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  • 1. Aijaz, Asim
    et al.
    Sarakinos, Kostas
    Lundin, Daniel
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Helmersson, Ulf
    A strategy for increased carbon ionization in magnetron sputtering discharges2012In: Diamond and related materials, ISSN 0925-9635, E-ISSN 1879-0062, Vol. 23, p. 1-4Article in journal (Refereed)
    Abstract [en]

    A strategy that facilitates a substantial increase of carbon ionization in magnetron sputtering discharges is presented in this work. The strategy is based on increasing the electron temperature in a high power impulse magnetron sputtering discharge by using Ne as the sputtering gas. This allows for the generation of an energetic C+ ion population and a substantial increase in the C+ ion flux as compared to a conventional Ar-HiPIMS process. A direct consequence of the ionization enhancement is demonstrated by an increase in the mass density of the grown films up to 2.8 g/cm(3); the density values achieved are substantially higher than those obtained from conventional magnetron sputtering methods.

  • 2.
    Alfvén, Hannes
    et al.
    KTH, Superseded Departments.
    Axnäs, Ingvar
    KTH, Superseded Departments.
    Brenning, Nils
    KTH, Superseded Departments.
    Lindqvist, Per-Arne
    KTH, Superseded Departments.
    Further Explorations of Cosmogonic Shadow Effects in the Saturnian Rings1985Report (Other academic)
  • 3.
    Alfvén, Hannes
    et al.
    KTH, Superseded Departments.
    Axnäs, Ingvar
    KTH, Superseded Departments.
    Brenning, Nils
    KTH, Superseded Departments.
    Lindqvist, Per-Arne
    KTH, Superseded Departments.
    Voyager Saturnian Ring Measurements and the Early History of the Solar System1985Report (Other academic)
  • 4.
    Appelgren, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Hurtig, Tomas
    Larsson, Anders
    Novac, Bucur
    Nyholm, Sten E.
    Modeling of a small helical magnetic flux compression generator2008In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 36, no 5, p. 2662-2672Article in journal (Refereed)
    Abstract [en]

     In order to gain experience in explosive pulsed power and to provide experimental data as the basis for computer modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency. The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high-power pulses for various loads. This paper presents a simulation model of this helical FCG. The model, which was implemented in Matlab-Simulink, uses analytical expressions for the generator inductance. The model of resistive losses takes into account the heating of the conductors and the diffusion of the magnetic field into the conductors. The simulation results are compared with experimental data from two experiments with identical generators but with different seed currents, influencing the resistive losses. The model is used to analyze the performance of the generator.

  • 5.
    Appelgren, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nyholm, Sten E.
    Small helical magnetic flux compression generators: experiments and analysis2008In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 36, no 5, p. 2673-2683Article in journal (Refereed)
    Abstract [en]

     In order to gain experience in explosive pulsed power and to provide experimental data for modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency (FOI). The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high power pulses for various loads. This paper presents the design of, and tests with, this helical FCG. The generator had an initial inductance of 23 mu H and was operated into a load of 0.2 mu H. The generator is charged with 0.27 kg of high explosives (PBXN-5). Various types of diagnostics were used to monitor the operation of the generator, including current probes, optical fibers, and piezo gauges. With seed currents of 5.7 and 11.2 kA, final currents of 269 and 436 kA were obtained, corresponding to current amplification factors of 47 and 39. The peak of the current was reached about 30 mu s after the time of crowbar. The two generators showed only small losses in terms of 2 pi-clocking. Using signals from optical fibers, the deflection angle of the armature could be determined to be 10 degrees in good agreement with hydrodynamic simulations of the detonation process and the detonation velocity to be 8.7 km/s in agreement with tabulated value.

