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  • 1.
    Abramson, Nils
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion.
    Boman, J.
    Bonnevier, Björn
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Plane intersections of rotational ellipsoids2006Ingår i: The American mathematical monthly, ISSN 0002-9890, E-ISSN 1930-0972, Vol. 113, nr 4, s. 336-339Artikel i tidskrift (Refereegranskat)
  • 2. Airila, M. I.
    et al.
    Aho-Mantila, L.
    Brezinsek, S.
    Coad, J. P.
    Kirschner, A.
    Likonen, J.
    Matveev, D.
    Rubel, Marek
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Strachan, J. D.
    Widdowson, A.
    Wiesen, S.
    ERO modelling of local deposition of injected C-13 tracer at the outer divertor of JET2009Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T138, s. 014021-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The 2004 tracer experiment of JET with the injection of (CH4)-C-13 into H-mode plasma at the outer divertor has been modelled with the Monte Carlo impurity transport code ERO. EDGE2D solutions for inter-ELM and ELM-peak phases were used as plasma backgrounds. Local two-dimensional (2D) deposition patterns at the vertical outer divertor target plate were obtained for comparison with post-mortem surface analyses. ERO also provides emission profiles for comparison with radially resolved spectroscopic measurements. Modelling indicates that enhanced re-erosion of deposited carbon layers is essential in explaining the amount of local deposition. Assuming negligible effective sticking of hydrocarbons, the measured local deposition of 20-34% is reproduced if re-erosion of deposits is enhanced by a factor of 2.5-7 compared to graphite erosion. If deposits are treated like the substrate, the modelled deposition is 55%. Deposition measurements at the shadowed area around injectors can be well explained by assuming negligible re-erosion but similar sticking behaviour there as on plasma-wetted surfaces.

  • 3. Allen, S. L.
    et al.
    Wampler, W. R.
    McLean, A. G.
    Whyte, D. G.
    West, W. P.
    Stangeby, P. C.
    Brooks, N. H.
    Rudakov, D. L.
    Phillips, V.
    Rubel, Marek J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Matthews, G. F.
    Nagy, A.
    Ellis, R.
    Bozek, A. S.
    C-13 transport studies in L-mode divertor plasmas on DIII-D2005Ingår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 337-39, nr 03-jan, s. 30-34Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    (CH4)-C-13 was injected with a toroidally-symmetric gas system into 22 identical lower-single-null L-mode discharges on DIII-D. The injection level was adjusted so that it did not significantly perturb the core or divertor plasmas, with a duration of similar to 3 s on each shot, for a total of similar to 300 T L of injected particles. The plasma shape remained very constant; the divertor strike points were controlled to similar to 1 cm at the divertor plate. At the beginning of the subsequent machine vent, 29 carbon tiles were removed for nuclear reaction analysis of C-13 content to determine regions of carbon deposition. It was found that only the tiles inboard of the inner strike point had appreciable 1 3 C above background. Visible spectroscopy measurements of the carbon injection and comparisons with modeling are consistent with carbon transport by means of scrape-off layer flow.

  • 4. Antoni, V.
    et al.
    Drake, James R.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Spada, E.
    Spolaore, M.
    Vianello, N.
    Bergsåker, Henric
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cavazzana, R.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Martines, E.
    Serianni, G.
    Coherent structures and anomalous transport in reversed field pinch plasmas2006Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T122, s. 1-7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The results leading to the identification of coherent structures emerging from the background turbulence in the edge region of the reversed field pinch experiments EXTRAP-T2R and RFX are reviewed. These structures have traits of vortices in velocity field and blobs in density, and the reconstruction of their spatial structure and of their time evolution is discussed focusing on the analysis tools applied. The role of these structures in the particle anomalous transport is addressed, showing that their collisions can contribute up to 50% the total particle losses. This process is shown to be responsible for bursts in particle flux and it is found to set a characteristic collision time, which is in agreement with the statistical properties of laminar times for particle flux bursts.

  • 5. Askinazi, L. G.
    et al.
    Kornev, V. A.
    Krikunov, S. V.
    Krupnik, L. I.
    Lebedev, S. V.
    Smirnov, A. I.
    Tendler, Michael
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Tukachinsky, A. S.
    Vildjunas, M. I.
    Zhubr, N. A.
    Plasma potential evolution in various operational modes in the TUMAN-3M tokamak2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, 2007, nr 3, s. 2010-2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    In a scenario with Counter-NBI it was found using HIBP that, due to the NBI effect (most probably, orbit loss with some heating and momentum impact), core plasma potential plasma gradually became more negative (for ∼200V). Strong positive perturbation of the core plasma potential was registered by the HIBP during the burst of peripheral MHDs with low m, n. If such a burst takes place in the H-mode (both ohmic and counter-NBI heated), the positive potential perturbation leads to H-mode termination. The most probable mechanism of the positive field build-up during MHD burst is though to be a loss of fast electrons along partly disturbed magnetic field lines near the island's separatrix [3,4]. This mechanism is similar to the ergodic divertor's action on the TEXTOR [6], where radial electric field modification by the electron loses was also discussed. A quantitative analysis of the subject may be found in [7]. Similar mechanism may be responsible for a positive perturbation of central plasma potential registered in the sawtooth crashes. The GAM with δφ/φ∼0.3 and δφ/φ≫δn/n∼0.05 where observed with HIBP in a core region of the TUMAN-3M r/a∼0.33 in the current ramp phase. Further studies are needed to reveal a possible connection between the GAM evolution and plasma confinement in the TUMAN-3M.

  • 6. Askinazi, L. G.
    et al.
    Kornev, V. A.
    Krikunov, S. V.
    Krupnik, L. I.
    Lebedev, S. V.
    Smirnov, A. I.
    Tendler, Michael
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Tukachinsky, A. S.
    Vildjunas, M. I.
    Zhubr, N. A.
    Radial electric field evolution in various operational modes in the TUMAN-3M tokamak2008Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 123, s. 012010-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Radial electric field evolution has been studied on the TUAMN-3M tokamak in different modes of operation: ohmic and NBI heating, L- and H-modes, with and without strong MHD activity. Peripheral radial electric field was measured using Langmuire probes, which were inserted up to 2cm inside LCFS, while core plasma potential evolution was measured using HIBP diagnostic. It was found, that in presence of strong MHD activity radial electric field in a vicinity of the island changed sign from negative to positive and could reach up to 4kV/m. Central plasma potential exhibited a positive perturbation of ∼700V during the MHD burst. This positive radial electric field might lead to H-mode termination, both in ohmic and NBI heating cases. Possible mechanism of the positive Er generation, namely the electron losses along ergodized magnetic field lines in the presence of MHD-island, is discussed. The same mechanism might be responsible for the positive potential spikes during a saw-tooth crash, also observed using HIBP. Another phenomenon observed using HIBP was quasi-coherent potential oscillations with the frequency close to one of the GAM. Possible location of these oscillations in the core region r/a ∼ 0.33 is discussed.

