Volume 22, Issue 10, October 2015
 LETTERS


A tunable microplasma gradientindex lens for millimeter waves
View Description Hide DescriptionThis work presents proof of concept of a novel application of field emission assisted (FEA) microplasmas that exploits the relatively high plasma number densities encountered in these devices. We hypothesize that the number density gradients and the resulting gradient in the microplasma relative permittivity/refractive index can be utilized as a tunable diverging lens with on/off ability to defocus waves in the Terahertz regime. Electron number density profiles obtained from onedimensional particleincell with Monte Carlo collisions simulations for a typical FEA microplasma are used to determine the relative permittivity and conductivity profiles. Frequency domain wave propagation simulations using these profiles show that submm waves can be controlled using the microplasma lens with the degree of defocusing depending on the wavelength. In spite of the nonzero conductivity, the medium is not significantly lossy at the frequencies considered.

Convergent ablation measurements of plastic ablators in gasfilled rugby hohlraums on OMEGA
View Description Hide DescriptionIndirectdrive implosions experiments were conducted on the Omega Laser Facility to test the performance of uniformly doped plastic ablators for Inertial Confinement Fusion. The first convergent ablation measurements in gasfilled rugby hohlraums are reported. Ignition relevant limb velocities in the range from 150 to 300 have been reached by varying the laser drive energy and the initial capsule aspect ratio. The measured capsule trajectory and implosion velocity are in good agreement with 2D integrated simulations and a zerodimensional modeling of the implosions. We demonstrate experimentally the scaling law for the maximum implosion velocity predicted by the improved rocket model [Y. Saillard, Nucl. Fusion 46, 1017 (2006)] in the highablation regime case.

 MAGNETIC RECONNECTION IN LABORATORY AND SPACE PLASMAS—III


Localized electron heating by strong guidefield magnetic reconnection
View Description Hide DescriptionLocalized electron heating of magnetic reconnection was studied under strong guidefield using two merging spherical tokamak plasmas in the University of Tokyo Spherical Tokamak experiment. Our new slidetype twodimensional Thomson scattering system is documented for the first time the electron heating localized around the Xpoint. Shape of the high electron temperature area does not agree with that of energy dissipation term . If we include a guidefield effect term for , the energy dissipation area becomes localized around the Xpoint, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the Xpoint.

Laboratory study of diffusion region with electron energization during high guide field reconnection
View Description Hide DescriptionFloating potential profile was measured around the Xpoint during high guide field reconnection in UTST merging experiment where the ratio of guide field ( ) to reconnecting magnetic field ( ) is . Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the Xpoint during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the Xpoint. While fieldaligned components of inductive electric field ( ) and electrostatic electric field ( ) cancel out with each other away from the Xpoint, exceeds around the Xpoint, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field and electric field profile, our particle trajectory analysis indicates that fast electrons with energies over 300 eV are produced within 1 μs around the Xpoint in the nonideal MHD region. These results indicate that production of fast electrons or electron heating are expected to be observed in the vicinity of the Xpoint.

Fluid vs. kinetic magnetic reconnection with strong guide fields
View Description Hide DescriptionThe fast rates of magnetic reconnection found in both nature and experiments are important to understand theoretically. Recently, it was demonstrated that twofluid magnetic reconnection remains fast in the strong guide field regime, regardless of the presence of fastdispersive waves. This conclusion is in agreement with recent results from kinetic simulations, and is in contradiction to the findings in an earlier twofluid study, where it was suggested that fastdispersive waves are necessary for fast reconnection. In this paper, we give a more detailed derivation of the analytic model presented in a recent letter and present additional simulation results to support the conclusions that the magnetic reconnection rate in this regime is independent of both collisional dissipation and systemsize. In particular, we present a detailed comparison between fluid and kinetic simulations, finding good agreement in both the reconnection rate and overall length of the current layer. Finally, we revisit the earlier twofluid study, which arrived at different conclusions, and suggest an alternative interpretation for the numerical results presented therein.

