# Physics of Plasmas

Published by American Institute of Physics

Online ISSN: 1089-7674

Print ISSN: 1070-664X

Published by American Institute of Physics

Online ISSN: 1089-7674

Print ISSN: 1070-664X

Publications

The flow structure of ions in a diverging magnetic field has been experimentally studied in an electron cyclotron resonance plasma. The flow velocity field of ions has been measured with directional Langmuir probes calibrated with the laser induced fluorescence spectroscopy. For low ion-temperature plasmas, it is concluded that the ion acceleration due to the axial electric field is important compared with that of gas dynamic effect. It has also been found that the detachment of ion stream line from the magnetic field line takes place when the parameter |f(ci)L(B)∕V(i)| becomes order unity, where f(ci), L(B), and V(i) are the ion cyclotron frequency, the characteristic scale length of magnetic field inhomogeneity, and the ion flow velocity, respectively. In the detachment region, a radial electric field is generated in the plasma and the ions move straight with the E×B rotation driven by the radial electric field.

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The material of limiter in HT-7 tokamak was changed from graphite to molybdenum in the last experimental campaign. The pitch angle scattering of runaway electrons due to anomalous Doppler resonance effects was observed. The experimental results agree very well with the stable boundary condition expected from the linear resistive theory but only agree with that from the nonlinear evolutionary of runaway-electron distribution theory in low electric field region. The current carried by runaway electrons is the same under different limiter conditions.

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A novel electron beam focusing scheme for medical X-ray sources is described in this paper. Most vacuum based medical X-ray sources today employ a tungsten filament operated in temperature limited regime, with electrostatic focusing tabs for limited range beam optics. This paper presents the electron beam optics designed for the first distributed X-ray source in the world for Computed Tomography (CT) applications. This distributed source includes 32 electron beamlets in a common vacuum chamber, with 32 circular dispenser cathodes operated in space charge limited regime, where the initial circular beam is transformed into an elliptical beam before being collected at the anode. The electron beam optics designed and validated here are at the heart of the first Inverse Geometry CT system, with potential benefits in terms of improved image quality and dramatic X-ray dose reduction for the patient.

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Measurements of ion energy distribution are performed in the accelerated plasma of a coaxial electromagnetic plasma gun operating in a gas-puff mode at relatively low discharge energy (900 J) and discharge potential (4 kV). The measurements are made using a Thomson-type mass and energy spectrometer with a gated microchannel plate and phosphor screen as the ion sensor. The parabolic ion trajectories are captured from the sensor screen with an intensified charge-coupled detector camera. The spectrometer was designed and calibrated using the Geant4 toolkit, accounting for the effects on the ion trajectories of spatial non-uniformities in the spectrometer magnetic and electric fields. Results for hydrogen gas puffs indicate the existence of a class of accelerated protons with energies well above the coaxial discharge potential (up to 24 keV). The Thomson analyzer confirms the presence of impurities of copper and iron, also of relatively high energies, which are likely erosion or sputter products from plasma-electrode interactions.

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Measurements are presented of the time-dependent current distribution inside a coaxial electromagnetic plasma gun. The measurements are carried out using an array of six axially distributed dual-Rogowski coils in a balanced circuit configuration. The radial current distributions indicate that operation in the gas-puff mode, i.e., the mode in which the electrode voltage is applied before injection of the gas, results in a stationary ionization front consistent with the presence of a plasma deflagration. The effects of varying the bank capacitance, transmission line inductance, and applied electrode voltage were studied over the range from 14 to 112 μF, 50 to 200 nH, and 1 to 3 kV, respectively.

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The propagation of ultra intense laser pulses through matter is connected
with the generation of strong moving magnetic fields in the propagation channel
as well as the formation of a thin ion filament along the axis of the channel.
Upon exiting the plasma the magnetic field displaces the electrons at the back
of the target, generating a quasistatic electric field that accelerates and
collimates ions from the filament. Two-dimensional Particle-in-Cell simulations
show that a 1 PW laser pulse tightly focused on a near-critical density target
is able to accelerate protons up to an energy of 1.3 GeV. Scaling laws and
optimal conditions for proton acceleration are established considering the
energy depletion of the laser pulse.

