Endre søk
Begrens søket
2345678 201 - 250 of 1191
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Treff pr side
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 201. Catarino, N.
    et al.
    Widdowson, A.
    Baron-Wiechec, A.
    Coad, J. P.
    Heinola, K.
    Rubel, Marek
    KTH.
    Alves, E.
    Time-resolved deposition in the remote region of the JET-ILW divertor: measurements and modelling2017Inngår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T170, artikkel-id 014059Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One crucial requirement for the development of fusion power is to know where, and how much, impurities collect in the machine, and how much of the fuelling isotope tritium will be trapped therein. The most relevant information on this issue comes from the operation of the Joint European Tokamak (JET), which is the world's largest operating tokamak and has the same interior plasma-facing materials as the next step machine, ITER. Much of the information gained so far has been from post-mortem analysis of samples collected after whole campaigns involving varied types of operation. This paper describes time-resolved measurements of the deposition rate using rotating collectors (RC) placed in remote areas of the JET divertor during the 2013-2014 campaign with the ITER-like Wall (ILW). These techniques allow the effects of different types of operation to be distinguished. Rotating collectors made of silicon discs housed behind an aperture are exposed to the plasma. Each time the magnetic field coils are ramped up for a discharge the disc rotates, providing a linear relationship between the exposed region and the discharge number. Post-mortem ion beam analyses provide information on the deposit composition as a function of the discharge number. The results show that the Be deposition average for the RC in the corners of the inner and outer divertor are 4.9 x 10(16) cm(-2) and 1.8 x 10(17) cm(-2), respectively, accumulated over an average of similar to 25 pulses. Data from the rotating collector below Tile 5 in the central region of divertor indicate a Be deposition rate of 9.3 x 10(15) cm(-2), per similar to 25 pulses.

  • 202.
    Causa, F.
    et al.
    CNR, Ist Fis Plasma Piero Caldirola, Via R Cozzi 53, I-20125 Milan, Italy.;CNR, Ist Fis Plasma, Via R Cozzi 53, I-20125 Milan, Italy..
    Ratynskaia, Svetlana
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Rymd- och plasmafysik.
    Tolias, Panagiotis
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Rymd- och plasmafysik.
    Zito, P.
    ENEA, Fus & Nucl Safety Dept, CR Frascati, Via E Fermi 45, I-00044 Rome, Italy..
    Analysis of runaway electron expulsion during tokamak instabilities detected by a single-channel Cherenkov probe in FTU2019Inngår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, nr 4, artikkel-id 046013Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The expulsion of runaway electrons (REs) during different types of tokamak instabilities is analysed by means of a Cherenkov probe inserted into the scrape-off layer of the FTU tokamak. One such type of instability, the well-known tearing mode, is involved in disruptive plasma termination events, during which the risk of RE avalanche multiplication is highest. The second type, known as anomalous Doppler instability, influences RE dynamics by enhancing pitch angle scattering. Three scenarios are analysed here, characterised by different RE generation rates and mechanisms. The main conclusions are drawn from correlations between the Cherenkov probe and other diagnostics. In particular, the Cherenkov probe permits the detection of fast electron expulsion with a high level of detail, presenting peaks with 100% signal contrast during tearing mode growth and rotation, and sub-peak structures reflecting the interplay between the magnetic island formed by the tearing mode, RE diffusion during island rotation and the geometry of obstacles in the vessel. Correlations between the Cherenkov signal, hard x-ray emission and electron cyclotron emission reveal the impulsive development of the anomalous Doppler instability with instability rise time in the microsecond scale resolved by the high time-resolution of the Cherenkov probe.

  • 203. Cazzola, E.
    et al.
    Innocenti, M. E.
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Goldman, M. V.
    Newman, D. L.
    Lapenta, G.
    On the electron dynamics during island coalescence in asymmetric magnetic reconnection2015Inngår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, nr 9, artikkel-id 092901Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present an analysis of the electron dynamics during rapid island merging in asymmetric magnetic reconnection. We consider a doubly periodic system with two asymmetric transitions. The upper layer is an asymmetric Harris sheet of finite width perturbed initially to promote a single reconnection site. The lower layer is a tangential discontinuity that promotes the formation of many X-points, separated by rapidly merging islands. Across both layers, the magnetic field and the density have a strong jump, but the pressure is held constant. Our analysis focuses on the consequences of electron energization during island coalescence. We focus first on the parallel and perpendicular components of the electron temperature to establish the presence of possible anisotropies and non-gyrotropies. Thanks to the direct comparison between the two different layers simulated, we can distinguish three main types of behavior characteristic of three different regions of interest. The first type represents the regions where traditional asymmetric reconnections take place without involving island merging. The second type of regions instead shows reconnection events between two merging islands. Finally, the third regions identify the regions between two diverging island and where typical signature of reconnection is not observed. Electrons in these latter regions additionally show a flat-top distribution resulting from the saturation of a two-stream instability generated by the two interacting electron beams from the two nearest reconnection points. Finally, the analysis of agyrotropy shows the presence of a distinct double structure laying all over the lower side facing the higher magnetic field region. This structure becomes quadrupolar in the proximity of the regions of the third type. The distinguishing features found for the three types of regions investigated provide clear indicators to the recently launched Magnetospheric Multiscale NASA mission for investigating magnetopause reconnection involving multiple islands.

  • 204. Cecconello, M.
    et al.
    Sangaroon, S.
    Conroy, S.
    Donato, M.
    Ericsson, G
    Marini-Bettolo, C.
    Ronchi, R.
    Ström, Petter
    Uppsala University, Sweden.
    Weiszflog, M.
    Wodniak, I.
    Turnyanskiy, M.
    Akers, R
    Cullen, A
    Fitzgerald, I
    McArdle, G
    Pacoto, C
    homas-Davies, N.
    The 2.5 MeV neutron flux monitor for MAST2014Inngår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 753, s. 72-83Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A proof-of-principle collimated Neutron flux Camera (NC) monitor for the measurement of the 2.45 MeV neutron emission from the deuterium–deuterium fusion reactions has been developed, installed and put into use at the Mega Ampere Spherical Tokamak (MAST). The NC measures the spatial and time resolved volume integrated neutron emissivity in deuterium fusion plasmas in the presence of auxiliary plasma heating along two equatorial and two diagonal lines of sight whose tangency radius can be varied between plasma discharges. This paper describes the NC design principles, their technical realization and its performances illustrated with experimental observations of different plasma scenarios. Neutron count rates in the range 0.1–1.5 MHz are routinely observed allowing time resolutions as high as 1 ms with a statistical uncertainty less than 10% and an energy threshold of 0.5 MeV. Examples of the effect of plasma instabilities on the neutron emission are presented. The good results obtained will be used for the design of the neutron flux camera monitor for MAST Upgrade.

