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Jun 2013

Volume 20, Issue 6 (partial)

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Preface to Special Topic Section: Advances in Magnetic Reconnection Research in Space and Laboratory Plasmas. Part II

Hantao Ji, Yasushi Ono, and Ryoji Matsumoto

Phys. Plasmas 20, 061101 (2013); http://dx.doi.org/10.1063/1.4811118 (3 pages)

Online Publication Date: 11 June 2013

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Abstract Unavailable
Show PACS
01.30.-y Physics literature and publications
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Vd Magnetic reconnection
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
95.30.Qd Magnetohydrodynamics and plasmas

A review of pressure anisotropy caused by electron trapping in collisionless plasma, and its implications for magnetic reconnection

Jan Egedal, Ari Le, and William Daughton

Phys. Plasmas 20, 061201 (2013); http://dx.doi.org/10.1063/1.4811092 (18 pages)

Online Publication Date: 12 June 2013

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From spacecraft data, it is evident that electron pressure anisotropy develops in collisionless plasmas. This is in contrast to the results of theoretical investigations, which suggest this anisotropy should be limited. Common for such theoretical studies is that the effects of electron trapping are not included; simply speaking, electron trapping is a non-linear effect and is, therefore, eliminated when utilizing the standard methods for linearizing the underlying kinetic equations. Here, we review our recent work on the anisotropy that develops when retaining the effects of electron trapping. A general analytic model is derived for the electron guiding center distribution math(v∥,v⊥) of an expanding flux tube. The model is consistent with anisotropic distributions observed by spacecraft, and is applied as a fluid closure yielding anisotropic equations of state for the parallel and perpendicular components (relative to the local magnetic field direction) of the electron pressure. In the context of reconnection, the new closure accounts for the strong pressure anisotropy that develops in the reconnection regions. It is shown that for generic reconnection in a collisionless plasma nearly all thermal electrons are trapped, and dominate the properties of the electron fluid. A new numerical code is developed implementing the anisotropic closure within the standard two-fluid framework. The code accurately reproduces the detailed structure of the reconnection region observed in fully kinetic simulations. These results emphasize the important role of pressure anisotropy for the reconnection process. In particular, for reconnection geometries characterized by small values of the normalized upstream electron pressure, βe∞, the pressure anisotropy becomes large with p∥≫p⊥ and strong parallel electric fields develop in conjunction with this anisotropy. The parallel electric fields can be sustained over large spatial scales and, therefore, become important for electron acceleration.
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52.35.Vd Magnetic reconnection
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
51.10.+y Kinetic and transport theory of gases
52.25.Dg Plasma kinetic equations
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Magnetic reconnection in a weakly ionized plasma

James E. Leake, Vyacheslav S. Lukin, and Mark G. Linton

Phys. Plasmas 20, 061202 (2013); http://dx.doi.org/10.1063/1.4811140 (12 pages)

Online Publication Date: 13 June 2013

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Magnetic reconnection in partially ionized plasmas is a ubiquitous phenomenon spanning the range from laboratory to intergalactic scales, yet it remains poorly understood and relatively little studied. Here, we present results from a self-consistent multi-fluid simulation of magnetic reconnection in a weakly ionized reacting plasma with a particular focus on the parameter regime of the solar chromosphere. The numerical model includes collisional transport, interaction and reactions between the species, and optically thin radiative losses. This model improves upon our previous work in Leake et al. [“Multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma,” Astrophys. J. 760, 109 (2012)] by considering realistic chromospheric transport coefficients, and by solving a generalized Ohm's law that accounts for finite ion-inertia and electron-neutral drag. We find that during the two dimensional reconnection of a Harris current sheet with an initial width larger than the neutral-ion collisional coupling scale, the current sheet thins until its width becomes less than this coupling scale, and the neutral and ion fluids decouple upstream from the reconnection site. During this process of decoupling, we observe reconnection faster than the single-fluid Sweet-Parker prediction, with recombination and plasma outflow both playing a role in determining the reconnection rate. As the current sheet thins further and elongates, it becomes unstable to the secondary tearing instability, and plasmoids are seen. The reconnection rate, outflows, and plasmoids observed in this simulation provide evidence that magnetic reconnection in the chromosphere could be responsible for jet-like transient phenomena such as spicules and chromospheric jets.
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95.30.Qd Magnetohydrodynamics and plasmas
96.60.Na Chromosphere
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Vd Magnetic reconnection
52.65.Kj Magnetohydrodynamic and fluid equation