…

Ion-to-magnetohydrodynamic scale physics of the transverse velocity shear layer and associated Kelvin-Helmholtz instability (KHI) in a homogeneous, collisionless plasma are investigated by means of full particle simulations. The shear layer is broadened to reach a kinetic equilibrium when its initial thickness is close to the gyrodiameter of ions crossing the layer, namely, of ion-kinetic scale. The broadened thickness is larger in B⋅Ω<0 case than in B⋅Ω>0 case, where Ω is the vorticity at the layer. This is because the convective electric field, which points out of (into) the layer for B⋅Ω<0 (B⋅Ω>0), extends (reduces) the gyrodiameters. Since the kinetic equilibrium is established before the KHI onset, the KHI growth rate depends on the broadened thickness. In the saturation phase of the KHI, the ion vortex flow is strengthened (weakened) for B⋅Ω<0 (B⋅Ω>0), due to ion centrifugal drift along the rotational plasma flow. In ion inertial scale vortices, this drift effect is crucial in altering the ion vortex size. These results indicate that the KHI at Mercury-like ion-scale magnetospheric boundaries could show clear dawn-dusk asymmetries in both its linear and nonlinear growth.

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The magnicon is a scanning-beam microwave amplifier that is being
developed as a high power, highly efficient microwave source for use in
powering the next generation of high gradient electron linear
accelerators. In this paper, we present a progress report on a new
thermionic magnicon experiment designed to produce more than 50 MW at
11.4 GHz, using a 210 A, 500 kV beam from an ultrahigh convergence
thermionic electron gun driven by a rep-rated modulator. This new design
has a predicted efficiency in excess of 60%

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Components of the wave magnetic field in a helicon discharge have been measured with a single-turn, coaxial magnetic probe. Left- and right-handed helical antennas, as well as plane-polarized antennas, were used; and the results were compared with the field patterns computed for a nonuniform plasma. The results show that the right-hand circularly polarized mode is preferentially excited with all antennas, even those designed to excite the left-hand mode. For right-hand excitation, the radial amplitude profiles are in excellent agreement with computations.

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Field emitter arrays (FEA) are promising electron sources for vacuum microelectronic microwave amplifiers and oscillators. In this paper, microelectronic analogs of "classic" microwave devices are considered. Traveling wave klystrons, klystrodes and cross-field amplifiers with FEA are described. Basic output parameters (gain, efficiency, etc.) of these devices are calculated. A model of a self-excited vacuum microtriode oscillator is developed. The oscillator is studied numerically, as well as by experimental investigation of the circuit analog model. Chaotic generation in the oscillator under external harmonic driving is analyzed. The rf interaction with the electron beam modulated by the field emission in a twystrode is investigated using a particle-in-cell code and a one-dimensional nonlinear code. (C) 2002 American Institute of Physics.

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In order to estimate the radiated power that can be expected from the next-generation z-pinch driver such as ZR at 28 MA, current-scaling experiments have been conducted on the 18-MA driver Z. We report on the current scaling of single 40-mm diameter tungsten 240-wire arrays with a fixed 110-ns implosion time. The wire diameter is decreased in proportion to the load current. The load current is reduced by reducing the charge voltage on the Marx banks. On one shot firing only 3 of the 4 levels of the Z machine further reduced the load current. The radiated energy scales as the current squared as expected but the radiated power scales as the current to the 3.5 power due to increased pinch instability at lower current. As the current is reduced the rise-time of the x-ray pulse increases and at the lowest current value of 10.4 MA a shoulder appears on the leading edge of the x-ray pulse. We will report on experiments in February 2002 which will attempt to image the pinch along the axis to determine the nature of the reduced stability at lower currents.

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Test particle motion is analyzed analytically and numerically in the field configuration consisting of the equilibrium self-electric and self-magnetic fields of a well-matched, thin, continuous, intense charged-particle beam and an applied periodic focusing solenoidal magnetic field. The self fields are determined self-consistently, assuming the beam to have a uniform-density, rigid-rotor Vlasov equilibrium distribution. Using the Hamilton-Jacobi method, the betatron oscillations of test particles in the average self fields and applied focusing field are analyzed, and the nonlinear resonances induced by periodic modulations in the self fields and applied field are determined. The Poincare surface-of-section method is used to analyze numerically the phase-space structure for test particle motion outside the outermost envelope of the beam over a wide range of system parameters. For vacuum phase advance sigma(v)=80 degrees, it is found that the phase-space structure is almost entirely regular at low beam intensity (phase advance sigma greater than or similar to 70 degrees, say), whereas at moderate beam intensity (30 degrees less than or similar to sigma less than or similar to 70 degrees), nonlinear resonances appear, the most pronounced of which is the third-order primary nonlinear resonance. As the beam intensity is further increased (sigma less than or similar to 30 degrees), the widths of the higher-order nonlinear resonances increase, and the chaotic region of phase space increases in size. Furthermore, the many chaotic layers associated with the separatrices of the primary and secondary nonlinear resonances are still divided by the remaining invariant Kolmogorov-Arnold-Moser surfaces, even at very high beam intensities. The implications of the rich nonlinear resonance structure and chaotic particle motion found in the present test-particle studies are discussed in the context of halo formation. (C) 1999 American Institute of Physics. [S1070-664X(99)03409-6].