  • 7. Bailescu, V.
    et al.
    Burcea, G.
    Balan, N.
    Dinuta, G.
    Serban, G.
    Lungu, C. P.
    Mustata, I.
    Lungu, A. M.
    Rubel, Marek
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Coad, P.
    Pedrick, L.
    Handley, R.
    Inconel tiles coated with beryllium by thermal evaporation2008Ingår i: EPS Conf. Plasma Phys., EPS - Europhys. Conf. Abstr., 2008, nr 3Konferensbidrag (Refereegranskat)
  • 8.
    Bergkvist, Tommy
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hellsten, Torbjörn A K
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Non-linear Alfvén eigenmode dynamics of a burning plasma in the presence of ion cyclotron resonance heating2006Ingår i: EPS Conf. Plasma Phys., EPS, 2006, s. 1792-1795Konferensbidrag (Refereegranskat)
    Abstract [en]

    Alfvén eigenmodes (AEs) excited by α particles in a burning plasma can degrade the heating efficiency by spatial redistribution of the resonant α particles. Changes of the orbit invariants in phase space by collisions and other waves, such as magnetosonic waves during ion cyclotron resonance heating (ICRH), lead to changes in the phase between the αs and AEs, causing a decorrelation of the interactions. ICRH lead to an increased decorrelation of the AE interactions and hence a stronger radial redistribution of the thermonuclear α particles by the AEs. Renewal of the distribution function by thermonuclear reactions and losses of α particles to the wall lead to a continuous drive of the AEs and a radial redistribution of the α particles. The redistribution results in a degradation of the heating efficiency.

  • 9.
    Bergkvist, Tommy
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Effects of ICRH on the dynamics of fast particle excited alfven eigenmodes2007Ingår i: Radio Frequency Power in Plasmas, American Institute of Physics (AIP), 2007, Vol. 933, s. 455-458Konferensbidrag (Refereegranskat)
    Abstract [en]

    ICRH is often used in experiments to simulate destabilization of Alfvén eigenmodes by thermonuclear α-particles. Whereas the slowing down distribution of α-particles is nearly isotropic, the ICRH creates an anisotropic distribution function with non-standard orbits. The ICRH does not only build up gradients in phase space, which destabilizes the AEs, but it also provides a strong phase decorrelation mechanism between ions and AEs. Renewal of the distribution function by thermonuclear reactions and losses of α-particles to the wall lead to a continuous drive of the AEs. Simulations of the non-linear dynamics of AEs and the impact they have on the heating profile due to particle redistribution are presented.

  • 10.
    Bergkvist, Tommy
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hellsten, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Holmström, Kerstin
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Non-linear Alfvén Eigenmode Dynamic of Burning Plasma in the Presence of Ion Cyclotron Resonance Heating2006Ingår i: 33rd EPS Plasma Physics Conference, 2006Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Alfvén eigenmodes (AEs) excited by a particles in a burning plasma can degrade theheating efficiency by spatial redistribution of the resonant a particles. Changes of the orbitinvariants in phase space by collisions and other waves, such as magnetosonic waves duringion cyclotron resonance heating (ICRH), lead to changes in the phase between the as andAEs, causing a decorrelation of the interactions. ICRH lead to an increased decorrelation ofthe AE interactions and hence a stronger radial redistribution of the thermonuclear a particlesby the AEs. Renewal of the distribution function by thermonuclear reactions and lossesof a particles to the wall lead to a continuous drive of the AEs and a radial redistributionof the a particles. The redistribution results in a degradation of the heating efficiency.

  • 11. Berthomier, M.
    et al.
    Fazakerley, A. N.
    Forsyth, C.
    Pottelette, R.
    Alexandrova, O.
    Anastasiadis, A.
    Aruliah, A.
    Blelly, P. -L
    Briand, C.
    Bruno, R.
    Canu, P.
    Cecconi, B.
    Chust, T.
    Daglis, I.
    Davies, J.
    Dunlop, M.
    Fontaine, D.
    Genot, V.
    Gustavsson, B.
    Haerendel, G.
    Hamrin, M.
    Hapgood, M.
    Hess, S.
    Kataria, D.
    Kauristie, K.
    Kemble, S.
    Khotyaintsev, Y.
    Koskinen, H.
    Lamy, L.
    Lanchester, B.
    Louarn, P.
    Lucek, E.
    Lundin, R.
    Maksimovic, M.
    Manninen, J.
    Marchaudon, A.
    Marghitu, O.
    Marklund, Göran T.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Milan, S.
    Moen, J.
    Mottez, F.
    Nilsson, H.
    Ostgaard, N.
    Owen, C. J.
    Parrot, M.
    Pedersen, A.
    Perry, C.
    Pincon, J. -L
    Pitout, F.
    Pulkkinen, T.
    Rae, I. J.
    Rezeau, L.
    Roux, A.
    Sandahl, I.
    Sandberg, I.
    Turunen, E.
    Vogt, J.
    Walsh, A.
    Watt, C. E. J.
    Wild, J. A.
    Yamauchi, M.
    Zarka, P.
    Zouganelis, I.
    Alfvén: magnetosphere-ionosphere connection explorers2012Ingår i: Experimental astronomy (Print), ISSN 0922-6435, E-ISSN 1572-9508, Vol. 33, nr 2-3, s. 445-489Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The aurorae are dynamic, luminous displays that grace the night skies of Earth's high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth's atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The "ideal magnetohydrodynamics" description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfv,n concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets.

  • 12. Beurskens, M N A
    et al.
    Osborne, T H
    Schneider, P A
    Wolfrum, E
    Frassinetti, Lorenzo
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Groebner, R
    Lomas, P
    Nunes, I
    Saarelma, S
    Scannell, R
    Snyder, P B
    Zarzoso, D
    Balboa, I
    Bray, B
    Brix, M
    Flanagan, J
    Giroud, C
    Giovannozzi, E
    Kempenaars, M
    Loarte, A
    de la Luna, E
    Maddison, G
    Maggi, C F
    McDonald, D
    Pasqualotto, R
    Saibene, G
    Sartori, R
    Solano, E
    Walsh, M
    Zabeo, L
    Team, D I I I-D
    Team, ASDEX Upgrade
    Contributors, J E T-E F D A
    H-mode pedestal scaling in DIII-D, ASDEX Upgrade, and JET2011Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 18, nr 5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Multidevice pedestal scaling experiments in the DIII-D, ASDEX Upgrade (AUG), and JET tokamaks are presented in order to test two plasma physics pedestal width models. The first model proposes a scaling of the pedestal width Delta/a proportional to rho*(1/2) to rho* based on the radial extent of the pedestal being set by the point where the linear turbulence growth rate exceeds the E x B velocity. In the multidevice experiment where rho* at the pedestal top was varied by a factor of four while other dimensionless parameters where kept fixed, it has been observed that the temperature pedestal width in real space coordinates scales with machine size, and that therefore the gyroradius scaling suggested by the model is not supported by the experiments. The density pedestal width is not invariant with rho* which after comparison with a simple neutral fuelling model may be attributed to variations in the neutral fuelling patterns. The second model, EPED1, is based on kinetic ballooning modes setting the limit of the radial extent of the pedestal region and leads to Delta(psi) proportional to beta p(1/2). All three devices show a scaling of the pedestal width in normalised poloidal flux as Delta(psi) proportional to beta p(1/2), as described by the kinetic ballooning model; however, on JET and AUG, this could not be distinguished from an interpretation where the pedestal is fixed in real space. Pedestal data from all three devices have been compared with the predictive pedestal model EPED1 and the model produces pedestal height values that match the experimental data well.

