Volume 23, Issue 8, August 2016
 LETTERS


Fluid moments of the nonlinear Landau collision operator
View Description Hide DescriptionAn important problem in plasma physics is the lack of an accurate and complete description of Coulomb collisions in associated fluid models. To shed light on the problem, this Letter introduces an integral identity involving the multivariate Hermite tensor polynomials and presents a method for computing exact expressions for the fluid moments of the nonlinear Landau collision operator. The proposed methodology provides a systematic and rigorous means of extending the validity of fluid models that have an underlying inversesquare force particle dynamics to arbitrary collisionality and flow.

Improved operation of a microwave pulse compressor with a lasertriggered highpressure gas plasma switch
View Description Hide DescriptionThe influence of laser beam parameters on the output pulses of a resonant microwave compressor with a lasertriggered plasma switch was investigated. The Sband compressor, consisting of a rectangular waveguidebased cavity and Hplane waveguide tee with a shorted side arm, was filled with pressurized dry air and pumped by 1.8μslong microwave pulses of up to 450 kW power. A Nd:YAG laser was used to ignite the gas discharge in the tee side arm for output pulse extraction. The laser beam (at 213 nm or 532 nm) was directed along the RF electric field lines. It was found that the compressor operated most effectively when the laser beam was focused at the center of the switch waveguide crosssection. In this case, the power extraction efficiency reached ∼47% at an output power of ∼14 MW, while when the laser beam was not focused the maximal extraction efficiency was only ∼20% at ∼6 MW output power. Focusing the laser beam resulted also in a dramatic decrease (down to <1 ns) in the delay of the output pulses' appearance with respect to the time of the beam's entrance into the switch, and the jitter of the output pulses' appearance was minimized. In addition, the quality of the output pulses' waveform was significantly improved.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Plasma electron hole kinematics. I. Momentum conservation
View Description Hide DescriptionWe analyse the kinematic properties of a plasma electron hole: a nonlinear selfsustained localized positive electric potential perturbation, trapping electrons, which behaves as a coherent entity. When a hole accelerates or grows in depth, ion and electron plasma momentum is changed both within the hole and outside, by an energization process we call jetting. We present a comprehensive analytic calculation of the momentum changes of an isolated general onedimensional hole. The conservation of the total momentum gives the hole's kinematics, determining its velocity evolution. Our results explain many features of the behavior of hole speed observed in numerical simulations, including selfacceleration at formation, and hole pushing and trapping by ion streams.

Plasma electron hole kinematics. II. Hole tracking ParticleInCell simulation
View Description Hide DescriptionThe kinematics of a 1D electron hole is studied using a novel ParticleInCell simulation code. A hole tracking technique enables us to follow the trajectory of a fastmoving solitary hole and study quantitatively hole acceleration and coupling to ions. We observe a transient at the initial stage of hole formation when the hole accelerates to several times the coldion sound speed. Artificially imposing slow ion speed changes on a fully formed hole causes its velocity to change even when the ion stream speed in the hole frame greatly exceeds the ion thermal speed, so there are no reflected ions. The behavior that we observe in numerical simulations agrees very well with our analytic theory of hole momentum conservation and the effects of “jetting.”

Multiregion relaxed Hall magnetohydrodynamics with flow
View Description Hide DescriptionThe recent formulations of multiregion relaxed magnetohydrodynamics (MRxMHD) have generalized the famous WoltjerTaylor states by incorporating a collection of “ideal barriers” that prevent global relaxation and flow. In this paper, we generalize MRxMHD with flow to include Hall effects, and thereby obtain the partially relaxed counterparts of the famous double Beltrami states as a special subset. The physical and mathematical consequences arising from the introduction of the Hall term are also presented. We demonstrate that our results (in the ideal MHD limit) constitute an important subset of ideal MHD equilibria, and we compare our approach against other variational principles proposed for deriving the partially relaxed states.