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Universal voltage‐current characteristics are presented for a planar diode, showing the general transition from the Fowler–Nordheim relation to the Child–Langmuir law. These curves are normalized to the intrinsic scales that are constructed from the Fowler–Nordheim coefficients A, B. They provide an immediate assessment of the importance of the space charge effects, once the gap voltage, gap spacing, and the Fowler–Nordheim coefficients are specified. An example in the parameter regime of vacuum microelectronics is presented.

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Two-dimensional electromagnetic (EM) particle-in-cell simulations were performed to study the effect of the displacement current and the self-magnetic field on the space-charge limited current density or the Child-Langmuir law of a short-pulse electron flow with a propagation distance of ζ and an emitting width of W from the classical regime to the relativistic regime. Numerical scaling of the two-dimensional electromagnetic Child-Langmuir (CL) law was constructed, and it scales with (ζ / W) and (ζ / W)<sup>2</sup> at the classical and relativistic regimes, respectively. Our findings reveal that the displacement current can considerably enhance the space-charge limited (SCL) current density as compared to the well-known two-dimensional electrostatic (ES) Child-Langmuir law even at the classical regime.

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A theory describing the influence of reflections on operation of gyrotrons with radial output is presented. The theory is used for evaluating the effect of reflections on the operation of the 170 GHz ITER coaxial cavity gyrotron, which is under development in cooperation between EUROATOM Associations (CRPP Lausanne, FZK Karlsruhe, and HUT Helsinki) together with European tube industry (Thales Electron Devices, Velizy, France). It is shown that for optimally chosen external magnetic field value and electron beam radius, possible reflections do not change the final steady-state operation, which corresponds to generation of a 2.2 MW millimeter-wave power. The effect of deviation of the magnetic field and the beam radius from optimal values on the device operation is also studied.

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Summary form only given. A general theory of multipactor between
parallel capacitor plates with orthogonal electric and magnetic fields
is given. Expressions are derived for the resonant phases at which the
rf-driven cascades occur; these reduce to previously derived expressions
in the limit of vanishing magnetic field. This work also obtains the
conditions governing the stability of the motion about those phases, as
well as a dynamic constraint from imposing the restriction that each
impact on a plate is the first impact that is allowed by the equations
of motion. Chaotic effects from the random ejection velocities of the
secondaries are addressed for the first time

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A novel crossed-field secondary-emission (CFSE) electron source that is capable of producing high current tubular electron beams is described. This new electron source is based on the mechanism of secondary emission multiplication of electron current in a magnetron-like device having smooth cylindrical electrodes. The input electron current may be as low as a few mA. The multiplication process starts at the negative slope of an applied voltage pulse. After initiation, the current is extracted from the diode region with no regard to the voltage pulse shape and as a consequence, the CFSE electron source can operate in a long pulse mode. At the diode voltage of ~40 kV for a diode gap of ~6 mm, the output current reaches a value of more than 100A. A further increase of current up to 1 kA is feasible

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Summary form only given. The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary-electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density 0.2-4.5times10<sup>9</sup> cm<sup>-3</sup>, electron temperature 0.14-0.21 eV, pressure 0.3-12 mtorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10-65 eV range as being mostly secondaries from ionization and those in the 0-10 eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of three from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.

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It is demonstrated that new modes of particle oscillations, associated with magnetic properties, can exist in a one-dimensional chain of charged magnetized particles in a plasma. The stability of equilibria, transitions between the different equilibria, and a critical dependence on the external fields and plasma parameters are investigated.

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Summary form only given, as follows. In low voltage gyrodevices
operating at high cyclotron harmonics the electrons are weakly coupled
to the synchronous harmonic of an electromagnetic field. To realize
efficient operation of a device it is necessary to compensate for this
weak coupling by increasing the resonator Q-factor. When the required
Q-factor is on the order of the ohmic Q-factor, the optimization of
gyrotron parameters should be modified by taking into account ohmic
losses of the microwave power. In the present paper this modification
has been done and the optimization of two- and three-cavity gyrodevices
operating at cyclotron harmonics has been considered

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More recently, N. Roy et al. [Phys. Plasmas \textbf{19}, 033705 (2012)] have
investigated the occurrence of nonlinear solitary and double-layers in an
ultrarelativistic dusty electron-positron-ion degenerate plasma using a Sagdeev
potential method. They have considered a full parametric examination on
Mach-number criteria for existence of such nonlinear excitations using the
specific degeneracy limits of Chandrasekhar equation of state (EoS) for
Fermi-Dirac plasmas. In this comment we point-out a misleading extension of
polytropic EoS to study the Fermi-Dirac relativistically degenerate plasmas.