  • 6.
    Axnäs, Ingvar
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Experiments on the Magnetic Field and Neutral Density Limits on CIV Interaction1988Report (Other academic)
  • 7.
    Axnäs, Ingvar
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    Laboratory Experiments on the Magnetic Field and Neutral Density Limits on CIV Interaction1990Report (Other academic)
  • 8.
    Axnäs, Ingvar
    et al.
    KTH, Superseded Departments.
    Brenning, Nils
    KTH, Superseded Departments.
    Gahm, G.
    KTH, Superseded Departments.
    Plasma processes in the excitation of Herbig-Haro objects1984Report (Other academic)
  • 9. Bohlmark, J.
    et al.
    Helmersson, U.
    VanZeeland, M.
    Axnäs, Ingvar
    KTH, Superseded Departments, Alfvén Laboratory.
    Alami, J.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    Measurement of the magnetic field change in a pulsed high current magnetron discharge2004In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 13, no 4, p. 654-661Article in journal (Refereed)
    Abstract [en]

    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 x 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 (similar to10(3) 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.

  • 10. Bohlmark, J.
    et al.
    Lattemann, M.
    Gudmundsson, J. T.
    Ehiasarian, A. P.
    Gonzalvo, Y. Aranda
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Helmersson, U.
    The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge2006In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 515, no 4, p. 1522-1526Article in journal (Refereed)
    Abstract [en]

    The energy distribution of sputtered and ionized metal atoms as well as ions from the sputtering gas is reported for a high power impulse magnetron sputtering (HIPIMS) discharge. High power pulses were applied to a conventional planar circular magnetron Ti target. The peak power on the target surface was 1-2 kW/cm(2) with a duty factor of about 0.5%. Time resolved, and time averaged ion energy distributions were recorded with an energy resolving quadrupole mass spectrometer. The ion energy distributions recorded for the HIPIMS discharge are broader with maximum detected energy of 100 eV and contain a larger fraction of highly energetic ions (about 50% with E-i > 20 eV) as compared to a conventional direct current magnetron sputtering discharge. The composition of the ion flux was also determined, and reveals a high metal fraction. During the most intense moment of the discharge, the ionic flux consisted of approximately 50% Ti1+, 24% Ti2+, 23% Ar1+, and 3% Ar2+ ions.

  • 11.
    Bohm, Martin
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Dynamic trapping: Neutralization of positive space charge in a collisionless magnetized plasma1990In: Physical Review Letters, ISSN 00319007, Vol. 65, p. 859-866Article in journal (Refereed)
    Abstract [en]

    It is shown by numerical simulations that in a collisionless plasma electron inertia leads to inefficient neutralization of positive space charge and allows large positive potentials (φ ≫ kTe/e) to be established and maintained on the time scale of ion motion. This is true even if the buildup of positive space charge is so slow that it corresponds to a small fraction of the random electron current of the surrounding plasma. A simple physical model clarifies the physics of the process and provides an analytical expression for the potential.

  • 12. Bohm, Martin
    et al.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Dynamic trapping of electrons in the porcupine ionospheric ion beam experiment1992In: Advances in Space Research, ISSN 02731177, Vol. 12, p. 9-14Article in journal (Refereed)
    Abstract [en]

    Electrons are needed to maintain quasineutrality in a case where positive ions are injected across the magnetic field into a limited volume in a magnetized plasma. In the absence of collisions, a positive potential builds up and traps the electrons which enter the region along the magnetic field. If the added density of ions exceeds the ambient density, large potential differences along the magnetic field can be maintained this way. The process explains several features of the Porcupine xenon ion beam injection experiment, where strong magnetic-field-aligned electric fields were measured in the vicinity of a xenon ion beam which was injected into the ambient ionosphere from a spinning subpayload. © 1992.