  • 205.
    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 pinch2005Konferansepaper (Fagfellevurdert)
  • 206.
    Cecconello, Marco
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Frassinetti, Lorenzo
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Olofsson, Erik
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Khan, Muhammad Waqas Mehmood
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Brunsell, Per
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Resistive tearing modes dynamics with plasma control in a reversed field pinch2008Inngår i: 35th EPS Conference on Plasma Physics 2008, EPS 2008 - Europhysics Conference Abstracts: Volume 32, Issue 1, 2008, s. 429-432Konferansepaper (Fagfellevurdert)
  • 207.
    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 pinch2006Inngår i: Proceedings of the 33rd European Physical Society Conference on Plasma Physics, 2006, s. 1680-1683Konferansepaper (Fagfellevurdert)
  • 208.
    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 modes2006Inngår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 48, nr 9, s. 1311-1331Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 209. Chapman, I. T.
    et al.
    Graves, J. P.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Jaeger, E. F.
    Jucker, M.
    Sauter, O.
    Sawtooth Control in ITER using Ion Cyclotron Resonance Heating2011Inngår i: Proceedings of the EPS Conference on Plasma Physics, 2011Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Numerical modelling of the effects of ion cyclotron resonance heating (ICRH) on the stability of the internal kink mode suggests that ICRH should be considered as an essential sawtooth control tool in ITER. Sawtooth control using ICRH is achieved by directly affecting the energy of the kink mode rather than through modification of the magnetic shear by driving localised currents. Consequently, ICRH can be seen as complementary to the planned electron cyclotron current drive actuator. Simulations of the ICRH distribution using independent RF codes give confidence in numerical predictions that the stabilising influence of the fusion-born alphas can be negated by appropriately tailored minority 3He ICRH heating in ITER.

  • 210. Chapman, I. T.
    et al.
    Graves, J. P.
    Lennholm, M.
    Faustin, J.
    Lerche, E.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. EUROfusion Consortium, England.
    Tholerus, Simon
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. EUROfusion Consortium, England.
    The merits of ion cyclotron resonance heating schemes for sawtooth control in tokamak plasmas2015Inngår i: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 81, nr 06, artikkel-id 365810601Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    JET experiments have compared the efficacy of low-and high field side ion cyclotron resonance heating (ICRH) as an actuator to deliberately minimise the sawtooth period. It is found that low-field side ICRH with low minority concentration is optimal for saw tooth control for two main reasons. Firstly, low-field side heating means that any toroidal phasing of the ICRH (-90 degrees, +90 degrees or dipole) has a destabilising effect on the sawteeth, meaning that dipole phasing can be employed, since tins is preferable due to less plasma wall interaction from Resonant Frequency (RI) sheaths. Secondly, the resonance position of the low field side ICRH does not have to be very accurately placed to achieve saw tooth control, relaxing the requirement for real-time control of the RF frequency. These empirical observations have been confirmed by hybrid kinetic-magnetohydrodynamic modelling, and suggest that the ICRH antenna design for ITER is well positioned to provide a control actuator capable of having a significant effect on the sawtooth behaviour.

  • 211. Chasapis, A.
    et al.
    Matthaeus, W. H.
    Parashar, T. N.
    Lecontel, O.
    Retinò, A.
    Breuillard, H.
    Khotyaintsev, Y.
    Vaivads, Andris
    Lavraud, B.
    Eriksson, E.
    Moore, T. E.
    Burch, J. L.
    Torbert, R. B.
    Lindqvist, Per-Arne
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ergun, R. E.
    Marklund, Göran
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Goodrich, K. A.
    Wilder, F. D.
    Chutter, M.
    Needell, J.
    Rau, D.
    Dors, I.
    Russell, C. T.
    Le, G.
    Magnes, W.
    Strangeway, R. J.
    Bromund, K. R.
    Leinweber, H. K.
    Plaschke, F.
    Fischer, D.
    Anderson, B. J.
    Pollock, C. J.
    Giles, B. L.
    Paterson, W. R.
    Dorelli, J.
    Gershman, D. J.
    Avanov, L.
    Saito, Y.
    Electron Heating at Kinetic Scales in Magnetosheath Turbulence2017Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 836, nr 2, artikkel-id 247Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a statistical study of coherent structures at kinetic scales, using data from the Magnetospheric Multiscale mission in the Earth's magnetosheath. We implemented the multi-spacecraft partial variance of increments (PVI) technique to detect these structures, which are associated with intermittency at kinetic scales. We examine the properties of the electron heating occurring within such structures. We find that, statistically, structures with a high PVI index are regions of significant electron heating. We also focus on one such structure, a current sheet, which shows some signatures consistent with magnetic reconnection. Strong parallel electron heating coincides with whistler emissions at the edges of the current sheet.

  • 212. Chen, L. -J
    et al.
    Wang, S.
    Wilson, L. B. , I I I
    Schwartz, S.
    Bessho, N.
    Moore, T.
    Gershman, D.
    Giles, B.
    Malaspina, D.
    Wilder, F. D.
    Ergun, R. E.
    Hesse, M.
    Lai, H.
    Russell, C.
    Strangeway, R.
    Torbert, R. B.
    Vinas, F. -A
    Burch, J.
    Lee, S.
    Pollock, C.
    Dorelli, J.
    Paterson, W.
    Ahmadi, N.
    Goodrich, K.
    Lavraud, B.
    Le Contel, O.
    Khotyaintsev, Yu.V.
    Lindqvist, Per-Arne
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Rymd- och plasmafysik.
    Boardsen, S.
    Wei, H.
    Le, A.
    Avanov, L.
    Electron Bulk Acceleration and Thermalization at Earth's Quasiperpendicular Bow Shock2018Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, nr 22, artikkel-id 225101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electron heating at Earth's quasiperpendicular bow shock has been surmised to be due to the combined effects of a quasistatic electric potential and scattering through wave-particle interaction. Here we report the observation of electron distribution functions indicating a new electron heating process occurring at the leading edge of the shock front. Incident solar wind electrons are accelerated parallel to the magnetic field toward downstream, reaching an electron-ion relative drift speed exceeding the electron thermal speed. The bulk acceleration is associated with an electric field pulse embedded in a whistler-mode wave. The high electron-ion relative drift is relaxed primarily through a nonlinear current-driven instability. The relaxed distributions contain a beam traveling toward the shock as a remnant of the accelerated electrons. Similar distribution functions prevail throughout the shock transition layer, suggesting that the observed acceleration and thermalization is essential to the cross-shock electron heating. © 2018 American Physical Society.