  • 13. Blixt, E. M.
    et al.
    Grydeland, T.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet. University of Southampton, United Kingdom .
    Hagfors, T.
    La Hoz, C.
    Lanchester, B. S.
    Løvhaug, U. P.
    Trondsen, T. S.
    Dynamic rayed aurora and enchanced ion-acoustic radar echoes2005Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, nr 1, s. 3-11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these Identified features are directly related to the presence of low energy (10-100 eV) precipitating electrons in addition to the higher energy population producing most of the associated light. The low energy contribution is vital for the formation of the enhanced ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes. In one event two imagers were used to observe the auroral rays simultaneously, one from the radar site and one 7 km away. The data from these imagers shows that the auroral rays and the strong backscattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.

  • 14. Blixt, E. M.
    et al.
    Grydeland, T.
    Ivchenko, Nickolay V.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hagfors, T.
    La Hoz, C.
    Lanchester, B. S.
    Lovhaug, U. P.
    Trondsen, T. S.
    Dynamic rayed aurora and enhanced ion-acoustic radar echoes2005Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, nr 1, s. 3-11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these identified features are directly related to the presence of low energy (10-100 eV) precipitating electrons in addition to the higher energy population producing most of the associated light. The low energy contribution is vital for the formation of the enhanced ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes. In one event two imagers were used to observe the auroral rays simultaneously, one from the radar site and one 7 kin away. The data from these imagers shows that the auroral rays and the strong backscattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.

  • 15.
    Blomberg, Lars G.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cumnock, Judy
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Alexeev, I.I.
    Belenkaya, E. S.
    Bobrovnikov, S. Y.
    Kalegaev, V. V.
    Transpolar aurora: time evolution, associated convection patterns, and a possible cause2005Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, nr 5, s. 1917-1930Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present two event studies illustrating the detailed relationships between plasma convection, field-aligned currents, and polar auroral emissions, as well as illustrating the influence of the Interplanetary Magnetic Field's y-component on theta aurora development. The transpolar are of the theta aurorae moves across the entire polar region and becomes part of the opposite side of the auroral oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the POLAR UVI instrument provides measurements of auroral emissions. Ionospheric electrostatic potential patterns are calculated at different times during the evolution of the theta aurora using the KTH model. These model patterns are compared to the convection predicted by mapping the magnetopause electric field to the ionosphere using the Paraboloid Model of the magnetosphere. The model predicts that parallel electric fields are set up along the magnetic field lines projecting to the transpolar aurora. Their possible role in the acceleration of the auroral electrons is discussed.

  • 16.
    Bogdanowicz, Janusz
    et al.
    IMEC, Kapeldreef 75, B-3000 Leuven, Belgium.
    Gilbert, Matthieu
    IMEC, Kapeldreef 75, B-3000 Leuven, Belgium.
    Innocenti, Nicolas
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Koelling, Sebastian
    IMEC, Kapeldreef 75, B-3000 Leuven, Belgium.
    Vanderheyden, Benoit
    4 Institut Montefiore, Sart-Tilman B28, B-4000 Liege, Belgium.
    Vandervorst, Wilfried
    IMEC, Kapeldreef 75, B-3000 Leuven, Belgium.
    Light absorption in conical silicon particles2013Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, nr 3, s. 3891-3896Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The problem of the absorption of light by a nanoscale dielectric cone is discussed. A simplified solution based on the analytical Mie theory of scattering and absorption by cylindrical objects is proposed and supported by the experimental observation of sharply localized holes in conical silicon tips after high-fluence irradiation. This study reveals that light couples with tapered objects dominantly at specific locations, where the local radius corresponds to one of the resonant radii of a cylindrical object, as predicted by Mie theory.

  • 17. Boswell, C. J.
    et al.
    Berk, H. L.
    Borba, D.
    Johnson, Tomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Nave, M. F. F.
    Pinches, S. D.
    Sharapov, S. E.
    Observation and explanation of the JET n = 0 chirping mode2005Konferensbidrag (Refereegranskat)
  • 18. Brezinsek, S.
    et al.
    Jachmich, S.
    Stamp, M. F.
    Meigs, A. G.
    Coenen, J. W.
    Krieger, K.
    Giroud, C.
    Groth, M.
    Philipps, V.
    Grünhagen, S.
    Smith, R.
    Van Rooij, G. J.
    Ivanova, Darya
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Matthews, G. F.
    Residual carbon content in the initial ITER-Like Wall experiments at JET2013Ingår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 438, nr Suppl., s. S303-S308Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The residual carbon content and carbon edge flux in JET have been assessed by three independent diagnostic techniques after start of plasma operation with the ITER-Like Wall (ILW) with beryllium first wall and tungsten divertor: (i) in-situ measurements with optical spectroscopy on low ionisation stages of carbon, (ii) charge-exchange recombination spectroscopy, and (iii) residual gas composition analysis in dedicated global gas balance experiments. Direct comparison experiments in L-mode discharges were carried out between references from the previously installed material configuration with plasma-facing components made of carbon-fibre composite (JET-CFC) and the JET-ILW. The temporal evolution of the C divertor flux since installation of the ILW has been studied in the ohmic phase of dedicated monitoring discharges which have been executed regularly throughout the experimental exploitation so far (60000 plasma seconds). The C flux behaviour in the divertor can be divided in three phases: initial fast drop, moderate reduction phase, and a long lasting phase with almost constant C flux. The Be flux in both divertor legs mirrors the behaviour of C. All experiments and diagnostic techniques demonstrate a strong reduction in C fluxes and C content of more than one order of magnitude with respect to JET-CFC which is in line with the reduction in long-term fuel retention due to co-deposition. There is no evidence of an increase in residual carbon in time, thus no indication that a damage of the thin tungsten coatings on CFC substrate in the divertor occurred.

