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  • 51. Kalered, E.
    et al.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Pilch, I.
    Caillault, L.
    Minéa, T.
    Ojamäe, L.
    On the work function and the charging of small (r ≤ 5 nm) nanoparticles in plasmas2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 1, article id 013702Article in journal (Refereed)
    Abstract [en]

    The growth of nanoparticles (NPs) in plasmas is an attractive technique where improved theoretical understanding is needed for quantitative modeling. The variation of the work function W with size for small NPs, rNP≤ 5 nm, is a key quantity for modeling of three NP charging processes that become increasingly important at a smaller size: electron field emission, thermionic electron emission, and electron impact detachment. Here we report the theoretical values of the work function in this size range. Density functional theory is used to calculate the work functions for a set of NP charge numbers, sizes, and shapes, using copper for a case study. An analytical approximation is shown to give quite accurate work functions provided that rNP > 0.4 nm, i.e., consisting of about >20 atoms, and provided also that the NPs have relaxed close to spherical shape. For smaller sizes, W deviates from the approximation, and also depends on the charge number. Some consequences of these results for nanoparticle charging are outlined. In particular, a decrease in W for NP radius below about 1 nm has fundamental consequences for their charge in a plasma environment, and thereby on the important processes of NP nucleation, early growth, and agglomeration.

  • 52. Kempf, Yann
    et al.
    Pokhotelov, Dimitry
    von Alfthan, Sebastian
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Palmroth, Minna
    Koskinen, Hannu E. J.
    Wave dispersion in the hybrid-Vlasov model: Verification of Vlasiator2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 11Article in journal (Refereed)
    Abstract [en]

    Vlasiator is a new hybrid-Vlasov plasma simulation code aimed at simulating the entire magnetosphere of the Earth. The code treats ions (protons) kinetically through Vlasov's equation in the six-dimensional phase space while electrons are a massless charge-neutralizing fluid [M. Palmroth et al., J. Atmos. Sol.-Terr. Phys. 99, 41 (2013); A. Sandroos et al., Parallel Comput. 39, 306 (2013)]. For first global simulations of the magnetosphere, it is critical to verify and validate the model by established methods. Here, as part of the verification of Vlasiator, we characterize the low-beta plasma wave modes described by this model and compare with the solution computed by the Waves in Homogeneous, Anisotropic Multicomponent Plasmas (WHAMP) code [K. Ronnmark, Kiruna Geophysical Institute Reports No. 179, 1982], using dispersion curves and surfaces produced with both programs. The match between the two fundamentally different approaches is excellent in the low-frequency, long wavelength range which is of interest in global magnetospheric simulations. The left-hand and right-hand polarized wave modes as well as the Bernstein modes in the Vlasiator simulations agree well with the WHAMP solutions. Vlasiator allows a direct investigation of the importance of the Hall term by including it in or excluding it from Ohm's law in simulations. This is illustrated showing examples of waves obtained using the ideal Ohm's law and Ohm's law including the Hall term. Our analysis emphasizes the role of the Hall term in Ohm's law in obtaining wave modes departing from ideal magnetohydrodynamics in the hybrid-Vlasov model. 

  • 53.
    Koen, Etienne
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Collier, A. B.
    Maharaj, S. K.
    Particle-in-cell simulations of beam-driven electrostatic waves in a plasma2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 4, p. 042101-Article in journal (Refereed)
    Abstract [en]

    Using a particle-in-cell simulation, the characteristics of electrostatic waves are investigated in a three-electron component plasma including an electron beam. A Maxwellian distribution is used to describe the electron velocities. Three electrostatic modes are excited, namely electron plasma, electron acoustic, and beam-driven waves. These modes have a broad frequency spectrum and have been associated with intense broadband electrostatic noise observed in the Earth's auroral zone. The simulation results compare well with analytical dispersion and growth rate relations. This agreement serves to validate the simulation technique.

  • 54.
    Koen, Etienne
    et al.
    KTH, School of Electrical Engineering (EES). Space Commercial Serv Holdings SCSH Grp, South Africa; SANSA, South Africa.
    Collier, Andrew B.
    Maharaj, Shimul K.
    A particle-in-cell approach to obliquely propagating electrostatic waves2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 9Article in journal (Refereed)
    Abstract [en]

    The electron-acoustic and beam-driven modes associated with electron beams have previouslybeen identified and studied numerically. These modes are associated with Broadband ElectrostaticNoise found in the Earth’s auroral and polar cusp regions. Using a 1-D spatial Particle-in-Cell sim-ulation, the electron-acoustic instability is studied for a magnetized plasma, which includes coolions, cool electrons and a hot, drifting electron beam. Both the weakly and strongly magnetizedregimes with varying wave propagation angle, h, with respect to the magnetic field are studied. Theamplitude and frequency of the electron-acoustic mode are found to decrease with increasing h.The amplitude of the electron-acoustic mode is found to significantly grow at intermediate wave-number ranges. It reaches a saturation level at the point, where a plateau forms in the hot electron velocity distribution after which the amplitude of the electron-acoustic mode decays

  • 55.
    Koen, Etienne
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. South African National Space Agency (SANSA), South Africa.
    Collier, Andrew B
    Maharaj, Shimul K
    Hellberg, Manfred A
    Particle-in-cell simulations of ion-acoustic waves with application to Saturn's magnetosphere2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 072122Article in journal (Refereed)
    Abstract [en]

    Using a particle-in-cell simulation, the dispersion and growth rate of the ion-acoustic mode areinvestigated for a plasma containing two ion and two electron components. The electron velocitiesare modelled by a combination of two kappa distributions, as found in Saturn’s magnetosphere.The ion components consist of adiabatic ions and an ultra-low density ion beam to drive a veryweak instability, thereby ensuring observable waves. The ion-acoustic mode is explored for a rangeof parameter values such as j, temperature ratio, and density ratio of the two electron components.The phase speed, frequency range, and growth rate of the mode are investigated. Simulations ofdouble-kappa two-temperature plasmas typical of the three regions of Saturn’s magnetosphere are also presented and analysed.

  • 56.
    Koen, Etienne J.
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Collier, A. B.
    Maharaj, S. K.
    A simulation approach of high-frequency electrostatic waves found in Saturn's magnetosphere2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 4, p. 042102-Article in journal (Refereed)
    Abstract [en]

    Using a particle-in-cell simulation, the characteristics of electron plasma and electron acoustic waves are investigated in plasmas containing an ion and two electron components. The electron velocities are modeled by a combination of two kappa distributions. The model applies to the extended plasma sheet region in Saturn's magnetosphere where the cool and hot electron velocities are found to have low indices, kappa(c) similar or equal to 2 and kappa(h) similar or equal to 4. For such low values of kappa(c) and kappa(h), the electron plasma and electron acoustic waves are coupled. The model predicts weakly damped electron plasma waves while electron acoustic waves should also be observable, although less prominent.

  • 57. Korovinskiy, D. B.
    et al.
    Divin, A. V.
    Erkaev, N. V.
    Semenov, V. S.
    Artemyev, A. V.
    Ivanova, V. V.
    Ivanov, I. B.
    Lapenta, G.
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz). KTH, School of Computer Science and Communication (CSC), Centres, Centre for High Performance Computing, PDC.
    Biernat, H. K.
    The double-gradient magnetic instability: Stabilizing effect of the guide field2015In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, no 1, article id 012904Article in journal (Refereed)
    Abstract [en]

    The role of the dawn-dusk magnetic field component in stabilizing of the magnetotail flapping oscillations is investigated in the double-gradient model framework (Erkaev et al., Phys. Rev. Lett. 99, 235003 (2007)), extended for the magnetotail-like configurations with non-zero guide field By. Contribution of the guide field is examined both analytically and by means of linearized 2-dimensional (2D) and non-linear 3-dimensional (3D) MHD modeling. All three approaches demonstrate the same properties of the instability: stabilization of current sheet oscillations for short wavelength modes, appearing of the typical (fastest growing) wavelength lambda(peak) of the order of the current sheet width, decrease of the peak growth rate with increasing B-y value, and total decay of the mode for B-y similar to 0: 5 in the lobe magnetic field units. Analytical solution and 2D numerical simulations claim also the shift of lambda(peak) toward the longer wavelengths with increasing guide field. This result is barely visible in 3D simulations. It may be accounted for the specific background magnetic configuration, the pattern of tail-like equilibrium provided by approximated solution of the conventional Grad-Shafranov equation. The configuration demonstrates drastically changing radius of curvature of magnetic field lines, R-c. This, in turn, favors the "double-gradient" mode (lambda > R-c) in one part of the sheet and classical "ballooning" instability (lambda < R-c) in another part, which may result in generation of a "combined" unstable mode. (C) 2015 AIP Publishing LLC.