The transfer between electron bulk kinetic energy and thermal energy in collisionless magnetic reconnection

San Lu, Quanming Lu, Can Huang, and Shui Wang

Phys. Plasmas 20, 061203 (2013); http://dx.doi.org/10.1063/1.4811119 (5 pages)

Online Publication Date: 14 June 2013

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By performing two-dimensional particle-in-cell simulations, we investigate the transfer between electron bulk kinetic and electron thermal energy in collisionless magnetic reconnection. In the vicinity of the X line, the electron bulk kinetic energy density is much larger than the electron thermal energy density. The evolution of the electron bulk kinetic energy is mainly determined by the work done by the electric field force and electron pressure gradient force. The work done by the electron gradient pressure force in the vicinity of the X line is changed to the electron enthalpy flux. In the magnetic island, the electron enthalpy flux is transferred to the electron thermal energy due to the compressibility of the plasma in the magnetic island. The compression of the plasma in the magnetic island is the consequence of the electromagnetic force acting on the plasma as the magnetic field lines release their tension after being reconnected. Therefore, we can observe that in the magnetic island the electron thermal energy density is much larger than the electron bulk kinetic energy density.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Rr Particle-in-cell method
52.25.Kn Thermodynamics of plasmas

The influence of intense electric fields on three-dimensional asymmetric magnetic reconnection

P. L. Pritchett

Phys. Plasmas 20, 061204 (2013); http://dx.doi.org/10.1063/1.4811123 (8 pages)

Online Publication Date: 14 June 2013

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A three-dimensional particle-in-cell simulation of magnetic reconnection in an asymmetric configuration without a guide field and with temperature ratio Ti/Te>1 demonstrates that intense perpendicular electric fields are produced on the low-density side of the current layer where there is a strong gradient in the plasma density. The simulation shows that the 3-D reconnection rate is unaffected by these intense electric fields, that the electron current layer near the X line remains coherent and does not break up, but that localized regions of strong energy dissipation exist along the low-density separatrices. Near the X line the dominant term in the generalized Ohm's law for the reconnection electric field remains the off-diagonal electron pressure gradient ∂Pexy/∂x. On the low-beta separatrix, however, the anomalous drag −〈δnδEy〉/〈n〉 makes an equally important contribution to that of the pressure gradient to the average Ey field.
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52.35.Vd Magnetic reconnection
52.65.Rr Particle-in-cell method
52.25.-b Plasma properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

The adiabatic phase mixing and heating of electrons in Buneman turbulence

H. Che, J. F. Drake, M. Swisdak, and M. L. Goldstein

Phys. Plasmas 20, 061205 (2013); http://dx.doi.org/10.1063/1.4811137 (4 pages)

Online Publication Date: 14 June 2013

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The nonlinear development of the strong Buneman instability and the associated fast electron heating in thin current layers with Ωe/ωpe<1 is explored. Phase mixing of the electrons in wave potential troughs and a rapid increase in temperature are observed during the saturation of the instability. We show that the motion of trapped electrons can be described using a Hamiltonian formalism in the adiabatic approximation. The process of separatrix crossing as electrons are trapped and de-trapped is irreversible and guarantees that the resulting electron energy gain is a true heating process.
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52.50.-b Plasma production and heating
02.60.Gf Algorithms for functional approximation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Effects of the non-uniform initial environment and the guide field on the plasmoid instability

Lei Ni, Jun Lin, and Nicholas A. Murphy

Phys. Plasmas 20, 061206 (2013); http://dx.doi.org/10.1063/1.4811144 (8 pages)