  • 19.
    Brunsell, Per R.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Kuldkepp, Mattias
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Menmuir, Sheena
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hedqvist, Anders
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Yadikin, Dimitry
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Rachlew, Elisabeth
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R2006Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 46, nr 11, s. 904-913Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Discharges in the thin shell reversed field pinch (RFP) device EXTRAP T2R without active feedback control are characterized by growth of non-resonant m = 1 unstable resistive wall modes (RWMs) in agreement with linear MHD theory. Resonant m = 1 tearing modes (TMs) exhibit initially fast rotation and the associated perturbed radial fields at the shell are small, but eventually TMs wall-lock and give rise to a growing radial field. The increase in the radial field at the wall due to growing RWMs and wall-locked TMs is correlated with an increase in the toroidal loop voltage, which leads to discharge termination after 3-4 wall times. An active magnetic feedback control system has been installed in EXTRAP T2R. A two-dimensional array of 128 active saddle coils (pair-connected into 64 independent m = 1 coils) is used with intelligent shell feedback control to suppress the m = 1 radial field at the shell. With feedback control, active stabilization of the full toroidal spectrum of 16 unstable m = 1 non-resonant RWMs is achieved, and TM wall locking is avoided. A three-fold extension of the pulse length, up to the power supply limit, is observed. Intelligent shell feedback control is able to maintain the plasma equilibrium for 10 wall times, with plasma confinement parameters sustained at values comparable to those obtained in thick shell devices of similar size.

  • 20.
    Brunsell, Per
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Yadikin, Dmitriy
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Marchiori, Giuseppe
    Feedback stabilization of resistive wall modes in a reversed-field pinch2005Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, nr 9, s. 092508-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An array of saddle coils having Nc =16 equally spaced positions along the toroidal direction has been installed for feedback control of resistive wall modes (RWMs) on the EXTRAP T2R reversed-field pinch [P. R. Brunsell, H. Bergsaker, M. Cecconello, Plasma Phys. Controlled Fusion 43, 1457 (2001)]. Using feedback, multiple nonresonant RWMs are simultaneously suppressed for three to four wall times. Feedback stabilization of RWMs results in a significant prolongation of the discharge duration. This is linked to a better sustainment of the plasma and tearing mode toroidal rotation with feedback. Due to the limited number of coils in the toroidal direction, pairs of modes with toroidal mode numbers n, n′ that fulfill the condition ∫n- n′ ∫ = Nc are coupled by the feedback action from the discrete coil array. With only one unstable mode in a pair of coupled modes, the suppression of the unstable mode is successful. If two modes are unstable in a coupled pair, two possibilities exist: partial suppression of both modes or, alternatively, complete stabilization of one target mode while the other is left unstable.

  • 21.
    Brunsell, Per
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Yadikin, Dmitriy
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Menmuir, Sheena
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Rachlew, Elisabeth
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Atom- och molekylfysik.
    Zanca, P.
    Active control of multiple resistive wall modes2005Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 47, nr 12 B, s. B25-B36Artikel i tidskrift (Refereegranskat)
    Abstract [en]

     A two-dimensional array of saddle coils at M-c poloidal and N-c toroidal positions is used on the EXTRAP T2R reversed-field pinch (Brunsell P R et al 2001 Plasma Phys. Control. Fusion 43 1457) to study active control of resistive wall modes (RWMs). Spontaneous growth of several RWMs with poloidal mode number m = 1 and different toroidal mode number n is observed experimentally, in agreement with linear MHD modelling. The measured plasma response to a controlled coil field and the plasma response computed using the linear circular cylinder MHD model are in quantitive agreement. Feedback control introduces a linear coupling of modes with toroidal mode numbers n, n' that fulfil the condition vertical bar n - n'vertical bar = N-c. Pairs of coupled unstable RWMs are present in feedback experiments with an array of Mc x Nc = 4 x 16 coils. Using intelligent shell feedback, the coupled modes are generally not controlled even though the field is suppressed at the active coils. A better suppression of coupled modes may be achieved in the case of rotating modes by using the mode control feedback scheme with individually set complex gains. In feedback with a larger array of Mc x Nc = 4 x 32 coils, the coupling effect largely disappears, and with this array, the main internal RWMs n = -11, -10, +5, +6 are all simultaneously suppressed throughout the discharge (7-8 wall times). With feedback there is a two-fold extension of the pulse length, compared to discharges without feedback.

  • 22.
    Carlqvist, Per
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    A remarkable double helix in the V838 Mon nebula2005Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 436, nr 1, s. 231-239Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the beginning of 2002 the previously unnoted star V838 Mon had a powerful outburst. The star is surrounded by pre-existing, dusty clouds which are illuminated by the star in an expanding, parabolic layer. Spectacular images captured by the Hubble Space Telescope Advanced Camera for Surveys show that the clouds are to a great extent built up by filaments and concentric shells. One of the most remarkable features is a filamentary structure forming a double helix. The structure, which has a projected length and width of similar to 9 '' and similar to 1.'' 4, respectively, points almost radially towards V838 Mon. In order to reveal the geometry of the double helix in some more detail, a three-dimensional computer model of the structure has been constructed. The model also assists in determining the expansion rate of the light echoes along the double helix. By means of the expansion rate and the tilt of the double helix the distance to V838 Mon is determined to be 2.4 +/- 0.5 kpc. A theory of the double helix, based on a magnetized and twisted filament, is presented. Dynamic and magnetic forces play an essential role in the shaping of the double helix. The theory is supported by a mechanical analogy model. Double helices in other cosmic environments are also discussed.