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Recently, Yan-Xia Xu, et al. in the article Ref. [Phys. Plasmas \textbf{18},
052301 (2011)] have studied the effects of various plasma parameters on
interaction of two ion-acoustic solitary waves in an unmagnetized
three-dimensional electron-positron-ion quantum plasma. They have used the
extended reductive perturbation technique, the so-called, extended
Poincare'-Lighthill-Kuo (PLK) technique, to deduce from the model governing the
quantum hydrodynamics (QHD) differential equations leading to the soliton
dynamical properties, namely, Korteweg-de Vries evolution equations (one for
each wave) and coupled differential equations describing the phase-shift in
trajectories of solitons due to the two dimensional collision. The variation of
the calculated collision phase-shifts are then numerically inspected in terms
of numerous plasma fractional parameters. In this comment we give some notes
specific to the validity of the results of above-mentioned article and refer to
important misconceptions about the use of the Fermi-temperature in quantum
plasmas, appearing in this article and many other recently published ones.

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Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb (Wigner) crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed. Comment: 19 pages, 6 figures

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Thomas has recently derived scaling laws for X-ray radiation from electrons
accelerated in plasma bubbles, as well as a threshold for the self-injection of
background electrons into the bubble [A. G. R. Thomas, Phys. Plasmas 17, 056708
(2010)]. To obtain this threshold, the equations of motion for a test electron
are studied within the frame of the bubble model, where the bubble is described
by prescribed electromagnetic fields and has a perfectly spherical shape. The
author affirms that any elliptical trajectory of the form x'^2/{\gamma}_p^2 +
y'^2 = R^2 is solution of the equations of motion (in the bubble frame), within
the approximation p'_y^2/p'_x^2 \ll 1. In addition, he highlights that his
result is different from the work of Kostyukov et al. [Phys. Rev. Lett. 103,
175003 (2009)], and explains the error committed by
Kostyukov-Nerush-Pukhov-Seredov (KNPS). In this comment, we show that
numerically integrated trajectories, based on the same equations than the
analytical work of Thomas, lead to a completely different result for the
self-injection threshold, the result published by KNPS [Phys. Rev. Lett. 103,
175003 (2009)]. We explain why the analytical analysis of Thomas fails and we
provide a discussion based on numerical simulations which show exactly where
the difference arises. We also show that the arguments of Thomas concerning the
error of KNPS do not hold, and that their analysis is mathematically correct.
Finally, we emphasize that if the KNPS threshold is found not to be verified in
PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of
the model itself, and not to an error in the mathematical derivation.

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Zaghloul [Phys. Plasmas 17, 062701 (2010); arXiv:1010.1161v1] reconsiders the
occupation probability formalism in plasma thermodynamics and claims
inconsistencies in previous models. I show that the origin of this incorrect
claim is an omission of the configurational factor from the partition function.
This arXiv version is supplemented with two appendices, where I add remarks and
comments on two more recent publications of the same author on the same
subject: on his response to this Comment [Phys. Plasmas 17, 124705 (2010)] and
on his criticism towards the Hummer and Mihalas's (1988) formalism [Phys.
Plasmas 17, 122903 (2010); arXiv:1010.1102v1].

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The neoclassical calculation of the helicon wave theory contains a fundamental flaw. Use is made of a proportional relationship between the magnetic field and its curl to derive the Helmholtz equation describing helicon wave propagation; however, by the fundamental theorem of Stokes, the curl of the magnetic field must be perpendicular to that portion of the field contributing to the local curl. Reexamination of the equations of motion indicates that only electromagnetic waves propagate through a stationary region of constant pressure in a fully ionized, neutral medium. Comment: 7 pages, 1 figure, to be published in Phys. Plasmas, http://link.aip.org/link/?PHPAEN/16/054701/1

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The geometric analysis of the gyromotion for charged particles in a time-dependent magnetic field by Liu and Qin [Phys. Plasmas 18, 072505 (2011)] is reformulated in terms of the spatial angles that represent the instantaneous orientation of the magnetic field. This new formulation, which includes the equation of motion for the pitch angle, clarifies the decomposition of the gyroangle-averaged equation of motion for the gyrophase into its dynamic and geometric contributions. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748568]

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It is pointed out that electron thermal fluctuations can couple with the ion
acoustic mode in an inhomogeneous plasma to generate a low frequency ion time
scale electromagnetic wave. This electromagnetic wave can become unstable if
the temperature and density gradients are parallel to each other which can be
the case in laser-plasmas similar to stellar cores. The comparisons of the
present theoretical model with the previous investigations are also presented.
The final result is applied to a classical laser induced plasma for
illustration.