  • 58.
    Korovinskiy, D. B.
    et al.
    Austrian Acad Sci, Space Res Inst, A-8042 Graz, Austria..
    Divin, A. V.
    St Petersburg State Univ, Earth Phys Dept, Petrodvorets 198504, Russia..
    Semenov, V. S.
    St Petersburg State Univ, Earth Phys Dept, Petrodvorets 198504, Russia..
    Erkaev, N. V.
    St Petersburg State Univ, Earth Phys Dept, Petrodvorets 198504, Russia.;Russian Acad Sci, Siberian Branch, Inst Computat Modelling, Krasnoyarsk 660036, Russia.;Siberian Fed Univ, Dept Appl Mech, Krasnoyarsk 660041, Russia..
    Ivanov, I. B.
    Petersburg Nucl Phys Inst, Theoret Phys Div, Gatchina 188300, Russia..
    Kiehas, S. A.
    Austrian Acad Sci, Space Res Inst, A-8042 Graz, Austria..
    Markidis, Stefano
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    The transition from "double-gradient" to ballooning unstable mode in bent magnetotail-like current sheet2019In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, no 10, article id 102901Article in journal (Refereed)
    Abstract [en]

    The magnetotail-like magnetoplasma configuration is examined for the stability to the transversal mode by means of linear 2.5- and nonlinear 3-dimensional MHD simulations. The exact two-dimensional Kan-like solution of the Vlasov-Maxwell equations is utilized for background equilibrium bent current sheets. Both linear and nonlinear simulations reveal the same features: the bent current sheet is unstable to perturbations with the wave vector pointing in the out-of-plane direction; the unstable mode is localized in the summer hemisphere; in-plane plasma flow is rotating from the earthward/tailward direction in the near-Earth region to the vertical direction in the tail. Rotation of the plasma velocity and variation of the background plasma parameters in longitudinal (Earth-Sun) direction allow considering the observed plasma motions as a transient mode from the so-called double-gradient (in distant tail) to the conventional ballooning (close to the Earth) instability. It is found that the mode localization is controlled by second derivatives of the total pressure in longitudinal and normal (north-south) directions. This feature is rendered by a newly developed quasi-two-dimensional analytical model of the transversal mode in the bent current sheet. Published under license by AIP Publishing.

  • 59.
    Kuldkepp, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Brunsell, Per R.
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Cecconello, Marco
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Dux, R.
    Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching.
    Menmuir, Sheena
    KTH, School of Engineering Sciences (SCI), Physics.
    Rachlew, Elisabeth
    KTH, School of Engineering Sciences (SCI), Physics.
    Measurements and modeling of transport and impurity radial profiles in the EXTRAP T2R reversed field pinch2006In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 13, no 9, p. 092506-Article in journal (Refereed)
    Abstract [en]

    Radial impurity profiles of oxygen in the rebuilt reversed field pinch EXTRAP T2R [P. R. Brunsell , Plasma Phys. Control. Fusion 43, 1457 (2001)] have been measured with a multichannel spectrometer. Absolute ion densities for oxygen peak between 1-4x10(10) cm(-3) for a central electron density of 1x10(13) cm(-3). Transport simulations with the one-dimensional transport code STRAHL with a diffusion coefficient of 20 m(2) s(-1) yield density profiles similar to those measured. Direct measurement of the ion profile evolution during pulsed poloidal current drive suggests that the diffusion coefficient is reduced by a factor similar to 2 in the core but remains unaffected toward the edge. Core transport is not significantly affected by the radial magnetic field growth seen at the edge in discharges without feedback control. This indicates that the mode core amplitude remains the same while the mode eigenfunction increases at the edge.

  • 60. Lanctot, M. J.
    et al.
    Park, J. -K
    Piovesan, P.
    Sun, Y.
    Buttery, R. J.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Grierson, B. A.
    Hanson, J. M.
    Haskey, S. R.
    In, Y.
    Jeon, Y. M.
    La Haye, R. J.
    Logan, N. C.
    Marrelli, L.
    Orlov, D. M.
    Paz-Soldan, C.
    Wang, H. H.
    Strait, E. J.
    Impact of toroidal and poloidal mode spectra on the control of non-axisymmetric fields in tokamaks2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 5, article id 056117Article in journal (Refereed)
    Abstract [en]

    In several tokamaks, non-axisymmetric magnetic field studies show that applied magnetic fields with a toroidal harmonic n = 2 can lead to disruptive n = 1 locked modes. In Ohmic plasmas, n = 2 magnetic reconnection thresholds in otherwise stable discharges are readily accessed at edge safety factors q similar to 3, low density, and low rotation. Similar to previous studies with n = 1 fields, the thresholds are correlated with the "overlap" field computed with the IPEC code. The overlap field quantifies the plasma-mediated coupling of the external field to the resonant field. Remarkably, the "critical overlap fields" at which magnetic islands form are similar for applied n = 1 and 2 fields. The critical overlap field increases with plasma density and edge safety factor but is independent of the toroidal field. Poloidal harmonics m> nq dominate the drive for resonant fields while m < nq harmonics have a negligible impact. This contrasts with previous results in H-mode discharges at high plasma pressure in which the toroidal angular momentum is sensitive to low poloidal harmonics. Together, these results highlight unique requirements for n > 1 field control including the need for multiple rows of coils to control selected plasma parameters for specific functions (e.g., rotation control or ELM suppression).

  • 61. Lapenta, Giovanni
    et al.
    Goldman, Martin
    Newman, David
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Propagation speed of rotation signals for field lines undergoing magnetic reconnection2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 10, p. 102113-Article in journal (Refereed)
    Abstract [en]

    Reconnection is associated with two bending of the magnetic field lines. Considering the usual plane of a 2D reconnection simulation, the first bending is in-plane and produces the needed topological changes by bringing oppositely directed filed lines in proximity. The second is typical of fast reconnection and is out of plane, leading to the formation of the Hall magnetic field. This second rotation has recently been observed to proceed at superAlfvenic speeds and to carry substantial energy fluxes (Shay et al., Phys. Rev. Lett. 107, 065001 (2011)). We revisit these rotations with a new diagnostics based on dispersing a multitude of virtual probes into a kinetic simulation, akin the approach of multi spacecraft missions. The results of the new diagnostics are compared with the theory of characteristics applied to the two fluid model. The comparison of virtual probes and the method of characteristics confirm the findings relative to the out of plane rotation and uncover the existence of two families of characteristics. Both are observed in the simulation. The early stage of reconnection develops on the slower compressional branch and the later faster phase develops on the faster torsional branch. The superAlfvenic signal is only relevant in the second phase.

  • 62. Lapenta, Giovanni
    et al.
    Goldman, Martin
    Newman, David
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Divin, Andrey
    Electromagnetic energy conversion in downstream fronts from three dimensional kinetic reconnection2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 5, p. 055702-Article in journal (Refereed)
    Abstract [en]

    The electromagnetic energy equation is analyzed term by term in a 3D simulation of kinetic reconnection previously reported by Vapirev et al. [J. Geophys. Res.: Space Phys. 118, 1435 (2013)]. The evolution presents the usual 2D-like topological structures caused by an initial perturbation independent of the third dimension. However, downstream of the reconnection site, where the jetting plasma encounters the yet unperturbed pre-existing plasma, a downstream front is formed and made unstable by the strong density gradient and the unfavorable local acceleration field. The energy exchange between plasma and fields is most intense at the instability, reaching several pW/m(3), alternating between load (energy going from fields to particles) and generator (energy going from particles to fields) regions. Energy exchange is instead purely that of a load at the reconnection site itself in a region focused around the x-line and elongated along the separatrix surfaces. Poynting fluxes are generated at all energy exchange regions and travel away from the reconnection site transporting an energy signal of the order of about S approximate to 10(-3)W/m(2). (C) 2014 AIP Publishing LLC.