  • 23. Castaldo, C.
    et al.
    Ratynskaia, Svetlana
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Pericoli, V.
    Rypdal, K.
    De Angelis, U.
    Morfill, G. E.
    Pieroni, L.
    Capobianco, G.
    De Angeli, M.
    Gabellieri, L.
    Giovannozzi, E.
    Maddaluno, G.
    Marmolino, C.
    Orsitto, F.
    Romano, A.
    Rufoloni, A.
    Tuccillo, A. A.
    Fast dust particles in tokamak plasmas: Detection and effects2007Ingår i: 34th EPS Conference on Plasma Physics 2007, EPS 2007 - Europhysics Conference Abstracts, European Physical Society , 2007, nr 2, s. 848-851Konferensbidrag (Refereegranskat)
  • 24.
    Cecconello, Marco
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Brunsell, Per R.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Yadikin, Dmitriy
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Drake, James Robert
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Rotation evolution of tearing modes during feedback stabilization of resistive wall modes in a reversed field pinch2005Konferensbidrag (Refereegranskat)
  • 25.
    Cecconello, Marco
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Kuldkepp, Mattias
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Menmuir, Sheena
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Hedqvist, Anders
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Brunsell, Per R.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Current profile modifications with active feedback stabilization of resistive wall modes in a reversed field pinch2006Ingår i: Proceedings of the 33rd European Physical Society Conference on Plasma Physics, 2006, s. 1680-1683Konferensbidrag (Refereegranskat)
  • 26.
    Cecconello, Marco
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Menmuir, Sheena
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Brunsell, Per R.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Kuldkepp, Mattias
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Rotation in a reversed field pinch with active feedback stabilization of resistive wall modes2006Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 48, nr 9, s. 1311-1331Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Active feedback stabilization of multiple resistive wall modes (RWMs) has been successfully proven in the EXTRAP T2R reversed field pinch. One of the features of plasma discharges operated with active feedback stabilization, in addition to the prolongation of the plasma discharge, is the sustainment of the plasma rotation. Sustained rotation is observed both for the internally resonant tearing modes (TMs) and the intrinsic impurity oxygen ions. Good quantitative agreement between the toroidal rotation velocities of both is found: the toroidal rotation is characterized by an acceleration phase followed, after one wall time, by a deceleration phase that is slower than in standard discharges. The TMs and the impurity ions rotate in the same poloidal direction with also similar velocities. Poloidal and toroidal velocities have comparable amplitudes and a simple model of their radial profile reproduces the main features of the helical angular phase velocity. RWMs feedback does not qualitatively change the TMs behaviour and typical phenomena such as the dynamo and the `slinky' are still observed. The improved sustainment of the plasma and TMs rotation occurs also when feedback only acts on internally non- resonant RWMs. This may be due to an indirect positive effect, through non- linear coupling between TMs and RWMs, of feedback on the TMs or to a reduced plasma- wall interaction affecting the plasma flow rotation. Electromagnetic torque calculations show that with active feedback stabilization the TMs amplitude remains well below the locking threshold condition for a thick shell. Finally, it is suggested that active feedback stabilization of RWMs and current profile control techniques can be employed simultaneously thus improving both the plasma duration and its confinement properties.

  • 27.
    Chen, Biao
    et al.
    Centre for Optical and Electromagnetic Research, Joint Laboratory of Optical Communications, Zhejiang University, Hangzhou 310027, China.
    Chen, Jiajia
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik (Stängd 20120101), Fotonik (Stängd 20120101).
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Efficient and fine scheduling algorithm for bandwidth allocation in Ethernet passive optical networks2006Ingår i: IEEE Journal of Selected Topics in Quantum Electronics, ISSN 1077-260X, E-ISSN 1558-4542, Vol. 12, nr 4, s. 653-660Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel fine scheduling algorithm is introduced for upstream bandwidth allocation in an Ethernet-based passive optical network. This scheduling algorithm consists of an inter optical network unit (ONU) scheduler at the optical line terminal (OLT) and an intra-ONU scheduler at each ONU. In the inter-ONU scheduling, a novel GATE/REPORT approach is introduced to eliminate the unused remainders without transmission delay and maximize the utilization of bandwidth. Our novel intra-ONU scheduler gives fair bandwidth allocation to the queues of different priorities for each user in a hierarchical and decentralized way. Numerical results have shown that our overall scheduling algorithm can fulfill various requirements of delay and throughput for the transmission of multimedia traffic for each end user..

  • 28. Chen, Biao
    et al.
    Guo, Changjian
    Chen, Jiajia
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik (Stängd 20120101), Fotonik (Stängd 20120101).
    Zhang, Linjian
    Jiang, Qiong
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Add/drop multiplexing and TDM signal transmission in an optical CDMA ring network2007Ingår i: Journal of Optical Networking, ISSN 1536-5379, Vol. 6, nr 8, s. 969-974Artikel i tidskrift (Refereegranskat)
  • 29.
    Chen, Daru
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    Yu, Zhangwei
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    Qin, S.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Switchable dual-wavelength Raman erbium-doped fibre laser2006Ingår i: Electronics Letters, ISSN 0013-5194, E-ISSN 1350-911X, Vol. 42, nr 4, s. 202-204Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A fibre Bragg grating feedback fibre laser with both Raman and erbium-doped fibre pumps is proposed. Dual-wavelength switching is achieved by controlling the power of the Raman pump. The characteristics of the dual-wavelength switching are studied experimentally, and the mechanism is explained physically.

  • 30.
    Chen, Jiajia
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik (Stängd 20120101), Fotonik (Stängd 20120101).
    Chen, Biao
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    A novel algorithm for intra-ONU bandwidth allocation in ethernet passive optical networks2005Ingår i: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 9, nr 9, s. 850-852Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel decentralized algorithm is introduced for the intra-ONU bandwidth allocation in an Ethernet Passive Optical Network (EPON). The algorithm is of low computational complexity, and can guarantee both the priority and fairness of the differentiated services. Simulation results are presented and compared with those of two existing bandwidth allocation algorithms.

  • 31.
    Chen, Jiajia
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik (Stängd 20120101), Fotonik (Stängd 20120101).
    Chen, Biao
    Centre for Optical Electromagnetic Research, Joint Laboratory of Optical Communications, Zhejiang University, Hangzhou 310027, China.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Self-protection scheme against failures of distributed fiber links in an Ethernet passive optical network2006Ingår i: Journal of Optical Networking, ISSN 1536-5379, Vol. 5, nr 9, s. 662-666Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel self-protection scheme for an Ethernet passive optical network is introduced and studied at both the physical and the media access control layers. The scheme is simple and fast and can provide 1: 1 protection and automatic traffic restoration against the fiber link failure between a remote node ( RN) and any optical network unit (ONU). Simulation results show that fiber failure does not degrade the transmission performance, and the restoration time depends mainly on the switch time of the physical layer. Our protection scheme saves many long fibers, does not influence other normal ONUs, and requires no active device in the RN.

  • 32. Coad, J. P.
    et al.
    Rubel, Marek J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Bekris, N.
    Brennan, D.
    Hole, D.
    Likonen, J.
    Vainonen-Ahlgren, E.
    Distribution of hydrogen isotopes, carbon and beryllium on in-vessel surfaces in the various jet divertors2005Ingår i: Fusion science and technology, ISSN 1536-1055, E-ISSN 1943-7641, Vol. 48, nr 1, s. 551-556Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    JET has operated with divertors of differing geometries since 1994. Impurities accumulated in the inner leg of all the divertors, and operation of the first (Mk I) divertor with beryllium tiles demonstrated that most are eroded from the main chamber walls and swept along the scrape-off layer to the inner divertor. Carbon deposited at the inner divertor is then locally transported to shadowed regions such as the inner louvres, where, for example, most of the tritium was trapped during the deuterium-tritium experiment (DTE1). Factors affecting these transport processes (e.g. temperature) are important for ITER, but are not well understood.