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At laser intensities above 1023W/cm2 the interaction of a laser with a plasma
is qualitatively different to the interactions at lower intensities. In this
intensity regime solid targets start to become relativistically underdense,
gamma-ray production by synchrotron emission starts to become an important
feature of the dynamics and, at even higher intensities, electron-positron pair
production by the non-linear Breit-Wheeler process starts to occur. Previous
work in this intensity regime has considered ion acceleration1,2, identified
different mechanisms for the underlying plasma physics of laser generation of
gamma-rays3,4,5 considered the effect of target parameters on gamma-ray
generation6 and considered the creation of solid density positronium plasma3.
However a complete linked understanding of the important new physics of this
regime is still lacking. In this paper, an analysis is presented of the effects
of target density, laser intensity, target preplasma properties and other
parameters on the conversion efficiency, spectrum and angular distribution of
gamma-rays by synchrotron emission. An analysis of the importance of
Breit-Wheeler pair production is also presented.

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We have studied the propagation of fast electrons through laser irradiated Ti
foils by monitoring the emission of hard X-rays and K-{\alpha} radiation from
bare foils and foils backed by a thick epoxy layer. Key observations include
strong refluxing of electrons and divergence of the electron beam in the foil
with evidence of magnetic field collimation. Our diagnostics have allowed us to
estimate the fast electron temperature and fraction of laser energy converted
to fast electrons. We have observed clear differences between the fast electron
temperatures observed with bare and epoxy backed targets which may be due to
the effects of refluxing.

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A minimal model for magnetic reconnection and, generally, low-frequency
dynamics in low-beta plasmas is proposed. The model combines analytical and
computational simplicity with physical realizability: it is a rigorous limit of
gyrokinetics for plasma beta of order the electron-ion mass ratio. The model
contains collisions and can be used both in the collisional and collisionless
reconnection regimes. It includes gyrokinetic ions (not assumed cold) and
allows for the topological rearrangement of the magnetic field lines by either
resistivity or electron inertia, whichever predominates. The two-fluid dynamics
are coupled to electron kinetics --- electrons are not assumed isothermal and
are described by a reduced drift-kinetic equation. The model therefore allows
for irreversibility and conversion of magnetic energy into electron heat via
parallel phase mixing in velocity space. An analysis of the exchanges between
various forms of free energy and its conversion into electron heat is provided.
It is shown how all relevant linear waves and regimes of the tearing
instability (collisionless, semicollisional and fully resistive) are recovered
in various limits of our model. An efficient way to simulate our equations
numerically is proposed, via the Hermite representation of the velocity space.
It is shown that small scales in velocity space will form, giving rise to a
shallow Hermite-space spectrum, whence it is inferred that, for steady-state or
sufficiently slow dynamics, the electron heating rate will remain finite in the
limit of vanishing collisionality.

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It was recently proposed that the electron-frame dissipation measure, the
energy transfer from the electromagnetic field to plasmas in the electron's
rest frame, identifies the dissipation region of collisionless magnetic
reconnection [Zenitani et al. Phys. Rev. Lett. 106, 195003 (2011)]. The measure
is further applied to the electron-scale structures of antiparallel
reconnection, by using two-dimensional particle-in-cell (PIC) simulations. The
size of the central dissipation region is controlled by the electron-ion mass
ratio, suggesting that electron physics is essential. A narrow electron jet
extends along the outflow direction until it reaches an electron shock. The jet
region appears to be anti-dissipative. At the shock, electron heating is
relevant to a magnetic cavity signature. The results are summarized to a
unified picture of the single dissipation region in a Hall magnetic geometry.

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Biskamp and Schwarz [Phys. Plasmas, 8, 3282 (2001)] have reported that the energy spectrum of two-dimensional magnetohydrodynamic turbulence is proportional to $k^{-3/2}$, which is a prediction of Iroshnikov-Kraichnan phenomenology. In this comment we report some earlier results which conclusively show that for two-dimensional magnetohydrodynamic turbulence, Kolmogorov-like phenomenology (spectral index 5/3) is better model than Iroshnikov-Kraichnan phenomenology; these results are based on energy flux analysis. Comment: 2 pages, Revtex, 1 figure (Phys. Plasmas, v9, p1484, 2002)

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The tangential layers are characterized by a bulk plasma velocity and a magnetic field that are perpendicular to the gradient direction. They have been extensively described in the frame of the Magneto-Hydro-Dynamic (MHD) theory. But the MHD theory does not look inside the transition region if the transition has a size of a few ion gyroradii. A series of kinetic tangential equilibria, valid for a collisionless plasma is presented. These equilibria are exact analytical solutions of the Maxwell-Vlasov equations. The particle distribution functions are sums of an infinite number of elementary functions parametrized by a vector potential. Examples of equilibria relevant to space plasmas are shown. A model for the deep and sharp density depletions observed in the auroral zone of the Earth is proposed. Tangential equilibria are also relevant for the study of planetary environments and of remote astrophysical plasmas.