Frozen flux violation, electron demagnetization and magnetic reconnection
View Description Hide DescriptionWe argue that the analogue in collisionless plasma of the collisional diffusion region of magnetic reconnection is properly defined in terms of the demagnetization of the plasma electrons that enable “frozen flux” slippage to occur. This condition differs from the violation of the “frozenin” condition, which only implies that two fluid effects are involved, rather than the necessary slippage of magnetic flux as viewed in the electron frame. Using 2D Particle In Cell (PIC) simulations, this approach properly finds the saddle point region of the flux function. Our demagnetization conditions are the dimensionless guiding center approximation expansion parameters for electrons which we show are observable and determined locally by the ratio of nonideal electric to magnetic field strengths. Proxies for frozen flux slippage are developed that (a) are measurable on a single spacecraft, (b) are dimensionless with theoretically justified threshold values of significance, and (c) are shown in 2D simulations to recover distinctions theoretically possible with the (unmeasurable) flux function. A new potentially observable dimensionless frozen flux rate, ΛΦ, differentiates significant from anecdotal frozen flux slippage. A single spacecraft observable, ϒ, is shown with PIC simulations to be essentially proportional to the unobservable local Maxwell frozen flux rate. This relationship theoretically establishes electron demagnetization in 3D as the general cause of frozen flux slippage. In simple 2D cases with an isolated central diffusion region surrounded by separatrices, these diagnostics uniquely identify the traditional diffusion region (without confusing it with the two fluid “iondiffusion” region) and clarify the role of the separatrices where frozen flux violations do occur but are not substantial. In the more complicated guide and asymmetric 2D cases, substantial flux slippage regions extend out along, but inside of, the preferred separatrices, demonstrating that ΛΦ ≠ 0 violations are present over significant distances (in ion inertial units) from the separator identified by the 2D flux function; these violations are, however, generally weaker than seen at known separators in 2D simulations.

Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case
View Description Hide DescriptionThe key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of inplane bipolar converging electrostatic electric field ( ) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole outofplane magnetic field, which causes the parallel electric field ( ) from to accelerate particles along the magnetic field. We explain the electron and ion dynamics and their velocity distributions and flow structures during the timedependent driven reconnection as they move from the upstream to the downstream. In particular, we address the following key physics issues: (1) the decoupling of electron and ion dynamics due to meandering orbits around the field reversal region and the generation of a pair of converging bipolar electrostatic electric field ( ) around the reconnection region; (2) the slowdown of electron and ion inflow velocities due to acceleration/deceleration of electrons and ions by as they move across the neutral sheet; (3) how the reconnection current layer is enhanced and how the orbit meandering particles are accelerated inside the reconnection region by ; (4) why the electron outflow velocity from the reconnection region reaches superAlfvenic speed and the ion outflow velocity reaches Alfvenic speed; (5) how the quadrupole magnetic field is produced and how is produced; (6) how electrons and ions are accelerated by around the separatrix region; (7) why electrons have a flattop parallel velocity distribution in the upstream just outside the reconnection region as observed in the magnetotail; (8) how electron and ion dynamics decouple and how the monopolar electrostatic electric field is produced around the separatrix region; (9) how ions gain energy as they move across the separatrix region into the downstream and how the ion velocity distribution is thermalized in the far downstream; and (10) how electrons move across the separatrix region and in the downstream and how the electron velocity distribution is thermalized in the far downstream. Finally, the main energy source for driving magnetic reconnection and particle acceleration/heating is the inductive electric field, which accelerates both electrons and ions around the reconnection current layer and separatrix regions.

