| 11. |
- Bozhko, Igor, et al.
(författare)
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Modes of Impulse Barrier Discharge to Water Surface in Atmospheric Air
- 2020
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 48:1, s. 156-162
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Tidskriftsartikel (refereegranskat)abstract
- The possibilities of creating a homogeneous discharge in air of atmospheric pressure between a flat metallicelectrode covered by a dielectric barrier and water surface are demonstrated. In the experiments, the discharge was initiated in the gas gaps by applying unipolar voltage pulses with fronts of ∼15 ns and duration of ∼100 ns. It was found that the discharge mode was affected by the pulse repetition rate. Its critical magnitude corresponding to the appearance of a homogeneous discharge was reduced from 500 to 50 s−1 if the length of the air gap was increased from 1 to 4 mm. The lifetime of a homogeneous discharge mode was affected by the total duration of the discharge. Thus, it was about several tens of seconds at a pulse repetition frequency of 100 s−1 and it reduced at increased frequencies. It is suggested that the transition from homogeneous to inhomogeneous modes is associated with local overheating of the gas due to significant energy dissipation in the discharge volume.
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| 12. |
- Bratman, V. L., et al.
(författare)
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Numerical Study of a Low-Voltage Gyrotron ("Gyrotrino") for DNP/NMR Spectroscopy
- 2017
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Ingår i: IEEE Transactions on Plasma Science. - : IEEE. - 0093-3813 .- 1939-9375. ; 45:4, s. 644-648
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Tidskriftsartikel (refereegranskat)abstract
- Feasibility of a gyrotron for the dynamic nuclear polarization (DNP) purpose, integrated with nuclear magnetic resonance (NMR) spectrometer inside a single cryomagnet, is analyzed on the basis of numerical simulations. The necessary condition for DNP is matching of the gyrotrino and DNP frequencies. This imposes a strong restriction on the gyrotron operating voltage, which should be less than 2 kV. The most part of the uniform magnetic field region in the cryomagnet is occupied by a sample with NMR probe, so there is a very limited space for the gyrotron cavity. This dictates a number of peculiarities for the gyrotrino design, in particular, the diffraction power output from the cathode end of the cavity and collecting of a thin electron beam in a strong magnetic field. According to simulations, the gyrotrino operating at the fundamental cyclotron resonance with a voltage of 1.5 kV can provide an output power of 10-20 W at a frequency of 264 GHz, which is suitable for many NMR-DNP experiments.
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| 13. |
- Brenning, Nils
(författare)
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Interaction between a dust cloud and a magnetized plasma in relative motion
- 2001
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Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 29:2, s. 302-306
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between a dust cloud and a magnetized plasma is investigated by use of an idealized model where the dust particles have uniform size, a uniform density within the dust cloud, and start with the same velocity across the magnetic field in the plasma's rest frame. The interaction is found to be governed by a dimensionless parameter K which is a function of dust cloud, and ambient plasma, parameters. For K much smaller than unity, the interaction goes on for typically 1/(2 piK) gyro times, with the particles in the dust cloud performing gyro motions with decreasing radius, For K close to unity, the dust motion is stopped on the order of a dust particle gyro time, For the case K much greater than 1, the plasma in the flux tube through the dust cloud is dragged across the magnetic field over a distance of the order of Kr-d, where r(d) is the dust gyro radius, before the motion is stopped. Some expected effects for a more realistic dust cloud with density gradients, and containing dust with a spread in size, are discussed. The results have bearing on dusty plasma in space, e.g., models of the formation of spokes in Saturn's ring system.
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| 14. |
- Böhlmark, Johan, et al.
(författare)
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Spatial electron density distribution in a high-power pulsed magnetron discharge
- 2005
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 33:2, s. 346-347
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Tidskriftsartikel (refereegranskat)abstract
- The spatial electron density distribution was measured as function of time in a high-power pulsed magnetron discharge. A Langmuir probe was positioned in various positions below the target and the electron density was mapped out. We recorded peak electron densities exceeding 1019 m-3 in a close vicinity of the target. The dynamics of the discharge showed a dense plasma expanding from the "race-track" axially into the vacuum chamber. We also record electrons trapped in a magnetic bottle where the magnetron magnetic field is zero, formed due to the unbalanced magnetron.
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| 15. |
- Cannas, Barbara, et al.
(författare)
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Recurrence Plots for Dynamic Analysis of Type-I ELMs at JET With a Carbon Wall
- 2019
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Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 47:4, s. 1871-1877
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the dynamic characteristics of type-I edge-localized modes (ELM) time series from the JET tokamak, the world's largest magnetic confinement plasma physics experiment, have been investigated through recurrence plots (RPs). The analysis has been focused on RPs of pedestal temperature, line averaged electron density, and outer divertor D-alpha time series during experiments with a carbon wall. The analysis of RPS shows the patterns similar to those characteristics of signals exhibiting type-2 intermittency, in particular, a characteristic kite-like shape; this gives useful hints to model the temperature signal as well as the D-alpha radiation time series, with simple nonlinear maps capturing the nearly periodic behavior of type-I ELMs.
