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  • Resultat 1-10 av 27
  • [1]23Nästa
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  • Appelgren, Patrik, et al. (författare)
  • Experimental Study of Electromagnetic Effects on Solid Copper Jets
  • 2010
  • Ingår i: Journal of applied mechanics. - 0021-8936. ; 77:1, s. 011010
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we present a study of the interaction between all electric current pulse and a solid copper jet. Experiments were performed using a dedicated pulsed power supply delivering a current pulse of such amplitude, rise little, and duration that the jet is efficiently affected. The copper jet was created by using a shaped charge warhead. All electrode configuration consisting of two aluminum plates with a separation distance of 150 mm was used. The discharge current pulse and the voltages at the capacitors and at the electrodes were measured to obtain data oil energy deposition in and the resistance of the jet and electrode contact region. X-ray diagnostics were used to radiograph the jet, and by analyzing the radiograph, the degree of disruption of the electrified jet could be obtained. It was found that a current pulse with an amplitude of 200-250 kA and a rise time of 16 mu s could strongly enhance the natural fragmentation of the jet. In this case, the initial electric energy was 100 kJ and about 90% of the electric energy was deposited in the jet and electrodes. At the exit of the electrode region, the jet fragments formed rings with a radial velocity of up to 200 m/s, depending oil the initial electric energy in the pulsed power supply. [DOI: 10.1115/1.3172251]
  • Appelgren, Patrik, et al. (författare)
  • Modeling of a small helical magnetic flux compression generator
  • 2008
  • Ingår i: IEEE Transactions on Plasma Science. - 0093-3813. ; 36:5, s. 2662-2672
  • Tidskriftsartikel (refereegranskat)abstract
    •  In order to gain experience in explosive pulsed power and to provide experimental data as the basis for computer modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency. The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high-power pulses for various loads. This paper presents a simulation model of this helical FCG. The model, which was implemented in Matlab-Simulink, uses analytical expressions for the generator inductance. The model of resistive losses takes into account the heating of the conductors and the diffusion of the magnetic field into the conductors. The simulation results are compared with experimental data from two experiments with identical generators but with different seed currents, influencing the resistive losses. The model is used to analyze the performance of the generator.
  • Appelgren, Patrik, et al. (författare)
  • Small Helical Magnetic Flux-Compression Generators : Experiments and Analysis
  • 2008
  • Ingår i: IEEE Transactions on Plasma Science. - 0093-3813. ; 36:5, s. 2673-2683
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to gain experience in explosive pulsed power and to provide experimental data for modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency (FOI). The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high power pulses for various loads. This paper presents the design of, and tests with, this helical FCG. The generator had an initial inductance of 23 mu H and was operated into a load of 0.2 mu H. The generator is charged with 0.27 kg of high explosives (PBXN-5). Various types of diagnostics were used to monitor the operation of the generator, including current probes, optical fibers, and piezo gauges. With seed currents of 5.7 and 11.2 kA, final currents of 269 and 436 kA were obtained, corresponding to current amplification factors of 47 and 39. The peak of the current was reached about 30 mu s after the time of crowbar. The two generators showed only small losses in terms of 2 pi-clocking. Using signals from optical fibers, the deflection angle of the armature could be determined to be 10 degrees in good agreement with hydrodynamic simulations of the detonation process and the detonation velocity to be 8.7 km/s in agreement with tabulated value.
  • Elfsberg, Mattias, et al. (författare)
  • Experimental Studies of Anode and Cathode Materials in a Repetitive Driven Axial Vircator
  • 2008
  • Ingår i: IEEE Transactions on Plasma Science. - 0093-3813. ; 36, s. 688-693
  • Tidskriftsartikel (refereegranskat)abstract
    •  Repetitive use of a high-power microwave (HPM) radiation source implies strong erosion on cathode and anode materials. Electrode material endurance has been studied in a series of experiments with an axial vircator powered by a compact Marx generator. The Marx generator operated in a 10 Hz repetitive mode with a burst of ten pulses. Velvet and graphite was used as electron-emitting materials, and they showed markedly different pulse characteristics. Three different anode materials were used; stainless steel mesh, stainless steel wires and molybdenum wires, which all had different influence on the pulse characteristics.