Online Publication Date: 14 June 2013

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Effects of non-uniform initial mass density and temperature on the plasmoid instability are studied via 2.5-dimensional resistive magnetohydrodynamic (MHD) simulations. Our results indicate that the development of the plasmoid instability is apparently prevented when the initial plasma density at the center of the current sheet is higher than that in the upstream region. As a result, the higher the plasma density at the center and the lower the plasma β in the upstream region, the higher the critical Lundquist number needed for triggering secondary instabilities. When β = 0.2, the critical Lundquist number is higher than 104. For the same Lundquist number, the magnetic reconnection rate is lower for the lower plasma β case. Oppositely, when the initial mass density is uniform and the Lundquist number is low, the magnetic reconnection rate turns out to be higher for the lower plasma β case. For the high Lundquist number case (>104) with uniform initial mass density, the magnetic reconnection is not affected by the initial plasma β and the temperature distribution. Our results indicate that the guide field has a limited impact on the plasmoid instability in resistive MHD.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Vd Magnetic reconnection
52.65.Kj Magnetohydrodynamic and fluid equation
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

On phase diagrams of magnetic reconnection

P. A. Cassak and J. F. Drake

Phys. Plasmas 20, 061207 (2013); http://dx.doi.org/10.1063/1.4811120 (8 pages)

Online Publication Date: 17 June 2013

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Recently, “phase diagrams” of magnetic reconnection were developed to graphically organize the present knowledge of what type, or phase, of reconnection is dominant in systems with given characteristic plasma parameters. Here, a number of considerations that require caution in using the diagrams are pointed out. First, two known properties of reconnection are omitted from the diagrams: the history dependence of reconnection and the absence of reconnection for small Lundquist number. Second, the phase diagrams mask a number of features. For one, the predicted transition to Hall reconnection should be thought of as an upper bound on the Lundquist number, and it may happen for considerably smaller values. Second, reconnection is never “slow,” it is always “fast” in the sense that the normalized reconnection rate is always at least 0.01. This has important implications for reconnection onset models. Finally, the definition of the relevant Lundquist number is nuanced and may differ greatly from the value based on characteristic scales. These considerations are important for applications of the phase diagrams. This is demonstrated by example for solar flares, where it is argued that it is unlikely that collisional reconnection can occur in the corona.
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52.35.Vd Magnetic reconnection
52.80.Hc Glow; corona
96.60.Hv Electric and magnetic fields, solar magnetism
96.60.qe Flares
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Development of multi-hierarchy simulation model with non-uniform space grids for collisionless driven reconnection

Shunsuke Usami, Ritoku Horiuchi, Hiroaki Ohtani, and Mitsue Den

Phys. Plasmas 20, 061208 (2013); http://dx.doi.org/10.1063/1.4811121 (8 pages)

Online Publication Date: 18 June 2013

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A multi-hierarchy simulation model aimed at magnetic reconnection studies has been developed, in which macroscopic and microscopic physics are solved self-consistently and simultaneously. In this work, the previous multi-hierarchy model by these authors is extended to a more realistic one with non-uniform space grids. Based on the domain decomposition method, the multi-hierarchy model consists of three parts: a magnetohydrodynamics algorithm to express the macroscopic global dynamics, a particle-in-cell algorithm to describe the microscopic kinetic physics, and an interface algorithm to interlock macro and micro hierarchies. For its verification, plasma flow injection is simulated in this multi-hierarchy model and it is confirmed that the interlocking method can describe the correct physics. Furthermore, this model is applied to collisionless driven reconnection in an open system. Magnetic reconnection is found to occur in a micro hierarchy by injecting plasma from a macro hierarchy.
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52.35.Vd Magnetic reconnection
52.65.Kj Magnetohydrodynamic and fluid equation
52.65.Rr Particle-in-cell method
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Excitation and propagation of electromagnetic fluctuations with ion-cyclotron range of frequency in magnetic reconnection laboratory experiment

Michiaki Inomoto, Akihiro Kuwahata, Hiroshi Tanabe, Yasushi Ono, and TS Group

Phys. Plasmas 20, 061209 (2013); http://dx.doi.org/10.1063/1.4811469 (7 pages)

Online Publication Date: 18 June 2013

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Large-amplitude electromagnetic fluctuations of ion-cyclotron-frequency range are detected in a laboratory experiment inside the diffusion region of a magnetic reconnection with a guide field. The fluctuations have properties similar to kinetic Alfvén waves propagating obliquely to the guide field. Temporary enhancement of the reconnection rate is observed during the occurrence of the fluctuations, suggesting a relationship between the modification in the local magnetic structure given by these fluctuations and the intermittent fast magnetic reconnection.
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52.35.Vd Magnetic reconnection
52.25.Gj Fluctuation and chaos phenomena
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Fi Transport properties
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back to top Basic Plasma Phenomena, Waves, Instabilities