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| 16. |
- Diaz, Oscar, 1980-, et al.
(författare)
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Numerical Modeling of Electrical Discharges in Long Air Gaps Tested With Positive Switching Impulses
- 2018
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 46:3, s. 611-621
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Tidskriftsartikel (refereegranskat)abstract
- The numerical modeling of electrical discharges occurring in atmospheric air has been in continuous development during the past decades in different fields, such as high-voltage techniques and lightning protection. Different methodologies have been proposed to represent the physical phenomena taking place at a single full discharge event, departing both from experimental and theoretical approaches. The implementation of these methodologies in numerical routines combined with the use of numerical methods to determine the electric potential distribution permits the creation of models whose predictions closely agree with the real case situations, where electrode arrangements might have nonsymmetric geometries. In this paper, we present an improved version of a simulation methodology for representing electrical discharges in long air gaps. This simulation methodology includes new elements like: 1) the 3-D leader channel tortuosity based on laboratory experimental measurements and 2) two new methods for the estimation of the electric charge contained in the so-called leader-corona region based on the electrostatic potential of fictitious potential rings representing the active region in front of the leader tip. Results from the simulation were compared with experimental records and a reasonably good agreement is found between them.
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| 17. |
- Dieckmann, Mark E, 1969-, et al.
(författare)
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Three-dimensional visualization of electron acceleration in a magnetized plasma
- 2002
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 30:1 I, s. 20-21
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Tidskriftsartikel (refereegranskat)abstract
- We examine wave-particle interactions in a magnetized plasma. We present snapshots of an animation of the three-dimensional electron phase space distribution produced by an electrostatic wave propagating across a magnetic field. The distribution function has been evolved by a particle in cell simulation. The electron phase space has been visualized by distributing the simulation electrons over an array representing phase space density and by volume rendering this array. The results are, due to the choice of initial plasma and wave parameters, of relevance for electron acceleration at astrophysical shocks.
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| 18. |
- Elfsberg, Mattias, et al.
(författare)
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Experimental Studies of Anode and Cathode Materials in a Repetitive Driven Axial Vircator
- 2008
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 36, s. 688-693
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Tidskriftsartikel (refereegranskat)abstract
- Repetitive use of a high-power microwave (HPM) radiation source implies strong erosion on cathode and anode materials. Electrode material endurance has been studied in a series of experiments with an axial vircator powered by a compact Marx generator. The Marx generator operated in a 10 Hz repetitive mode with a burst of ten pulses. Velvet and graphite was used as electron-emitting materials, and they showed markedly different pulse characteristics. Three different anode materials were used; stainless steel mesh, stainless steel wires and molybdenum wires, which all had different influence on the pulse characteristics.
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| 19. |
- Engwall, Erik, 1977-, et al.
(författare)
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Double-Probe Measurements in Cold Tenuous Space Plasma Flows
- 2006
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Ingår i: IEEE Transactions on Plasma Science. - : Nuclear & Plasma Sciences Society. - 0093-3813 .- 1939-9375. ; 34:5:2, s. 2071-2077
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Tidskriftsartikel (refereegranskat)abstract
- Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population.
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| 20. |
- Eriksson, Anders, et al.
(författare)
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Influence of Ar and N2 Pressure on Plasma Chemistry, Ion Energy, and Thin Film Composition during Filtered Arc Deposition from Ti3SiC2 Cathodes
- 2014
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Ingår i: IEEE Transactions on Plasma Science. - : IEEE Press. - 0093-3813 .- 1939-9375. ; 42:11, s. 3498-3507
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Tidskriftsartikel (refereegranskat)abstract
- Arc plasma from Ti3SiC2 compound cathodes used in a filtered dc arc system has been characterized with respect to plasma chemistry and charge-state resolved ion energies. In vacuum, the plasma composition is dominated by Ti ions, with concentrations of 84.3, 9.3, and 6.4 at% for Ti, Si, and C ions, respectively. The reduced amount of Si and most notably C compared with the cathode composition is confirmed by analysis of film composition in corresponding growth experiments. The deposition of light-element deficient films is thus related to plasma generation or filter transport. The ion energy distributions in vacuum range up to 140, 90, and 70 eV for Ti, Si, and C, respectively. Corresponding average ion energies of 48, 36, and 27 eV are reduced upon introduction of gas, down to around 5 eV at 0.6 Pa Ar or 0.3 Pa N2 for all species. In vacuum, the charge state distributions of Si and C are shifted to higher values compared with corresponding elemental cathodes, likely caused by changed effective electron temperature of the plasma stemming from compound cathode material and/or by electron impact ionization in the filter. The average ion charge states are reduced upon addition of Ar, ranging between 1.97 and 1.48 for Ti, 1.91 and 1.46 for Si, and 1.25 and 1.02 for C. Similar effects are observed upon introduction of N2, though with more efficient charge state reduction with pressure. It is conceivable that the pressure-induced changes in ion energy and charge state are crucial for the film synthesis from a microstructure evolution point of view, as they affect the ion-surface interactions through supply of energy, especially when substrate biasing is employed during arc deposition from a compound cathode.
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