Sharpfront wave of strong magnetic field diffusion in solid metal
View Description Hide DescriptionWhen a strong magnetic field diffuses into a solid metal, if the metal's resistance possesses an abrupt rise at some critical temperature and the magnetic field strength is above some critical value, the magnetic field will diffuse into the metal in the form of a sharpfront wave. Formulas for the critical conditions under which a sharpfront magnetic diffusion wave emerges and a formula for the wavefront velocity are derived in this work.

FokkerPlanck equation in the presence of a uniform magnetic field
View Description Hide DescriptionThe FokkerPlanck equation in the presence of a uniform magnetic field is derived which has the same form as the case of no magnetic field but with different FokkerPlanck coefficients. The coefficients are calculated explicitly within the binary collision model, which are free from infinite sums of Bessel functions. They can be used to investigate relaxation and transport phenomena conveniently. The kinetic equation is also manipulated into the Landau form from which it is straightforward to compare with previous results and prove the conservation laws.

Excitation of nonlinear ion acoustic waves in CH plasmas
View Description Hide DescriptionExcitation of nonlinear ion acoustic wave (IAW) by an external electric field is demonstrated by Vlasov simulation. The frequency calculated by the dispersion relation with no damping is verified much closer to the resonance frequency of the smallamplitude nonlinear IAW than that calculated by the linear dispersion relation. When the wave number increases, the linear Landau damping of the fast mode (its phase velocity is greater than any ion's thermal velocity) increases obviously in the region of in which the fast mode is weakly damped mode. As a result, the deviation between the frequency calculated by the linear dispersion relation and that by the dispersion relation with no damping becomes larger with increasing. When is not large, such as , the nonlinear IAW can be excited by the driver with the linear frequency of the modes. However, when is large, such as , the linear frequency cannot be applied to exciting the nonlinear IAW, while the frequency calculated by the dispersion relation with no damping can be applied to exciting the nonlinear IAW.

Particleincell simulation study of the scaling of asymmetric magnetic reconnection with inplane flow shear
View Description Hide DescriptionWe investigate magnetic reconnection in systems simultaneously containing asymmetric (antiparallel) magnetic fields, asymmetric plasma densities and temperatures, and arbitrary inplane bulk flow of plasma in the upstream regions. Such configurations are common in the highlatitudes of Earth's magnetopause and in tokamaks. We investigate the convection speed of the Xline, the scaling of the reconnection rate, and the condition for which the flow suppresses reconnection as a function of upstream flow speeds. We use twodimensional particleincell simulations to capture the mixing of plasma in the outflow regions better than is possible in fluid modeling. We perform simulations with asymmetric magnetic fields, simulations with asymmetric densities, and simulations with magnetopauselike parameters where both are asymmetric. For flow speeds below the predicted cutoff velocity, we find good scaling agreement with the theory presented in Doss et al. [J. Geophys. Res. 120, 7748 (2015)]. Applications to planetary magnetospheres, tokamaks, and the solar wind are discussed.

Particleincell simulation of twodimensional electron velocity shear driven instability in relativistic domain
View Description Hide DescriptionWe carry out particleincell simulations to study the instabilities associated with a 2D sheared electron flow configuration against a neutralizing background of ions. Both weak and strong relativistic flow velocities are considered. In the weakly relativistic case, we observe the development of electromagnetic KelvinHelmholtz instability with similar characteristics as that predicted by the electron Magnetohydrodynamic (EMHD) model. On the contrary, in a strong relativistic case, the compressibility effects of electron fluid dominate and introduce upper hybrid electrostatic oscillations transverse to the flow which are very distinct from EMHD fluid behavior. In the nonlinear regime, both weak and strong relativistic cases lead to turbulence with broad power law spectrum.

Investigation of nonextensivity trapped electrons effect on the solitary ionacoustic wave using fractional Schamel equation
View Description Hide DescriptionIonacoustic (IA) solitary wave propagation is investigated by solving the fractional Schamel equation (FSE) in a homogenous system of unmagnetized plasma. This plasma consists of the nonextensive trapped electrons and cold fluid ions. The effects of the nonextensive qparameter, electron trapping, and fractional parameter have been studied. The FSE is derived by using the semiinverse and Agrawal's methods. The analytical results show that an increase in the amount of electron trapping and nonextensive qparameter increases the soliton ionacoustic amplitude in agreement with the previously obtained results. However, it is viceversa for the fractional parameter. This feature leads to the fact that the fractional parameter may be used to increase the IA soliton amplitude instead of increasing electron trapping and nonextensive parameters.