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Thermally excited phonon spectra of 2D Yukawa solids and liquids in the presence of an external magnetic field are studied using computer simulations. Special attention is paid to the variation of wave spectra in terms of several key parameters, such as the strength of coupling, the screening parameter, and the intensity of the magnetic field. In addition, comparisons are made with several analytical theories, including random-phase approximation, quasi-localized charge approximation, and harmonic approximation, and the validity of those theories is discussed in the present context. Comment: 21 pages 11 figures

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We derive an effective coupling parameter for two-dimensional Yukawa systems
based on the height of the first maximum of the pair distribution function. Two
variants -- one valid in the high-coupling range, the other for arbitrary
couplings of the liquid -- are derived. Comparison to previous approaches to
Yukawa coupling parameters shows that the present concept is more general and
more accurate.
Using, in addition, dynamical information contained in the velocity
autocorrelation function, we outline a reference data method that can be
employed as a non-invasive measurement scheme of the plasma parameters -- the
coupling strength and the screening length. This approach requires only input
from a time-series of configuration snapshots and particle velocities with no
recourse to additional information about the system. Our results should be
directly applicable as a simple, yet reliable diagnostic method for a variety
of experiments, including dusty plasmas, colloidal suspensions and ions in
traps, and can be employed to facilitate comparisons between experiments,
theory and simulations.

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Nonlinear gyrokinetics provides a suitable framework to describe
short-wavelength turbulence in magnetized laboratory and astrophysical plasmas.
In the electrostatic limit, this system is known to exhibit a free energy
cascade towards small scales in (perpendicular) real and/or velocity space. The
dissipation of free energy is always due to collisions (no matter how weak the
collisionality), but may be spread out across a wide range of scales. Here, we
focus on freely-decaying 2D electrostatic turbulence on sub-ion-gyroradius
scales. An existing scaling theory for the turbulent cascade in the weakly
collisional limit is generalized to the moderately collisional regime. In this
context, non-universal power law scalings due to multiscale dissipation are
predicted, and this prediction is confirmed by means of direct numerical
simulations.

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A possible solution to the unexplained high intensity hard x-ray (HXR)
emission observable during solar flares was investigated via 3D fully
relativistic, electromagnetic particle-in-cell (PIC) simulations with realistic
ion to electron mass ratio. A beam of accelerated electrons was injected into a
magnetised, Maxwellian, homogeneous and inhomogeneous background plasma. The
electron distribution function was unstable to the beam-plasma instability and
was shown to generate Langmuir waves, while relaxing to plateau formation. In
order to estimate the role of the background density gradient on an unbound
(infinite spatial extent) beam, three different scenarios were investigated: a)
a uniform density background; b) a weak density gradient, n_R/n_L=3; c) a
strong gradient case, n_R/n_L=10, where n_R and n_L denote background electron
densities on the left and right edges of the simulation box respectively. The
strong gradient case produced the largest fraction of electrons beyond 15 v_th.
Further, two cases (uniform and strong gradient background) with spatially
localized beam injections were performed aiming to show drifts of the generated
Langmuir wave wavenumbers, as suggested in previous studies. For the strong
gradient case, the Langmuir wave power is shown to drift to smaller
wavenumbers, as found in previous quasi-linear simulations.

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This paper presents simulations of isolated 3D filaments in a slab geometry obtained using a newly developed 3D reduced fluid code, written using the BOUT++ framework. First, systematic scans were performed to investigate how the dynamics of a filament are affected by its amplitude, perpendicular size, and parallel extent. The perpendicular size of the filament was found to have a strong influence on its motions, as it determined the relative importance of parallel currents to polarization and viscous currents, whilst drift-wave instabilities were observed if the initial amplitude of the blob was increased sufficiently. Next, the 3D simulations were compared to 2D simulations using different parallel closures; namely, the
sheath dissipation closure, which neglects parallel gradients, and the
vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure was found to not replicate the 3D perpendicular dynamics and overestimated the initial radial acceleration of all the filaments studied. In contrast, a more satisfactory comparison with the sheath dissipation closure was obtained, even in the presence of significant parallel gradients, where the closure is no longer valid. Specifically, it captured the contrasting dynamics of filaments with different perpendicular sizes that were observed in the 3D simulations which the vorticity advection closure failed to replicate. However, neither closure successfully replicated the
Boltzmann spinning effects nor the associated poloidal drift of the blob that was observed in the 3D simulations. Although the sheath dissipation closure was concluded to be more successful in replicating the 3D dynamics, it is emphasized that the vorticity advection closure may still be relevant for situations where the parallel current is inhibited from closing through the sheath due to effects such as strong magnetic shear around X points or increased resistivity near the targets.