  • 213. Chen, Yuxi
    et al.
    Toth, Gabor
    Cassak, Paul
    Jia, Xianzhe
    Gombosi, Tamas I.
    Slavin, James A.
    Markidis, Stefano
    KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Peng, Ivy Bo
    KTH.
    Jordanova, Vania K.
    Henderson, Michael G.
    Global Three-Dimensional Simulation of Earth's Dayside Reconnection Using a Two-Way Coupled Magnetohydrodynamics With Embedded Particle-in-Cell Model: Initial Results2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 10, s. 10318-10335Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We perform a three-dimensional (3-D) global simulation of Earth's magnetosphere with kinetic reconnection physics to study the flux transfer events (FTEs) and dayside magnetic reconnection with the recently developed magnetohydrodynamics with embedded particle-in-cell model. During the 1 h long simulation, the FTEs are generated quasi-periodically near the subsolar point and move toward the poles. We find that the magnetic field signature of FTEs at their early formation stage is similar to a "crater FTE," which is characterized by a magnetic field strength dip at the FTE center. After the FTE core field grows to a significant value, it becomes an FTE with typical flux rope structure. When an FTE moves across the cusp, reconnection between the FTE field lines and the cusp field lines can dissipate the FTE. The kinetic features are also captured by our model. A crescent electron phase space distribution is found near the reconnection site. A similar distribution is found for ions at the location where the Larmor electric field appears. The lower hybrid drift instability (LHDI) along the current sheet direction also arises at the interface of magnetosheath and magnetosphere plasma. The LHDI electric field is about 8 mV/m, and its dominant wavelength relative to the electron gyroradius agrees reasonably with Magnetospheric Multiscale (MMS) observations.

  • 214. Cherigier-Kovacic, L.
    et al.
    Ström, Petter
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Lejeune, A.
    Doveil, F.
    Electric field induced Lyman-alpha emission of a hydrogen beam for electric field measurements2015Inngår i: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 86, nr 6, artikkel-id 063504Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electric field induced Lyman-alpha emission is a new way of measuring weak electric fields in vacuum and in a plasma. It is based on the emission of Lyman-alpha radiation (121.6 nm) by a low-energy metastable H atom beam due to Stark-quenching of the 2s level induced by the field. In this paper, we describe the technique in detail. Test measurements have been performed in vacuum between two plates polarized at a controlled voltage. The intensity of emitted radiation, proportional to the square of the field modulus, has been recorded by a lock-in technique, which gives an excellent signal to noise ratio. These measurements provide an in situ calibration that can be used to obtain the absolute value of the electric field. A diagnostic of this type can help to address a long standing challenge in plasma physics, namely, the problem of measuring electric fields without disturbing the equilibrium of the system that is being studied.

  • 215. Chust, T.
    et al.
    Roux, A.
    Berthomier, M.
    Bouabdellah, A.
    Coillot, C.
    Ruocco, S.
    Alison, D.
    Soucek, J.
    Santolik, O.
    Vaivads, Andris
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Maksimovic, M.
    Bale, S. D.
    A low frequency receiver for the Solar Orbiter mission2006Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The Low Frequency Receiver (LFR) is one of the main subsystems of the Radio and Plasma Wave (RPW) experiment that we wish to submit in response to a possible Announcement of Opportunity for the Solar Orbiter payload. It will be connected to two different sensor units: an electric antenna unit and a magnetic search coil unit that will be optimized to perform both quasi-DC and high frequency measurements. The LFR is dedicated to analyse and process onboard the low frequency signals from a fraction of a Hertz up to -10 kHz, covering in situ measurements of the electromagnetic waves of the solar wind and extended corona. Due to the telemetry constraints different strategies for analysing and transmitting the data have to be defined, implying different onboard working modes. The design and the technological characteristics of the LFR are presented.

  • 216. Citrin, J.
    et al.
    Garcia, J.
    Görier, T.
    Jenko, F.
    Mantica, P.
    Told, D.
    Bourdelle, C.
    Hatch, D. R.
    Hogeweij, G. M. D.
    Johnson, Thomas
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Pueschel, M. J.
    Schneider, M.
    Electromagnetic stabilization of tokamak microturbulence in a high-β regime2014Inngår i: 41st EPS Conference on Plasma Physics, EPS 2014, European Physical Society (EPS) , 2014Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

  • 217. Coad, J. P.
    et al.
    Esser, H. -G
    Likonen, J.
    Mayer, M.
    Neill, G.
    Philipps, V.
    Rubel, Marek J.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Vince, J.
    Diagnostics for studying deposition and erosion processes in JET2005Inngår i: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 74, nr 1-4, s. 745-749Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Estimates of erosion, deposition and H-isotope retention in JET from previous divertor campaigns have relied on analysis of in-vessel components removed at shutdowns. The components analysed have also provided an incomplete coverage of the vessel. In 2004, new diagnostics are being installed to give a more complete picture (such as smart tiles) and to provide some time resolution. The latter includes further quartz microbalances (QMB), following the successful operation of a prototype in 2002-2004 [H.-G. Esser, G. Neill, P. Coad, G.F. Matthews, D. Jolovic, D. Wilson, M. Freisinger, V. Philipps, Quartz microbalance: a time-resolved diagnostic to measure material deposition in JET, Fusion Eng. Des. 66-68 (2003) 855-860; H.-G. Esser, V. Philipps, M. Freisinger, G.F. Matthews, J.P. Coad, G.F. Neill, JET EFDA Contributors, Effect of plasma configuration on carbon migration measured in the inner divertor of JET using quartz microbalance, J. Nucl. Mater. 337-339 (2005) 84-87], which will also have temperature control. Other diagnostics include rotating collectors and deposition monitors [M. Mayer, V. Rohde, P. Coad, P. Wienhold, ASDEX Upgrade Team, JET EFDA Contributors, Carbon erosion and migration in fusion devices, Phys. Scr. T111 (2004) 55-59]. Units are also being installed to provide information on mirrors for ITER.