  • 63. Loarte, A.
    et al.
    Saibene, G.
    Sartori, R.
    Eich, T.
    Kallenbach, A.
    Suttrop, W.
    Kempenaars, M.
    Beurskens, M.
    de Baar, M.
    Lönnroth, J.
    Lomas, P. J.
    Matthews, G.
    Fundamenski, W.
    Parail, V.
    Becoulet, M.
    Monier-Garbet, P.
    de la Luna, E.
    Goncalves, B.
    Silva, C.
    Corre, Yann
    KTH, Superseded Departments, Physics.
    Characterization of pedestal parameters and edge localized mode energy losses in the Joint European Torus and predictions for the International Thermonuclear Experimental Reactor2004In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, no 5, p. 2668-2678Article in journal (Refereed)
    Abstract [en]

    This paper presents the experimental characterization of pedestal parameters, edge localized mode (ELM) energy, and particle losses from the main plasma and the corresponding ELM energy fluxes on plasma facing components for a series of dedicated experiments in the Joint European Torus (JET). From these experiments, it is demonstrated that the simple hypothesis relating the peeling-ballooning linear instability to ELM energy losses is not valid. Contrary to previous observations at lower triangularities, small energy losses at low collisionality have been obtained in regimes at high plasma triangularity and q(95)similar to4.5, indicating that the edge plasma magnetohydrodynamic stability is linked with the transport mechanisms that lead to the loss of energy by conduction during type I ELMs. Measurements of the ELM energy fluxes on the divertor target show that their time scale is linked to the ion transport along the field and the formation of a high energy sheath, in agreement with kinetic modeling of ELMs. Higher density ELMs, of a convective nature, lead to overall much longer time scales for the ELM energy flux, with more than 80% of the ELM energy flux arriving after the surface divertor temperature has reached its maximum value. On the contrary, for low density ELMs, of a conductive nature, up to 40% of the energy flux arrives at the divertor target before the surface divertor temperature has reached its maximum value. These large and more conductive ELMs may lead to up to similar to50% of the ELM energy reaching the main wall plasma facing components instead of the divertor target. The extrapolation to the International Thermonuclear Experimental Reactor of the obtained results is described and the main uncertainties discussed.

  • 64.
    Lucco Castello, Federico
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Hansen, Jesper Schmidt
    Dyre, Jeppe C.
    Isomorph invariance and thermodynamics of repulsive dense bi-Yukawa one-component plasmas2019In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, no 5, article id 053705Article in journal (Refereed)
    Abstract [en]

    In numerous realizations of complex plasmas, dust-dust interactions are characterized by two screening lengths and are thus better described by a combination of Yukawa potentials. The present work investigates the static correlations and the thermodynamics of repulsive dense bi-Yukawa fluids based on the fact that such strongly coupled systems exhibit isomorph invariance. The strong virial-potential energy correlations are demonstrated with the aid of molecular dynamics simulations, an accurate analytical expression for the isomorph family of curves is obtained, and an empirical expression for the fluid-solid phase-coexistence line is proposed. The isomorph-based empirically modified hypernetted-chain approach, grounded on the ansatz of isomorph invariant bridge functions, is then extended to such systems and the resulting structural properties show an excellent agreement with the results of computer simulations. A simple and accurate closed-form expression is obtained for the excess internal energy of dense bi-Yukawa fluids by capitalizing on the compact parameterization offered by the Rosenfeld-Tarazona decomposition in combination with the Rosenfeld scaling, which opens up the energy route to thermodynamics.

  • 65.
    Malmberg, J-A
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brunsell, P R
    KTH, Superseded Departments, Alfvén Laboratory.
    Yagi, Y
    Koguchi, H
    Locked modes in two reversed-field pinch devices of different size and shell system2000In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 7, no 10, p. 4184-4196Article in journal (Refereed)
  • 66.
    Malmberg, Jenny A.
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brzozowski, Jerzy H.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brunsell, Per R.
    KTH, Superseded Departments, Alfvén Laboratory.
    Cecconello, Marco
    KTH, Superseded Departments, Alfvén Laboratory.
    Drake, James R.
    KTH, Superseded Departments, Alfvén Laboratory.
    Mode- and plasma rotation in a resistive shell reversed-field pinch2004In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, no 2, p. 647-658Article in journal (Refereed)
    Abstract [en]

    Mode rotation studies in a resistive shell reversed-field pinch, EXTRAP T2R [P. R. Brunsell , Plasma Phys. Control. Fusion 43, 1 (2001)] are presented. The phase relations and nonlinear coupling of the resonant modes are characterized and compared with that expected from modeling based on the hypothesis that mode dynamics can be described by a quasi stationary force balance including electromagnetic and viscous forces. Both m=0 and m=1 resonant modes are studied. The m=1 modes have rotation velocities corresponding to the plasma flow velocity (20-60 km/s) in the core region. The rotation velocity decreases towards the end of the discharge, although the plasma flow velocity does not decrease. A rotating phase locked m=1 structure is observed with a velocity of about 60 km/s. The m=0 modes accelerate throughout the discharges and reach velocities as high as 150-250 km/s. The observed m=0 phase locking is consistent with theory for certain conditions, but there are several conditions when the dynamics are not described. This is not unexpected because the assumption of quasi stationarity for the mode spectra is not fulfilled for many conditions. Localized m=0 perturbations are formed in correlation with highly transient discrete dynamo events. These perturbations form at the location of the m=1 phase locked structure, but rotate with a different velocity as they spread out in the toroidal direction. (C) 2004 American Institute of Physics.

  • 67.
    Malmberg, Jenny-Ann
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brunsell, Per
    KTH, Superseded Departments, Alfvén Laboratory.
    Resistive wall instabilities and tearing mode dynamics in the EXTRAP T2R thin shell reversed-field pinch2002In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 9, no 1, p. 212-221Article in journal (Refereed)
    Abstract [en]

    Observations of resistive wall instabilities and tearing mode dynamics in the EXTRAP T2R thin shell (tau (w)=6 ms) reversed field pinch are described. A nonresonant mode (m=1,n=-10) with the same handedness as the internal field grows nearly exponentially with an average growth time of about 2.6 ms (less than 1/2 of the shell time) consistent with linear stability theory. The externally nonresonant unstable modes (m=1,n >0), predicted by linear stability theory, are observed to have only low amplitudes (in the normal low-Theta operation mode of the device). The radial field of the dominant internally resonant tearing modes (m=1,n=-15 to n=-12) remain low due to spontaneous fast mode rotation, corresponding to angular phase velocities up to 280 krad/s. Phase aligned mode structures are observed to rotate toroidally with an average angular velocity of 40 krad/s, in the opposite direction of the plasma current. Toward the end of the discharge, the radial field of the internally resonant modes grows as the modes slow down and become wall-locked, in agreement with nonlinear computations. Fast rotation of the internally resonant modes has been observed only recently and is attributed to a change of the front-end system (vacuum vessel, shell, and TF coil) of the device.