A study on the steadystate solutions of a relativistic Bursian diode in the presence of a transverse magnetic field
View Description Hide DescriptionA comprehensive study on the steady states of a planar vacuum diode driven by a cold relativistic electron beam in the presence of an external transverse magnetic field is presented. The regimes, where no electrons are turned around by the external magnetic field and where they are reflected back to the emitter by the magnetic field, are both considered in a generalized way. The problem is solved by two methods: with the Euler and the Lagrange formulation. Taking nonrelativistic limit, the solutions are compared with the similar ones which were obtained for the Bursian diode with a nonrelativistic electron beam in previous work [Pramanik et al., Phys. Plasmas 22, 112108 (2015)]. It is shown that, at a moderate value of the relativistic factor of the injected beam, the region of the ambiguous solutions located to the right of the SCL bifurcation point (space charge limit) in the nonrelativistic regime disappears. In addition, the dependencies of the characteristic bifurcation points and the transmitted current on the Larmor frequency as well as on the relativistic factor are explored.

PIC simulation of compressive and rarefactive dust ionacoustic solitary waves
View Description Hide DescriptionThe nonlinear propagations of dust ionacoustic solitary waves in a collisionless fourcomponent unmagnetized dusty plasma system containing nonextensive electrons, inertial negative ions, Maxwellian positive ions, and negatively charged static dust grains have been investigated by the particleincell method. By comparing the simulation results with those obtained from the traditional reductive perturbation method, it is observed that the rarefactive KdV solitons propagate stably at a low amplitude, and when the amplitude is increased, the prime wave form evolves and then gradually breaks into several small amplitude solitary waves near the tail of soliton structure. The compressive KdV solitons propagate unstably and oscillation arises near the tail of soliton structure. The finite amplitude rarefactive and compressive Gardner solitons seem to propagate stably.

Development of core ion temperature gradients and edge sheared flows in a helicon plasma device investigated by laser induced fluorescence measurements
View Description Hide DescriptionWe report experimental observation of ion heating and subsequent development of a prominent ion temperature gradient in the core of a linear magnetized plasma device, and the controlled shear decorrelation experiment. Simultaneously, we also observe the development of strong sheared flows at the edge of the device. Both the ion temperature and the azimuthal velocity profiles are quite flat at low magnetic fields. As the magnetic field is increased, the core ion temperature increases, producing centrally peaked ion temperature profiles and therefore strong radial gradients in the ion temperature. Similarly, we observe the development of large azimuthal flows at the edge, with increasing magnetic field, leading to strong radially sheared plasma flows. The ion velocities and temperatures are derived from laser induced fluorescence measurements of Doppler resolved velocity distribution functions of argon ions. These features are consistent with the previous observations of simultaneously existing radially separated multiple plasma instabilities that exhibit complex plasma dynamics in a very simple plasma system. The ion temperature gradients in the core and the radially sheared azimuthal velocities at the edge point to mechanisms that can drive the multiple plasma instabilities, that were reported earlier.

Effects of alphaproton drift velocity on alpha particle firehose instability
View Description Hide DescriptionIn situ measurements have shown that the lessabundant alpha particles are characterized by temperature anisotropy which could drive the anisotropydriven kinetic instabilities in the solar wind. In the collisionless plasma, the differential alphaproton flow velocity usually has a nonzero value of the order of the local Alfvén velocity. The presence of such differential flow may affect the properties of dispersion relations for anisotropydriven instabilities. Based upon linear Vlasov dispersion theory in a homogeneous plasma, the present study investigates the effects of the alphaproton drift velocity on the parallel and oblique firehose instabilities driven by an excessive parallel temperature anisotropy of alpha particles, where the parallel and oblique represent directions of fluctuation propagation relative to the background magnetic field. It is found that for oblique firehose mode as well as parallel mode, the dispersion properties are affected by the presence of the alphaproton drift velocity, which in turn results in the increase of the maximum growth rates as Vd increases and consequently leads to the modification of the marginal stability conditions in the parameter space . We discuss the relevance of our results to the measured temperature anisotropy of alpha particles in the solar wind context.

