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Träfflista för sökning "WFRF:(Brenning Nils) srt2:(2000-2004)"

Sökning: WFRF:(Brenning Nils) > (2000-2004)

  • Resultat 1-6 av 6
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1.
  • Brenning, Nils, et al. (författare)
  • Dynamic trapping and skidding of dense plasma clouds
  • 2004
  • Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 70:03-feb, s. 153-156
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the possibility that the mechanism dynamic trapping can play a role in decoupling dense plasma clouds injected in a thinner ambient plasma, by establishing strong magnetic-field-aligned electric fields in the vicinity or in the edge of the cloud. Dynamic trapping has previously been shown to allow such fields to be established and maintained on the time scale of ion motion, also for arbitrarily low current densities. A model is presented of how such fields could arise and decouple injected plasma clouds, a mechanism which we call dynamic decoupling. A dimensionless parameter. the dynamic decoupling factor F-DD, is derived which gives an estimate of the importance of the process. One possible application is the CRRES ionospheric injection experiments where anomalous skidding has recently been reported. However. the dynamic decoupling mechanism might also play a role in naturally occurring situations, e.g. the impulsive penetration of plasmoids from the solar wind into the Earth's magnetosphere.
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2.
  • Brenning, Nils (författare)
  • Interaction between a dust cloud and a magnetized plasma in relative motion
  • 2001
  • Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 29:2, s. 302-306
  • Tidskriftsartikel (refereegranskat)abstract
    • The interaction between a dust cloud and a magnetized plasma is investigated by use of an idealized model where the dust particles have uniform size, a uniform density within the dust cloud, and start with the same velocity across the magnetic field in the plasma's rest frame. The interaction is found to be governed by a dimensionless parameter K which is a function of dust cloud, and ambient plasma, parameters. For K much smaller than unity, the interaction goes on for typically 1/(2 piK) gyro times, with the particles in the dust cloud performing gyro motions with decreasing radius, For K close to unity, the dust motion is stopped on the order of a dust particle gyro time, For the case K much greater than 1, the plasma in the flux tube through the dust cloud is dragged across the magnetic field over a distance of the order of Kr-d, where r(d) is the dust gyro radius, before the motion is stopped. Some expected effects for a more realistic dust cloud with density gradients, and containing dust with a spread in size, are discussed. The results have bearing on dusty plasma in space, e.g., models of the formation of spokes in Saturn's ring system.
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3.
  • Böhlmark, Johan, et al. (författare)
  • Measurement of the magnetic field change in a pulsed high current magnetron discharge
  • 2004
  • Ingår i: Plasma Sources Science and Technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 13:4, s. 654-661
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we present a study of how the magnetic field of a circular planar magnetron is affected when it is exposed to a pulsed high current discharge. Spatially resolved magnetic field measurements are presented and the magnetic disturbance is quantified for different process parameters. The magnetic field is severely deformed by the discharge and we record changes of several millitesla, depending on the spatial location of the measurement. The shape of the deformation reveals the presence of azimuthally drifting electrons close to the target surface. Time resolved measurements show a transition between two types of magnetic perturbations. There is an early stage that is in phase with the axial discharge current and a late stage that is not in phase with the discharge current. The later part of the magnetic field deformation is seen as a travelling magnetic wave. We explain the magnetic perturbations by a combination of E × B drifting electrons and currents driven by plasma pressure gradients and the shape of the magnetic field. A plasma pressure wave is also recorded by a single tip Langmuir probe and the velocity (~103 m s−1) of the expanding plasma agrees well with the observed velocity of the magnetic wave. We note that the axial (discharge) current density is much too high compared to the azimuthal current density to be explained by classical collision terms, and an anomalous charge transport mechanism is required.
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4.
  • Gunell, Herbert, et al. (författare)
  • Experiments on anomalous electron currents to a positive probe in a magnetized plasma stream
  • 2000
  • Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 27:2, s. 161-164
  • Tidskriftsartikel (refereegranskat)abstract
    • The first laboratory experiments are reported of anomalously high currents drawn by an electron collecting probe in a magnetized plasma stream. The currents exceed the theoretical values by Parker and Murphy [Parker and Murphy, 1967] by more than a factor of three. Measurements of the potential pattern around the probe indicate that the enhanced currents are possible because electrons E x B-drift into a local field-aligned current channel.
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5.
  • Hurtig, Tomas, et al. (författare)
  • The penetration of plasma clouds across magnetic boundaries : The role of high frequency oscillations
  • 2004
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 11:7, s. L33-L36
  • Tidskriftsartikel (refereegranskat)abstract
    • Experiments are reported where a collision-free plasma cloud penetrates a magnetic barrier by self-polarization. Two closely related effects, both fundamental for the penetration mechanism, are studied quantitatively: anomalous fast magnetic field penetration (two orders of magnitude faster than classical), and anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion). It is concluded that they are both mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process.
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6.
  • Hurtig, T., et al. (författare)
  • Three-dimensional electrostatic particle-in-cell simulation with open boundaries applied to a plasma beam entering a curved magnetic field
  • 2003
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 10:11, s. 4291-4305
  • Tidskriftsartikel (refereegranskat)abstract
    • Three-dimensional electrostatic particle-in-cell simulations of a laboratory experiment with an elongated plasma cloud entering a curved magnetic field are presented. A moving grid is used to follow the plasma motion from a region with longitudinal magnetic field, through a transition region where the field curves, and into a region where the magnetic field has a constant angle of 45degrees to the flow direction. In order to isolate the physics from disturbing boundary effects a method to create open boundary conditions has been implemented. As a result the boundaries are essentially moved to infinity. The simulation reproduces and gives physical insight into several experimental results concerning the plasma's macroscopic behavior in the transition region, which have earlier been only partly understood. First, the deformation of the plasma from a cylinder to a slab; second, the formation of strong currents along the sides of the plasma cloud in the transition region, which continue into field-aligned currents in the (upstream) flow-parallel field region, and close across the magnetic field both in the front and in the back of the penetrating cloud; and, third, the formation of a potential structure including (in the transition region) magnetic-field-aligned electric fields, and (both in, and downstream of, the transition region) a potential trough structure in the plasma's rest frame. It is found that all these macroscopic phenomena are intimately linked and can be understood within one consistent physical picture. The basic driving mechanism is the azimuthal electric field that is induced when, in the plasma's rest frame, the transverse magnetic field grows in time. The plasma's response is complicated by the fact that penetrating plasma clouds are in a parameter range where currents are not related to electric fields by a local conductivity: the ion motion is instead determined by the macroscopic potential structure.
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