Observation and numerical modeling of chromospheric evaporation during the impulsive phase of a solar flare
View Description Hide DescriptionWe have studied the chromospheric evaporation flow during the impulsive phase of the flare by using the Hinode/EUV Imaging Spectrometer observation and 1D hydrodynamic numerical simulation coupled to the timedependent ionization. The observation clearly shows that the strong redshift can be observed at the base of the flaring loop only during the impulsive phase. We performed two different numerical simulations to reproduce the strong downflows in FeXII and FeXV during the impulsive phase. By changing the thermal conduction coefficient, we carried out the numerical calculation of chromospheric evaporation in the thermal conduction dominant regime (conductivity coefficient κ 0 = classical value) and the enthalpy flux dominant regime (κ 0 = 0.1 × classical value). The chromospheric evaporation calculation in the enthalpy flux dominant regime could reproduce the strong redshift at the base of the flare during the impulsive phase. This result might indicate that the thermal conduction can be strongly suppressed in some cases of flare. We also find that timedependent ionization effect is important to reproduce the strong downflows in Fe XII and Fe XV.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Excitation of surface modes by electron beam in semibounded quantum plasma
View Description Hide DescriptionThe excitation of the TM surface modes due to the interaction of electron beam with a semibounded quantum magnetized plasma is investigated. The generated current and the perturbed densities of the electron beam and plasma are obtained. The wave equation that describes the excited fields has been solved to obtain the dispersion relation for these modes. It is found that the quantum effects play important role for frequencies less and bigger than plasma frequency such that the phase velocity of modes increases with increasing the quantum effects compared to the classical case. It has also been displayed that in the absence of external magnetic field, the surface modes appear in the all regions of the wavelength while they have been only excited for high wavenumber in the presence of the magnetic field. Besides, it has been shown that the dispersion curves of the modes depend essentially on the density ratio of beam and plasma.

Nonlinear TrivelpieceGould waves: Frequency, functional form, and stability
View Description Hide DescriptionThis paper considers the frequency, spatial form, and stability of nonlinear TrivelpieceGould (TG) waves on a cylindrical plasma column of length L and radius rp , treating both traveling waves and standing waves, and focussing on the regime of experimental interest in which . In this regime, TG waves are weakly dispersive, allowing strong modecoupling between Fourier harmonics. The mode coupling implies that linear theory for such waves is a poor approximation even at fairly small amplitude, and nonlinear theories that include a small number of harmonics, such as threewave parametric resonance theory, also fail to fully capture the stability properties of the system. It is found that nonlinear standing waves suffer jumps in their functional form as their amplitude is varied continuously. The jumps are caused by nonlinear resonances between the standing wave and nearly linear waves whose frequencies and wave numbers are harmonics of the standing wave. Also, the standing waves are found to be unstable to a multiwave version of threewave parametric resonance, with an amplitude required for instability onset that is much larger than expected from three wave theory. It is found that traveling waves are linearly stable for all amplitudes that could be studied, in contradiction to threewave theory.

The electron forewake: Shadowing and driftenergization as flowing magnetized plasma encounters an obstacle
View Description Hide DescriptionFlow of magnetized plasma past an obstacle creates a traditional wake, but also a forewake region arising from shadowing of electrons. The electron forewakes resulting from supersonic flows past insulating and floatingpotential obstacles are explored with 2D electrostatic particleincell simulations, using a physical ion to electron mass ratio. Driftenergization is discovered to give rise to modifications to the electron velocitydistribution, including a slopereversal, providing a novel drive of forewake instability. The slopereversal is present at certain locations in all the simulations, and appears to be quite robustly generated. Wings of enhanced electron density are observed in some of the simulations, also associated with driftenergization. In the simulations with a floatingpotential obstacle, the specific potential structure behind that obstacle allows fast electrons to cross the wake, giving rise to a more traditional shadowingdriven twostream instability. Fluctuations associated with such instability are observed in the simulations, but this instabilitymechanism is expected to be more sensitive to the plasma parameters than that associated with the slopereversal.

Statically screened ion potential and Bohm potential in a quantum plasma
View Description Hide DescriptionThe effective potential Φ of a classical ion in a weakly correlated quantum plasma in thermodynamic equilibrium at finite temperature is well described by the random phase approximation screened Coulomb potential. Additionally, collision effects can be included via a relaxation time ansatz (Mermin dielectric function). These potentials are used to study the quality of various statically screened potentials that were recently proposed by Shukla and Eliasson (SE) [Phys. Rev. Lett. 108, 165007 (2012)], AkbariMoghanjoughi (AM) [Phys. Plasmas 22, 022103 (2015)], and Stanton and Murillo (SM) [Phys. Rev. E 91, 033104 (2015)] starting from quantum hydrodynamic (QHD) theory. Our analysis reveals that the SE potential is qualitatively different from the full potential, whereas the SM potential (at any temperature) and the AM potential (at zero temperature) are significantly more accurate. This confirms the correctness of the recently derived [Michta et al., Contrib. Plasma Phys. 55, 437 (2015)] prefactor 1/9 in front of the Bohm term of QHD for fermions.