  • 218.
    Coad, J. P.
    et al.
    Culham Sci Ctr, Culham Ctr Fus Energy, Abingdon OX14 3DB, Oxon, England..
    Rubel, Marek
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Fusionsplasmafysik.
    Likonen, J.
    VTT Tech Res Ctr Finland, POB 1000, FIN-02044 Espoo, Finland..
    Bekris, N.
    Karlsruhe Inst Technol, D-76021 Karlsruhe, Germany..
    Brezinsek, S.
    Forschungszentrum Juelich, Inst Energieforsch Plasmaphys, D-52425 Julich, Germany..
    Matthew, G. F.
    Culham Sci Ctr, Culham Ctr Fus Energy, Abingdon OX14 3DB, Oxon, England..
    Mayer, M.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany..
    Widdowson, A. M.
    Culham Sci Ctr, Culham Ctr Fus Energy, Abingdon OX14 3DB, Oxon, England..
    Material migration and fuel retention studies during the JET carbon divertor campaigns2019Inngår i: Fusion engineering and design, ISSN 0920-3796, E-ISSN 1873-7196, Vol. 138, s. 78-108Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The first divertor was installed in the JET machine between 1992 and 1994 and was operated with carbon tiles and then beryllium tiles in 1994-5. Post-mortem studies after these first experiments demonstrated that most of the impurities deposited in the divertor originate in the main chamber, and that asymmetric deposition patterns generally favouring the inner divertor region result from drift in the scrape-off layer. A new monolithic divertor structure was installed in 1996 which produced heavy deposition at shadowed areas in the inner divertor corner, which is where the majority of the tritium was trapped by co-deposition during the deuterium-tritium experiment in 1997. Different divertor geometries have been tested since such as the Gas-Box and High-Delta divertors; a principle objective has been to predict plasma behaviour, transport and tritium retention in ITER. Transport modelling experiments were carried out at the end of four campaigns by puffing C-13-labelled methane, and a range of diagnostics such as quartz-microbalance and rotating collectors have been installed to add time resolution to the post-mortem analyses. The study of material migration after D-D and D-T campaigns clearly revealed important consequences of fuel retention in the presence of carbon walls. They gave a strong impulse to make a fundamental change of wall materials. In 2010 the carbon divertor and wall tiles were removed and replaced with tiles with Be or W surfaces for the ITER-Like Wall Project.

  • 219.
    Coda, S.
    et al.
    Ecole Polytech Fed Lausanne, Swiss Plasma Ctr, CH-1015 Lausanne, Switzerland..
    Frassinetti, Lorenzo
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Fusionsplasmafysik.
    Zuin, M.
    Consorzio RFX, Corso Stati Uniti 4, I-35127 Padua, Italy..
    et al.,
    Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond2019Inngår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 59, nr 11, artikkel-id 112023Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device's unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly noninductive II-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power `starvation' reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached 1,-mode phase, increasing the outer connection length reduces the in-out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variableconfiguration baffles and possibly divertor ptunping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECR and 1 MW neutral beam injection heating will be added.

  • 220. Coenen, J. W.
    et al.
    Arnoux, G.
    Bazylev, B.
    Matthews, G. F.
    Autricque, A.
    Balboa, I.
    Clever, M.
    Dejarnac, R.
    Coffey, I.
    Corre, Y.
    Devaux, S.
    Frassinetti, Lorenzo
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Gauthier, E.
    Horacek, J.
    Jachmich, S.
    Komm, M.
    Knaup, M.
    Krieger, K.
    Marsen, S.
    Meigs, A.
    Mertens, Ph.
    Pitts, R. A.
    Puetterich, T.
    Rack, M.
    Stamp, M.
    Sergienko, G.
    Tamain, P.
    Thompson, V.
    ELM-induced transient tungsten melting in the JET divertor2015Inngår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 55, nr 2, artikkel-id 023010Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The original goals of the JET ITER-like wall included the study of the impact of an all W divertor on plasma operation (Coenen et al 2013 Nucl. Fusion 53 073043) and fuel retention (Brezinsek et al 2013 Nucl. Fusion 53 083023). ITER has recently decided to install a full-tungsten (W) divertor from the start of operations. One of the key inputs required in support of this decision was the study of the possibility of W melting and melt splashing during transients. Damage of this type can lead to modifications of surface topology which could lead to higher disruption frequency or compromise subsequent plasma operation. Although every effort will be made to avoid leading edges, ITER plasma stored energies are sufficient that transients can drive shallow melting on the top surfaces of components. JET is able to produce ELMs large enough to allow access to transient melting in a regime of relevance to ITER. Transient W melt experiments were performed in JET using a dedicated divertor module and a sequence of I-P = 3.0 MA/B-T = 2.9 T H-mode pulses with an input power of P-IN = 23 MW, a stored energy of similar to 6 MJ and regular type I ELMs at Delta W-ELM = 0.3 MJ and f(ELM) similar to 30 Hz. By moving the outer strike point onto a dedicated leading edge in the W divertor the base temperature was raised within similar to 1 s to a level allowing transient, ELM-driven melting during the subsequent 0.5 s. Such ELMs (delta W similar to 300 kJ per ELM) are comparable to mitigated ELMs expected in ITER (Pitts et al 2011 J. Nucl. Mater. 415 (Suppl.) S957-64). Although significant material losses in terms of ejections into the plasma were not observed, there is indirect evidence that some small droplets (similar to 80 mu m) were released. Almost 1 mm (similar to 6 mm(3)) of W was moved by similar to 150 ELMs within 7 subsequent discharges. The impact on the main plasma parameters was minor and no disruptions occurred. The W-melt gradually moved along the leading edge towards the high-field side, driven by j x B forces. The evaporation rate determined from spectroscopy is 100 times less than expected from steady state melting and is thus consistent only with transient melting during the individual ELMs. Analysis of IR data and spectroscopy together with modelling using the MEMOS code Bazylev et al 2009 J. Nucl. Mater. 390-391 810-13 point to transient melting as the main process. 3D MEMOS simulations on the consequences of multiple ELMs on damage of tungsten castellated armour have been performed. These experiments provide the first experimental evidence for the absence of significant melt splashing at transient events resembling mitigated ELMs on ITER and establish a key experimental benchmark for the MEMOS code.