  • 33.
    Collier, Andrew
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    VLF and ULF Waves Associated with Magnetospheric Substorms2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    A magnetospheric substorm is manifested in a variety of phenomena observed both in space and on the ground. Two electromagnetic signatures are the Substorm Chorus Event (SCE) and Pi2 pulsations.

    The SCE is a Very Low Frequency (VLF) radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into an observer’s field of view from an injection site around midnight. The ascending frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts and the modification of the resonance conditions resulting from the radial component of the E × B drift. Two numerical models have been developed which simulate the production of a SCE. One accounts for both radial and azimuthal electron drifts but treats the wave-particle interaction in an approximate fashion, while the other retains only the azimuthal drift but rigorously calculates both the electron anisotropy and the wave growth rate. Results from the latter model indicate that the injected electron population should have an enhanced high-energy tail in order to produce a realistic SCE.

    Pi2 are damped Ultra Low Frequency (ULF) pulsations with periods between 40 and 150 s. The impulsive metamorphosis of the nightside inner magnetosphere during the onset of the substorm expansion phase is accompanied by a broad spectrum of magnetohydrodynamic (MHD) waves. Over a limited range of local times around midnight these waves excite field line resonances (FLRs) on field lines connected with the auroral zone. Compressional waves propagate into the inner magnetosphere, where they generate cavity mode resonances. The uniform frequency of Pi2 pulsations at middle and low latitudes is a consequence of these cavity modes. A number of Pi2 events were identified at times when the Cluster constellation was located in the nightside inner magnetosphere. Electric and magnetic field data from Cluster were used to establish the existence of both cavity and field line resonances during these events. The associated Poynting flux indicated negligible radial or field-aligned energy flow but an appreciable azimuthal flux directed away from midnight.

  • 34.
    Collier, Andrew
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Hughes, Arthur Robert W
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Lichtenberger, J
    Steinbach, P
    Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations2006Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, nr 2, s. 529-542Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Lightning Imaging Sensor (LIS) data have been analysed to ascertain the statistical pattern of lightning occurrence over southern Africa. The diurnal and seasonal variations are mapped in detail. The highest flash rates (107.2 km(-2) y(-1)) occur close to the equator but maxima are also found over Madagascar (32.1 km(-2) y(-1)) and South Africa (26.4 km(-2) y(-1)). A feature of the statistics is a relatively steady contribution from over the ocean off the east coast of South Africa that appears to be associated with the Agulhas current. Lightning statistics are of intrinsic meteorological interest but they also relate to the occurrence of whistlers in the conjugate region. Whistler observations are made at Tihany, Hungary. Statistics reveal that the period of most frequent whistler occurrence does not correspond to the maximum in lightning activity in the conjugate region but is strongly influenced by ionospheric illumination and other factors. The whistler/flash ratio, R, shows remarkable variations during the year and has a peak that is narrowly confined to February and March.

  • 35.
    Cumnock, Judy A.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    High-latitude aurora during steady northward interplanetary magnetic field and changing IMF B-y2005Ingår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, nr A2, s. A02304-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-latitude transpolar arcs (TPAs) are often observed during northward interplanetary magnetic field (IMF); of these, theta aurora are seen when transpolar arcs move in the dawn or dusk direction across the entire polar region in response to IMF By changes. Periods of study were chosen when By changes sign during steady northward IMF in order to determine the influence of IMF B-x, B-y, the strength of the IMF, the solar wind, and Earth dipole tilt on the occurrence and motion of high-latitude TPAs forming theta aurora. For a 4.5-year period there are 55 events for which IMF B-z is northward for at least 2 hours before and at least 3 hours after a B-y sign change. Of these, 19 occurred when the Polar satellite was over the Northern Hemisphere for the duration of the event. We find that for northward IMF and a B-y sign change theta aurora are almost always formed in the Northern Hemisphere, regardless of B-x and dipole tilt. This implies that theta aurorae form simultaneously in both hemispheres. IMF B-y does not appear to influence the intensity and duration of the arc. Strongest UV emissions occur in the summer hemisphere. Evolution time has a fairly complex dependence on solar wind parameters.

  • 36.
    Cumnock, Judy A.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Blomberg, Lars G.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Alexeev, I. I.
    Belenkaya, E. S.
    Bobrovnikov, S. Yu.
    Kalegaev, V. V.
    Simultaneous polar aurorae and modelled convection patterns in both hemispheres2006Ingår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 38, nr 8, s. 1685-1693Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an event study illustrating the relationships between plasma convection and polar auroral emissions, as well as illustrating the influence of the interplanetary magnetic field's y-component on theta aurora development in both hemispheres. Transpolar arcs (TPAs) are often observed during northward IMF with duskside (dawnside) formation of the TPA and dawnward (duskward) motion occurring when B-y changes from positive to negative in the northern (southern) hemisphere. POLAR UVI provides images in the northern hemisphere while DMSP provides ionospheric plasma flow and precipitating particle data in both hemispheres. Concurrent solar wind plasma and interplanetary magnetic field measurements are provided by the ACE satellite. Utilizing the satellite data as inputs, the Royal Institute of Technology (KTH) numerical model provides the high-latitude ionospheric electrostatic potential patterns in both hemispheres calculated at different times during the evolution of the theta aurora resulting from a variety of field-aligned current configurations associated with the changing global aurora. These model patterns are compared to the convection predicted by mapping the magnetopause electric field to the ionosphere using the Moscow State University's (MSU) paraboloid model of the magnetosphere. The model predicts that parallel electric fields are set up along the magnetic field lines projecting to the transpolar aurora. Their possible role in the acceleration of the auroral electrons is discussed.

  • 37.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Lanchester, Betty S.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Whiter, Daniel K.
    Variations in energy, flux, and brightness of pulsating aurora measured at high time resolution2017Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 35, nr 3, s. 493-503Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-resolution multispectral optical and incoherent scatter radar data are used to study the variability of pulsating aurora. Two events have been analysed, and the data combined with electron transport and ion chemistry modelling provide estimates of the energy and energy flux during both the ON and OFF periods of the pulsations. Both the energy and energy flux are found to be reduced during each OFF period compared with the ON period, and the estimates indicate that it is the number flux of foremost higher-energy electrons that is reduced. The energies are found never to drop below a few kilo-electronvolts during the OFF periods for these events. The high-resolution optical data show the occurrence of dips in brightness below the diffuse background level immediately after the ON period has ended. Each dip lasts for about a second, with a reduction in brightness of up to 70% before the intensity increases to a steady background level again. A different kind of variation is also detected in the OFF period emissions during the second event, where a slower decrease in the background diffuse emission is seen with its brightness minimum just before the ON period, for a series of pulsations. Since the dips in the emission level during OFF are dependent on the switching between ON and OFF, this could indicate a common mechanism for the precipitation during the ON and OFF phases. A statistical analysis of brightness rise, fall, and ON times for the pulsations is also performed. It is found that the pulsations are often asymmetric, with either a slower increase of brightness or a slower fall.