Temporal evolution of bubble tip velocity in classical Rayleigh-Taylor instability at arbitrary Atwood numbers

W. H. Liu, L. F. Wang, W. H. Ye, and X. T. He

Phys. Plasmas 20, 062101 (2013); http://dx.doi.org/10.1063/1.4801505 (11 pages)

Online Publication Date: 11 June 2013

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In this research, the temporal evolution of the bubble tip velocity in Rayleigh-Taylor instability (RTI) at arbitrary Atwood numbers and different initial perturbation velocities with a discontinuous profile in irrotational, incompressible, and inviscid fluids (i.e., classical RTI) is investigated. Potential models from Layzer [Astrophys. J. 122, 1 (1955)] and perturbation velocity potentials from Goncharov [Phys. Rev. Lett. 88, 134502 (2002)] are introduced. It is found that the temporal evolution of bubble tip velocity [u(t)] depends essentially on the initial perturbation velocity [u(0)]. First, when the u(0)<C(1)uasp, the bubble tip velocity increases smoothly up to the asymptotic velocity (uasp) or terminal velocity. Second, when C(1)uasp ≤ u(0)<C(2)uasp, the bubble tip velocity increases quickly, reaching a maximum velocity and then drops slowly to the uasp. Third, when C(2)uasp ≤ u(0)<C(3)uasp, the bubble tip velocity decays rapidly to a minimum velocity and then increases gradually toward the uasp. Finally, when u(0) ≥ C(3)uasp, the bubble tip velocity decays monotonically to the uasp. Here, the critical coefficients C(1),C(2), and C(3), which depend sensitively on the Atwood number (A) and the initial perturbation amplitude of the bubble tip [h(0)], are determined by a numerical approach. The model proposed here agrees with hydrodynamic simulations. Thus, it should be included in applications where the bubble tip velocity plays an important role, such as the design of the ignition target of inertial confinement fusion where the Richtmyer-Meshkov instability (RMI) can create the seed of RTI with u(0) ∼ uasp, and stellar formation and evolution in astrophysics where the deflagration wave front propagating outwardly from the star is subject to the combined RMI and RTI.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
02.60.-x Numerical approximation and analysis
28.52.Cx Fueling, heating and ignition
28.52.Fa Materials

Ion acoustic shock waves in plasmas with warm ions and kappa distributed electrons and positrons

S. Hussain, S. Mahmood, and Hafeez Ur-Rehman

Phys. Plasmas 20, 062102 (2013); http://dx.doi.org/10.1063/1.4810793 (7 pages)

Online Publication Date: 12 June 2013

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The monotonic and oscillatory ion acoustic shock waves are investigated in electron-positron-ion plasmas (e-p-i) with warm ions (adiabatically heated) and nonthermal kappa distributed electrons and positrons. The dissipation effects are included in the model due to kinematic viscosity of the ions. Using reductive perturbation technique, the Kadomtsev-Petviashvili-Burgers (KPB) equation is derived containing dispersion, dissipation, and diffraction effects (due to perturbation in the transverse direction) in e-p-i plasmas. The analytical solution of KPB equation is obtained by employing tangent hyperbolic (Tanh) method. The analytical condition for the propagation of oscillatory and monotonic shock structures are also discussed in detail. The numerical results of two dimensional monotonic shock structures are obtained for graphical representation. The dependence of shock structures on positron equilibrium density, ion temperature, nonthermal spectral index kappa, and the kinematic viscosity of ions are also discussed.
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52.35.Tc Shock waves and discontinuities
52.25.-b Plasma properties
52.27.Cm Multicomponent and negative-ion plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Experimental evidence of ion acoustic soliton chain formation and validation of nonlinear fluid theory

Amar Kakad, Yoshiharu Omura, and Bharati Kakad

Phys. Plasmas 20, 062103 (2013); http://dx.doi.org/10.1063/1.4810794 (13 pages)