  • 68.
    Manas, P.
    et al.
    Max Planck Inst Plasma Phys, D-85748 Garching, Germany.;Aix Marseille Univ, CNRS, PIIM UMR7345, F-13397 Marseille, France..
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Elevant, Thomas
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Garcia-Carrasco, Alvaro
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Ivanova, Darya
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Menmuir, Sheena
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Rachlew, Elisabeth
    KTH, School of Engineering Sciences (SCI), Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Ström, Petter
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Tholerus, Emmi
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Weckmann, Armin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Zychor, I.
    Inst Plasma Phys & Laser Microfus, PL-01497 Warsaw, Poland..
    et al.,
    Gyrokinetic modeling of impurity peaking in JET H-mode plasmas2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 6, article id 062511Article in journal (Refereed)
    Abstract [en]

    Quantitative comparisons are presented between gyrokinetic simulations and experimental values of the carbon impurity peaking factor in a database of JET H-modes during the carbon wall era. These plasmas feature strong NBI heating and hence high values of toroidal rotation and corresponding gradient. Furthermore, the carbon profiles present particularly interesting shapes for fusion devices, i.e., hollow in the core and peaked near the edge. Dependencies of the experimental carbon peaking factor (R/L-nC) on plasma parameters are investigated via multilinear regressions. A marked correlation between R/L-nC and the normalised toroidal rotation gradient is observed in the core, which suggests an important role of the rotation in establishing hollow carbon profiles. The carbon peaking factor is then computed with the gyrokinetic code GKW, using a quasi-linear approach, supported by a few non-linear simulations. The comparison of the quasi-linear predictions to the experimental values at mid-radius reveals two main regimes. At low normalised collisionality, nu*, and T-e/T-i < 1, the gyrokinetic simulations quantitatively recover experimental carbon density profiles, provided that rotodiffusion is taken into account. In contrast, at higher nu* and T-e/T-i > 1, the very hollow experimental carbon density profiles are never predicted by the simulations and the carbon density peaking is systematically over estimated. This points to a possible missing ingredient in this regime.

  • 69. Marchand, R.
    et al.
    Miyake, Y.
    Usui, H.
    Deca, J.
    Lapenta, G.
    Mateo-Velez, J. C.
    Ergun, R. E.
    Sturner, A.
    Genot, V.
    Hilgers, A.
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Cross-comparison of spacecraft-environment interaction model predictions applied to Solar Probe Plus near perihelion2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 6, p. 062901-Article in journal (Refereed)
    Abstract [en]

    Five spacecraft-plasma models are used to simulate the interaction of a simplified geometry Solar Probe Plus (SPP) satellite with the space environment under representative solar wind conditions near perihelion. By considering similarities and differences between results obtained with different numerical approaches under well defined conditions, the consistency and validity of our models can be assessed. The impact on model predictions of physical effects of importance in the SPP mission is also considered by comparing results obtained with and without these effects. Simulation results are presented and compared with increasing levels of complexity in the physics of interaction between solar environment and the SPP spacecraft. The comparisons focus particularly on spacecraft floating potentials, contributions to the currents collected and emitted by the spacecraft, and on the potential and density spatial profiles near the satellite. The physical effects considered include spacecraft charging, photoelectron and secondary electron emission, and the presence of a background magnetic field. Model predictions obtained with our different computational approaches are found to be in agreement within 2% when the same physical processes are taken into account and treated similarly. The comparisons thus indicate that, with the correct description of important physical effects, our simulation models should have the required skill to predict details of satellite-plasma interaction physics under relevant conditions, with a good level of confidence. Our models concur in predicting a negative floating potential V-fl similar to -10V for SPP at perihelion. They also predict a "saturated emission regime" whereby most emitted photo-and secondary electron will be reflected by a potential barrier near the surface, back to the spacecraft where they will be recollected.

  • 70.
    Markidis, Stefano
    et al.
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Henri, P.
    Lapenta, G.
    Divin, A.
    Goldman, M.
    Newman, D.
    Laure, Erwin
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz). KTH, School of Computer Science and Communication (CSC), Centres, Centre for High Performance Computing, PDC.
    Kinetic simulations of plasmoid chain dynamics2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 8, p. 082105-Article in journal (Refereed)
    Abstract [en]

    The dynamics of a plasmoid chain is studied with three dimensional Particle-in-Cell simulations. The evolution of the system with and without a uniform guide field, whose strength is 1/3 the asymptotic magnetic field, is investigated. The plasmoid chain forms by spontaneous magnetic reconnection: the tearing instability rapidly disrupts the initial current sheet generating several small-scale plasmoids that rapidly grow in size coalescing and kinking. The plasmoid kink is mainly driven by the coalescence process. It is found that the presence of guide field strongly influences the evolution of the plasmoid chain. Without a guide field, a main reconnection site dominates and smaller reconnection regions are included in larger ones, leading to an hierarchical structure of the plasmoid-dominated current sheet. On the contrary in presence of a guide field, plasmoids have approximately the same size and the hierarchical structure does not emerge, a strong core magnetic field develops in the center of the plasmoid in the direction of the existing guide field, and bump-on-tail instability, leading to the formation of electron holes, is detected in proximity of the plasmoids.

  • 71.
    Markidis, Stefano
    et al.
    KTH, School of Computer Science and Communication (CSC), Centres, Centre for High Performance Computing, PDC.
    Lapenta, G.
    Divin, A.
    Goldman, M.
    Newman, D.
    Andersson, L.
    Three dimensional density cavities in guide field collisionless magnetic reconnection2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 3, p. 032119-Article in journal (Refereed)
    Abstract [en]

    Particle-in-cell simulations of collisionless magnetic reconnection with a guide field reveal for the first time the three dimensional features of the low density regions along the magnetic reconnection separatrices, the so-called cavities. It is found that structures with further lower density develop within the cavities. Because their appearance is similar to the rib shape, these formations are here called low density ribs. Their location remains approximately fixed in time and their density progressively decreases, as electron currents along the cavities evacuate them. They develop along the magnetic field lines and are supported by a strong perpendicular electric field that oscillates in space. In addition, bipolar parallel electric field structures form as isolated spheres between the cavities and the outflow plasma, along the direction of the low density ribs and of magnetic field lines.

  • 72. Marrelli, L.
    et al.
    Frassinetti, Lorenzo
    Consorzio RFX.
    Martin, P.
    Sarff, J.S.
    Reduced intermittency in the magnetic turbulence of reversed field pinch plasmas2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 3, p. 030701-Article in journal (Refereed)
    Abstract [en]

    The statistical temporal properties of broadband magnetic turbulence in the edge of reversed field pinch (RFP) plasmas are significantly altered when global magnetohydrodynamic tearing modes and magnetic relaxation are reduced. Standard RFP plasmas, having relatively large tearing fluctuations, exhibit broadband intermittent bursts of magnetic fluctuations in the bandwidth f < 1.5 MHz. When the global tearing is reduced via parallel current drive in the edge region, the magnetic turbulence is much less intermittent and has statistical behavior typical of self-similar turbulence (like that expected in self-organized criticality systems). A connection between intermittency and long wavelength plasma instabilities is therefore implied.

  • 73.
    Modestov, Mikhail
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Bychkov, V.
    Brodin, G.
    Marklund, M.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Astronomy, Sweden .
    Evolution of the magnetic field generated by the Kelvin-Helmholtz instability2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 7, p. 072126-Article in journal (Refereed)
    Abstract [en]

    The Kelvin-Helmholtz instability in an ionized plasma is studied with a focus on the magnetic field generation via the Biermann battery (baroclinic) mechanism. The problem is solved by using direct numerical simulations of two counter-directed flows in 2D geometry. The simulations demonstrate the formation of eddies and their further interaction and merging resulting in a large single vortex. In contrast to general belief, it is found that the instability generated magnetic field may exhibit significantly different structures from the vorticity field, despite the mathematically identical equations controlling the magnetic field and vorticity evolution. At later stages of the nonlinear instability development, the magnetic field may keep growing even after the hydrodynamic vortex strength has reached its maximum and started decaying due to dissipation.