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The manner in which the rate of magnetic reconnection scales with the
Lundquist number in realistic three-dimensional (3D) geometries is still an
unsolved problem. It has been demonstrated that in 2D rapid non-linear tearing
allows the reconnection rate to become almost independent of the Lundquist
number (the `plasmoid instability'). Here we present the first study of an
analogous instability in a fully 3D geometry, defined by a magnetic null point.
The 3D null current layer is found to be susceptible to an analogous
instability, but is marginally more stable than an equivalent 2D
Sweet-Parker-like layer. Tearing of the sheet creates a thin boundary layer
around the separatrix surface, contained within a flux envelope with a
hyperbolic structure that mimics a spine-fan topology. Efficient mixing of flux
between the two topological domains occurs as the flux rope structures created
during the tearing process evolve within this envelope. This leads to a
substantial increase in the rate of reconnection between the two domains.

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We present large scale 3D particle-in-cell (PIC) simulations to examine
particle energization in magnetic reconnection of relativistic
electron-positron (pair) plasmas. The initial configuration is set up as a
relativistic Harris equilibrium without a guide field. These simulations are
large enough to accommodate a sufficient number of tearing and kink modes.
Contrary to the non-relativistic limit, the linear tearing instability is
faster than the linear kink instability, at least in our specific parameters.
We find that the magnetic energy dissipation is first facilitated by the
tearing instability and followed by the secondary kink instability. Particles
are mostly energized inside the magnetic islands during the tearing stage due
to the spatially varying electric fields produced by the outflows from
reconnection. Secondary kink instability leads to additional particle
acceleration. Accelerated particles are, however, observed to be thermalized
quickly. The large amplitude of the vertical magnetic field resulting from the
tearing modes by the secondary kink modes further help thermalizing the
non-thermal particles generated from the secondary kink instability.
Implications of these results for astrophysics are briefly discussed.

…

We report the results of a 3D particle-in-cell (PIC) simulation carried out
to study the early-stage evolution of the shock formed when an unmagnetized
relativistic jet interacts with an ambient electron-ion plasma. Full-shock
structures associated with the interaction are observed in the ambient frame.
When open boundaries are employed in the direction of the jet; the forward
shock is seen as a hybrid structure consisting of an electrostatic shock
combined with a double layer, while the reverse shock is seen as a double
layer. The ambient ions show two distinct features across the forward shock: a
population penetrating into the shocked region from the precursor region and an
accelerated population escaping from the shocked region into the precursor
region. This behavior is a signature of a combination of an electrostatic shock
and a double layer. Jet electrons are seen to be electrostatically trapped
between the forward and reverse shock structures showing a ring-like
distribution in a phase-space plot, while ambient electrons are thermalized and
become essentially isotropic in the shocked region. The magnetic energy density
grows to a few percent of the jet kinetic energy density at both the forward
and the reverse shock transition layers in a rather short time scale. We see
little disturbance of the jet ions over this time scale.

…

Previous studies [Malara et al ApJ, 533, 523 (2000)] considered
small-amplitude Alfven wave (AW) packets in Arnold-Beltrami-Childress (ABC)
magnetic field using WKB approximation. In this work linearly polarised Alfven
wave dynamics in ABC magnetic field via direct 3D MHD numerical simulation is
studied for the first time. Gaussian AW pulse with length-scale much shorter
than ABC domain length and harmonic AW with wavelength equal to ABC domain
length are studied for four different resistivities. While it is found that AWs
dissipate quickly in the ABC field, surprisingly, AW perturbation energy
increases in time. In the case of the harmonic AW perturbation energy growth is
transient in time, attaining peaks in both velocity and magnetic perturbation
energies within timescales much smaller than resistive time. In the case of the
Gaussian AW pulse velocity perturbation energy growth is still transient in
time, attaining a peak within few resistive times, while magnetic perturbation
energy continues to grow. It is also shown that the total magnetic energy
decreases in time and this is governed by the resistive evolution of the
background ABC magnetic field rather than AW damping. On contrary, when
background magnetic field is uniform, the total magnetic energy decrease is
prescribed by AW damping, because there is no resistive evolution of the
background. By considering runs with different amplitudes and by analysing
perturbation spectra, possible dynamo action by AW perturbation-induced
peristaltic flow and inverse cascade of magnetic energy have been excluded.
Therefore, the perturbation energy growth is attributed to a new instability.
The growth rate appears to be dependent on the value of the resistivity and
spatial scale of the AW disturbance. Thus, when going beyond WKB approximation,
AW damping, described by full MHD equations, does not guarantee decrease of
perturbation energy.