The critical intensity of Alfvén waves for electroncyclotron maser to favor the Omode emission
View Description Hide DescriptionThe presence of Alfvén waves (AWs) has been found to significantly affect electroncyclotron maser (ECM), which is a powerful emission mechanism in astrophysical plasmas. A conventional ECM driven by powerlaw electrons with a lowerenergy cutoff generally prefers Xmode emission to Omode. In particular, the ECM possibly favors Omode because it is dependent on the relative intensity of the present AWs, , where Bw and B 0 are the field strength of AWs and the ambient magnetic field, respectively. This paper, for the first time, quantitatively investigates the critical relative intensity of AWs, above which the ECM becomes to favor the Omode emission. It is found that the critical intensity depends on velocity distribution function features of energetic electrons, as well as on ambient plasma parameters. In principle, the critical intensity is in the order of ξ ∼ 10^{−2} for powerlaw electrons with a lower energy cutoff, when the fundamental wave in Xmode is suppressed. Moreover, the incorporation of the loss cone distribution caused by the magnetic mirror effect can considerably lower the critical intensity of AWs. This study contributes to the understanding of solar type I radio storm emissions, which are dominated by the Omode.
 Nonlinear Phenomena, Turbulence, Transport

Alfven solitary waves in nonrelativistic, relativistic, and ultrarelativistic degenerate quantum plasma
View Description Hide DescriptionNonlinear circularly polarized Alfvén waves are studied in magnetized nonrelativistic, relativistic, and ultrarelativistic degenerate Fermi plasmas. Using the quantum hydrodynamic model, Zakharov equations are derived and the Sagdeev potential approach is used to investigate the properties of the electromagnetic solitary structures. It is seen that the amplitude increases with the increase of electron density in the relativistic and ultrarelativistic cases but decreases in the nonrelativistic case. Both right and left handed waves are considered, and it is seen that supersonic, subsonic, and super and subAlfvénic solitary structures are obtained for different polarizations and under different relativistic regimes.

Electronscale dissipative electrostatic solitons in multispecies plasmas
View Description Hide DescriptionThe linear and nonlinear properties of smallamplitude electronacoustic solitary waves are investigated via the fluid dynamical approach. A threecomponent plasma is considered, composed of hot electrons, cold electrons, and ions (considered stationary at the scale of interest). A dissipative (wave damping) effect is assumed due to electronneutral collisions. The background (hot) electrons are characterized by an energetic (excessively superthermal) population and are thus modeled via a κtype nonthermal distribution. The linear characteristics of electronacoustic excitations are discussed, for different values of the plasma parameters (superthermality index κ and cold versus hot electron population concentration β). Large wavelengths (beyond a threshold value) are shown to be overdamped. The reductive perturbation technique is used to derive a dissipative Korteweg deVries (KdV) equation for smallamplitude electrostatic potential disturbances. These are expressed by exact solutions in the form of dissipative solitary waves, whose dynamics is investigated analytically and numerically. Our results should be useful in elucidating the behavior of space and experimental plasmas characterized by a coexistence of electron populations at different temperatures, where electronneutral collisions are of relevance.

Threedimensional gyrokinetic simulation of the relaxation of a magnetized temperature filament
View Description Hide DescriptionAn electromagnetic, 3D gyrokinetic particle code is used to study the relaxation of a magnetized electron temperature filament embedded in a large, uniform plasma of lower temperature. The study provides insight into the role played by unstable driftAlfvén waves observed in a basic electron heat transport experiment [D. C. Pace et al., Phys. Plasmas 15, 122304 (2008)] in which anomalous crossfield transport has been documented. The simulation exhibits the early growth of temperaturegradientdriven, driftAlfvén fluctuations that closely match the eigenmodes predicted by linear theory. At the onset of saturation, the unstable fluctuations display a spiral spatial pattern, similar to that observed in the laboratory, which causes the rearrangement of the temperature profile. After saturation of the linear instability, the system exhibits a markedly different behavior depending on the inclusion in the computation of modes without variation along the magnetic field, i.e., kz = 0. In their absence, the initial filament evolves into a broadened temperature profile, selfconsistent with undamped, finite amplitude driftAlfvén waves. But the inclusion of kz = 0 modes causes the destruction of the filament and damping of the driftAlfvén modes leading to a final state consisting of undamped convective cells and multiple, smallerscale filaments.
 Magnetically Confined Plasmas, Heating, Confinement