Online Publication Date: 14 June 2013

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We perform one-dimensional fluid simulation of ion acoustic (IA) solitons propagating parallel to the magnetic field in electron-ion plasmas by assuming a large system length. To model the initial density perturbations (IDP), we employ a KdV soliton type solution. Our simulation demonstrates that the generation mechanism of IA solitons depends on the wavelength of the IDP. The short wavelength IDP evolve into two oppositely propagating identical IA solitons, whereas the long wavelength IDP develop into two indistinguishable chains of multiple IA solitons through a wave breaking process. The wave breaking occurs close to the time when electrostatic energy exceeds half of the kinetic energy of the electron fluid. The wave breaking amplitude and time of its initiation are found to be dependent on characteristics of the IDP. The strength of the IDP controls the number of IA solitons in the solitary chains. The speed, width, and amplitude of IA solitons estimated during their stable propagation in the simulation are in good agreement with the nonlinear fluid theory. This fluid simulation is the first to confirm the validity of the general nonlinear fluid theory, which is widely used in the study of solitary waves in laboratory and space plasmas.
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52.35.Sb Solitons; BGK modes
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Stimulated Raman back scattering of extraordinary electromagnetic waves from periodically magnetized nanoparticle lattice

A. Chakhmachi

Phys. Plasmas 20, 062104 (2013); http://dx.doi.org/10.1063/1.4810803 (8 pages)

Online Publication Date: 17 June 2013

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Stimulated Raman back scattering of extraordinary electromagnetic waves from the nanoparticle lattice is investigated in the presence of the static magnetic field. In the context of macroscopic theory, dispersion relation and growth rate of extraordinary mode for different values of static magnetic field and lattice parameters are derived and analyzed. It is found that when the static magnetic field is off, dispersion relation has two branches. These branches are related to the plasmonic and body wave branches of the plane polarized wave. Low frequency branch of the pump wave is not involved in the instability while the other branch is not stable, and the growth rate of Raman back scattered wave has one peak. If the electrons have cyclotron frequency by static magnetic field, dispersion has three branches. These branches are related to the plasmonic and body wave branches of left and right hand circularly polarized waves. In this situation, it is found that low frequency lower branch of the pump wave is stable while other branches are not stable, and the growth rate of Raman back scattered wave has three peaks. Numerical study of growth rate in various cyclotron frequencies shows that the growth rate increases and the instability band width decreases with increasing static magnetic field.
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52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering
02.10.-v Logic, set theory, and algebra
52.25.Xz Magnetized plasmas
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Error-field penetration in reversed magnetic shear configurations

H. H. Wang, Z. X. Wang, X. Q. Wang, and X. G. Wang

Phys. Plasmas 20, 062105 (2013); http://dx.doi.org/10.1063/1.4811391 (6 pages)

Online Publication Date: 17 June 2013

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Error-field penetration in reversed magnetic shear (RMS) configurations is numerically investigated by using a two-dimensional resistive magnetohydrodynamic model in slab geometry. To explore different dynamic processes in locked modes, three equilibrium states are adopted. Stable, marginal, and unstable current profiles for double tearing modes are designed by varying the current intensity between two resonant surfaces separated by a certain distance. Further, the dynamic characteristics of locked modes in the three RMS states are identified, and the relevant physics mechanisms are elucidated. The scaling behavior of critical perturbation value with initial plasma velocity is numerically obtained, which obeys previously established relevant analytical theory in the viscoresistive regime.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Tn Ideal and resistive MHD modes; kinetic modes
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
02.60.Cb Numerical simulation; solution of equations
back to top Magnetically Confined Plasmas, Heating, Confinement

Characterization of heat loads from mitigated and unmitigated vertical displacement events in DIII-D

E. M. Hollmann, N. Commaux, N. W. Eidietis, D. A. Humphreys, T. J. Jernigan, C. J. Lasnier, R. A. Moyer, R. A. Pitts, M. Sugihara, E. J. Strait, J. Watkins, and J. C. Wesley

Phys. Plasmas 20, 062501 (2013); http://dx.doi.org/10.1063/1.4810792 (9 pages)

Online Publication Date: 10 June 2013

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Experiments have been conducted on the DIII-D tokamak to study the distribution and repeatability of heat loads and vessel currents resulting from vertical displacement events (VDEs). For unmitigated VDEs, the radiated power fraction appears to be of order 50%, with the remaining power dominantly conducted to the vessel walls. Shot-to-shot scatter in heat loads measured at one toroidal location is not large (<±50%), suggesting that toroidal asymmetries in conducted heat loads are not large. Conducted heat loads are clearly observed during the current quench (CQ) of both mitigated and unmitigated disruptions. Significant poloidal asymmetries in heat loads and radiated power are often observed in the experiments but are not yet understood. Energy dissipated resistively in the conducting walls during the CQ appears to be small (<5%). The mitigating effect of neon massive gas injection (MGI) as a function of MGI trigger delay has also been studied. Improved mitigation is observed as the MGI trigger delay is decreased. For sufficiently early MGI mitigation, close to 100% radiated energy and a reduction of roughly a factor 2 in vessel forces is achieved.
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52.50.Gj Plasma heating by particle beams
52.55.Fa Tokamaks, spherical tokamaks
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.40.Mj Particle beam interactions in plasmas

Destabilization of low-n peeling modes by trapped energetic particles

G. Z. Hao, Y. Q. Liu, A. K. Wang, G. Matsunaga, M. Okabayashi, Z. Z. Mou, and X. M. Qiu

Phys. Plasmas 20, 062502 (2013); http://dx.doi.org/10.1063/1.4811382 (10 pages)

Online Publication Date: 18 June 2013

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The kinetic effect of trapped energetic particles (EPs), arising from perpendicular neutral beam injection, on the stable low-n peeling modes in tokamak plasmas is investigated, through numerical solution of the mode's dispersion relation derived from an energy principle. A resistive-wall peeling mode with m/n = 6/1, with m and n being the poloidal and toroidal mode numbers, respectively, is destabilized by trapped EPs as the EPs' pressure exceeds a critical value βc*, which is sensitive to the pitch angle of trapped EPs. The dependence of βc* on the particle pitch angle is eventually determined by the bounce average of the mode eigenfunction. Peeling modes with higher m and n numbers can also be destabilized by trapped EPs. Depending on the wall distance, either a resistive-wall peeling mode or an ideal-kink peeling mode can be destabilized by EPs.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Fi Transport properties
02.10.Ud Linear algebra
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Gj Plasma heating by particle beams
52.55.Fa Tokamaks, spherical tokamaks
back to top Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter

Diagnosing suprathermal ion populations in Z-pinch plasmas using fusion neutron spectra

P. F. Knapp, D. B. Sinars, and K. D. Hahn

Phys. Plasmas 20, 062701 (2013); http://dx.doi.org/10.1063/1.4810805 (11 pages)

Online Publication Date: 18 June 2013

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The existence of suprathermal ion populations gives rise to significant broadening of and modifications to the fusion neutron spectrum. We show that when this population takes the form of a power-law at high energies, specific changes occur to the spectrum which are diagnosable. In particular, the usual Gaussian spectral shape produced by a thermal plasma is replaced by a Lorentz-like spectrum with broad wings extending far from the spectral peak. Additionally, it is found that the full width at half maximum of the spectrum depends on both the ion temperature and the power-law exponent. This causes the use of the spectral width for determination of the ion temperature to be unreliable. We show that these changes are distinguishable from other broadening mechanisms, such as temporal and motional broadening, and that detailed fitting of the spectral shape is a promising method for extracting information about the state of the ions.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.70.Nc Particle measurements
back to top Ionospheric, Solar-System, and Astrophysical Plasmas

Mirror force induced wave dispersion in Alfvén waves

P. A. Damiano and J. R. Johnson

Phys. Plasmas 20, 062901 (2013); http://dx.doi.org/10.1063/1.4810788 (5 pages)

Online Publication Date: 12 June 2013

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Recent hybrid MHD-kinetic electron simulations of global scale standing shear Alfvén waves along the Earth's closed dipolar magnetic field lines show that the upward parallel current region within these waves saturates and broadens perpendicular to the ambient magnetic field and that this broadening increases with the electron temperature. Using resistive MHD simulations, with a parallel Ohm's law derived from the linear Knight relation (which expresses the current-voltage relationship along an auroral field line), we explore the nature of this broadening in the context of the increased perpendicular Poynting flux resulting from the increased parallel electric field associated with mirror force effects. This increased Poynting flux facilitates wave energy dispersion across field lines which in-turn allows for electron acceleration to carry the field aligned current on adjacent field lines. This mirror force driven dispersion can dominate over that associated with electron inertial effects for global scale waves.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Tn Ideal and resistive MHD modes; kinetic modes
94.30.cq MHD waves, plasma waves, and instabilities
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
back to top Lasers, Particle Beams, Accelerators, Radiation Generation