Quantum mechanical expansion of variance of a particle in a weakly nonuniform electric and magnetic field
View Description Hide DescriptionWe have solved the Heisenberg equation of motion for the time evolution of the position and momentum operators for a nonrelativistic spinless charged particle in the presence of a weakly nonuniform electric and magnetic field. It is shown that the drift velocity operator obtained in this study agrees with the classical counterpart, and that, using the time dependent operators, the variances in position and momentum grow with time. The expansion rate of variance in position and momentum are dependent on the magnetic gradient scale length, however, independent of the electric gradient scale length. In the presence of a weakly nonuniform electric and magnetic field, the theoretical expansion rates of variance expansion are in good agreement with the numerical analysis. It is analytically shown that the variance in position reaches the square of the interparticle separation, which is the characteristic time much shorter than the proton collision time of plasma fusion. After this time, the wavefunctions of the neighboring particles would overlap, as a result, the conventional classical analysis may lose its validity. The broad distribution of individual particle in space means that their Coulomb interactions with other particles become weaker than that expected in classical mechanics.

Optical Tamm states in onedimensional superconducting photonic crystal
View Description Hide DescriptionIn this study, we investigate localized and resonant optical waves associated with a semiinfinite superlattice made out of superconductordielectric bilayers and terminated with a cap layer. Both transverse electric and transverse magnetic waves are considered. These surface modes are analogous to the socalled Tamm states associated with electronic states found at the surface of materials. The surface guided modes induced by the cap layer strongly depend on whether the superlattice ends with a superconductor or a dielectric layer, the thickness of the surface layer, the temperature of the superconductor layer as well as on the polarization of the waves. Different kinds of surface modes are found and their properties examined. These structures can be used to realize the highly sensitive photonic crystal sensors.

Nonlinear absorption of short intense laser pulse in multispecies plasma
View Description Hide DescriptionIn the present paper, the detailed investigation concerning the effect of inclusion of heavy negative ions into the finite background plasma on the laser absorption has been carried out by employing particleincell simulation method. For this purpose, in this configuration, the laser energy absorption relying on the nonlinear phenomena such as phasemixing, wavebreaking, and scattering has been studied in the RamanBrillouin regime. It is shown that the inclusion of heavy negative ions suppresses the scattering while increases the phasemixing time. Moreover, it is illustrated that this inclusion can increase the laser absorption in finite plasma environment, after saturation. The obtained results are expected to be relevant to the experiments on the mass spectrometry with laser desorption techniques as well as on the laserplasma interaction with application to particles acceleration.

Pulsating jetlike structures in magnetized plasma
View Description Hide DescriptionThe formation of pulsating jetlike structures has been studied in the scope of the nonhydrostatic model of a magnetized plasma with horizontally nonuniform density. We discuss two mechanisms which are capable of stopping the gravitational spreading appearing to grace the RayleighTaylor instability and to lead to the formation of stationary or oscillating localized structures. One of them is caused by the Coriolis effect in the rotating frames, and another is connected with the Lorentz effect for magnetized fluids. Magnetized jets/drops with a positive buoyancy must oscillate in transversal size and can manifest themselves as “radio pulsars.” The estimates of their frequencies are made for conditions typical for the neutron star's ocean.

Modified electron acoustic field and energy applied to observation data
View Description Hide DescriptionImproved electrostatic acoustic field and energy have been debated in vortex trapped hot electrons and fluid of cold electrons with pressure term plasmas. The perturbed higherorder modifiedKortewegde Vries equation (PhomKdV) has been worked out. The effect of trapping and electron temperatures on the electrofield and energy properties in auroral plasmas has been inspected.