  • 74. Neu, R.
    et al.
    Arnoux, G.
    Beurskens, M.
    Bobkov, V.
    Brezinsek, S.
    Bucalossi, J.
    Calabro, G.
    Challis, C.
    Coenen, J. W.
    De La Luna, E.
    De Vries, P. C.
    Dux, R.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Giroud, C.
    Groth, M.
    Hobirk, J.
    Joffrin, E.
    Lang, P.
    Lehnen, M.
    Lerche, E.
    Loarer, T.
    Lomas, P.
    Maddison, G.
    Maggi, C.
    Matthews, G.
    Marsen, S.
    Mayoral, M. -L
    Meigs, A.
    Mertens, P.
    Nunes, I.
    Philipps, V.
    Pütterich, T.
    Rimini, F.
    Sertoli, M.
    Sieglin, B.
    Sips, A. C. C.
    Van Eester, D.
    Van Rooij, G.
    First operation with the JET International Thermonuclear Experimental Reactor-like wall2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 5, p. 056111-1-056111-13Article in journal (Refereed)
    Abstract [en]

    To consolidate International Thermonuclear Experimental Reactor (ITER) design choices and prepare for its operation, Joint European Torus (JET) has implemented ITER's plasma facing materials, namely, Be for the main wall and W in the divertor. In addition, protection systems, diagnostics, and the vertical stability control were upgraded and the heating capability of the neutral beams was increased to over 30 MW. First results confirm the expected benefits and the limitations of all metal plasma facing components (PFCs) but also yield understanding of operational issues directly relating to ITER. H-retention is lower by at least a factor of 10 in all operational scenarios compared to that with C PFCs. The lower C content (≈ factor 10) has led to much lower radiation during the plasma burn-through phase eliminating breakdown failures. Similarly, the intrinsic radiation observed during disruptions is very low, leading to high power loads and to a slow current quench. Massive gas injection using a D2/Ar mixture restores levels of radiation and vessel forces similar to those of mitigated disruptions with the C wall. Dedicated L-H transition experiments indicate a 30% power threshold reduction, a distinct minimum density, and a pronounced shape dependence. The L-mode density limit was found to be up to 30% higher than for C allowing stable detached divertor operation over a larger density range. Stable H-modes as well as the hybrid scenario could be re-established only when using gas puff levels of a few 1021 es-1. On average, the confinement is lower with the new PFCs, but nevertheless, H factors up to 1 (H-Mode) and 1.3 (at β N ≈ 3, hybrids) have been achieved with W concentrations well below the maximum acceptable level.

  • 75.
    Oliver, H. J. C.
    et al.
    Univ Texas Austin, Inst Fus Studies, Austin, TX 78712 USA.;Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England..
    Bergsåker, Henric
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Bykov, Igor
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Garcia-Carrasco, Alvaro
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Hellsten, Torbjörn
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Johnson, Thomas
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Menmuir, Sheena
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Petersson, Per
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Rachlew, Elisabeth
    KTH, School of Engineering Sciences (SCI), Physics.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Rubel, Marek
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Stefanikova, Estera
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Ström, Petter
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Tholerus, Emmi
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Tolias, Panagiotis
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Olivares, Pablo Vallejos
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics.
    Weckmann, Armin
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Zhou, Yushun
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Fusion Plasma Physics. KTH, Fusion Plasma Phys, EES, SE-10044 Stockholm, Sweden..
    Zychor, I.
    Natl Ctr Nucl Res, PL-05400 Otwock, Poland..
    Axisymmetric global Alfven eigenmodes within the ellipticity-induced frequency gap in the Joint European Torus2017In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 12, article id 122505Article in journal (Refereed)
    Abstract [en]

    Alfven eigenmodes (AEs) with toroidal mode number n = 0 (i.e., axisymmetric) have been observed in the ellipticity-induced frequency range in the Joint European Torus. The axisymmetric modes are of interest because they can be used to diagnose fast particle energy distributions at the mode location. The modes were identified as global Alfven eigenmodes (GAEs), with the ellipticity of the plasma cross-section preventing strong continuum damping of the modes. The MHD codes CSCAS, MISHKA, and AEGIS were used to compute the n = 0 Alfven continuum, eigenmode structure, and continuum damping. For zero ellipticity, a single mode exists at a frequency below the Alfven continuum branch. This mode has two dominant poloidal harmonics with poloidal mode numbers m = +/- 1 that have the same polarity; therefore, it is an even mode. For finite ellipticity, the continuum branch splits into two branches and the single GAE splits into two modes. An even mode exists below the minimum of the top continuum branch, and the frequency of this mode coincides with the experimentally observed AE frequency. The other mode is found below the lower continuum branch with opposite signs between the two poloidal harmonics (an odd mode structure). This mode was not excited in our experiment. Analytical theory for the n = 0 GAE in an elliptical cylinder shows the n = 0 Alfven continuum agrees with the numerical modelling.

  • 76.
    Olson, Jonas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Dust-driven and plasma-driven currents in the inner magnetosphere of Saturn2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 4, p. 042903-Article in journal (Refereed)
    Abstract [en]

    General equations for dust-driven currents and current systems J(D) in magnetized plasmas are derived and, as a concrete example, applied to the E ring of Saturn at radial distances 3R(S) < R < 5R(S). An azimuthal ring current J(D,phi) acts as a current generator and is coupled to two secondary dust-driven current systems down to the ionosphere of Saturn, both rotating with the magnetospheric plasma. One of these closes across the polar cap, and the other over a limited range in latitude. These dust-driven current systems are embedded in three systems of plasma-driven currents J(p): a ring current, a cross-polar-cap current system, and an ion pickup current system. Both the J(D) and the J(p) current systems have been quantitatively assessed from a data set for the E ring of Saturn in which the unknown distribution of small dust is treated by a power law extrapolation from the known distribution of larger dust. From data on the magnetic perturbations during a crossing of the equatorial plane, an approximate constraint on the fraction of the electrons that can be trapped on the dust is derived. For this amount of electron capture, it is demonstrated that all three types of dust-driven currents are, within somewhat more than an order of magnitude, of the same strength as the corresponding types of plasma-driven currents. Considering also that both plasma and dust densities vary with the geyser activity at the south pole of Enceladus, it is concluded that both the dust-driven and the plasma-driven contributions to the current system associated with the E ring need to be retained for a complete description.

  • 77.
    Olson, Jonas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    The magnetospheric clock of Saturn-A self-organized plasma dynamo2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 8, p. 082901-Article in journal (Refereed)
    Abstract [en]

    The plasma in the inner magnetosphere of Saturn is characterized by large-amplitude azimuthal density variations in the equatorial plane, with approximately a sinusoidal dependence on the azimuthal angle [D. A. Gurnett et al., Science 316, 442 (2007)]. This structure rotates with close to the period of the planet itself and has been proposed to steer other nonaxisymmetric phenomena, e. g., the Saturn kilometric radiation SKR [W. S. Kurth et al., Geophys. Res. Lett. 34, L02201 (2007)], and inner-magnetosphere magnetic field perturbations [D. J. Southwood and M. G. Kivelson, J. Geophys. Res. 112(A12), A12222 (2007)]. There is today no consensus regarding the basic driving mechanism. We here propose it to be a plasma dynamo, located in the neutral gas torus of Enceladus but coupled both inwards, through electric currents along the magnetic field lines down to the planet, and outwards through the plasma flow pattern there. Such a dynamo mechanism is shown to self-regulate towards a state that, with realistic parameters, can reproduce the observed configuration of the magnetosphere. This state is characterized by three quantities: the Pedersen conductivity in the polar cap, the ionization time constant in the neutral gas torus, and a parameter characterizing the plasma flow pattern. A particularly interesting property of the dynamo is that regular (i.e., constant-amplitude, sinusoidal) variations in the last parameter can lead to complicated, non-periodic, oscillations around the steady-state configuration.

  • 78.
    Olson, Jonas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Miloch, W.J.
    Department of Physics and Technology, University of Tromsö, Norway.
    Ratynskaia, Svetlana
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Yaroshenko, V.
    Max-Planck-Institute für Extraterrestrische Physik, Germany.
    Potential structure around th Cassini spacecraft near the orbit of Enceladus2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 10Article in journal (Refereed)
    Abstract [en]

    We present the results of numerical simulations of the potential structure around an object in a streaming plasma with parameters relevant for the Cassini spacecraft passing through Saturn's plasma disk near the orbit of Enceladus. Two-and three-dimensional particle-in-cell codes have been used allowing the potential of the simulated spacecraft body to develop self-consistently through the collection of charge by its surface. The dependence of the density and potential profiles on ambient plasma density, electron temperature, and ion drift speed is discussed. The spacecraft floating potential values, found in the simulations, are compared to those deduced from the analysis of Cassini Langmuir probe characteristics.