  • 13.
    Bohm, Martin
    et al.
    KTH, Superseded Departments.
    Brenning, Nils
    KTH, Superseded Departments.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments.
    Dynamic Trapping of Electrons in the Porcupine Ionospheric Ion Beam Experiment1990Report (Other academic)
  • 14.
    Bolin, Odd
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    A Numerical Study of the Electrodynamical Interaction Between Comet Shoemaker-Levy 9 and Jupiter1994Report (Other academic)
  • 15.
    Bolin, Odd
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    One-Dimensional Numerical Simulations of the Low-Frequency Electric Fields in the CRIT I and CRIT II Rocket Experiments1992Report (Other academic)
  • 16.
    Brenning, Nils
    KTH, Superseded Departments.
    A Necessary Condition for the Critical Ionization Velocity Interaction1982Report (Other academic)
  • 17.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    An Appendix to the Paper Te determination in low-density plasmas from the HeI 3889 Å and 5016 Å line intensities1981Report (Other academic)
  • 18.
    Brenning, Nils
    KTH, Superseded Departments.
    An Improved Microwave Interferometer Technique for Plasma Density Measurements1983Report (Other academic)
  • 19.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    Current Limitation in Alfvén Wings1995Report (Other academic)
  • 20.
    Brenning, Nils
    KTH, Superseded Departments.
    Electron Temperature Determination from the He I 3889Å and 5016Å Line Intensities1979Report (Other academic)
  • 21.
    Brenning, Nils
    KTH, Superseded Departments.
    Electron Temperature Measurements in Low Density Plasmas by Helium Spectroscopy1977Report (Other academic)
  • 22.
    Brenning, Nils
    KTH, Superseded Departments.
    Electron temperature measurements in low density plasmas by helium spectroscopy II - parameter limits for validity of different methods1978Report (Other academic)
  • 23.
    Brenning, Nils
    KTH, Superseded Departments.
    Experiments on the Critical Ionization Velocity Interaction in Weak Magnetic Fields1980Report (Other academic)
  • 24.
    Brenning, Nils
    KTH, Superseded Departments.
    "Horizontal" Thermal Equilibrium due to Excitation Transfer Between Excited States of Neutral He in Transient Plasma1978Report (Other academic)
  • 25.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory.
    Interaction between a dust cloud and a magnetized plasma in relative motion2001In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 29, no 2, p. 302-306Article in journal (Refereed)
    Abstract [en]

    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.

  • 26.
    Brenning, Nils
    KTH, Superseded Departments.
    On the Role of the Ionization Frequency to Gyrofrequency Ratio in the Critical Ionization Velocity Interaction1986Report (Other academic)
  • 27.
    Brenning, Nils
    KTH, Superseded Departments.
    On the Role of the Magnetic Field Strength in Critical Ionization Velocity Interaction1985Report (Other academic)
  • 28.
    Brenning, Nils
    KTH, Superseded Departments.
    On the Spoke Structure in Critical Velocity Rotating Plasmas1989Report (Other academic)
  • 29.
    Brenning, Nils
    KTH, Superseded Departments.
    Review of Impact Experiments on the Critical Ionization Velocity1982Report (Other academic)
  • 30.
    Brenning, Nils
    KTH, Superseded Departments.
    Testing a Very Good Microwave Interferometer1986Report (Other academic)
  • 31.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Axnas, I.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, D.
    Helmerson, U.
    A bulk plasma model for dc and HiPIMS magnetrons2008In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 17, no 4Article in journal (Refereed)
    Abstract [en]

    A plasma discharge model has been developed for the bulk plasma (also called the extended presheath) in sputtering magnetrons. It can be used both for high power impulse magnetron sputtering (HiPIMS) and conventional dc sputtering magnetrons. Demonstration calculations are made for the parameters of the HiPIMS sputtering magnetron at Link "oping University, and also benchmarked against results in the literature on dc magnetrons. New insight is obtained regarding the structure and time development of the currents, the electric fields and the potential profiles. The transverse resistivity eta(perpendicular to) has been identified as having fundamental importance both for the potential profiles and for the motion of ionized target material through the bulk plasma. New findings are that in the HiPIMS mode, as a consequence of a high value of eta(perpendicular to), (1) there can be an electric field reversal that in our case extends 0.01-0.04m from the target, (2) the electric field in the bulk plasma is typically an order of magnitude weaker than in dc magnetrons, (3) in the region of electric field reversal the azimuthal current is diamagnetic in nature, i.e. mainly driven by the electron pressure gradient, and actually somewhat reduced by the electron Hall current which here has a reversed direction and (4) the azimuthal current above the racetrack can, through resistive friction, significantly influence the motion of the ionized fraction of the sputtered material and deflect it sideways, away from the target and towards the walls of the magnetron.