Gyrokinetic study of the impact of the electron to ion heating ratio on the turbulent diffusion of highly charged impurities
View Description Hide DescriptionA gyrokinetic study based on numerical and analytical calculations is presented, which computes the dependence of the turbulent diffusion of highly charged impurities on the ratio of the electron to the ion heat flux of the plasma. Nonlinear simulations show that the size of the turbulent diffusion of heavy impurities can vary by one order of magnitude with fixed total heat flux and is an extremely sensitive function of the electron to ion heat flux ratio. Numerical linear calculations are found to reproduce the nonlinear results. Thereby, a quasilinear analytical approach is used to explain the origin of this dependence.

Transport theory in the collisional boundary layer regime for finite aspect ratio tokamaks with broken symmetry
View Description Hide DescriptionTransport theory in the collisional boundary layer regime for tokamaks with broken symmetry is extended to include the effects of the finite aspect ratio and finite plasma . Here, is the ratio of the plasma thermal pressure to the magnetic field pressure. Transport fluxes are calculated on the perturbed magnetic surface where plasma pressure is constant. The extension of the theory to finite aspect ratio tokamaks is made possible because the perturbed particle distribution that contributes to the transport fluxes in the collisional boundary layer regime is localized in the pitch angle space. Invoking the fluxforce relation, the transport fluxes can be used for modeling toroidal plasma flow in tokamaks.
 Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter

Instability growth for magnetized liner inertial fusion seeded by electrothermal, electrochoric, and material strength effects
View Description Hide DescriptionA critical limitation of magnetically imploded systems such as magnetized liner inertial fusion (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is the magnetoRayleighTaylor (MRT) instability which primarily disrupts the outer surface of the liner. MagLIFrelevant experiments have showed large amplitude multimode MRT instability growth growing from surface roughness [McBride et al., Phys. Rev. Lett. 109, 135004 (2012)], which is only reproduced by 3D simulations using our MHD code Gorgon when an artificially azimuthally correlated initialisation is added. We have shown that the missing azimuthal correlation could be provided by a combination of the electrothermal instability (ETI) and an “electrochoric” instability (ECI); describing, respectively, the tendency of current to correlate azimuthally early in time due to temperature dependent Ohmic heating; and an amplification of the ETI driven by density dependent resistivity around vapourisation. We developed and implemented a material strength model in Gorgon to improve simulation of the solid phase of liner implosions which, when applied to simulations exhibiting the ETI and ECI, gave a significant increase in wavelength and amplitude. Full circumference simulations of the MRT instability provided a significant improvement on previous randomly initialised results and approached agreement with experiment.
 Radiation: Emission, Absorption, Transport

Polarization of Balmer alpha radiation resulting from H^{+}+H collisions in Debye plasmas
View Description Hide DescriptionThe linear polarization degree of Balmer alpha radiation resulting from collisions in a hot, dense weakly coupled plasma is studied in the energy range by adopting the DebyeHückel potential to represent the screened interaction between charged plasma particles. Due to the energy splitting of nl hydrogen states in the short range DebyeHuckel potential, the Balmer alpha radiation contains three components corresponding to 3s2p, 3p2s, and 3d2p transitions, of which only the last two can be linearly polarized. For calculation of 3lm excitation and electron capture cross sections, the twocenter expansion atomic orbital close coupling method is used for a number of Debye screening lengths. The effects of plasma screening on the 3lm cross sections are manifested in significant changes of their magnitudes and energy behavior with respect to the ones in the unscreened case, producing significant changes in the polarization degree of Balmer 3p2s and 3d2p lines.