  • 221. Coenen, J. W.
    et al.
    Arnoux, G.
    Bazylev, B.
    Matthews, G. F.
    Jachmich, S.
    Balboa, I.
    Clever, M.
    Dejarnac, R.
    Coffey, I.
    Corre, Y.
    Devaux, S.
    Frassinetti, Lorenzo
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Gauthier, E.
    Horacek, J.
    Knaup, M.
    Komm, M.
    Krieger, K.
    Marsen, S.
    Meigs, A.
    Mertens, Ph.
    Pitts, R. A.
    Puetterich, T.
    Rack, M.
    Stamp, M.
    Sergienko, G.
    Tamain, P.
    Thompson, V.
    ELM induced tungsten melting and its impact on tokamak operation2015Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 463, s. 78-84Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In JET-ILW dedicated melt exposures were performed using a sequence of 3MA/2.9T H-Mode JET pulses with an input power of P-IN = 23 MW, a stored energy of similar to 6 MJ and regular type I ELMs at Delta W-ELM = 0.3 MJ and f(ELM) similar to 30 Hz. In order to assess the risk of starting ITER operations with a full W divertor, one of the task was to measure the consequences of W transients melting due to ELMs. JET is the only tokamak able to produce transients/ ELMs large enough (>300 kJ per ELM) to facilitate melting of tungsten. Such ELMs are comparable to mitigated ELMs expected in ITER. By moving the outer strike point (OSP) onto a dedicated leading edge the base temperature was raised within similar to 1 s to allow transient ELM-driven melting during the subsequent 0.5 s. Almost 1 mm (similar to 6 mm(3)) of W was moved by similar to 150 ELMs within 5 subsequent discharges. Significant material losses in terms of ejections into the plasma were not observed. There is indirect evidence that some small droplets (similar to 80 mu m) were ejected. The impact on the main plasma parameters is minor and no disruptions occurred. The W-melt gradually moved along the lamella edge towards the high field side, driven by j x B forces. The evaporation rate determined is 100 times less than expected from steady state melting and thus only consistent with transient melting during individual ELMs. IR data, spectroscopy, as well as melt modeling point to transient melting. Although the type of damage studied in these JET experiments is unlikely to be experienced in ITER, the results do strongly support the design strategy to avoid exposed edges in the ITER divertor. The JET experiments required a surface at normal incidence and considerable pre-heating to produce tungsten melting. They provide unique experimental evidence for the absence of significant melt splashing at events resembling mitigated ELMs on ITER and establish a unique experimental benchmark for the simulations being used to study transient shallow melting on ITER W divertor PFUs.

  • 222.
    Collier, Andrew
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    VLF and ULF Waves Associated with Magnetospheric Substorms2006Doktoravhandling, med artikler (Annet vitenskapelig)
    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.

  • 223.
    Collier, Andrew
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Hughes, Arthur Robert W
    University of KwaZulu-Natal, Durban, South Africa.
    Blomberg, Lars G.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Sutcliffe, P. R.
    Evidence of standing waves during a Pi2 pulsation event observed on Cluster2006Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, nr 10, s. 2719-2733Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Observations of Pi2 pulsations at middle and low latitudes have been explained in terms of cavity mode resonances, whereas transients associated with field-aligned currents appear to be responsible for the high latitude Pi2 signature. Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on 21 January 2003, when three of the satellites were within the plasmasphere (L=4.7,4.5 and 4.6) while the fourth was on the plasmapause or in the plasmatrough (L=6.6). Simultaneous pulsations at ground observatories and the injection of particles at geosynchronous orbit corroborate the occurrence of a substorm. Evidence of a cavity mode resonance is established by considering the phase relationship between the orthogonal electric and magnetic field components associated with radial and field-aligned standing waves. The relative phase between satellites located on either side of the geomagnetic equator indicates that the field-aligned oscillation is an odd harmonic. Finite azimuthal Poynting flux suggests that the cavity is effectively open ended and the azimuthal wave number is estimated as m similar to 13.5.

  • 224.
    Cumnock, J. A.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik. KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Sharber, J. R.
    Heelis, R. A.
    Blomberg, Lars G.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Germany, G. A.
    Spann, J. F.
    Coley, W. R.
    Interplanetary magnetic field control of theta aurora development2002Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 107, nr A7Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    [1] We ascertain the influence of the B-y component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF B-z, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF B-y. It is important to note that since transpolar arcs can persist for 20-30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to snapshots'' of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF B-y is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in B-z is needed for theta aurora formation.

  • 225.
    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-y2005Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 110, nr A2, s. A02304-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 226.
    Cumnock, Judy A.
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Blomberg, Lars G.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Transpolar arc evolution and associated potential patterns2004Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, nr 4, s. 1213-1231Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present two event studies encompassing detailed relationships between plasma convection, field-aligned current, auroral emission, and particle precipitation boundaries. We illustrate the influence of the Interplanetary Magnetic Field B, component on theta aurora development by showing two events during which the theta originates on both the dawn and dusk sides of the aurora] oval. Both theta then move across the entire polar region and become part of the opposite side of the aurora] oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the Polar UVI instrument provides measurements of auroral emissions. Utilizing satellite data as inputs, the Royal Institute of Technology model provides the high-latitude ionospheric electrostatic potential pattern 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.

  • 227.
    Cumnock, Judy A.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Blomberg, Lars G.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Kullen, Anita
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Karlsson, Tomas
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Sundberg, K. Å. Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Small-scale characteristics of extremely high latitude aurora2009Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, nr 9, s. 3335-3347Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We examine 14 cases of an interesting type of extremely high latitude aurora as identified in the precipitating particles measured by the DMSP F13 satellite. In particular we investigate structures within large-scale arcs for which the particle signatures are made up of a group of multiple distinct thin arcs. These cases are chosen without regard to IMF orientation and are part of a group of 87 events where DMSP F13 SSJ/4 measures emissions which occur near the noon-midnight meridian and are spatially separated from both the dawnside and duskside auroral ovals by wide regions with precipitating particles typical of the polar cap. For 73 of these events the high-latitude aurora consists of a continuous region of precipitating particles. We focus on the remaining 14 of these events where the particle signatures show multiple distinct thin arcs. These events occur during northward or weakly southward IMF conditions and follow a change in IMF B-y. Correlations are seen between the field-aligned currents and plasma flows associated with the arcs, implying local closure of the FACs. Strong correlations are seen only in the sunlit hemisphere. The convection associated with the multiple thin arcs is localized and has little influence on the large-scale convection. This also implies that the sunward flow along the arcs is unrelated to the overall ionospheric convection.