  • 32.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Axnas, I.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tennfors, E.
    Koepke, M.
    Radiation from an electron beam in a magnetized plasma: Whistler mode wave packets2006In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A11Article in journal (Refereed)
    Abstract [en]

    Experimental studies are reported of oscillations and radiation that is spontaneously excited by an electron beam which is shot along a diverging magnetic field into a plasma from a hot cathode. In the present study we focus on oscillations below the electron gyrofrequency, where we find that whistler mode radiation appears in the form of bursts, or wave packets, each with typically 0.1-1 mu s time duration, and which together cover typically a few percent of the full time. Wave packets are found in a broad frequency range of 7-40 MHz, while each individual wave packet is dominated by a single frequency. There is propagation along two routes: at the group velocity resonance cone angle, away from the central channel where the waves are excited, and in a channel along the magnetic field. Features of the whistler mode wave packets that are studied include (1) the statistics of amplitudes, frequencies, and time durations; (2) the propagation and decay of wave packets with different frequencies; (3) the group and phase velocities; and (4) how the wave packet production varies with the energy, and the current density, in the electron beam.

  • 33.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Axnäs, Ingvar
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Koepke, Mark
    KTH.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Tennfors, Einar
    KTH, School of Electrical Engineering (EES), Fusion Plasma Physics.
    Radiation from an electron beam in magnetized plasma: excitation of a whistler mode wave packet by interacting, higher-frequency, electrostatic-wave eigenmodes2017In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 59, no 12, article id 124006Article in journal (Refereed)
    Abstract [en]

    Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in the laboratory by an electron beam from a hot cathode, appear transiently, each with a time duration tau around similar to 1 mu s. The wave packets have a center frequency f(W) that is broadly distributed in the range 7 MHz < f(W) < 40 MHz. They are excited in a region with separate electrostatic (es) plasma oscillations at values of f(hf), 200 MHz < f(hf) < 500 MHz, that are hypothesized to match eigenmode frequencies of an axially localized hf es field in a well-defined region attached to the cathode. Features of these es-eigenmodes that are studied include: the mode competition at times of transitions from one dominating es-eigenmode to another, the amplitude and spectral distribution of simultaneously occurring es-eigenmodes that do not lead to a transition, and the correlation of these features with the excitation of whistler mode waves. It is concluded that transient coupling of es-eigenmode pairs at f(hf) such that vertical bar f(1, hf) - f(2, hf)vertical bar = f(W) < f(ge) can explain both the transient lifetime and the frequency spectra of the whistler-mode wave packets (f(W)) as observed in lab. The generalization of the results to bursty whistler-mode excitation in space from electron beams, created on the high potential side of double layers, is discussed.

  • 34.
    Brenning, Nils
    et al.
    KTH, Superseded Departments.
    Bohm, Martin
    KTH, Superseded Departments.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments.
    Dynamic Trapping of Electrons in Space Plasmas1989Report (Other academic)
  • 35.
    Brenning, Nils
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory.
    Dynamic trapping and skidding of dense plasma clouds2004In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 70, no 03-feb, p. 153-156Article in journal (Refereed)
    Abstract [en]

    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.

  • 36.
    Brenning, Nils
    et al.
    KTH, Superseded Departments.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments.
    Bohm, Martin
    KTH, Superseded Departments.
    An Extension of the Boltzmann Relation to Collisionless Magnetized Plasma1990Report (Other academic)
  • 37.
    Brenning, Nils
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Haerendel, G.
    Kelley, M.C.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Pfaff, R.
    Providakes, J.
    Stenbaek-Nielsen, H.C.
    Swenson, C.
    Torbert, R.
    Wescott, E.M.
    Interpretation of the Electric Fields Measured in an Ionospheric Critical Ionization Velocity Experiment1991In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 96, p. 9719-9733Article in journal (Refereed)
    Abstract [en]

    This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV) mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.

  • 38.
    Brenning, Nils
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Haerendel, G.
    Kelley, M.C.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Pfaff, R.
    Providakes, J.
    Stenbaek-Nielsen, H.C.
    Swenson, C.
    Torbert, R.B.
    Wescott, E.M.
    Critical ionization velocity interaction in the CRIT I rocket experiment1990In: Advances in Space Research, ISSN 02731177, Vol. 10, p. 63-66Article in journal (Refereed)
    Abstract [en]

    In the rocket experiment CRIT I, launched from Wallops Island on 13 May 1986, two identical Barium shaped charges were fired from distances of 1.3 km and 3.6 km towards the main experiment payload, which was separated from a sub-payload by a couple of km along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in ionospheric critical velocity experiments is investigated in the light of the CRIT I results. It is concluded that both the "homogeneous" and the "ionizing front" models can be applied, in different parts of the stream. It is also possible that a third, entirely different, mechanism may contribute to the electron heating. This mechanism involves direct energization of electrons in the magnetic-field-aligned component of the DC electric field. © 1989.