  • 38.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Schlatter, Nicola Manuel
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Roth, Lorenz
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Karlsson, Alexander
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Relation of anomalous F region radar echoes in the high-latitude ionosphere to auroral precipitation2017Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 35, nr 3, s. 475-479Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Non-thermal echoes in incoherent scatter radar observations are occasionally seen in the high-latitude ionosphere. Such anomalous echoes are a manifestation of plasma instabilities on spatial scales matching the radar wavelength. Here we investigate the occurrence of a class of spatially localized anomalous echoes with an enhanced zero Doppler frequency feature and their relation to auroral particle precipitation. The ionization profile of the E region is used to parametrize the precipitation, with nmE and hmE being the E region peak electron density and the altitude of the peak, respectively. We find the occurrence rate of the echoes to generally increase with nmE and decrease with hmE, thereby indicating a correlation between the echoes and high-energy flux precipitation of particles with a high characteristic energy. The highest occurrence rate of > 20% is found for hmE = 109 km and nmE D 10(11.9) m(-3), averaged over the radar observation volume.

  • 39.
    Dahlin, Jon-Erik
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Numerical studies of current profile control in the reversed-field pinch2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The Reversed-Field Pinch (RFP) is one of the major alternatives for realizing energy production from thermonuclear fusion. Compared to alternative configurations (such as the tokamak and the stellarator) it has some advantages that suggest that an RFP reactor may be more economic. However, the conventional RFP is flawed with anomalously large energy and particle transport (which leads to unacceptably low energy confinement) due to a phenomenon called the "RFP dynam".

    The dynamo is driven by the gradient in the plasma current in the plasma core, and it has been shown that flattening of the plasma current profile quenches the dynamo and increases confinement. Various forms of current profile control schemes have been developed and tested in both numerical simulations and experiments.

    In this thesis an automatic current profile control routine has been developed for the three-dimensional, non-linear resistive magnetohydrodynamic computer code DEBSP. The routine utilizes active feedback of the dynamo associated fluctuating electric field, and is optimized for replacing it with an externally supplied field while maintaining field reversal. By introducing a semi-automatic feedback scheme, the number of free parameters is reduced, making a parameter scan feasible. A scaling study was performed and scaling laws for the confinement of the advanced RFP (an RFP with enhanced confinement due to current profile control) have been obtained.

    The conclusions from this research project are that energy confinement is enhanced substantially in the advanced RFP and that poloidal beta values are possible beyond the previous theoretical limit beta βΘ < ½. Scalings toward the reactor regime indicate strongly enhanced confinement as compared to conventional RFP scenarios, but the question of reactor viability remains open.

  • 40.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    A novel feedback algorithm for simulating controlled dynamics and confinement in the advanced reversed-field pinch2005Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, nr 6, s. 062502-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the advanced reversed-field pinch (RFP), the current density profile is externally controlled to diminish tearing instabilities. Thus the scaling of energy confinement time with plasma current and density is improved substantially as compared to the conventional RFP. This may be numerically simulated by introducing an ad hoc electric field, adjusted to generate a tearing mode stable parallel current density profile. In the present work a current profile control algorithm, based on feedback of the fluctuating electric field in Ohm's law, is introduced into the resistive magnetohydrodynamic code DEBSP [D. D. Schnack and D. C. Baxter, J. Comput. Phys. 55, 485 (1984); D. D. Schnack, D. C. Barnes, Z. Mikic, D. S. Marneal, E. J. Caramana, and R. A. Nebel, Comput. Phys. Commun. 43, 17 (1986)]. The resulting radial magnetic field is decreased considerably, causing an increase in energy confinement time and poloidal beta. It is found that the parallel current density profile spontaneously becomes hollow, and that a formation, being related to persisting resistive g modes, appears close to the reversal surface.

  • 41.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Improved Computer Simulations of Energy Confinement in the Advanced Reversed-field Pinch2006Ingår i: 33rd EPS Conference on Plasma Phys, 2006Konferensbidrag (Refereegranskat)
    Abstract [en]

    A revised algorithm for numerical simulations of the advanced reversed-field pinch (RFP) is presented. The results show improved scalings of magnetic fluctuations, energy confinement time τE and poloidal beta βθ with basic initial parameters as compared to what has been presented by the authors in earlier studies of the advanced RFP. The improved behaviour of the advanced RFP stems from the introduction of current profile control (CPC), implemented through a scheme of active feedback of the electric dynamo field. The work, which has an optimistic approach and sweeps over a large parameter domain reaching into the reactor relevant region, is theoretical and claims to answer the question of how far CPC can bring the RFP concept in principle. Experimental implementation is thus a later concern. With this scheme, a state with strongly suppressed tearing mode activity is achieved, which allows for a theoretical study of pressure driven resistive g-modes. This is a task that has been very hard to perform in the past, since tearing modes have always dominated the RFP dynamics. Thus it is now possible, for the first time, to investigate whether pressure driven modes, which are persistent in the RFP, are fatal for the confinement of a high-beta RFP configuration or if they can be accepted in a future reactor.

  • 42.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Numerical studies of confinement scalings for the dynamo-free reversed-field pinch2007Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, nr 1, s. 9-16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the reversed-field pinch (RFP), tearing modes associated with the dynamo are responsible for reduced energy- and particle confinement. In this study, it is observed that by implementing current profile control (CPC) in the RFP, a dynamo-free state can be achieved. The effect of CPC in the RFP is examined by the use of numerical simulations, and scaling laws are presented for confinement parameters. The model is nonlinear MHD in 3D including finite resistivity and pressure. A linear regression analysis is performed on simulation data from a series of computer runs for a set of initial parameter values. Scaling laws are determined for radial magnetic field, energy confinement time, poloidal beta and temperature. Confinement is improved substantially as compared with the conventional RFP - the temperature reaches reactor relevant levels by ohmic heating alone. It is observed that the configuration spontaneously develops into a quasi single helicity state. The CPC scheme is designed to eliminate the fluctuating electric dynamo field Ef ≤ -〈v × B〉, using feedback of an externally imposed electric field. The focus of this study is on obtaining principal theoretical optimization of confinement in the RFP by implementing CPC and to formulate scaling laws for confinement parameters, thus investigating the reactor viability of the concept.

  • 43.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Ultra-high beta in numerical simulations of a tearing-mode reduced reversed-field pinch2007Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, nr 9, s. 1184-1188Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the advanced reversed-field pinch (RFP), current profile control (CPC) enables energy confinement time and poloidal beta to increase substantially as compared with the conventional RFP due to reduced magnetic field stochasticity. Numerical simulations using the three-dimensional non-linear resistive MHD-code DEBSP are performed showing that the poloidal beta is not limited to the m ≤ 0 stability criterion βθ < 1/2. Instead, as tearing modes are diminished, it may approach unity. The beta criterion is theoretically analysed and a new, more general, criterion is derived. Analytic estimates of the resistive tearing and g-mode growth rates are derived for m ≤ 0, and it is shown that both tearing and g-mode growth rates decrease significantly as CPC is employed. Furthermore, quasi-steady state operation with increased confinement due to active control of the current profile is numerically demonstrated for the advanced RFP for a scenario with βθ < 1/2.

  • 44.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Anderson, Jay
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Numerical studies of active current profile control in the reversed-field pinch2007Ingår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 49, nr 2, s. 183-195Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Quenching of the reversed-field pinch (RFP) dynamo is observed in numerical simulations using current profile control. A novel algorithm employing active feedback of the dynamo field has been utilized. The quasi-steady state achieved represents an important improvement as compared with earlier numerical work and may indicate a direction for the design of future experiments. Both earlier and the novel schemes of feedback control result in quasi-single helicity states. The energy confinement time and poloidal beta are observed to be substantially increased, as compared with the conventional RFP, in both the cases. Different techniques for experimental implementation are discussed.

  • 45.
    Dai, Daoxin
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    He, Jian
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    He, Sailing
    Elimination of multimode effects in a silicon-on-insulator etched diffraction grating demultiplexer with bi-level taper structure2005Ingår i: IEEE Journal of Selected Topics in Quantum Electronics, ISSN 1077-260X, E-ISSN 1558-4542, Vol. 11, nr 2, s. 439-443Artikel i tidskrift (Refereegranskat)
    Abstract [en]

     Multimode effects in the free propagation region (FPR) of an etched diffraction grating (EDG) demultiplexer based on silicon-on-insulator are analyzed. The insertion loss and the crosstalk increase due to these undesired multimode effects. A bi-level taper structure between the FPR and the input/output waveguides is proposed. It is shown that such a taper structure can reduce the multimode effects to an almost negligible level. At the same time, the 3-dB passband width is enlarged by increasing the rib width. No additional fabrication process is needed for an EDG with such a design.

  • 46.
    Dai, Daoxin
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    He, Jian-Jun
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Compact silicon-on-insulator-based multimode interference coupler with bilevel taper structures2005Ingår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 44, nr 24, s. 5036-5041Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 × 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.

  • 47.
    Dai, Daoxin
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Reduction of multimode effects in an SOI-based etched diffraction grating demultiplexer2005Ingår i: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 247, nr 4-6, s. 281-290Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Multimode effects in the free propagation region (FPR) of a silicon-on-insulator (SOI)-based etched diffraction grating (EDG) demultiplexer are analyzed. In a conventional design, the power coupled to the higher order modes is considerable, which introduces a significant excess loss and crosstalk. It is shown that the multimode effects in an SOI EDG demultiplexer are much more detrimental than in an arrayed waveguide grating demultiplexer. Several methods for reducing the multimode effects are discussed. In particular, a laterally tapered structure between the FPR and the input/output waveguides is proposed as a simple and effective method for reducing the power coupled to the higher order modes and consequently reducing the multimode effects. The taper width is optimized to minimize the crosstalk.

  • 48. De Temmerman, G.
    et al.
    Rubel, Marek J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Coad, J. P.
    Pitts, R. A.
    Drake, James Robert
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Mirror test for ITER: Optical characterisation of metal mirrors in divertor tokamaks2005Ingår i: 32nd EPS Conference on Plasma Physics 2005, EPS 2005, Held with the 8th International Workshop on Fast Ignition of Fusion Targets: Europhysics Conference Abstracts, 2005, s. 586-589Konferensbidrag (Refereegranskat)
  • 49.
    Drake, James Robert
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Brunsell, Per
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Yadikin, Dimitry
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cecconello, Marco
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Malmberg, Jenny
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Liu, Y.
    Experimental and theoretical studies of active control of resistive wall mode growth in the EXTRAP T2R reversed-field pinch2005Ingår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 45, nr 7, s. 557-564Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Active feedback control of resistive wall modes (RWMs) has been demonstrated in the EXTRAP T2R reversed-field pinch experiment. The control system includes a sensor consisting of an array of magnetic coils (measuring mode harmonics) and an actuator consisting of a saddle coil array (producing control harmonics). Closed-loop (feedback) experiments using a digital controller based on a real time Fourier transform of sensor data have been studied for cases where the feedback gain was constant and real for all harmonics (corresponding to an intelligent-shell) and cases where the feedback gain could be set for selected harmonics, with both real and complex values (targeted harmonics). The growth of the dominant RWMs can be reduced by feedback for both the intelligent-shell and targeted-harmonic control systems. Because the number of toroidal positions of the saddle coils in the array is half the number of the sensors, it is predicted and observed experimentally that the control harmonic spectrum has sidebands. Individual unstable harmonics can be controlled with real gains. However if there are two unstable mode harmonics coupled by the sideband effect, control is much less effective with real gains. According to the theory, complex gains give better results for (slowly) rotating RWMs, and experiments support this prediction. In addition, open loop experiments have been used to observe the effects of resonant field errors applied to unstable, marginally stable and robustly stable modes. The observed effects of field errors are consistent with the thin-wall model, where mode growth is proportional to the resonant field error amplitude and the wall penetration time for that mode harmonic.

  • 50. Ergun, R. E.
    et al.
    Chen, L. -J
    Wilder, F. D.
    Ahmadi, N.
    Eriksson, S.
    Usanova, M. E.
    Goodrich, K. A.
    Holmes, J. C.
    Sturner, A. P.
    Malaspina, D. M.
    Newman, D. L.
    Torbert, R. B.
    Argall, M. R.
    Lindqvist, Per-Arne
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Burch, J. L.
    Webster, J. M.
    Drake, J. F.
    Price, L.
    Cassak, P. A.
    Swisdak, M.
    Shay, M. A.
    Graham, D. B.
    Strangeway, R. J.
    Russell, C. T.
    Giles, B. L.
    Dorelli, J. C.
    Gershman, D.
    Avanov, L.
    Hesse, M.
    Lavraud, B.
    Le Contel, O.
    Retino, A.
    Phan, T. D.
    Goldman, M. V.
    Stawarz, J. E.
    Schwartz, S. J.
    Eastwood, J. P.
    Hwang, K. -J
    Nakamura, R.
    Wang, S.
    Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause2017Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 7, s. 2978-2986Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large-amplitude parallel electric fields (E-||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large-amplitude (similar to 100mV/m) E-|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.

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