Probing electron acceleration and x-ray emission in laser-plasma accelerators

C. Thaury, K. Ta Phuoc, S. Corde, P. Brijesh, G. Lambert, S. P. D. Mangles, M. S. Bloom, S. Kneip, and V. Malka

Phys. Plasmas 20, 063101 (2013); http://dx.doi.org/10.1063/1.4810791 (4 pages)

Online Publication Date: 10 June 2013

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While laser-plasma accelerators have demonstrated a strong potential in the acceleration of electrons up to giga-electronvolt energies, few experimental tools for studying the acceleration physics have been developed. In this paper, we demonstrate a method for probing the acceleration process. A second laser beam, propagating perpendicular to the main beam, is focused on the gas jet few nanosecond before the main beam creates the accelerating plasma wave. This second beam is intense enough to ionize the gas and form a density depletion, which will locally inhibit the acceleration. The position of the density depletion is scanned along the interaction length to probe the electron injection and acceleration, and the betatron X-ray emission. To illustrate the potential of the method, the variation of the injection position with the plasma density is studied.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.38.Ph X-ray, γ-ray, and particle generation
29.20.df Betatrons
29.20.Ej Linear accelerators
52.25.Jm Ionization of plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Plasma expansion into a waveguide created by a linearly polarized femtosecond laser pulse

N. Lemos, T. Grismayer, L. Cardoso, G. Figueira, R. Issac, D. A. Jaroszynski, and J. M. Dias

Phys. Plasmas 20, 063102 (2013); http://dx.doi.org/10.1063/1.4810797 (9 pages)

Online Publication Date: 12 June 2013

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We demonstrate the efficient generation of 4 mm and 8 mm long plasma waveguides in hydrogen and helium. These waveguides have matching spots sizes for 13 to 34 μm laser beams. The plasma waveguides are created by ultra-short laser pulses (sub-picosecond) of moderate intensities, ∼ 1015–1016 W cm−2, that heat the plasma to initial temperatures of tens of eV in order to create a hot plasma column that will expand into a plasma waveguide. We have determined that the main heating mechanism when using fs laser pulses and plasma densities ∼ 1018–19 cm−3 is Above Threshold Ionization. Detailed time and space electron density measurements are presented for the laser produced plasma waveguides.
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52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)
52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Jm Ionization of plasmas

Laser red shifting based characterization of wakefield excitation in a laser-plasma accelerator

S. Shiraishi, C. Benedetti, A. J. Gonsalves, K. Nakamura, B. H. Shaw, T. Sokollik, J. van Tilborg, C. G. R. Geddes, C. B. Schroeder, Cs. Tóth, E. Esarey, and W. P. Leemans

Phys. Plasmas 20, 063103 (2013); http://dx.doi.org/10.1063/1.4810802 (11 pages)

Online Publication Date: 14 June 2013

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Optical spectra of a drive laser exiting a channel guided laser-plasma accelerator (LPA) are analyzed through experiments and simulations to infer the magnitude of the excited wakefields. The experiments are performed at sufficiently low intensity levels and plasma densities to avoid electron beam generation via self-trapping. Spectral redshifting of the laser light is studied as an indicator of the efficiency of laser energy transfer into the plasma through the generation of coherent plasma wakefields. Influences of input laser energy, plasma density, temporal and spatial laser profiles, and laser focal location in a plasma channel are analyzed. Energy transfer is found to be sensitive to details of laser pulse shape and focal location. The experimental conditions for these critical parameters are modeled and included in particle-in-cell simulations. Simulations reproduce the redshift of the laser within uncertainties of the experiments and produce an estimate of the wake amplitudes in the experiments as a function of amount of redshift. The results support the practical use of laser redshifting to quantify the longitudinally averaged accelerating field that a particle would experience in an LPA powered below the self-trapping limit.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
29.20.Ej Linear accelerators
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Hb Self-focussing, channeling, and filamentation in plasmas

Electrons trajectories around a bubble regime in intense laser plasma interaction

Ding Lu, Xue-Yan Zhao, Bai-Song Xie, Muhammad Ali Bake, Hai-Bo Sang, and Hai-Cheng Wu

Phys. Plasmas 20, 063104 (2013); http://dx.doi.org/10.1063/1.4811387 (6 pages)

Online Publication Date: 17 June 2013

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Some typical electrons trajectories around a bubble regime in intense laser plasma interaction are investigated theoretically. By considering a modification of the fields and ellipsoid bubble shape due to the presence of residual electrons in the bubble regime, we study in detail the electrons nonlinear dynamics with or without laser pulse. To examine the electron dynamical behaviors, a set of typical electrons, which locate initially at the front of the bubble, on the transverse edge and at the bottom of the bubble respectively, are chosen for study. It is found that the range of trapped electrons in the case with laser pulse is a little narrower than that without laser pulse. The partial phase portraits for electrons around the bubble are presented numerically and their characteristic behaviors are discussed theoretically. Implication of our results on the high quality electron beam generation is also discussed briefly.
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52.38.Ph X-ray, γ-ray, and particle generation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.20.Dq Particle orbits
02.60.Cb Numerical simulation; solution of equations
back to top Low-Temperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Kinetic particle simulation of discharge and wall erosion of a Hall thruster

Shinatora Cho, Kimiya Komurasaki, and Yoshihiro Arakawa

Phys. Plasmas 20, 063501 (2013); http://dx.doi.org/10.1063/1.4810798 (12 pages)

Online Publication Date: 12 June 2013

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The primary lifetime limiting factor of Hall thrusters is the wall erosion caused by the ion induced sputtering, which is predominated by dielectric wall sheath and pre-sheath. However, so far only fluid or hybrid simulation models were applied to wall erosion and lifetime studies in which this non-quasi-neutral and non-equilibrium area cannot be treated directly. Thus, in this study, a 2D fully kinetic particle-in-cell model was presented for Hall thruster discharge and lifetime simulation. Because the fully kinetic lifetime simulation was yet to be achieved so far due to the high computational cost, the semi-implicit field solver and the technique of mass ratio manipulation was employed to accelerate the computation. However, other artificial manipulations like permittivity or geometry scaling were not used in order to avoid unrecoverable change of physics. Additionally, a new physics recovering model for the mass ratio was presented for better preservation of electron mobility at the weakly magnetically confined plasma region. The validity of the presented model was examined by various parametric studies, and the thrust performance and wall erosion rate of a laboratory model magnetic layer type Hall thruster was modeled for different operation conditions. The simulation results successfully reproduced the measurement results with typically less than 10% discrepancy without tuning any numerical parameters. It is also shown that the computational cost was reduced to the level that the Hall thruster fully kinetic lifetime simulation is feasible.
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52.77.-j Plasma applications
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.40.Hf Plasma-material interactions; boundary layer effects
52.40.Kh Plasma sheaths
52.65.Rr Particle-in-cell method
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Increasing energy coupling into plasma waves by tailoring the laser radial focal spot distribution in a laser wakefield accelerator

G. Genoud, M. S. Bloom, J. Vieira, M. Burza, Z. Najmudin, A. Persson, L. O. Silva, K. Svensson, C.-G. Wahlström, and S. P. D. Mangles

Phys. Plasmas 20, 064501 (2013); http://dx.doi.org/10.1063/1.4810795 (4 pages)

Online Publication Date: 10 June 2013

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By controlling the focal spot quality with a deformable mirror, we are able to show that increasing the fraction of pulse energy contained within the central part of the focal spot, while keeping the total energy and central spot size constant, significantly increases the amount of energy transferred to the wakefield: Our measurements show that the laser loses significantly more laser energy and undergoes greater redshifting and that more charge is produced in the accelerated beam. Three dimensional particle in cell simulations performed with accurate representations of the measured focal spot intensity distribution confirm that energy in the wings of the focal spot is effectively wasted. Even though self-focusing occurs, energy in the wings of the focal spot distribution is not coupled into the wakefield, emphasising the vital importance of high quality focal spot profiles in experiments.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Rr Particle-in-cell method
29.20.Ej Linear accelerators
52.38.Hb Self-focussing, channeling, and filamentation in plasmas
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