  • 39.
    Brenning, Nils
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Fälthammar, Carl-Gunne
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Haerendel, G.
    Kelley, M.C.
    Marklund, Göran
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Pfaff, R.
    Providakes, J.
    Stenbaek-Nielsen, H.C.
    Swenson, C.
    Wescott, E.M.
    Electrodynamic interaction between the CRIT I ionized barium streams and the ambient ionosphere1990In: Advances in Space Research, ISSN 02731177, Vol. 10, p. 67-70Article in journal (Refereed)
    Abstract [en]

    In the CRIT I Critical Velocity experiment, launched from Wallops Island on 13 May, 1986, two fast barium streams were ejected by means of shaped charges. Their electrodynamic interaction with the ambient ionosphere is discussed. An outstanding feature of the DC electric field observed within the streams was a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. One interpretation of the DC electric field data is that the internal electric fields of the streams is not greatly modified by Birkeland currents, i.e. a state is established, where the transverse currents are to a first approximation divergence-free. It is argued that this interpretation can explain why a reversal of the strong explosion-directed electric field was observed in the first explosion but not in the second (more distant one). © 1989.

  • 40.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Linköping University, Sweden; Université Paris-Sud, France.
    Gudmundsson, Jon Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, D.
    Minea, T.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Université Paris-Sud, France.
    Helmersson, U.
    The role of Ohmic heating in dc magnetron sputtering2016In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 25, no 6, article id 065024Article in journal (Refereed)
    Abstract [en]

    Sustaining a plasma in a magnetron discharge requires energization of the plasma electrons. In this work, Ohmic heating of electrons outside the cathode sheath is demonstrated to be typically of the same order as sheath energization, and a simple physical explanation is given. We propose a generalized Thornton equation that includes both sheath energization and Ohmic heating of electrons. The secondary electron emission yield gamma(SE) is identified as the key parameter determining the relative importance of the two processes. For a conventional 5 cm diameter planar dc magnetron, Ohmic heating is found to be more important than sheath energization for secondary electron emission yields below around 0.1.

  • 41.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Laboratoire de Physique des Gaz et Plasmas—LPGP, UMR 8578 CNRS, Université Paris-Saclay, France; Plasma and Coatings Physics Division, IFM-Materials Physics, Linköping University, Sweden.
    Gudmundsson, Jon Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Laboratoire de Physique des Gaz et Plasmas—LPGP, UMR 8578 CNRS, Université Paris-Sud, Université Paris-Saclay, France; Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Petty, T. J.
    Universite Paris Sud.
    Minea, Tiberiu
    Unicersite Paris Sud.
    Lundin, Daniel
    Universite Paris Sud.
    A unified treatment of self-sputtering, process gas recycling, and runaway for high power impulse sputtering magnetrons2017In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 26, no 12, article id 125003Article in journal (Refereed)
    Abstract [en]

    The combined processes of self-sputter (SS)-recycling and process gas recycling in high power impulse magnetron sputtering (HiPIMS) discharges are analyzed using the generalized recycling model (GRM). The study uses experimental data from discharges with current densities from the direct current magnetron sputtering range to the HiPIMS range, and using targets with self-sputter yields Y-SS from approximate to 0.1 to 2.6. The GRM analysis reveals that, above a critical current density of the order of J(crit) approximate to 0.2 A cm(-2), a combination of self-sputter recycling and gas-recycling is generally the case. The relative contributions of these recycling mechanisms, in turn, influence both the electron energy distribution and the stability of the discharges. For high self-sputter yields, above Y-SS approximate to 1, the discharges become dominated by SS-recycling, contain few hot secondary electrons from sheath energization, and have a relatively low electron temperature T-e. Here, stable plateau values of the discharge current develop during long pulses, and these values increase monotonically with the applied voltage. For low self-sputter yields, below Y-SS approximate to 0.2, the discharges above J(crit) are dominated by process gas recycling, have a significant sheath energization of secondary electrons and a higher T-e, and the current evolution is generally less stable. For intermediate values of YSS the discharge character gradually shifts between these two types. All of these discharges can, at sufficiently high discharge voltage, give currents that increase rapidly in time. For such cases we propose that a distinction should be made between 'unlimited' runaway and 'limited' runaway: in unlimited runaway the current can, in principle, increase without a limit for a fixed discharge voltage, while in limited runaway it can only grow towards finite, albeit very high, levels. For unlimited runway Y-SS > 1 is found to be a necessary criterion, independent of the amount of gas-recycling in the discharge.

  • 42.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Huo, Chunqing
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, Daniel
    Plasma and Coatings Physics Division, Linköping, Sweden.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Vitelaru, Catalin
    Stancu, Gabriel
    Minea, Tiberiu
    Helmersson, Ulf
    Understanding deposition rate loss in high power impulse magnetron sputtering: I. Ionization-driven electric fields2012In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 21, no 2, p. 025005-Article in journal (Refereed)
    Abstract [en]

    The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields E-z directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields E-z in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a 'sweet spot' in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product Pi(Townsend) is proposed. A parametric analysis of Pi(Townsend) shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength.

  • 43.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Hurtig, T.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Conditions for plasmoid penetration across abrupt magnetic barriers2005In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 12, no 1Article in journal (Refereed)
    Abstract [en]

    The penetration of plasma clouds, or plasmoids, across abrupt magnetic barriers (of the scale less than a few ion gyro radii, using the plasmoid directed velocity) is studied. The insight gained earlier, from detailed experimental and computer simulation investigations of a case study, is generalized into other parameter regimes. It is concluded for what parameters a plasi-noid should be expected to penetrate the magnetic barrier through self-polarization, penetrate through magnetic expulsion, or be rejected from the barrier. The scaling parameters are n(e), upsilon(o), B-perpendicular to, m(i), T-i, and the width w of the plasmoid. The scaling is based on a model for strongly driven, nonlinear magnetic field diffusion into a plasma which is a generalization of the earlier laboratory findings. The results are applied to experiments earlier reported in the literature, and also to the proposed application of impulsive penetration of plasmoids from the solar wind into the Earth's magnetosphere.

  • 44.
    Brenning, Nils
    et al.
    KTH, Superseded Departments.
    Lindberg, Lennart
    KTH, Superseded Departments.
    Eriksson, A.
    Energization of Electrons in a Plasma Beam Entering a Curved Magnetic Field1980Report (Other academic)
  • 45.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, Daniel
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Alfven's critical ionization velocity observed in high power impulse magnetron sputtering discharges2012In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 19, no 9, p. 093505-Article in journal (Refereed)
    Abstract [en]

    Azimuthally rotating dense plasma structures, spokes, have recently been detected in several high power impulse magnetron sputtering (HiPIMS) devices used for thin film deposition and surface treatment, and are thought to be important for plasma buildup, energizing of electrons, as well as cross-B transport of charged particles. In this work, the drift velocities of these spokes are shown to be strongly correlated with the critical ionization velocity, CIV, proposed by Alfven. It is proposed as the most promising approach in combining the CIV and HiPIMS research fields is to focus on the role of spokes in the process of electron energization.

  • 46.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Lundin, Daniel
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Minea, T.
    Costin, C.
    Vitelaru, C.
    Spokes and charged particle transport in HiPIMS magnetrons2013In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 8, p. 084005-Article in journal (Refereed)
    Abstract [en]

    Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E x B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented.