…

The electromagnetic energy equation is analyzed term by term in a 3D
simulation of kinetic reconnection previously reported by
\citet{vapirev2013formation}. 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 (DF) 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
$\mathbf S \approx 10^{-3} W/m^2$.

…

The filamentation (Weibel) instability plays a key role in the formation of
collisionless shocks which are thought to produce Gamma-Ray-Bursts and
High-Energy-Cosmic-Rays in astrophysical environments. While it has been known
for long that a flow-aligned magnetic field can completely quench the
instability, it was recently proved in 2D that in the cold regime, such
cancelation is possible if and only if the field is perfectly aligned. Here,
this result is finally extended to a 3D geometry. Calculations are conducted
for symmetric and asymmetric counter-streaming relativistic plasma shells. 2D
results are retrieved in 3D: the instability can never be completely canceled
for an oblique magnetic field. In addition, the maximum growth-rate is always
larger for wave vectors lying in the plan defined by the flow and the oblique
field. On the one hand, this bears consequences on the orientation of the
generated filaments. On the other hand, it certifies 2D simulations of the
problem can be performed without missing the most unstable filamentation modes.

…

We report here, for the first time, an observed instability of a
Kelvin-Helmholtz (KH) nature occurring in a fully three-dimensional (3D)
current-vortex sheet at the fan plane of a 3D magnetic null point. The
current-vortex layer forms self-consistently in response to foot point driving
around the spine lines of the null. The layer first becomes unstable at an
intermediate distance from the null point, with the instability being
characterized by a rippling of the fan surface and a filamentation of the
current density and vorticity in the shear layer. Owing to the 3D geometry of
the shear layer, a branching of the current filaments and vortices is observed.
The instability results in a mixing of plasma between the two topologically
distinct regions of magnetic flux on either side of the fan separatrix surface,
as flux is reconnected across this surface. We make a preliminary investigation
of the scaling of the system with the dissipation parameters. Our results
indicate that the fan plane separatrix surface is an ideal candidate for the
formation of current-vortex sheets in complex magnetic fields and, therefore,
the enhanced heating and connectivity change associated with the instabilities
of such layers.

…

In this work the dynamic magnetic field within a tearing-unstable
three-dimensional (3D) current sheet about a magnetic null point is described
in detail. We focus on the evolution of the magnetic null points and flux ropes
that are formed during the tearing process. Generally, we find that both
magnetic structures are created prolifically within the layer and are
non-trivially related. We examine how nulls are created and annihilated during
bifurcation processes, and describe how they evolve within the current layer.
The type of null bifurcation first observed is associated with the formation of
pairs of flux ropes within the current layer. We also find that new nulls form
within these flux ropes, both following internal reconnection and as adjacent
flux ropes interact. The flux ropes exhibit a complex evolution, driven by a
combination of ideal kinking and their interaction with the outflow jets from
the main layer. The finite size of the unstable layer also allows us to
consider the wider effects of flux rope generation. We find that the unstable
current layer acts as a source of torsional MHD waves and dynamic braiding of
magnetic fields. The implications of these results to several areas of
heliophysics are discussed.

…

Asymmetric current sheets are likely to be prevalent in both astrophysical
and laboratory plasmas with complex three dimensional (3D) magnetic topologies.
This work presents kinematic analytical models for spine and fan reconnection
at a symmetric 3D null with asymmetric current sheets. Asymmetric fan
reconnection is characterized by an asymmetric reconnection of flux past each
spine line and a bulk flow of plasma across the null point. In contrast,
asymmetric spine reconnection is inherently equal and opposite in how flux is
reconnected across the fan plane. The higher modes of spine reconnection also
include localized wedges of vortical flux transport in each half of the fan. In
this situation, two definitions for reconnection rate become appropriate: a
local reconnection rate quantifying how much flux is genuinely reconnected
across the fan plane and a global rate associated with the net flux driven
across each semi-plane. Through a scaling analysis it is shown that when the
ohmic dissipation in the layer is assumed to be constant, the increase in the
local rate bleeds from the global rate as the sheet deformation is increased.
Both models suggest that asymmetry in the current sheet dimensions will have a
profound effect on the reconnection rate and manner of flux transport in
reconnection involving 3D nulls.

…

The objective of this article is to report the parallel implementation of the
3D molecular dynamic simulation code for laser cluster interaction. The
benchmarking of the code has been done by comparing the simulation results with
some of the experiments performed across the globe. Scaling laws for the
computational time is being established by varying the number of processor
cores and number of macroparticles used. The capabilities of the code are
highlighted by implementing various diagnostic tools.

…

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