  • Fu, Ying, et al. (författare)
  • Energy band structure and spectral gain characteristics of dilute-nitride zinc blende InGaNAs quantum wells embedded in GaAs and GaNAs barriers
  • 2006
  • Ingår i: Journal of Applied Physics. - 00218979. ; 100:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The spectral gain characteristics of dilute-nitride zinc blende InxGa1-xNyAs1-y quantum wells embedded in GaNy1As1-y1 barriers have been investigated experimentally and theoretically. Two samples, both with the gain peak at 1300 nm, were studied for comparison. One has a high nitrogen concentration in the quantum well with the surrounding barriers being pure GaAs. The other has a lower and uniform nitrogen concentration in the quantum well and the barriers (GaNAs barriers). Measurements show the redshift of the gain peak induced by the incorporation of nitrogen and difference in the spectral gain characteristics. The energy band structures and spectral gain characteristics are analyzed theoretically using the standard eight-band k center dot p theory. It is shown that the introduction of nitrogen atoms in the GaAs barriers reduces the barrier height for the central quantum well so that the energy sublevels in the conduction band becomes condensed. The condensation of the conduction-band energy sublevels reduces the peak gain and makes the gain spectrum narrower, in agreement with measurements.
  • Han, T.T., et al. (författare)
  • Structural analysis of dilute-nitride zinc blende InxGa1?xNyAs1?y cluster by a semiempirical quantum chemistry study
  • 2007
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 101:12, s. 123707-1-123707-6
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the total energy of a series of dilute-nitride zinc blende Inx Ga1-x Ny As1-y cluster configurations using a semiempirical quantum chemistry method. It was found that In-N bonding is favorable from an energy point of view when the N atom is substitutional (replacing an As atom at the regular zinc blende lattice site) and the In mole fraction is smaller than 25%. In-N bonding is always favorable when the N atom is interstitial. Furthermore, an analysis of the incorporation of N-N pairs showed that substitutional incorporation is favored over interstitial. In addition, the dissociation of a N-N pair was found to depend on the local environment, being either In rich or In-free, along the dissociation trajectory when the average In mole fraction is high. The theoretical results are in agreement with experimental results.
  • Hrastinski, Stefan, et al. (författare)
  • Encouraging participation in an intra-organizational online idea community : A case study of a Swedish municipality
  • 2011
  • Ingår i: First Monday. - 1396-0466. ; 16:10
  • Tidskriftsartikel (refereegranskat)abstract
    • Online communities have flourished in organizations in recent years, but large numbers of them fail. A deeper understanding of how participation can be promoted in online idea communities (OIC) is essential, because the most common reason of failure is low levels of participation. In this paper, we investigated how participation could be encouraged in an intra–organizational OIC. Our case organization was Sollentuna municipality in Sweden. They decided to introduce an OIC to collect ideas from staff as input when developing the municipality’s forthcoming IT strategy. We used the theory of online identity–based communities (Ren, et al., 2007) as a lens and a mixed research method comprising interviews and an analysis of the content of the OIC. While this theoretical perspective certainly has merits on its own, the study topic at hand made it necessary to complement the theory with more specific design principles that take the unique characteristics of intra–organiztional OIC into account. These included that managers were expected by staff to be core members of the community, frequent and complementing promotion activities were necessary in order for the OIC to be used, very low entry barriers were expected, and the employees expected the discussion to be focused, both in terms of content and time. The design principles need to be tested and further developed, by conducting studies on other OIC.
  • Larsson, Anders Olof, 1979-, et al. (författare)
  • Blogs and Blogging : Current trends and future directions
  • 2011
  • Ingår i: First Monday. - 1396-0466. ; 16:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Adopting an interdisciplinary scope, this paper presents a review of research on blogs and blogging within the social sciences and the humanities. It maps out what kind of research has been completed, how it has been performed and what gaps that might need to be filled in this relatively new area of research. More specifically, the paper will analyze all articles on blogs and blogging published until 2009 and indexed by the ISI Web of Knowledge.
  • Lindberg, Hans, 1977-, et al. (författare)
  • Improved spectral properties of an optically pumped semiconductor disk laser using a thin diamond heat spreader as an intracavity filter
  • 2005
  • Ingår i: IEEE Photonics Technology Letters. - 1041-1135. ; 17:7, s. 1363-1365
  • Tidskriftsartikel (refereegranskat)abstract
    • This letter describes an optically pumped high-power InP-based semiconductor disk laser with a thin (50 mu m) diamond heat spreader bonded to the surface of the gain chip. The diamond heat spreader performs the multiple functions of heat removal, spectral filtering, and wavelength stabilization by utilizing the high thermal conductivity and the low thermooptic coefficient of diamond, along with the large free-spectral range of a thin intracavity etalon. A pump-power-limited output power of 680 mW at 1.55 mu m is demonstrated at a heat sink temperature of -30 degrees C, and 140 mW at room temperature. The spectral width was measured to be less than 0.08 nm and the spectral drift with temperature and pump power as low as 0.03 nm/degrees C and 0.14 nm/W, respectively.
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