  • 79.
    Peng, I. Bo
    et al.
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Laure, Erwin
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Johlander, A.
    Vaivads, A.
    Khotyaintsev, Y.
    Henri, P.
    Lapenta, G.
    Kinetic structures of quasi-perpendicular shocks in global particle-in-cell simulations2015In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, no 9, article id 092109Article in journal (Refereed)
    Abstract [en]

    We carried out global Particle-in-Cell simulations of the interaction between the solar wind and a magnetosphere to study the kinetic collisionless physics in super-critical quasi-perpendicular shocks. After an initial simulation transient, a collisionless bow shock forms as a result of the interaction of the solar wind and a planet magnetic dipole. The shock ramp has a thickness of approximately one ion skin depth and is followed by a trailing wave train in the shock downstream. At the downstream edge of the bow shock, whistler waves propagate along the magnetic field lines and the presence of electron cyclotron waves has been identified. A small part of the solar wind ion population is specularly reflected by the shock while a larger part is deflected and heated by the shock. Solar wind ions and electrons are heated in the perpendicular directions. Ions are accelerated in the perpendicular direction in the trailing wave train region. This work is an initial effort to study the electron and ion kinetic effects developed near the bow shock in a realistic magnetic field configuration.

  • 80.
    Raadu, Michael A.
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Shafiq, Muhammad
    KTH, Superseded Departments, Alfvén Laboratory.
    Shielding of a slowly moving test charge in a dusty plasma with dynamical grain charging2003In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 10, no 9, p. 3484-3491Article in journal (Refereed)
    Abstract [en]

    The dynamical charging of grains in a dusty plasma enhances the shielding of test charges. Time scales for charging are determined by the ambient plasma parameters and the grain dimensions. They can be very short, approaching the ion plasma period for grain sizes of the order of an electron Debye length. For a slowly moving test charge the response potential is found as a power series in the test charge velocity. Collisional effects are included. Analytical expressions for the response potential, valid for all radial distances, are found up to second order in the test charge velocity. The first-order dynamical charging term is shown to be the consequence of the delay in the shielding due to the dynamics of the charging process. The remaining first-order terms are given by analytical expressions that yield the well known asymptotic power law forms for large distances.

  • 81. Ram, Abhay K.
    et al.
    Dasgupta, Brahmananda
    Krishnamurthy, V.
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Anomalous diffusion of field lines and charged particles in Arnold-Beltrami-Childress force-free magnetic fields2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 7, p. 072309-Article in journal (Refereed)
    Abstract [en]

    The cosmic magnetic fields in regions of low plasma pressure and large currents, such as in interstellar space and gaseous nebulae, are force-free in the sense that the Lorentz force vanishes. The three-dimensional Arnold-Beltrami-Childress (ABC) field is an example of a force-free, helical magnetic field. In fluid dynamics, ABC flows are steady state solutions of the Euler equation. The ABC magnetic field lines exhibit a complex and varied structure that is a mix of regular and chaotic trajectories in phase space. The characteristic features of field line trajectories are illustrated through the phase space distribution of finite-distance and asymptotic-distance Lyapunov exponents. In regions of chaotic trajectories, an ensemble-averaged variance of the distance between field lines reveals anomalous diffusion-in fact, superdiffusion-of the field lines. The motion of charged particles in the force-free ABC magnetic fields is different from the flow of passive scalars in ABC flows. The particles do not necessarily follow the field lines and display a variety of dynamical behavior depending on their energy, and their initial pitch-angle. There is an overlap, in space, of the regions in which the field lines and the particle orbits are chaotic. The time evolution of an ensemble of particles, in such regions, can be divided into three categories. For short times, the motion of the particles is essentially ballistic; the ensemble-averaged, mean square displacement is approximately proportional to t(2), where t is the time of evolution. The intermediate time region is defined by a decay of the velocity autocorrelation function-this being a measure of the time after which the collective dynamics is independent of the initial conditions. For longer times, the particles undergo superdiffusion-the mean square displacement is proportional to t(alpha), where alpha > 1, and is weakly dependent on the energy of the particles. These super-diffusive characteristics, both of magnetic field lines and of particles moving in these fields, strongly suggest that theories of transport in three-dimensional chaotic magnetic fields need a shift from the usual paradigm of quasilinear diffusion.

  • 82.
    Ratynskaia, Svetlana
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Regnoli, Giorgio
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Klumov, B.
    Rypdal, K.
    Grain transport in three-dimensional soft dusty plasma states2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 3, p. 034502-Article in journal (Refereed)
    Abstract [en]

    Three-dimensional molecular dynamics simulations of aggregates of dust grains interacting via repulsive Yukawa forces, and subject to heating by neutrals and realistic boundary conditions, suggest that grain transport is ubiquitously anomalous for soft states at the transition between liquid and solid. These anomalies include self-similar, stretched-Gaussian, probability density distributions of grain displacements and superdiffusive displacement scaling on the short time scales for which the mean displacements sigma(tau) are less than the interparticle distance Delta. However, the details of the anomalous transport on these shorter time scales depend on system stiffness and confinement conditions. On time scales for which sigma(tau) >= Delta, humps can develop on the distribution at integer multiples of Delta, an effect of cooperative hopping of grains on the lattice. Relaxation toward Gaussian displacement distributions and normal diffusion takes place on time scales for which sigma(tau) > Delta. The simulations indicate that qualitative features previously found for hexatic states of two-dimensional aggregates by simulation and experiment are also present in three-dimensional configurations. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3334377]

  • 83. Ratynskaia, Svetlana V.
    et al.
    de Angelis, U.
    Khrapak, S.
    Klumov, B.
    Morfill, G. E.
    Electrostatic interaction between dust particles in weakly ionized complex plasmas2006In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 13, no 10Article in journal (Refereed)
    Abstract [en]

    The electrostatic potential around a dust particle in a complex plasma is calculated, taking into account ion-neutral collisions and collective effects, in a range of plasma parameters relevant for typical laboratory experiments. The existence of attractive wells and dependence of their shape on the main experimental control parameters, neutral gas pressure, and dust number density, is investigated.

  • 84. Ratynskaia, Svetlana V.
    et al.
    Demidov, V. I.
    Rypdal, K.
    Probe measurements of low-frequency plasma potential and electric field fluctuations in a magnetized plasma2002In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 9, no 10, p. 4135-4143Article in journal (Refereed)
    Abstract [en]

    A system of two cylindrical probes aligned along the magnetic field, and equipped with insulating end plugs, is proposed for measurement of low-frequency fluctuations of the electrostatic field in a magnetized plasma. It is demonstrated by modeling and experiments that the plug probe floats close to the plasma potential. The electric field component in a given direction is obtained by subtracting the plasma potentials obtained on two spatially separated plug probes. The probe system is applied to low-frequency electrostatic fluctuations in a simple magnetized torus, and reveals the presence of global oscillations, large scale propagating structures (m=1 modes), and developed turbulence with power-law spectra. Two different mode branches for the fluctuations are identified by comparing results from plug probes with results from conventional probes. Sources of errors arising from applying floating potential of conventional probes for electric field measurements are pointed out and discussed.

  • 85. Ratynskaia, Svetlana V.
    et al.
    Knapek, C.
    Rypdal, K.
    Khrapak, S.
    Morfill, G.
    Statistics of particle transport in a two-dimensional dusty plasma cluster2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 2Article in journal (Refereed)
    Abstract [en]

    Statistical analysis is performed on long time series of dust particle trajectories in a two-dimensional dusty plasma cluster. Particle transport is found to be superdiffusive on all time scales until the range of particle displacements approaches the size of the cluster. Analysis of probability distribution functions and rescaled range analysis of the position increments show that the signal is non-Gaussian self-similar with Hurst exponent H=0.6, indicating that the superdiffusion is caused by long-range dependencies in the system. Investigation of temporal and spatial characteristics of persistent particle slips demonstrates that they are associated with collective events present on all time scales and responsible for the non-Gaussianity and long-memory effects.

  • 86. Ratynskaia, Svetlana V.
    et al.
    Kompaneets, R.
    Ivlev, A. V.
    Knapek, C.
    Morfill, G. E.
    Transport in strongly coupled two-dimensional complex plasmas: Role of the interaction potential2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 1Article in journal (Refereed)
    Abstract [en]

    Observations of particle motion in a complex plasma monolayer in rf discharge have revealed anomalous dependence on neutral gas pressure; an immobile system at high gas pressures, then, decreasing the pressure, a state with viscoelastic vortical fluid motions and, hence, dramatically enhanced transport, followed again by an ordered state. While the inhibited transport at higher pressures can be attributed to dissipation on neutrals, the transition of the system back to an ordered state at lower pressures has been explained by an increase of the dust-dust viscous dissipation due to significant modifications of the interaction potential between the particles.