  • 228.
    Cumnock, Judy A.
    et al.
    Center for Space Sciences, University of Texas at Dallas, Richardson, TX .
    Le, G.
    Imber, S.
    Slavin, J. A.
    Zhang, Y.
    Paxton, L. J.
    Space Technology 5 multipoint observations of transpolar arc related field-aligned currents2011Inngår i: Journal of Geophysical Research - Space Physics, ISSN 0148-0227, Vol. 116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present two transpolar arc events where for the first time we are able to analyze changes in field-aligned currents associated with high-latitude transpolar auroral arcs on time scales of a few minutes. This is accomplished through the use of highly accurate multipoint magnetic field measurements provided by the Space Technology 5 mission, which consists of three microsatellites in low-Earth orbit. In the first event we examine measurements of an arc that is part of a highly dynamic auroral pattern, that of a hook-shaped arc. In the second event, a more stable dusk oval-aligned arc is analyzed. These events illustrate the dynamic nature of arc formation and show the usefulness of high-resolution multipoint measurements. Minimum variance analysis is used to determine the appropriateness of the infinite current sheet approximation and to calculate arc alignment angles which are then compared with those estimated from UV images or precipitating particle data.

  • 229.
    Cumnock, Judy
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Baker, J. B. H.
    Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA.
    Blomberg, Lars
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    On the stability of high-latitude plasma convection during northward IMF: an event study2007Rapport (Annet vitenskapelig)
    Abstract [en]

    We investigate the stability of the ionospheric convection pattern during northward IMF by studying an event where two DMSP satellites repeatedly traversed the Southern polar region. The Cluster satellite’s ionospheric footprint is located near the DMSP satellite tracks, moving slowly in the noon-midnight direction. TIMED/GUVI data confirm the presence of auroral activity at high latitude. SuperDARN plasma velocity data partially complete the picture. From the event studied we conclude that whereas the DMSP satellites observe local variations in the convection pattern between consecutive passes, Cluster confirms the existence of persistent sunward convection in the high-latitude ionosphere on a time scale of several hours.

  • 230.
    Cumnock, Judy
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Blomberg, Lars
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    The Mercury Environment: A Literature Survey2003Rapport (Annet vitenskapelig)
    Abstract [en]

    A literature survey was conducted focusing primarily on the plasma environment of planet Mercury, and secondarily on its neutral atmosphere and the electrical properties of the planetary surface. An extensive literature list, with narrative comments for selected publications is presented.

  • 231.
    Cumnock, Judy
    et al.
    Center for Space Sciences, University of Texas at Dallas, Richardson, TX.
    Heelis, R.A.
    Hairston, M.R.
    Response of the Ionospheric Convection Pattern to a Rotation of the Interplanetary Magnetic Field on January 14, 19881992Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 97, s. 19449-19460Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large-scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When B(y) and B(z) are positive, but B(y) is greater than B(z), strong evidence exists for dayside merging in a manner similar to that expected when B(z) is negative. This signature is diminished as B(y) decreases and becomes smaller than B(z) resulting in a four-cell convection pattern displaced toward the sunward side of the dawn-dusk meridian. In this case the sign of B(y) affects the relative sizes of the two highest-latitude cells. In the southern hemisphere the dusk side high-latitude cell is dominant for B(y) positive and the dawnside high-latitude cell is dominant for B(y) negative. The relative importance of possible electric field sources in the low-latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.

  • 232.
    Cumnock, Judy
    et al.
    Center for Space Sciences, University of Texas at Dallas, Richardson, TX.
    Heelis, R.A.
    Hairston, M.R.
    Newell, P.T.
    High-Latitude Ionospheric Convection Pattern During Steady Northward Interplanetary Magnetic Field1995Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 100, s. 14537-14555Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The DMSP F8 satellite’s coverage of Earth’s polar regions provides horizontal ion drift velocities along the dawn-dusk meridian at approximately 835 km altitude in each hemisphere during the similar to 100 min orbital period. We examine the ionospheric convection signatures observed by this spacecraft in the summer and winter hemispheres during periods when the interplanetary magnetic field (IMF) is directed northward for at least 45 min prior to the satellite entering the polar region and remains northward throughout the polar pass. These convection signatures can be readily categorized by the number of sunward and antisunward flow regions and by their potential distributions. Here we describe the most frequently identifiable and reproducible features of the convection pattern that exist during steady northward IMF conditions. In addition to IMF B-z, the influences on the convection pattern of the IMF B-z/\textbackslashB-y\textbackslash ratio, season, latitude, and solar wind velocity are all considered. The ratio B-z/\textbackslashB-y\textbackslash provides a first order organization of the signatures that occur on the dayside of the dawn-dusk meridian. Sunward flow at highest latitudes on the dayside of the dawn-dusk meridian is the dominant feature seen in the large-scale convection signature during steady northward IMF; however, sunward flow at highest latitudes does not imply the existence of a particular number of convection cells.

  • 233.
    Cumnock, Judy
    et al.
    Center for Space Sciences, University of Texas at Dallas, Richardson, TX.
    Sharber, J R
    Heelis, R A
    Hairston, M R
    Craven, J D
    Evolution of the global aurora during positive IMF B-z and varying IMF B-y conditions1997Inngår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol. 102, s. 17489-17497Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The DE 1 imaging instrumentation provides a full view of the entire amoral oval every 12 min for several hours during each orbit. We examined five examples of global evolution of the aurora that occurred during the northern hemisphere winter of 1981-1982 when the a component of the interplanetary magnetic field was positive and the y component was changing sign. Evolution of an expanded amoral emission region into a theta aurora appears to require a change in the sign of B-y during northward interplanetary magnetic field (IMF). Theta aurora are formed both from expanded duskside emission regions (B-y changes from positive to negative) and dawnside emission regions (B-y changes from negative to positive), however the dawnside-originating and duskside-originating evolutions are not mirror images. The persistence of a theta aurora after its formation suggests that there may be no clear relationship between the theta aurora pattern and the instantaneous configuration of the IMF.