  • 47.
    Brenning, Nils
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Merlino, R. L.
    Lundin, D.
    Raadu, Michael A.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Helmersson, U.
    Faster-than-Bohm Cross-B Electron Transport in Strongly Pulsed Plasmas2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 22Article in journal (Refereed)
    Abstract [en]

    We report the empirical discovery of an exceptionally high cross-B electron transport rate in magnetized plasmas, in which transverse currents are driven with abruptly applied high power. Experiments in three different magnetic geometries are analyzed, covering several orders of magnitude in plasma density, magnetic field strength, and ion mass. It is demonstrated that a suitable normalization parameter is the dimensionless product of the electron (angular) gyrofrequency and the effective electron-ion momentum transfer time, omega(ge)tau(EFF), by which all of diffusion, cross-resistivity, cross-B current conduction, and magnetic field diffusion can be expressed. The experiments show a remarkable consistency and yield close to a factor of 5 greater than the Bohm-equivalent values of diffusion coefficient D-perpendicular to, magnetic-diffusion coefficient D-B, Pedersen conductivity sigma(P), and transverse resistivity eta(perpendicular to).

  • 48.
    Brenning, Nils
    et al.
    KTH, Superseded Departments.
    Swenson, C.
    Kelley, M. C.
    Providakes, J.
    Torbert, R.
    The Collective Gyration of a Heavy Ion Cloud in a Magnetized Plasma1990Report (Other academic)
  • 49.
    Ekeroth, Sebastian
    et al.
    Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden..
    Munger, E. Peter
    Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden..
    Boyd, Robert
    Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden..
    Ekspong, Joakim
    Umea Univ, Dept Phys, SE-90187 Umea, Sweden..
    Wagberg, Thomas
    Umea Univ, Dept Phys, SE-90187 Umea, Sweden..
    Edman, Ludvig
    Umea Univ, Dept Phys, SE-90187 Umea, Sweden..
    Brenning, Nils
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics. Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden.
    Helmersson, Ulf
    Linkoping Univ, Dept Phys, SE-58183 Linkoping, Sweden..
    Catalytic Nanotruss Structures Realized by Magnetic Self-Assembly in Pulsed Plasma2018In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 18, no 5, p. 3132-3137Article in journal (Refereed)
    Abstract [en]

    Tunable nanostructures that feature a high surface area are firmly attached to a conducting substrate and can be fabricated efficiently over significant areas, which are of interest for a wide variety of applications in, for instance, energy storage and catalysis. We present a novel approach to fabricate Fe nanoparticles using a pulsed-plasma process and their subsequent guidance and self-organization into well-defined nanostructures on a substrate of choice by the use of an external magnetic field. A systematic analysis and study of the growth procedure demonstrate that nondesired nanoparticle agglomeration in the plasma phase is hindered by electrostatic repulsion, that a polydisperse nanoparticle distribution is a consequence of the magnetic collection, and that the formation of highly networked nanotruss structures is a direct result of the polydisperse nanoparticle distribution. The nanoparticles in the nanotruss are strongly connected, and their outer surfaces are covered with a 2 nm layer of iron oxide. A 10 mu m thick nanotruss structure was grown on a lightweight, flexible and conducting carbon-paper substrate, which enabled the efficient production of H-2 gas from water splitting at a low overpotential of 210 mV and at a current density of 10 mA/cm(2).

  • 50.
    Fälthammar, Carl-Gunne
    et al.
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Magnetosphere-ionosphere interactions as a key to the plasma Univers1995In: IEEE Transactions on Plasma Science, ISSN 0093-3813, E-ISSN 1939-9375, Vol. 23, p. 2-9Article in journal (Refereed)
    Abstract [en]

    Almost all known matter in the universe is in a state, the plasma state, that is rare on Earth, and whose physical properties are still incompletely understood. Its complexity is such that a reliable understanding must build on empirical knowledge. While laboratory experiments are still an important source of such knowledge, Earth’s magnetospere-ionosphere system, made accessible by space technology, vastly widens the parameter ranges in which plasma phenomena can be studied. This system contains all three main categories of plasma present in the universe. Furthermore, the interaction between the magnetosphere and the ionosphere excites a wealth of plasma physical phenomena of fundamental importance. These include, among others, formation of magnetic-field aligned electric fields, acceleration of charged particles, release of magnetically stored energy, formation of filamentary and cellular structures, as well as unexpected chemical separation processes. What has been learned, and what stilt remains to be learned, from study of the magnetosphere-ionosphere system should therefore provide a much improved basis for understanding of our universe.

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