  • 87.
    Ratynskaiy, Svetlana
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    De Angeli, M.
    Lazzaro, E.
    Marmolino, C.
    de Angelis, U.
    Castaldo, C.
    Cremona, A.
    Laguardia, L.
    Gervasini, G.
    Grosso, G.
    Plasma fluctuation spectra as a diagnostic tool for submicron dust2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 4, p. 043703-Article in journal (Refereed)
    Abstract [en]

    It is shown that the measurements of density fluctuation spectra in dusty plasmas can constitute a basis for in situ diagnostic of invisible submicron dust. The self-consistent kinetic theory that includes the charging processes and the natural density fluctuations of the dust particles predicts modifications of the spectra due to the presence of dust. A laboratory experiment was carried out where submicron dust was produced in a gas phase and diagnosed by surface analysis of samples and by measurements of its influence on the plasma density fluctuation spectra. Quantitative comparison of the latter with the theory yields information on dust density, size, and distribution in agreement with the results of the surface analysis. The method can be applied to various plasma environments in laboratory and space.

  • 88. Regnoli, G.
    et al.
    Bergsåker, Henric
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Tennfors, E.
    Zonca, F.
    Martines, E.
    Serianni, G.
    Spolaore, M.
    Vianello, N.
    Cecconello, Marco
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Antoni, V.
    Cavazzana, R.
    Malmberg, J. A.
    Observations of toroidicity-induced Alfven eigenmodes in a reversed field pinch plasma2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 4Article in journal (Refereed)
    Abstract [en]

    High frequency peaks in the spectra of magnetic field signals have been detected at the edge of Extrap-T2R [P. R. Brunsell, H. Bergshker, M. Cecconello, J. R. Drake, R. M. Gravestijn, A. Hedqvist, and J-A. Malmberg, Plasma Phys. Controlled Fusion, 43, 1457 (2001)]. The measured fluctuation is found to be mainly polarized along the toroidal direction, with high toroidal periodicity n and Alfvenic scaling (f proportional to B / root m(i)n(i)). Calculations for, a reversed field pinch plasma predict the existence of an edge resonant, high frequency, high-n number toroidicity-induced Alfven eigenmode with the,observed frequency scaling. In addition, gas puffing experiments show that edge density fluctuations are responsible for the rapid. changes of mode frequency. Finally a coupling with the electron drift turbulence is proposed as drive mechanism for the eigenmode.

  • 89. Restante, A. L.
    et al.
    Markidis, Stefano
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Lapenta, G.
    Intrator, T.
    Geometrical investigation of the kinetic evolution of the magnetic field in a periodic flux rope2013In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 20, no 8, p. 082501-Article in journal (Refereed)
    Abstract [en]

    Flux ropes are bundles of magnetic field wrapped around an axis. Many laboratory, space, and astrophysics processes can be represented using this idealized concept. Here, a massively parallel 3D kinetic simulation of a periodic flux rope undergoing the kink instability is studied. The focus is on the topology of the magnetic field and its geometric structures. The analysis considers various techniques such as Poincare maps and the quasi-separatrix layer (QSL). These are used to highlight regions with expansion or compression and changes in the connectivity of magnetic field lines and consequently to outline regions where heating and current may be generated due to magnetic reconnection. The present study is, to our knowledge, the first QSL analysis of a fully kinetic 3D particle in cell simulation and focuses the existing QSL method of analysis to periodic systems.

  • 90. Russell, P. G. F.
    et al.
    Arber, T. D.
    Coppins, M.
    Scheffel, Jan
    KTH.
    Linear Stability of the Collisionless, Large Larmor Radius Z-pinch1997In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 4, p. 2322-Article in journal (Refereed)
    Abstract [en]

    The Vlasov fluid model is used to study the m=0 and m=1 internal and free boundary modes in a collisionless, large Larmor radius Z pinch. Two methods (initial value and variational) are employed, and give good agreement. The growth rate can be reduced from its zero Larmor radius value by a factor of up to 10 for m=1, and up to 3 for m=0. Stability thresholds and the role of resonant ions are discussed.

  • 91. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Fluctuation threshold and profile resilience in weakly ionized plasma in a curved, unsheared magnetic field2004In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, no 10, p. 4623-4633Article in journal (Refereed)
    Abstract [en]

    A study of electrostatic fluctuations in a weakly ionized plasma confined by a purely toroidal magnetic field B is presented. At low B field the plasma is quiescent and transport is provided by stationary ExB flows along open equipotential surfaces. For B above a certain threshold, strong electrostatic fluctuations are excited, and the electron pressure profile on the low-field side is resilient with scale length much shorter than the threshold scale length for flute instability. At threshold the fluctuations exhibit the signatures of a monochromatic drift mode, and above threshold they are dominated by flute modes growing on the low-field side. Threshold behavior and profile resilience are explained as a two-stage process, where drift waves act as a seed for the unstable flute modes, and where a negative feedback mechanism involving anomalous transport regulates the electron pressure profile to maintain the imposed particle flux.

  • 92. Rypdal, K.
    et al.
    Ratynskaia, Svetlana V.
    Statistics of low-frequency plasma fluctuations in a simple magnetized torus2003In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 10, no 7, p. 2686-2695Article in journal (Refereed)
    Abstract [en]

    Statistical analysis is performed on time series of plasma potential and electron pressure fluctuations for two particular plasma states in a simple magnetized torus. In spite of the occurrence of cyclic trends in the signals, the analysis demonstrates the existence of long-range dependence on much longer time-scales than the plasma confinement time, and both Gaussian and non-Gaussian self-affinity is demonstrated in the probability distribution functions (PDFs) of increasingly coarse grained time series. Non-Gaussian PDFs for electron pressure fluctuations p are believed to arise from a nonlinear relationship between p and the plasma potential V. Non-Gaussian PDFs for V are well approximated by distributions derived from extreme value statistics.

  • 93. Saarelma, S.
    et al.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Bilkova, P.
    Challis, C. D.
    Chankin, A.
    Fridström, Richard
    KTH, School of Electrical Engineering and Computer Science (EECS), Fusion Plasma Physics.
    Garzotti, L.
    Horvath, L.
    Maggi, C. F.
    Contributors, J E T
    Self-consistent pedestal prediction for JET-ILW in preparation of the DT campaign2019In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, no 7, article id 072501Article in journal (Refereed)
    Abstract [en]

    The self-consistent core-pedestal prediction model of a combination of EPED1 type pedestal prediction and a simple stiff core transport model is able to predict Type I ELMy (edge localized mode) pedestals of a large JET-ILW (ITER-like wall) database at the similar accuracy as is obtained when the experimental global plasma β is used as input. The neutral penetration model [R. J. Groebner et al., Phys. Plasmas 9, 2134 (2002)] with corrections that take into account variations due to gas fueling and plasma triangularity is able to predict the pedestal density with an average error of 15%. The prediction of the pedestal pressure in hydrogen plasma that has higher core heat diffusivity compared to a deuterium plasma with similar heating and fueling agrees with the experiment when the isotope effect on the stability, the increased diffusivity, and outward radial shift of the pedestal are included in the prediction. However, the neutral penetration model that successfully predicts the deuterium pedestal densities fails to predict the isotope effect on the pedestal density in hydrogen plasmas.

  • 94. Saarelma, S.
    et al.
    Järvinen, A.
    Beurskens, M.
    Challis, C.
    Frassinetti, Lorenzo
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Giroud, C.
    Groth, M.
    Leyland, M.
    Maggi, C.
    Simpson, J.
    The effects of impurities and core pressure on pedestal stability in Joint European Torus (JET)2015In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 22, no 5, article id 056115Article in journal (Refereed)
    Abstract [en]

    The H-mode pedestal plays an important role in determining global confinement in tokamaks. In high triangularity H-mode experiments in Joint European Torus with the ITER-like wall (JET-ILW), significantly higher pedestal temperature and global confinement have been achieved with nitrogen seeding. The experimentally observed increase in pedestal height is inconsistent with the stability calculations using the experimental profiles. Numerically, we find that the consistency with stability improvement can be restored if we assume a shift of the pedestal inwards and increased ion dilution due to the impurity seeding. Significantly better confinement and pedestal height have been observed in JET-ILW plasmas when the core pressure is increased. The enhanced pedestal height can be linked to an improvement in edge stability arising from an increase in the Shafranov-shift, higher edge current, and pedestal widening in flux space.

  • 95. Sandquist, P.
    et al.
    Sharapov, S. E.
    Lisak, M.
    Johnson, Thomas J.
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Bidirectional tornado modes on the joint european torus2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 12Article in journal (Refereed)
    Abstract [en]

    In discharges on the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] with safety factor q(0)< 1 and high-power ion cyclotron resonance heating (ICRH), monster sawtooth crashes are preceded by frequency sweeping tornado modes in the toroidal Alfven eigenmode frequency range. A suite of equilibrium and spectral magnetohydrodynamical codes is used for explaining the observed evolution of the tornado mode frequency and for identifying temporal evolution of the safety factor inside the q=1 radius just before sawtooth crashes. In some cases, the tornado modes are observed simultaneously with both positive and negative toroidal mode numbers. Hence, a free energy source other than the radial gradient of the energetic ion pressure exciting these modes is sought. The distribution function of the ICRH-accelerated ions is assessed with the SELFO code [J. Hedin , Nucl. Fusion 42, 527 (2002)] and energetic particle drive due to the velocity space anisotropy of ICRH-accelerated ions is considered analytically as the possible source for excitation of bidirectional tornado modes.

  • 96.
    Scheffel, Jan
    et al.
    KTH.
    Liu, Donghui
    Magnetic Fluctuation Driven Cross-field Particle Transport in the RFP1997In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 4, p. 3620-Article in journal (Refereed)
    Abstract [en]

    Electrostatic and electromagnetic fluctuationsgenerally cause cross-field particle transport in confined plasmas. Thus core localized turbulence must be kept at low levels for sufficient energy confinement in magnetic fusion plasmas. Reversed-field pinch (RFP) equilibria can, theoretically, be completely stable to ideal and resistive (tearing) magnetohydrodynamic(MHD) modes at zero beta. Unstable resistive interchange modes are, however, always present at experimentally relevant values of the poloidal beta βθ.An analytical quasilinear, ambipolar diffusion model is here used to model associated particle transport. The results indicate that core density fluctuations should not exceed a level of about 1% for plasmas of fusion interest. Parameters of experimentally relevant stationary states of the RFP were adjusted to minimize growth rates, using a fully resistive linearized MHDstability code. Density gradient effects are included through employing a parabolic density profile. The scaling of particle diffusion (D(r)∝λ**2*n**0.5*T/aB, where λ is the mode width) is such that the effects of particle transport are milder in present day RFP experiments than in future reactor-relevant plasmas.

  • 97. Severo, J. H. F.
    et al.
    Nascimento, I. C.
    Tsypin, V. S.
    Kuznetsov, Y. K.
    Saettone, E. A.
    Vannucci, A.
    Galvao, R. M. O.
    Tendler, Michael
    KTH, Superseded Departments, Alfvén Laboratory.
    Mikhailovskii, A. B.
    Magnetic islands and plasma rotation in the Tokamak Chauffage Alfven Bresilien tokamak2004In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 11, no 2, p. 846-848Article in journal (Refereed)
    Abstract [en]

    Collisional plasma rotation in the Tokamak Chauffage Alfven Bresilien (TCABR) tokamak [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvao, Nucl. Fusion 43, 1047 (2003)] has been experimentally studied. It was found that the measured plasma poloidal rotation velocity agrees within error limits with neoclassical theoretical predictions, and toroidal velocity with experimental results obtained in analogous tokamaks, almost everywhere along the minor radius r, except for measurements at r/asimilar or equal to0.56 and r/asimilar or equal to0.89 (the minor radius of TCABR tokamak a=18 cm). For the first point, the measured plasma rotation velocities are higher than the velocity of the background plasma, respectively similar to30% and similar to10% for the poloidal and toroidal rotation velocities. Using a set of 22 Mirnov coils displaced poloidally, magnetic field perturbations were measured in shots adjusted to reproduce the ones of the previous plasma rotation measurements, and the results confirm that in the region r/asimilar or equal to0.89 the plasma rotates together with the magnetic island (3,1).

  • 98.
    Shafiq, Mohammad
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Raadu, Michael
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Effect of grain charging dynamics on the wake potential of a moving test charge in a dusty plasma2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 1, p. 012102-Article in journal (Refereed)
    Abstract [en]

    The response potential of a dusty (complex) plasma to a moving test charge strongly depends on its velocity. For a test charge moving with a velocity exceeding the dust-acoustic speed, a distinctive wake-field is produced trailing behind the test charge. Here the response to a fast moving test charge, when dispersion effects are small and the dust behaves as a cold plasma component, is considered. The effects of dynamical grain charging are included, and the cases with and without these effects are analyzed and compared. The plasma dielectric function is chosen assuming that all grains are of the same size and includes a response term for charging dynamics. The wake field potential is found either explicitly in terms of known functions or by using numerical methods for the integral expression. Maximum response is found on the wake cone with apex angle determined by the ratio between the dust acoustic velocity and the test charge velocity. The structure of the wake field stretches in the direction of the test charge velocity when this increases. The functional form of the field is given by separately changing the length scales parallel and perpendicular to the velocity. The potential on the axis gives an electric field close behind the test charge that can attract charges with the same sign. The grain charging dynamics leads to a spatial damping and a phase shift in the potential response.

  • 99.
    Shafiq, Mohammad
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Test charge response for a dusty plasma with both grain size distribution and dynamical charging2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 1, p. 012105-Article in journal (Refereed)
    Abstract [en]

    The form of the grain size distribution strongly influences the linear dielectric response of a dusty plasma. For a class of size distributions and a thermal velocity distribution, there is an equivalence to a Lorentzian distribution of monosized particles. The electrostatic response to a slowly moving test charge can then be found. Dynamical charging of grains in a dusty plasma leads to an enhanced time-dependent shielding of a test charge. Here the combined effect of both grain size distribution and dynamical grain charging on the response to a slowly moving test charge is analyzed. The dynamical charging contribution to the plasma dielectric has a complicated dependence on the parameters for the size distribution and on the charging rate. However, this dependence can be expressed in terms of known functions. Series expansions are used to derive the potential response to a slowly moving test charge. Previously known results may be recovered as special limiting cases of this investigation. The analytical expression for the plasma dielectric may be used for more general cases and is applicable to the study of electrostatic waves.

  • 100.
    Sishtla, Chaitanya Prasad
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Divin, Andrey
    St Petersburg State Univ, Dept Phys, St Petersburg 198504, Russia..
    Deca, Jan
    Univ Colorado, LASP, Boulder, CO 80303 USA.;NASA, Inst Modeling Plasma Atmospheres & Cosm Dust, SSERVI, Moffett Field, CA 94035 USA..
    Olshevsky, Viacheslav
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Markidis, Stefano
    KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Computational Science and Technology (CST).
    Electron trapping in the coma of a weakly outgassing comet2019In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 26, no 10, article id 102904Article in journal (Refereed)
    Abstract [en]

    Measurements from the Rosetta mission have shown a multitude of nonthermal electron distributions in the cometary environment, challenging the previously assumed plasma interaction mechanisms near a cometary nucleus. In this paper, we discuss electron trapping near a weakly outgassing comet from a fully kinetic (particle-in-cell) perspective. Using the electromagnetic fields derived from the simulation, we characterize the trajectories of trapped electrons in the potential well surrounding the cometary nucleus and identify the distinguishing features in their respective velocity and pitch angle distributions. Our analysis allows us to define a clear boundary in velocity phase space between the distributions of trapped and passing electrons. Published under license by AIP Publishing.

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