  • 234.
    Cumnock, Judy
    et al.
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Spann, J. F.
    Germany, G. A.
    Blomberg, Lars
    KTH, Tidigare Institutioner, Alfvénlaboratoriet.
    Coley, W. R.
    Clauer, C. R.
    Brittnacher, M. J.
    POLAR UVI Observations of Auroral Oval Intensifications During a Transpolar Arc Event on December 7, 19962000Rapport (Annet vitenskapelig)
    Abstract [en]

    The evolution of the northern hemisphere aurora is examined during a time when the IMF makes three brief southward excursions after a change in the sign of By during an extended period of northward IMF. POLAR UVI provides images of the aurora while DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density.

    Three different intensifications located in the nightside auroral oval occur during northward turnings of the IMF after brief periods of southward IMF. Spatial expansion, intensity of emissions and their duration are related to the length of time the IMF is southward prior to the northward turning. Thus the longer the period of enhanced magnetospheric convection the more intense the ionospheric response. Observations of a transpolar arc indicate that when the transpolar arc reaches highest latitudes it is located on a spatially narrow region of closed field lines, which extends along the noon-midnight meridian.

    UV observations indicate a connection between the transpolar arc and the nightside auroral enhancements. Precipitating particles associated with both features are attributed to a plasma sheet boundary layer source in the magnetotail implying a magnetospheric connection between the transpolar arc and the nightside auroral oval intensification.

  • 235.
    Dahlberg, Erling
    KTH, Tidigare Institutioner.
    Electromagnetic Shielding Formulae: (Addendum to TRITA-EPP-75-27)1979Rapport (Annet vitenskapelig)
  • 236.
    Dahlgren, Hanna
    KTH, Tidigare Institutioner, Alfvénlaboratoriet. KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Filamentary structures in planetary nebulae2005Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
  • 237.
    Dahlgren, Hanna
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Fine-scale morphology and spectral characteristics of active aurora2008Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Ground-based and in-situ observations of the aurora demonstrate an extreme richness in fine structure, with spatial scales down to tens of metres and time variations occurring on a fraction of a second. To further our understanding of the aurora, it is esssential to understand the mechanisms responsible for the small-scale structuring, since this is an intrinsic property of the auroral plasma. Still many questions about dynamics and structuring of aurora on small scales remain unanswered. In this thesis the low-light optical instrument ASK (Auroral Structure and Kinetics) is used to image small-scale structures in the aurora at very high spatial and temporal resolution. ASK is a multi-spectral instrument, imaging the aurora in three selected emission lines simultaneously. This provides information on the energy of the precipitating electrons. The SIF (Spectrographic Imaging Facility) instrument has been used in conjunction with ASK, to give a more complete picture of the spectral characteristics of the aurora, and to determine the contamination of the emission lines by other emissions. Data from ASK and SIF is used to study the relation between the morphology and dynamics of small-scale structures in the aurora and the energy of the precipitating electrons. By comparing electron density profiles provided by EISCAT (European Incoherent SCATter) measurements with modeling results, information on characteristic energy and energy flux of the precipitating electrons can be obtained. One of the ASK channels is imaging a metastable O+ emission, which has a lifetime of 5 s. By tracing the afterglow in this channel optically a direct measure of the E × B drift and thus of the local ionospheric electric fields is provided.

  • 238.
    Dahlgren, Hanna
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Multi-spectral analysis of fine scale aurora2010Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The Aurora Borealis is the visible manifestation of the complex plasma interaction between the solar wind and the Earth's magnetosphere and ionosphere. Ground based and in situ measurements demonstrate a prevalence of dynamic fine structure within auroral displays, with spatial scales down to tens of metres and time variations occurring on a fraction of a second.The fine-scale morphology is related to structuring of auroral currents and electric fields and detailed spatial, spectral and temporal observations of the aurora are crucial in understanding the electrodynamic processes taking place in the ionosphere and in its coupling to the magnetosphere.

    In this thesis, the low-light optical instrument ASK (Auroral Structure and Kinetics) is used to image small-scale structures in the aurora at very high spatial and temporal resolution. ASK is a multi-spectral instrument, imaging the aurora in three selected emissions simultaneously. This provides information on the energy of the precipitating electrons. The SIF (Spectrographic Imaging Facility) instrument has been used in conjunction with ASK, to give a more complete picture of the spectral characteristics of the aurora, and to determine the degree of contaminating emissions present in the same spectral interval as the emission lines observed by ASK.

    Data from ASK and SIF are used to study the relation between the morphology and dynamics of small-scale structures in the aurora and the energy of the precipitating electrons. By comparing electron density profiles provided by EISCAT (European Incoherent SCATter) radar measurements with modeling results, information on the characteristic energy and the energy flux of the precipitating electrons can be obtained. One of the ASK channels is imaging a metastable O+ emission, which has a lifetime of about 5 s. By tracing the afterglow in this channel optically a direct measure of the E x B drift is obtained from which the local ionospheric electric field can be calculated. ASK data has also been used to analyse the properties of a distorted auroral arc, in which auroral structuring was found to take place simultaneously at different spatial scales. The smallest features, 'ruffs', are undulations found to develop on the edge of an auroral curl, fold or shear. Detailed optical studies of black aurora, including both the type which is associated with plasma shear motions and no or weak shear motions were conducted from ASK data, to investigate the spectral properties and fine scale morphology of the black structures and to shed light on the processes behind this phenomenon.

  • 239.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Aikio, Anita
    Department of physical sciences, University of Oulu.
    Kaila, Kari
    Department of physical sciences, University of Oulu.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Lanchester, Betty
    Space Environment Physics Group, University of Southampton.
    Whiter, Daniel
    Space Environment Physics Group, University of Southampton.
    Marklund, Göran
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Simultaneous observations of small multi-scale structures in an auroral arc2010Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 72, s. 633-637Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Auroral arcs can develop small-scale distortions known as vortex streets or curls. Other common and somewhat larger spatially periodic distortions are auroral folds. In this event study we present simultaneous wide and narrow field imager observations of a third kind of structuring, on even smaller spatial scales. Boundary undulations, or “ruffs”, have been observed to form on the edge of an auroral arc and they occur superimposed on curls, folds or at times of auroral shear. The undulations typically have wavelengths of less than 3 km and amplitudes of less than 800 m. They are observed to move on the edge of the arc, with velocities of about 11 km/s. These observations, with multi-scale deformations, reveal a much more intricate structuring of auroral arcs than previously found.

