm. The performance of the multislit assembly is compared to other imaging geometries with different scatter rejection schemes by using the scatter detective quantum efficiency.

  • 18150.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Spectral shaping for photon counting digital mammography2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 580, no 2, p. 1046-1049Article in journal (Refereed)
    Abstract [en]

    The spectral shaping properties of conventional filters have been evaluated for a photon counting digital mammography system, and the result has been compared with the theoretical spectrum from a multi-prism X-ray lens (MPL). The absorption filters and the MPL were evaluated using a theoretical model of the system which has been verified experimentally. The spectral shaping performance is quantified with the spectral quantum efficiency (SQE), calculated as the polychromatic signal-difference-to-noise ratio (SDNR) squared over the optimal monochromatic SDNR squared at the same average glandular dose. The MPL increases the SQE by 25% compared to the investigated absorption filter when compared with a Tungsten anode. This translates into a potential dose reduction of 20% at maintained SDNR.

360361362363364365366 18101 - 18150 of 18264
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