  • 240.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Lanchester, B. S.
    Sullivan, J.
    Whiter, D.
    Marklund, Göran
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Strømme, A.
    Using spectral characteristics to interpret auroral imaging in the 731.9 nm 0+ line2008Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, nr 7, s. 1905-1917Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simultaneous observations were made of dynamic aurora during substorm activity on 26 January 2006 with three high spatial and temporal resolution instruments: the ASK (Auroral Structure and Kinetics) instrument, SIF (Spectrographic Imaging Facility) and ESR (EISCAT Svalbard Radar), all located on Svalbard (78° N, 16.2° E). One of the narrow field of view ASK cameras is designed to detect O+ ion emission at 731.9 nm. From the spectrographic data we have been able to determine the amount of contaminating N2 and OH emission detected in the same filter. This is of great importance to further studies using the ASK instrument, when the O+ ion emission will be used to detect flows and afterglows in active aurora. The ratio of O+ to N2 emission is dependent on the energy spectra of electron precipitation, and was found to be related to changes in the morphology of the small-scale aurora. The ESR measured height profiles of electron densities, which allowed estimates to be made of the energy spectrum of the precipitation during the events studied with optical data from ASK and SIF. It was found that the higher energy precipitation corresponded to discrete and dynamic features, including curls, and low energy precipitation corresponded to auroral signatures that were dominated by rays. The evolution of these changes on time scales of seconds is of importance to theories of auroral acceleration mechanisms.

  • 241.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Sullivan, J.
    Lanchester, B. S.
    Marklund, Göran
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Whiter, D.
    Morphology and dynamics of aurora at fine scale: first results for the ASK instrument2008Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, nr 5, s. 1041-1048Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ASK instrument (Auroral Structure and Kinetics) is a narrow field auroral imager, providing simultaneous images of aurora in three different spectral bands at multiple frames per second resolution. The three emission species studied are O-2(+) (5620 angstrom), O+ (7319 angstrom) and O (7774 angstrom). ASK was installed and operated for the first time in an observational campaign on Svalbard, from December 2005 to March 2006. The measurements were supported by data from the Spectrographic Imaging Facility (SIF). The relation between the morphology and dynamics of the visible aurora and its spectral characteristics is studied for selected events from this period. In these events it is found that dynamic aurora is coupled to high energy electron precipitation. By studying the O-2(+)/O intensity ratio we find that some auroral filaments are caused by higher energy precipitation within regions of lower energy precipitation, whereas other filaments are the result of a higher particle flux compared to the surroundings.

  • 242.
    Dahlgren, Hanna
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Ivchenko, Nickolay V.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Lanchester, B. S.
    School of Physics and Astronomy, University of Southampton.
    Ashrafi, M.
    School of Physics and Astronomy, University of Southampton.
    Whiter, D.
    School of Physics and Astronomy, University of Southampton.
    Marklund, Göran T.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Sullivan, J.
    School of Physics and Astronomy, University of Southampton.
    First direct optical observations of plasma flows using afterglow of O+ in discrete aurora2009Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 71, nr 2, s. 228-238Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Imaging of active structured aurora in the forbidden O+ ion line at 732.0 nm provides a possibility of direct observation of plasma drifts in the topside ionosphere. The metastable O+ P-2 state has a radiative lifetime of 5 s, so the oxygen ions can be detected after the precipitation creating them has ceased. The decay time of the O+ emission is studied and modelled with a time-dependent electron transport and ion chemistry model. Four examples are given of O+ afterglow observed with the multi-spectral imager, auroral structure and kinetics (ASK), which was located near Tromso, Norway, in 2006. Estimates are given of drift velocities resulting from the analysis of the afterglow motions. Bulk plasma velocities of 340 and 720 m/s directed eastwards were found for two afterglowing arc filaments, corresponding to southward electric fields of 18 and 40 mV/m, respectively.

  • 243.
    Dahlin, Jon-Erik
    KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Numerical studies of current profile control in the reversed-field pinch2006Doktoravhandling, med artikler (Annet vitenskapelig)
    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.

  • 244.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Advanced Reversed-field Pinch Scaling Laws2005Inngår i: 32nd EPS Conference on Plasma Physics, Tarragona, Spain 27 June-1 July, P-1.118, 2005, 2005Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A series of resistive magnetohydrodynamic numerical simulations are performed to generate scaling laws for energy confinement time τE and poloidal beta βp for the advanced reversed field-pinch (RFP). Strongly improved scaling with basic initial parameters is obtained as compared to the conventional RFP. Early results indicate an improved scaling of τE with plasma current I and line density N compared to the conventional RFP. The improved behaviour of the advanced RFP as compared to the conventional, uncontrolled RFP stems from the introduction of current profile control (CPC). In the present numerical simulations, CPC is performed by implementation of a parameter free automatic feedback algorithm, optimised to reduce the fluctuation caused v × B electric field. The scheme introduces an ad-hoc electric field within the plasma volume, automatically adjusted to dynamically control the plasma into more quiescent behaviour by eliminating current driven tearing mode instabilities and reducing resistive interchange modes.

  • 245.
    Dahlin, Jon-Erik
    et al.
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Scheffel, Jan
    KTH, Tidigare Institutioner                               , Alfvénlaboratoriet.
    Feedback current profile control in the advanced RFP2004Inngår i: Proceedings of the 31st EPS plasma physics conference, 2004Konferansepaper (Fagfellevurdert)
  • 246.
    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 Pinch2006Inngår i: 33rd EPS Conference on Plasma Phys, 2006Konferansepaper (Fagfellevurdert)
    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.

  • 247.
    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 pinch2007Inngår i: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 47, nr 1, s. 9-16Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 248.
    Dahlin, Jon-Erik
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Scheffel, Jan
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik.
    Scaling Laws of Confinement Parameters for the Advanced Reversed-field Pinch2005Inngår i: 47th APS Division of Plasma Physics Meeting, Denver, Colorado, 24-28 October, 2005, 2005Konferansepaper (Fagfellevurdert)
  • 249.
    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 pinch2007Inngår i: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 49, nr 2, s. 183-195Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 250. Daida, J. M.
    et al.
    Richey, C. J.
    Clauer, C. R.
    Baker, J. B.
    Cumnock, Judy A.
    Center for Space Sciences, University of Texas at Dallas, Richardson, TX .
    Brittnacher, M. J.
    Automating the Analysis of POLAR UVI Images Using a Hybrid Genetic Algorithm1998Inngår i: IEEE International Conference on Evolutionary Computation, 1998, s. 201-206Konferansepaper (Fagfellevurdert)
2345678 201 - 250 of 1191
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf