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October 2009

Volume 16, Issue 10,  Articles (10xxxx)

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Cover image from Takanobu Amano and Masahiro Hoshino, Phys. Plasmas 16, 102901 (2009).

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LETTERS

On the effect of rear-surface dielectric coatings on laser-driven proton acceleration

S. Betti, C. A. Cecchetti, E. Förster, A. Gamucci, A. Giulietti, D. Giulietti, T. Kämpfer, P. Köster, L. Labate, T. Levato, A. Lübcke, I. Uschmann, F. Zamponi, and L. A. Gizzi

Phys. Plasmas 16, 100701 (2009) (4 pages)

Online Publication Date: 29 October 2009

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Laser-driven ion acceleration has been experimentally investigated by irradiating, with tightly focused femtosecond laser pulses at 5×1019  W/cm2, thin metal foils, which have been back-coated with a µm thick dielectric layer. The observation we report shows the production of MeV proton bunches with an unexpected highly uniform spatial cross section.
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52.38.Kd, 41.75.Jv, 52.25.Fi

Magnetic multipole induced zero-rotation frequency bounce-resonant loss in a Penning–Malmberg trap used for antihydrogen trapping

G. B. Andresen et al. (ALPHA Collaboration)

Phys. Plasmas 16, 100702 (2009) (4 pages)

Online Publication Date: 30 October 2009

Full Text: PDF (471 kB)

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In many antihydrogen trapping schemes, antiprotons held in a short-well Penning–Malmberg trap are released into a longer well. This process necessarily causes the bounce-averaged rotation frequency [overline Omega ]r of the antiprotons around the trap axis to pass through zero. In the presence of a transverse magnetic multipole, experiments and simulations show that many antiprotons (over 30% in some cases) can be lost to a hitherto unidentified bounce-resonant process when [overline Omega ]r is close to zero.
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52.55.Lf
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ARTICLES

Basic Plasma Phenomena, Waves, Instabilities

Gyrokinetic deltaf particle simulations of toroidicity-induced Alfvén eigenmode

Jianying Lang, Yang Chen, Scott E. Parker, and Guo-Yong Fu

Phys. Plasmas 16, 102101 (2009) (11 pages)

Online Publication Date: 7 October 2009

Full Text: PDF (486 kB)

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Gyrokinetic deltaf particle simulation is used to investigate toroidicity-induced Alfvén eigenmodes (TAEs). Both thermal ions and energetic particles are fully kinetic, but a reduced fluid model is used for the electrons. Simulation of a single n=2 global TAE is carefully analyzed and benchmarked with an eigenmode analysis, and a very good agreement is achieved in both mode structure and mode frequency. The instability of the mode in the presence of energetic particles is demonstrated. In particular, gyrokinetic simulations demonstrate the kinetic damping effect of thermal ions, where the finite radial structure of kinetic Alfvén waves is well resolved and the damping rate is compared to and found to agree well with analytical theory.
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52.65.Tt, 52.35.Hr, 52.55.Fa, 52.35.Py

Pitch-angle diffusion of ions via nonresonant interaction with Alfvénic turbulence

P. H. Yoon, C. B. Wang, and C. S. Wu

Phys. Plasmas 16, 102102 (2009) (6 pages)

Online Publication Date: 8 October 2009

Full Text: PDF (209 kB)

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The present discussion revisits the problem of nonresonant heating of ions by Alfvénic turbulence. It is shown that in the limit of weak Alfvénic turbulence it is appropriate to describe the nonresonant heating of protons as perpendicular pseudoheating. However, in a more general situation it is demonstrated that the more appropriate view of the nonresonant heating process is the pitch-angle scattering in the wave frame. The purpose of this paper is to generalize the earlier theory to the case in which the energy density of the turbulent Alfvén waves is not necessarily very low. For weakly turbulent situation the present analysis confirms the earlier finding by Wu and Yoon [Phys. Rev. Lett. 99, 075001 (2007)], according to whom the nonresonant Alfvén wave heating is described as leading to perpendicular pseudoheating of the protons. However, for more general situation the present paper demonstrates that pitch-angle scattering plays the principal role in the Alfvén wave pseudoheating process, and thereby shows that the perpendicular heating discussed by Wu and Yoon is kinetic in nature, not attributable to fluid motion.
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52.35.Ra, 52.35.Mw

Bandgap characteristics of one-dimensional plasma photonic crystal

Yan Yin, Han Xu, M. Y. Yu, Yan-yun Ma, Hong-bin Zhuo, Cheng-lin Tian, and Fu-qiu Shao

Phys. Plasmas 16, 102103 (2009) (5 pages)

Online Publication Date: 8 October 2009

Full Text: PDF (636 kB)

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When two pump laser pulses intersect in an underdense plasma, plasma Bragg grating (PBG) is induced by the slow-varying ponderomotive force [Z. M. Sheng et al., Appl. Phys. B: Lasers Opt. 77, 673 (2003)]. Such a PBG can be considered as a one-dimensional (1D) plasma photonic crystal (PPC). Here the bandgap characteristic of 1D PPC composed of plasma layers of different densities is investigated theoretically and numerically. It is found that when the maximum density is lower than the critical density of the pump laser, there is only one normal-incidence bandgap. When the maximum density is higher than the critical density of the pump laser, high-order bandgaps are found. The theoretical results are verified by 1D particle-in-cell simulations.
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52.38.-r, 42.70.Qs, 52.65.Rr, 42.79.Dj

Weakly nonlinear ablative Rayleigh–Taylor instability at preheated ablation front

Zhengfeng Fan, Jisheng Luo, and Wenhua Ye

Phys. Plasmas 16, 102104 (2009) (9 pages)

Online Publication Date: 9 October 2009

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Stuart's weakly nonlinear theory is generalized to study single-mode ablative Rayleigh–Taylor instability (ARTI) at a broad ablation front caused by preheating. The thickness effect of the ablation front is considered and the spatial amplitude distributions of density, temperature, and velocity for harmonic modes are obtained in the present model. It is confirmed that the modified Lindl formula [W. H. Ye et al., Phys. Rev. E 65, 057401 (2002)] is valid for predicting the linear growth rate when the ablation front is broad. It is shown by the present model that the mass ablation of the shell is enhanced obviously due to the generation of harmonics while the harmonics' effect on the mass asymmetry of the shell is weaker than the expectation given by the classical theory. It is also indicated by the present model that ARTI is stabilized by the nonlinear correction for all modes. This conclusion is physical and different from the sharp boundary model where ARTI is enhanced by the nonlinear correction for the short wavelength case. The reason for this difference is due to the thickness effect of the ablation front.
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52.35.Py, 52.57.Fg

Particle-in-cell and hypernetted chain models of two-component, two-temperature coupled classical plasmas

D. V. Rose, T. C. Genoni, D. R. Welch, R. E. Clark, R. B. Campbell, T. A. Mehlhorn, and D. G. Flicker

Phys. Plasmas 16, 102105 (2009) (6 pages)

Online Publication Date: 15 October 2009

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Three-dimensional simulations of moderately to strongly coupled electron-ion and multicomponent classical plasmas using the particle-in-cell method are presented. The simulations resolve sub-Debye-length interparticle spacing to accurately model the dynamics of these systems. We consider realistic mass ratios and quasiequilibrium conditions with different component temperatures which are relevant on short time scales. The simulation results are in very good agreement with classical hypernetted chain calculations for dense electron-ion and ion-ion plasmas. Our results demonstrate the feasibility and utility of large-scale particle-in-cell simulations for the modeling and analysis of multicomponent moderately and strongly coupled plasmas.
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52.27.Gr, 52.27.Aj, 52.65.Rr, 52.65.Yy

A simple, analytical model of collisionless magnetic reconnection in a pair plasma

Michael Hesse, Seiji Zenitani, Masha Kuznetsova, and Alex Klimas

Phys. Plasmas 16, 102106 (2009) (10 pages)

Online Publication Date: 15 October 2009

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A set of conservation equations is utilized to derive balance equations in the reconnection diffusion region of a symmetric pair plasma. The reconnection electric field is assumed to have the function to maintain the current density in the diffusion region and to impart thermal energy to the plasma by means of quasiviscous dissipation. Using these assumptions it is possible to derive a simple set of equations for diffusion region parameters in dependence on inflow conditions and on plasma compressibility. These equations are solved by means of a simple, iterative procedure. The solutions show expected features such as dominance of enthalpy flux in the reconnection outflow, as well as combination of adiabatic and quasiviscous heating. Furthermore, the model predicts a maximum reconnection electric field of E*=0.4, normalized to the parameters at the inflow edge of the diffusion region.
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52.30.Cv, 52.25.Fi, 52.25.Kn

Single-fluid stability of stationary plasma equilibria with velocity shear and magnetic shear

Akira Miura

Phys. Plasmas 16, 102107 (2009) (11 pages)

Online Publication Date: 15 October 2009

Full Text: PDF (208 kB)

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By using incompressible single-fluid equations with a generalized Ohm's law neglecting the electron inertia, a linear eigenmode equation for a magnetic field perturbation is derived for stationary equilibria in a slab geometry with velocity and magnetic shears. The general eigenmode equation contains a fourth-order derivative of the perturbation in the highest order and contains Alfvén and whistler mode components for a homogeneous plasma. The ratio of the characteristic ion inertia length to the characteristic inhomogeneity scale length is chosen as a small parameter for expansion. Neglecting whistler mode in the lowest order, the eigenmode equation becomes a second-order differential equation similar to the ideal magnetohydrodynamic eigenmode equation except for the fact that the unperturbed perpendicular velocity contains both electric and ion diamagnetic drifts. A sufficient condition for stability against the Kelvin–Helmholtz instability driven by shear in the ion diamagnetic drift velocity is derived and then applied to tokamaks.
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52.35.Bj, 52.35.Kt, 52.35.Qz, 52.55.Fa

Full Coulomb collision operator in the moment expansion

Jeong-Young Ji and Eric D. Held

Phys. Plasmas 16, 102108 (2009) (8 pages)

Online Publication Date: 19 October 2009

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The full Coulomb collision operator and its moments including nonlinear terms are analytically calculated in the moment expansion. In coupling nonlinear terms, the product formula which expresses a product of two harmonic tensors as a series of single harmonic tensors is derived. The collision operators and moments are written in explicit formulas for arbitrary moments and for arbitrary temperature and mass ratios. These expressions easily reduce to formulas for the small mass-ratio approximation or for like species.
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52.20.-j, 52.25.Dg

Magnetothermal instability of plasmas in a horizontal magnetic field

Haijun Ren, Zhengwei Wu, Jintao Cao, and Paul K. Chu

Phys. Plasmas 16, 102109 (2009) (5 pages)

Online Publication Date: 29 October 2009

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The linear buoyancy instability in a magnetized plasma, generally referred to as magnetothermal instability (MTI), is investigated by considering anisotropic heat conduction. The external magnetic field is assumed to be horizontal and background heat flux is not taken into account. The general dispersion relationship of the convective instability is derived. The growth rate of the MTI in fixed boundary condition is presented and discussed. The effect of density spacial gradient on the MTI is investigated. The magnetic field is shown to suppress the MTI and even quench the instability when the magnetic field is strong enough. Under the standard Wentzel–Kramaers–Brillouin approximation, our results could be simplified to a brief form reported by one previous paper [E. Quataert, Astrophys. J. 673, 758 (2008)].
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52.35.Py, 52.25.Fi, 52.30.Cv

Quantum effects on the dispersion of ion acoustic waves

A. Mushtaq and D. B. Melrose

Phys. Plasmas 16, 102110 (2009) (5 pages)

Online Publication Date: 30 October 2009

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The longitudinal response function for an isotropic, nonrelativistic, thermal plasma with the quantum recoil included exactly is used to generalize the dispersion relation for ion acoustic waves and the absorption coefficient for Landau damping to include the quantum recoil. The results are compared to recent treatment of the dispersion relation derived using a fluid theory with the quantum effects included through the Bohm potential.
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52.25.Dg, 52.35.Dm

Extension of the electron dissipation region in collisionless Hall magnetohydrodynamics reconnection

Brian P. Sullivan, A. Bhattacharjee, and Yi-Min Huang

Phys. Plasmas 16, 102111 (2009) (11 pages)

Online Publication Date: 30 October 2009

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This paper presents Sweet–Parker type scaling arguments in the context of hyper-resistive Hall magnetohydrodynamics. Numerical experiments suggest that both cusplike and modestly more extended geometries are realizable. However, the length of the electron dissipation region, which is taken as a parameter by several recent studies, is found to depend explicitly on the level of hyper-resistivity. Furthermore, although hyper-resistivity can produce more extended electron dissipation regions, the length of the region remains smaller than one ion skin depth for the largest values of hyper-resistivity considered here, significantly shorter than current sheets seen in many recent kinetic studies. The length of the electron dissipation region is found to depend on electron inertia as well, scaling like (me/mi)3/8. However, the thickness of the region appears to scale similarly, so that the aspect ratio is at most very weakly dependent on (me/mi).
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52.30.Cv, 52.35.Vd

Nonlinear Phenomena, Turbulence, Transport

Nonlocal transport model in equilibrium two-component plasmas

Zhen Zheng, W. Rozmus, V. Yu. Bychenkov, A. V. Brantov, and C. E. Capjack

Phys. Plasmas 16, 102301 (2009) (18 pages)

Online Publication Date: 2 October 2009

Full Text: PDF (684 kB)

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The full set of linearized Fokker–Planck kinetic equations with Landau collision terms have been solved as an initial-value problem for equilibrium electron-ion plasmas. This work is a generalization of the nonlocal transport theory by Bychenkov et al. [Phys. Rev. Lett. 75, 4405 (1995)] to both electron and ion components with frequency-dependent responses. Transport closure relations have been obtained for the complete set of electron and ion fluid equations which are valid over the full range of particle collisionality and for an arbitrary ionic charge. The well-known limits of collisionless and strongly collisional plasma transport theory have been recovered. Practical fits based on the numerical calculations have been introduced for nonlocal and frequency-dependent ion transport coefficients.
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52.25.Fi, 52.65.Ff, 52.25.Dg, 52.20.Hv, 52.20.Fs

Dressed electrostatic solitary excitations in three component pair-plasmas: Application in isothermal pair-plasma with stationary ions

A. Esfandyari-Kalejahi, M. Akbari-Moghanjoughi, and B. Haddadpour-Khiaban

Phys. Plasmas 16, 102302 (2009) (9 pages)

Online Publication Date: 9 October 2009

Full Text: PDF (358 kB)

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In this work electrostatic solitary waves in a three component pair-plasma consisting of hot isothermal electrons (or negative fullerene ions), positrons (or positive fullerene ions), and stationary positive ions (say, dust particulates) are studied. Using reductive perturbation method, plasma fluid equations are reduced to a Korteweg–de Vries (KdV) equation. Considering the higher-order nonlinearity, a linear inhomogeneous equation is derived, and the stationary solutions of these coupled equations are achieved by applying the renormalization procedure of Kodama–Taniuti. It is observed that in the linear approximation and applying Fourier analysis, two electrostatic modes, namely, upper or optical and lower or acoustic modes, are present. However, the application of reductive perturbation technique confirms that only acoustic-electrostatic mode can propagate in such plasma as KdV soliton, the amplitude and width of which are studied regarding to plasma parameters sigma (positron-to-electron temperature ratio) and delta (stationary cold ions-to-electron density ratio). It is also observed that the higher-order nonlinearity leads to deformation of the soliton structure from bell-shaped to W-shaped depending on the variation in values of the plasma parameters sigma and delta. It is revealed that KdV-type solitary waves cannot propagate in three component pair-plasma when the pair-species temperature is equal.
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52.30.Ex, 52.35.-g, 52.35.Fp, 52.35.Mw

Coupled flows and oscillations in asymmetric rotating plasmas

A. R. Karimov, L. Stenflo, and M. Y. Yu

Phys. Plasmas 16, 102303 (2009) (4 pages)

Online Publication Date: 12 October 2009

Full Text: PDF (159 kB)

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Nonlinear coupling among the radial, axial, and azimuthal flows in an asymmetric cold rotating plasma is considered nonperturbatively. Exact solutions describing an expanding or contracting plasma with oscillations are then obtained. It is shown that despite the flow asymmetry the energy in the radial and axial flow components can be transferred to the azimuthal component but not the vice versa, and that flow oscillations need not be accompanied by density oscillations.
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52.35.Mw, 52.27.Jt, 52.35.We, 94.30.Tz

Orbit-averaged guiding-center Fokker–Planck operator

A. J. Brizard, J. Decker, Y. Peysson, and F.-X. Duthoit

Phys. Plasmas 16, 102304 (2009) (9 pages)

Online Publication Date: 12 October 2009

Full Text: PDF (182 kB)

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A general orbit-averaged guiding-center Fokker–Planck operator suitable for the numerical analysis of transport processes in axisymmetric magnetized plasmas is presented. The orbit-averaged guiding-center operator describes transport processes in a three-dimensional guiding-center invariant space: the orbit-averaged magnetic-flux invariant [overline psi ], the minimum-B pitch-angle coordinate xi0, and the momentum magnitude p.
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52.25.Fi, 52.65.Ff

Nonlinear electrostatic excitations in a weakly relativistic electron-positron-ion rotating magnetoplasma

S. K. El-Labany, W. M. Moslem, and E. I. El-Awady

Phys. Plasmas 16, 102305 (2009) (9 pages)

Online Publication Date: 13 October 2009

Full Text: PDF (3058 kB)

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By using a hydrodynamic model, Zakharov–Kuznetsov (ZK) equation for a rotating magnetoplasma consisting of weakly relativistic electrons and positrons as well as stationary positive ions is derived. The existence condition as well as the pulse-shaped localized solution of the ZK equation are obtained. Numerical analysis reveal that the profile and the phase velocity of the nonlinear pulses are significantly affected by the ion concentration, the plasma streaming speeds, as well as the positron-to-electron temperature ratio. Furthermore, the bending instability of the nonlinear excitations is investigated using the direct-k perturbation expansion method.
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52.35.Mw, 52.35.Fp, 52.35.Py, 52.27.Ny, 52.30.Cv

Streamer-induced transport in electron temperature gradient turbulence

T. Hauff and F. Jenko

Phys. Plasmas 16, 102306 (2009) (6 pages)

Online Publication Date: 15 October 2009

Full Text: PDF (169 kB)

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The question if and how streamers (i.e., radially elongated vortices) can lead to an enhancement of the electron heat transport in electron temperature gradient turbulence is addressed. To this aim, the electrons are treated as passive tracers, and their decorrelation mechanisms with respect to the advecting electrostatic potential are studied. A substantial transport enhancement is found in a wide region of parameter space.
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52.25.Fi, 52.25.Xz, 52.35.Ra, 52.35.We, 52.30.-q

Continuous wavelet transform based time-scale and multifractal analysis of the nonlinear oscillations in a hollow cathode glow discharge plasma

Md. Nurujjaman, Ramesh Narayanan, and A. N. Sekar Iyengar

Phys. Plasmas 16, 102307 (2009) (6 pages)

Online Publication Date: 15 October 2009

Full Text: PDF (2149 kB)

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Continuous wavelet transform (CWT) based time-scale and multifractal analyses have been carried out on the anode glow related nonlinear floating potential fluctuations in a hollow cathode glow discharge plasma. CWT has been used to obtain the contour and ridge plots. Scale shift (or inversely frequency shift), which is a typical nonlinear behavior, has been detected from the undulating contours. From the ridge plots, we have identified the presence of nonlinearity and degree of chaoticity. Using the wavelet transform modulus maxima technique we have obtained the multifractal spectrum for the fluctuations at different discharge voltages and the spectrum was observed to become a monofractal for periodic signals. These multifractal spectra were also used to estimate different quantities such as the correlation and fractal dimension, degree of multifractality, and complexity parameters. These estimations have been found to be consistent with the nonlinear time series analysis.
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52.35.Mw, 52.25.Gj, 52.35.Fp, 52.80.Hc

Runaway electron transport via tokamak microturbulence

T. Hauff and F. Jenko

Phys. Plasmas 16, 102308 (2009) (8 pages)

Online Publication Date: 27 October 2009

Full Text: PDF (416 kB)

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The mechanisms found for the magnetic transport of fast ions in the work of Hauff et al. [Phys. Rev. Lett. 102, 075004 (2009)] are extended to the diffusion of runaway electrons. Due to their smaller mass and larger energy, they behave strongly relativistically, for which reason the scaling laws defined previously have to be modified. It is found that due to these changes, the regime of constant magnetic transport does not exist anymore, but diffusivity scales with E−1 for magnetic transport, or even with E−2 in the case that finite gyroradius effects become important. It is shown that the modified analytical approaches are able to explain the surprisingly small values found in experiments, although it cannot be excluded that possibly other reduction mechanisms are present at the same time.
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52.35.Ra, 52.25.Fi, 52.55.-s, 52.55.Fa

Evolution of Alfvénic wave envelopes in spin-1/2 quantum Hall-magnetohydrodynamic plasmas

A. P. Misra, N. K. Ghosh, and P. K. Shukla

Phys. Plasmas 16, 102309 (2009) (6 pages)

Online Publication Date: 28 October 2009

Full Text: PDF (238 kB)

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The one-dimensional oblique propagation of large amplitude magnetohydrodynamic (MHD) waves in a high-beta quantum Hall-MHD plasma is studied with electron spin-1/2 effects. The plasma beta becomes high by the condition for the nonrelativistic fluid model to be valid and the condition for the collective effects to be important in quantum plasmas. Such a high-beta value is a prerequisite for large perturbations of the perpendicular magnetic field comparable with the longitudinal magnetic field. It is shown that the nonlinear evolution of such waves is described by a derivative nonlinear Schrödinger (DNLS) equation. It is found that the DNLS equation does not depend on the higher order quantum coupling associated with the Bohm potential, rather the pressure such as spin force plays the crucial role. Such an evolution equation is shown to admit spin-modified localized envelope solitons whose width L is reduced by epsilon2/v<sub>B</sub><sup>2</sup> and the amplitude increases with increasing epsilon2/v<sub>B</sub><sup>2</sup> values, where epsilon is the temperature normalized Zeeman energy and v<sub>B</sub><sup>2</sup> is the electron thermal energy normalized by the Alfvén wave energy. Moreover, the MHD waves are found to be modulationally unstable for a wave number exceeding its critical value, which typically depends on epsilon2/v<sub>B</sub><sup>2</sup>. The growth rate of the modulational instability is also investigated. Furthermore, the effect of dissipation due to plasma resistivity is shown to exhibit envelope shocklike structures instead of envelope solitons. The present nonlinear excitations can account for large scale structures in dense astrophysical plasma environments.
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52.35.Bj, 52.30.Cv, 52.35.Sb, 52.35.Py, 52.25.Fi

Magnetically Confined Plasmas, Heating, Confinement

Transport simulations of reversed field pinch plasmas: An integrated model coupling magnetohydrodynamics and impurity physics

I. Predebon, R. Paccagnella, M. E. Puiatti, and M. Valisa

Phys. Plasmas 16, 102501 (2009) (10 pages)

Online Publication Date: 2 October 2009

Full Text: PDF (804 kB)

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An integrated one-dimensional model for the description of plasma transport in the reversed field pinch is presented. The model unifies magnetohydrodynamics and particle/energy transport in a single framework, with the inclusion of impurity physics. Up to now, the applications of the code concern the chaotic regimes of the reversed field pinch, with the intent to clarify the role of the turbulent dynamo for the discharge sustainment and the relative weight of the source/sink terms in the energy balance equations.
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52.25.Fi, 52.25.Gj, 52.25.Vy, 52.25.Ya

Advanced techniques for neoclassical tearing mode control in DIII-D

F. A. G. Volpe, M. E. Austin, R. J. La Haye, J. Lohr, R. Prater, E. J. Strait, and A. S. Welander

Phys. Plasmas 16, 102502 (2009) (14 pages)

Online Publication Date: 6 October 2009

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Two techniques were developed at DIII-D [J. L. Luxon, Nucl. Fusion 42, 64 (2002)] to tackle ITER-specific aspects of neoclassical tearing mode (NTM) control, namely, (1) the relatively small size of the rotating islands, smaller than the electron cyclotron current drive (ECCD) deposition region, and (2) the increased tendency of the islands, compared to present devices, to lock to the wall or to the residual error field, in a position not necessarily accessible to ECCD. Modulated ECCD is known to suppress small islands more efficiently, when “broad,” than continuous ECCD. At DIII-D, a NTM of poloidal/toroidal mode numbers m/n=3/2 was completely stabilized by a new technique where oblique electron cyclotron emission acted at the same time as an indicator of good alignment between ECCD and the island, and as a waveform generator, for modulation in synch and in phase with the island O-point. In another experiment, after locking in an unfavorable position, a 2/1 island was steered by externally generated magnetic perturbations, brought in the view of the gyrotrons and partly stabilized by ECCD in the island O-point. Magnetic perturbations were also used to sustain and control the mode rotation, which has the potential for an easier ECCD modulation.
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52.55.Fa, 52.50.Sw

Properties of microturbulence in toroidal plasmas with reversed magnetic shear

Wenjun Deng and Zhihong Lin

Phys. Plasmas 16, 102503 (2009) (9 pages)

Online Publication Date: 8 October 2009

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Electrostatic drift wave turbulence in tokamak plasmas with reversed magnetic shear is studied using global gyrokinetic particle simulations. The linear eigenmode of the ion temperature gradient (ITG) instability exhibits a mode gap around the minimum safety factor (qmin) region, particularly when qmin is an integer, due to the rarefaction of rational surfaces. The collisionless trapped electron mode (CTEM) instability is suppressed in the negative-shear region due to the reversal of the toroidal precessional drift of trapped electrons. However, after nonlinear saturation, the ITG gap is filled up by the turbulence spreading and the CTEM fluctuation propagates into the stable negative-shear region. The steady state turbulence occupies the whole volume without any identifiable gap or coherent structures of the heat conductivity, perturbed temperature, or zonal flows in the qmin location or the reversed shear region. Our finding indicates that the electrostatic drift wave turbulence itself does not support either linear or nonlinear mechanism for the formation of internal transport barriers in the reversed magnetic shear when qmin crossing an integer.
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52.35.Ra, 52.25.Gj, 52.35.Fp, 52.35.Kt, 52.35.Mw, 52.35.Qz

Interpretation of particle pinches and diffusion coefficients in the edge pedestal of DIII-D H-mode plasmas

W. M. Stacey and R. J. Groebner

Phys. Plasmas 16, 102504 (2009) (6 pages)

Online Publication Date: 15 October 2009

Full Text: PDF (555 kB)

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A procedure is described for evaluating particle pinches to be used in interpreting particle diffusion coefficients from measured density and temperature profiles in the edge pedestal of tokamak plasmas. Application to the interpretation of two DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)]. discharges yields new information about particle pinches and particle diffusion coefficient profiles in the edge pedestal.
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52.25.Fi, 52.35.Qz, 52.55.Ez, 52.55.Fa, 52.40.Hf

Unified theory of resistive and inertial ballooning modes in three-dimensional configurations

T. Rafiq, C. C. Hegna, J. D. Callen, and A. H. Kritz

Phys. Plasmas 16, 102505 (2009) (9 pages)

Online Publication Date: 28 October 2009

Full Text: PDF (174 kB)

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Analytic results for the stability of resistive ballooning modes (RBMs) and electron inertial ballooning modes are obtained using a two-scale analysis. This work generalizes previous calculations used for axisymmetric s−alpha geometry [R. H. Hastie, J. J. Ramos, and F. Porcelli, Phys. Plasmas 10, 4405 (2003)] to general three-dimensional geometry. A unified theory is developed for RBMs and inertial ballooning modes, in which the effects of both ideal magnetohydrodynamic free energy (as measured by the asymptotic matching parameter Delta[prime]) and geodesic curvature drives in the nonideal layer are included in the dispersion relation. This unified theory can be applied to determine the stability of drift-resistive-inertial ballooning modes in the low temperature edge regions of tokamak and stellarator plasmas where steep density gradients exist.
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52.35.Py, 52.30.Cv, 52.55.Fa, 52.55.Hc

Resonant excitation of shear Alfvén perturbations by trapped energetic ions in a tokamak

I. G. Abel, B. N. Breizman, S. E. Sharapov, and JET-EFDA Contributors

Phys. Plasmas 16, 102506 (2009) (14 pages)

Online Publication Date: 30 October 2009

Full Text: PDF (632 kB)

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A new analytic expression is derived for the resonant drive of high n Alfvénic modes by particles accelerated to high energy by ion cyclotron resonance heating. This derivation includes finite orbit effects, and the formalism is completely nonperturbative. The high-n limit is used to calculate the complex particle response integrals along the orbits explicitly. This new theory is applied to downward sweeping Alfvén cascade quasimodes completing the theory of these modes and making testable predictions. These predictions are found to be consistent with experiments carried out on the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)].
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52.35.Bj, 52.50.Qt, 52.50.Sw, 52.55.Fa

Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter

Fast ignition of inertial fusion targets by laser-driven carbon beams

J. J. Honrubia, J. C. Fernández, M. Temporal, B. M. Hegelich, and J. Meyer-ter-Vehn

Phys. Plasmas 16, 102701 (2009) (7 pages)

Online Publication Date: 7 October 2009

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Two-dimensional simulations of ion beam driven fast ignition are presented. Ignition energies of protons with Maxwellian spectrum and carbon ions with quasimonoenergetic and Maxwellian energy distributions are evaluated. The effect of the coronal plasma surrounding the compressed deuterium-tritium is studied for three different fuel density distributions. It is found that quasimonoenergetic ions have better coupling with the compressed deuterium-tritium and substantially lower ignition energies. Comparison of quasimonoenergetic carbon ions and relativistic electrons as ignitor beams shows similar laser energy requirements, provided that a laser to quasimonoenergetic carbon ion conversion efficiency around 10% can be achieved.
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52.38.Kd, 52.65.Ww, 52.57.Kk

Azimuthally correlated ablation between z-pinch wire cores

J. C. Zier, J. D. Douglass, I. C. Blesener, K. S. Blesener, D. A. Chalenski, R. M. Gilgenbach, J. B. Greenly, D. A. Hammer, P. F. Knapp, B. R. Kusse, Y. Y. Lau, R. D. McBride, W. Syed, and E. P. Yu

Phys. Plasmas 16, 102702 (2009) (6 pages)

Online Publication Date: 8 October 2009

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Azimuthally correlated wire core ablation was compared for closely spaced versus widely spaced wires in a 1 MA Z-pinch. X-ray point-projection diagnostics revealed that 240  µm spaced wires exhibited a correlation coefficient approaching unity in both real space and in k-space. This correlated ablation between wires at a fixed axial location is believed to occur due to an enhanced, localized Joule heating. Wires separated by 2.47 mm or greater were uncorrelated in real space, but correlated in k-space, indicating the ablation structure between wires was shifted in phase.
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52.50.Lp, 52.58.Lq, 52.70.La

Study of fast-electron transport in laser-illuminated spherical targets

B. Yaakobi, O. V. Gotchev, R. Betti, and C. Stoeckl

Phys. Plasmas 16, 102703 (2009) (6 pages)

Online Publication Date: 15 October 2009

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The transport and scattering of fast electrons created by the two-plasmon-decay instability are studied by comparing the hard x-ray signal from two identically irradiated targets: a 1-mm-diam solid Cu sphere and a 1-mm-diam solid CH sphere, both coated with a 15  µm layer of CH. Comparing the results with Monte Carlo code simulations shows the role of scattering in the higher-Z Cu target. We find evidence that the fast electrons are created with a wide angular divergence and that higher-energy electrons transmitted through the target are reflected back into the target. Because of scattering, the fast-electron energy deposition (preheat) in Cu is about half that in CH, namely, ~0.15% of the laser energy for Cu as compared with ~0.30% for CH. Embedded high-Z layers in imploding fusion targets, because of the scattering, could provide protection against preheat.
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52.35.Py, 52.80.Qj, 52.25.Fi, 52.65.Pp, 52.57.Fg, 52.50.Jm

Ionospheric, Solar-System, and Astrophysical Plasmas

FREE

Nonlinear evolution of Buneman instability and its implication for electron acceleration in high Mach number collisionless perpendicular shocks

Takanobu Amano and Masahiro Hoshino

Phys. Plasmas 16, 102901 (2009) (13 pages)

Online Publication Date: 1 October 2009

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Nonlinear evolution of the Buneman instability and its application to electron acceleration in collisionless shocks are discussed. Two-dimensional particle-in-cell simulations show that the saturation level of the instability is reduced from one-dimensional simulation results. It is demonstrated that the reduced saturation level is due to the resonant wave-particle interactions with large amplitude obliquely propagating waves. A new estimate for the saturation level is given by considering the interactions with oblique modes. The effects of the large amplitude oblique modes on electron shock surfing acceleration that is mainly controlled by the Buneman instability are also investigated. Two-dimensional particle-in-cell simulations of the shock transition region are performed by adopting a local model with the periodic boundary condition. The results indicate that the presence of oblique modes introduces a stochastic behavior to the trajectories of energetic electrons. The maximum energy is limited by the finite lifetime of the instability in the present periodic model. However, this will not be the case in the realistic shock transition region. The application to realistic shocks with Mach numbers typical of supernova remnants is also discussed.
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52.35.Py, 52.35.Tc, 52.65.Rr, 94.30.cq, 52.35.Mw

Kinetic theory for the ion humps at the foot of the Earth's bow shock

D. Jovanović and V. V. Krasnoselskikh

Phys. Plasmas 16, 102902 (2009) (8 pages)

Online Publication Date: 5 October 2009

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The nonlinear kinetic theory is presented for the ion acoustic perturbations at the foot of the Earth's quasiperpendicular bow shock, that is characterized by weakly magnetized electrons and unmagnetized ions. The streaming ions, due to the reflection of the solar wind ions from the shock, provide the free energy source for the linear instability of the acoustic wave. In the fully nonlinear regime, a coherent localized solution is found in the form of a stationary ion hump, which is traveling with the velocity close to the phase velocity of the linear mode. The structure is supported by the nonlinearities coming from the increased population of the resonant beam ions, trapped in the self-consistent potential. As their size in the direction perpendicular to the local magnetic field is somewhat smaller that the electron Larmor radius and much larger that the Debye length, their spatial properties are determined by the effects of the magnetic field on weakly magnetized electrons. These coherent structures provide a theoretical explanation for the bipolar electric pulses, observed upstream of the shock by Polar and Cluster satellite missions.
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52.25.Dg, 52.35.Tc, 52.35.Fp, 52.35.Mw, 96.50.Ci, 96.50.Fm

Quasi-thermal noise in space plasma: “kappa” distributions

G. Le Chat, K. Issautier, N. Meyer-Vernet, I. Zouganelis, M. Maksimovic, and M. Moncuquet

Phys. Plasmas 16, 102903 (2009) (6 pages)

Online Publication Date: 9 October 2009

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The transport of energy in collisionless plasmas, especially in space plasmas, is far from being understood. Measuring the temperature of the electrons and their nonthermal properties can give important clues to understand the transport properties. Quasi-thermal noise (QTN) spectroscopy is a reliable tool for measuring accurately the electron density and temperature since it is less sensitive to the spacecraft perturbations than particle detectors. This work models the plasma QTN using a generalized Lorentzian (“kappa”) distribution function for the electrons. This noise is produced by the quasi-thermal fluctuations of the electrons and by the Doppler-shifted thermal fluctuations of the ions. A sum of two Maxwellian functions has mainly been used for modeling the QTN of the electrons, but the observations have shown that the electrons are better fitted by a kappa distribution function. Pioneer work on QTN calculation only considered integer values of kappa. This paper extends these calculations to real values of kappa and gives the analytic expressions and numerical calculations of the QTN with a kappa distribution function. This paper shows some generic properties and gives some practical consequences for plasma wave measurements in space.
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94.05.-a, 52.25.Kn, 52.25.Dg, 52.25.Fi

Kinetic Alfvén waves turbulence in the Earth's magnetosphere

Sachin Kumar and R. P. Sharma

Phys. Plasmas 16, 102904 (2009) (6 pages)

Online Publication Date: 28 October 2009

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The numerical simulations of the model equation governing the nonlinear dynamics of kinetic Alfvén waves in the intermediate-beta plasmas are performed. When the nonlinearity arises due to the ponderomotive force driven density perturbations of kinetic Alfvén waves, the model equation turns out to be a modified nonlinear Schrödinger equation. This has been solved numerically by using appropriate boundary conditions. The coherent, damped magnetic filaments with turbulent spectra have been observed. Our results reveal the interesting change in spectral index because of the damping effect. The steeper power spectra follow ~k−3.4 scaling. Using the Fokker–Planck equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons in these turbulent structures. These turbulent structures can be responsible for plasma heating in Earth's magnetosphere.
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52.35.Bj, 52.35.Ra, 52.35.Mw, 94.30.cg

Lasers, Particle Beams, Accelerators, Radiation Generation

Universal scaling of the electron distribution function in one-dimensional simulations of relativistic laser-plasma interactions

M. Sherlock

Phys. Plasmas 16, 103101 (2009) (6 pages)

Online Publication Date: 2 October 2009

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A formula based on simulations is given for the distribution function of energetic electrons which stream in the forward direction when a relativistically intense laser pulse irradiates a short-scale-length dense plasma at normal incidence. If the energy absorption fraction is known, the entire function can be characterized in terms of the laser intensity I. A number of important characteristics can then be obtained, including the fast-electron energy, current density, and angle of ejection. An objective means for calculating the fast-electron energy is proposed which gives a value of ~0.6 of the vacuum oscillatory energy. This reduction in energy is shown to be consistent with the energy required to draw a return current. These results could be used to provide the characteristics of the electron source in theory and simulation of intense laser-solid interactions.
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52.38.Dx, 02.60.Nm, 52.25.Dg, 52.25.Fi, 52.65.Ff, 52.25.Os

Two-dimensional axisymmetric Child–Langmuir scaling law

Benjamin Ragan-Kelley, John Verboncoeur, and Yang Feng

Phys. Plasmas 16, 103102 (2009) (6 pages)

Online Publication Date: 6 October 2009

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The classical one-dimensional (1D) Child–Langmuir law was previously extended to two dimensions by numerical calculation in planar geometries. By considering an axisymmetric cylindrical system with axial emission from a circular cathode of radius r, outer drift tube radius R>r, and gap length L, we further examine the space charge limit in two dimensions. Simulations were done with no applied magnetic field as well as with a large (100 T) longitudinal magnetic field to restrict motion of particles to 1D. The ratio of the observed current density limit JCL2 to the theoretical 1D value JCL1 is found to be a monotonically decreasing function of the ratio of emission radius to gap separation r/L. This result is in agreement with the planar results, where the emission area is proportional to the cathode width W. The drift tube in axisymmetric systems is shown to have a small but measurable effect on the space charge limit. Strong beam edge effects are observed with J(r)/J(0) approaching 3.5. Two-dimensional axisymmetric electrostatic particle-in-cell simulations were used to produce these results.
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52.80.-s, 52.65.Rr

Betatron x-ray generation from electrons accelerated in a plasma cavity in the presence of laser fields

A. G. R. Thomas and K. Krushelnick

Phys. Plasmas 16, 103103 (2009) (12 pages)

Online Publication Date: 9 October 2009

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X-ray generation by many charged particles experiencing accelerations similar to those in laser wakefield accelerator experiments, including the effects of the interaction of the laser pulse with trapped electrons, as well as betatron oscillations in an electron cavity, is directly evaluated. Semianalytic calculations of high energy photons are performed by solving classical spectral integrals for x rays produced by the combined action of a laser pulse and the fields of an electron cavity in an underdense plasma. Angularly resolved power spectra for electron bunches accelerated in the combined electromagnetic fields due to a Gaussian laser field and a paraboloid potential due to an electron cavity are calculated using a semianalytic numerical algorithm to explicitly calculate the well known spectral integrals. The laser polarizes the resulting x-ray radiation. In addition to the high energy photons due to the betatron oscillations, lower energy radiation is emitted in a conical emission pattern due to the coherent addition of radiation from the linear acceleration of the electrons in the wakefield.
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41.60.-m, 52.38.-r, 41.75.Jv, 52.65.Cc

Time-resolved measurements on reflectivity of an ultrafast laser-induced plasma mirror

Yi Cai, Wentao Wang, Changquan Xia, Jiansheng Liu, Li Liu, Cheng Wang, Yi Xu, Yuxin Leng, Ruxin Li, and Zhizhan Xu

Phys. Plasmas 16, 103104 (2009) (6 pages)

Online Publication Date: 20 October 2009

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Using a linearly chirped laser pulse to irradiate antireflection coated targets, the time-varying reflectivity of a plasma mirror (PM) has been measured at various laser intensities from 1012 to 1017  W/cm2. The onset of plasma generation as well as the formation process of a PM with the highest reflectivity has been observed. The rise time of the PM's reflectivity reaching up to the maximum varies from 300–500 fs at lower laser intensities but goes up to 900 fs at higher intensity of >1016  W/cm2. This long rise time can be attributed to a slowly rising shoulder of the laser pulse, which will trigger the generation of preplasma well in advance of the laser peak. The detailed measurements on both time-integrated and time-resolved reflectivity of a PM, which is induced by p- and s-polarized laser pulses, respectively, indicate that an s-polarized pulse is favorable to obtain the maximal reflectivity and the best contrast improvement as well. This difference can be attributed to the weaker absorption and a smaller plasma scale length generated for s-polarization.
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52.50.Jm

Terahertz generation via optical rectification of x-mode laser in a rippled density magnetized plasma

Lalita Bhasin and V. K. Tripathi

Phys. Plasmas 16, 103105 (2009) (6 pages)

Online Publication Date: 21 October 2009

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A scheme of resonant terahertz radiation generation by the optical rectification of a picosecond laser pulse in rippled density magnetized plasma is examined. The x-mode laser pulse, propagating perpendicular to dc magnetic field, exerts a quasistatic ponderomotive force on electrons, imparting them a drift with finite transverse component. The drift velocity beats with the density ripple to produce a current, resonantly driving terahertz radiation at frequency comparable to the inverse pulse duration. The terahertz power scales as the square of density ripple amplitude and rises with magnetic field strength. In the case when magnetic field is azimuthal with appropriate r-dependence, the terahertz power has a ring shape intensity distribution. At laser intensity of ~3×1015  W/cm2, in a 0.01% critical density plasma with 30 kG magnetic field and 30% density ripple, one may have power conversion efficiency of 0.04%.
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52.50.Jm, 52.55.-s, 52.25.-b

Characteristics of a velvet cathode under high repetition rate pulse operation

Tao Xun, Jian-De Zhang, Han-Wu Yang, Zi-Cheng Zhang, and Yu-Wei Fan

Phys. Plasmas 16, 103106 (2009) (6 pages)

Online Publication Date: 30 October 2009

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As commonly used material for cold cathodes, velvet works well in single shot and low repetition rate (rep-rate) high-power microwave (HPM) sources. In order to determine the feasibility of velvet cathodes under high rep-rate operation, a series of experiments are carried out on a high-power diode, driven by a ~300  kV, ~6  ns, ~100  Omega, and 1–300 Hz rep-rate pulser, Torch 02. Characteristics of vacuum compatibility and cathode lifetime under different pulse rep-rate are focused on in this paper. Results of time-resolved pressure history, diode performance, shot-to-shot reproducibility, and velvet microstructure changes are presented. As the rep-rate increases, the equilibrium pressure grows hyperlinearly and the velvet lifetime decreases sharply. At 300 Hz, the pressure in the given diode exceeded 1 Pa, and the utility shots decreased to 2000 pulses for nonstop mode. While, until the velvet begins to degrade, the pulse-to-pulse instability of diode voltage and current is quite small, even under high rep-rate conditions. Possible reasons for the operation limits are discussed, and methods to improve the performance of a rep-rate velvet cathode are also suggested. These results may be of interest to the repetitive HPM systems with cold cathodes.
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52.59.Mv, 84.40.-x, 82.45.Fk, 52.50.Dg

Stabilization of the potential multi-steady-state absolute instabilities in a gyrotron traveling-wave amplifier

Chao-Hai Du and Pu-Kun Liu

Phys. Plasmas 16, 103107 (2009) (7 pages)

Online Publication Date: 30 October 2009

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The problem of spurious oscillations induced by absolute instabilities is the most challenging one that hinders the development of the millimeter-wave gyrotron traveling-wave amplifiers (gyro-TWTs). A spurious oscillation exists as a high order axial mode (HOAM) in the interaction circuit. This paper is devoted to demonstrating the complicated steady states of these HOAMs and exploring corresponding techniques to stabilize these potential multi-steady-state absolute instabilities. The stability-oriented design principle is conveyed in a start-to-end design flow of a Ka-band TE11 mode gyro-TWT. Strong magnetic tapering near the downstream port, which is capable of cutting short the effective interaction circuit of a spurious oscillation and simultaneously boosting the amplification performance, is for the first time proposed to further improve the system stability. It is also found that an ideal prebunched electron beam in the linear stage is the necessary condition to efficient amplification in the nonlinear stage, suggesting that it is feasible to design a stable prebunching stage to replace the distributed-loss-loaded linear stage. The stability-oriented design principle provides more explicit reference for future design of a zero-drive stable gyro-TWT.
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52.35.Py, 84.30.Le, 84.40.Fe, 84.40.Ik, 52.35.Fp

Efficient generation of quasimonoenergetic ions by Coulomb explosions of optimized nanostructured clusters

M. Murakami and K. Mima

Phys. Plasmas 16, 103108 (2009) (6 pages)

Online Publication Date: 30 October 2009

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Coulomb explosion of spherical ion clusters is studied, which are composed of homogeneous two-species (light and heavy) ions. A simple analytical model is developed to describe the explosion performance in terms of two dimensionless parameters, the charge-over-mass ratio, and the charge density ratio. One-dimensional kinetic numerical model is performed to compare with the analytical model and to evaluate the energy coupling efficiency of quasimonoenergetic ion generation. It is crucial to preform an iso-Coulomb-potential profile of the light ions in the cluster for efficient generation of quasimonoenergetic ions. By controlling the radial density profiles of the light and heavy ions, the overall coupling efficiency (equal to the summed kinetic energy of the light ions in the highest 1% energy band divided by total kinetic energy of both ions) is optimized to be >30%–40% when about 90% of the total number of light ions is contained in the thin 1% energy band.
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52.38.Kd, 52.40.Kh, 52.50.Jm

Radiation: Emission, Absorption, Transport

Ion flux mapping in an inertial-electrostatic confinement device using a chordwire diagnostic

S. Krupakar Murali, J. F. Santarius, and G. L. Kulcinski

Phys. Plasmas 16, 103301 (2009) (7 pages)

Online Publication Date: 23 October 2009

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Theoretical study of ion microchannels that form in an inertial-electrostatic confinement (IEC) device has helped understand the behavior of ions of various energies within a microchannel and has also predicted that smaller grids produce a more converged core. However, such theoretical work has many limitations that make experimental work indispensable. In the present paper the experimental measurements of ion flow patterns into the cathode grid and their consequences (using a “chordwire” diagnostic that intercepts ions streaming into the IEC core) are reported. Experimental measurements also have quantified the interruption of the ion flow due to the adverse influence of the high voltage stalk. In addition, the chordwire arrangement can be used to study the radiation damage of materials.
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52.30.-q, 52.55.-s, 52.70.-m, 52.75.-d

Magnetic field and electromagnetic wave properties of carbon monoxide with high-pressure disproportionation single-walled carbon nanotubes

S. B. Tooski

Phys. Plasmas 16, 103302 (2009) (7 pages)

Online Publication Date: 27 October 2009

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A double-fluid theory is used to find the electromagnetic wave absorption of carbon monoxide with iron-catalyzed high-pressure disproportionation (HiPco)-grown single-walled carbon nanotubes (SWNTs). The electromagnetic wave absorption of carbon monoxide with HiPco SWNTs is obtained and is studied numerically. The absorption is then deduced and their functional dependence on the number density, collision frequency, cyclotron frequency, and angle of propagation is studied. The double-fluid theory predicts that there is an electromagnetic frequency dependency on the energy absorption properties of the system under investigation. The calculation results show that effects of magnetic field strength and the angle of microwave propagation on the absorption coefficient as well as the frequency band of resonant absorption are very significant.
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61.48.De, 61.82.Rx, 81.07.De

Low-Temperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Optical emission spectroscopy for simultaneous measurement of plasma electron density and temperature in a low-pressure microwave induced plasma

N. Konjević, S. Jovićević, and M. Ivković

Phys. Plasmas 16, 103501 (2009) (6 pages)

Online Publication Date: 5 October 2009

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The simple optical emission spectroscopy technique for diagnostics of low pressure microwave induced plasma (MIP) in hydrogen or in MIP seeded with hydrogen is described and tested. This technique uses the Boltzmann plot of relative line intensities along Balmer spectral series in conjunction with the criterion for partial local thermodynamic equilibrium for low electron density (Ne) plasma diagnostics. The proposed technique is tested in a low pressure MIP discharge for simultaneous determination of electron density Ne (1017–1018  m−3) and temperature Te.
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07.57.-c, 32.30.Jc, 32.70.Jz, 32.70.-n, 52.50.Dg, 52.75.Hn, 84.40.-x

Numerical simulation of high-current vacuum arc characteristics under combined action of axial magnetic field and external magnetic field from bus bar

Lijun Wang, Shenli Jia, Ke Liu, Liuhuo Wang, and Zongqian Shi

Phys. Plasmas 16, 103502 (2009) (8 pages)

Online Publication Date: 15 October 2009

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In this paper, the two-dimensional high-current vacuum arc (HCVA) model under the combined action of axial magnetic field (AMF) and external magnetic field from bus bar (EMFBB) is established. Based on this model, the influence of AMF and EMFBB on HCVA characteristics can be simulated and analyzed. Simulation results show that the HCVA column will be deflected by the Lorentz force generated by EMFBB and higher arc current. Moreover, the deflection level will be increased with the increase in external EMFBB strength. For HCVA, due to the smaller axial velocity near cathode side, the deflection of plasma parameters (such as ion number density, ion temperature, electron temperature, plasma pressure, and so on) near cathode side is more significant than that near anode side. The current deflection near cathode side toward direction of Lorentz force is more significant than that near anode side.
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52.80.Mg, 52.80.Vp, 84.70.+p

Sheath and presheath in plasma with warm ions

Scott Robertson

Phys. Plasmas 16, 103503 (2009) (6 pages)

Online Publication Date: 15 October 2009

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Numerical solutions are found for the potential profile of the presheath and sheath in plasma with a Maxwellian ion source distribution. Solutions are obtained for ratios of the electron temperature to ion source temperature from 0.1 to 10 for plasmas that are 20, 50, and 100 Debye lengths from the midplane to the wall. With increasing ion source temperature, the potential profile falls less rapidly in the presheath and the current density of ions to the wall increases and approaches the random ion current calculated for the source temperature. Ion distribution functions at the midplane, the sheath-presheath boundary, and at the wall are found and are more sharply peaked than Maxwellian distributions. At the midplane, the effective ion temperature is much less than the ion source temperature.
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52.40.Kh, 52.75.Xx

Experimental and theoretical study on effects of magnetic field topology on near wall conductivity in a Hall thruster

Daren Yu (于达仁), Hong Li (李鸿), Zhiwen Wu (武志文), Zhongxi Ning (宁中喜), and Guojun Yan (闫国军)

Phys. Plasmas 16, 103504 (2009) (8 pages)

Online Publication Date: 23 October 2009

Full Text: PDF (603 kB)

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An experiment has been made to investigate the effect of curved magnetic field topology on near wall conductivity in the ion acceleration region of Hall thrusters. The experimental results show that the electron current due to near wall conductivity is of the minimum in the case of focused topology and increases in the cases of both less-focus and over-focus topologies. This finding cannot be explained properly by the magnetic mirror effect, which is the one and only reported effect related to the magnetic field curvature so far. Based on the analysis of interaction between the plasma and the wall, a new physical effect is proposed. The difference of magnetic field topology causes different electric potential distribution, leads to different ion flux to the wall, results in the change of sheath property and secondary electron emission, and finally affects the electron current due to near wall conductivity. This effect is further justified by the agreement between the experiment and simulation which is performed with a particle-in-cell model. Therefore, we conclude that the ion flow injection is a significant effect to near wall conductivity in the scope of curved magnetic field topology besides the magnetic mirror effect. Moreover, we find that the focus topology of magnetic field is favorable to obtain a high thruster performance from both the ion acceleration aspect and the electron conduction aspect and so is useful practically for thruster optimization.
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52.75.Di

Parameters of a collisional radio-frequency sheath and dust characteristics resulting from the microparticle levitation

V. V. Yaroshenko, T. Antonova, H. M. Thomas, and G. E. Morfill

Phys. Plasmas 16, 103505 (2009) (7 pages)

Online Publication Date: 27 October 2009

Full Text: PDF (315 kB)

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The screening length, the time-average electric field, and the particle charge as well as the local vertical gradients of these quantities are determined experimentally within a sheath of a capacitively coupled rf, 13.56 MHz, discharge at enhanced argon gas pressures of 30, 55, and 100 Pa. The parameters are derived directly from comparative measurements of levitation positions of the particles of different sizes and variations in the levitation heights caused by formation of new dust layers. The electrostatic effect of the horizontally extended dust layers on the sheath electric field is investigated.
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52.40.Kh, 52.80.Pi, 52.27.Lw, 52.20.-j

Oscillations of the collisionless sheath at grazing incidence of the magnetic field

M. Shoucri, H. Gerhauser, and K. H. Finken

Phys. Plasmas 16, 103506 (2009) (16 pages)

Online Publication Date: 28 October 2009

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An Eulerian Vlasov code is used to study the physics of a collisionless sheath with grazing angles of incidence of the magnetic field. The electrons are treated with a parallel-B kinetic equation. In the case where the ion gyroradius is large compared to the Debye length, the electrons, frozen by the magnetic field line, have to move rapidly along the magnetic field B in their attempt to follow the ions gyrating perpendicular to B. Below a critical angle of incidence of the magnetic field, the large gyroradius ions can be scraped off at the plasma-wall interface of the sheath. The electrons now determine the characteristic time for information propagation, with low frequency oscillations appearing in the system. In the case where the ion gyroradius is reduced to the same order as the Debye length, the results evolve closer to those of the classical sheath, and the amplitude of the low frequency oscillations is reduced.
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52.40.Kh, 52.25.Dg, 52.30.Gz, 52.35.Fp

Dusty Plasmas

Shock wave in a two-dimensional dusty plasma crystal

Samiran Ghosh

Phys. Plasmas 16, 103701 (2009) (6 pages)

Online Publication Date: 8 October 2009

Full Text: PDF (199 kB)

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Two-dimensional (2D) shock structures of longitudinal dust lattice wave (LDLW) in a hexagonal Yukawa crystal are studied. The nonlinear evolution equation derived for dusty plasma crystal is found to be a 2D Burgers' equation, where the Burgers' term, i.e., the dissipation is provided by “hydrodynamic damping” due to irreversible processes that take place within the system. Analytical and numerical solutions of this equation on the basis of crystal experimental parameters show the development of compressional shock structures of LDLW in 2D dusty plasma crystal. The shock strength decreases (increases) with the increase in lattice parameter kappa (angle of propagation of the nonlinear wave). The results are discussed in the context of 2D monolayer hexagonal dusty plasma crystal experiments.
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52.27.Lw, 52.27.Jt, 52.35.Tc

Effect of ion temperature on oblique propagation of large amplitude solitary kinetic Alfvén waves

Prasanta Chatterjee, Taraknath Saha, S. V. Muniandy, and C. S. Wong

Phys. Plasmas 16, 103702 (2009) (6 pages)

Online Publication Date: 9 October 2009

Full Text: PDF (121 kB)

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The effect of ion temperature on the existence of large amplitude solitary kinetic Alfvén waves and double layers are investigated in a two-fluid model using Sagdeev's pseudopotential technique. The ion temperature, obliqueness parameter, and the ratio of cyclotron frequency to ion frequency are found to play significant roles in the formation and the shape of solitary kinetic Alfvén waves and double layers. Hump-type solitary waves and dip-type double layers are also observed. Conditions for the existence of solitary waves and double layers are discussed.
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52.35.Bj, 52.35.Sb, 52.35.Mw, 52.40.Kh, 52.25.-b

Simulation study of the magnetized sheath of a dusty plasma

G. Foroutan, H. Mehdipour, and H. Zahed

Phys. Plasmas 16, 103703 (2009) (11 pages)

Online Publication Date: 12 October 2009

Full Text: PDF (454 kB)

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Numerical solutions of stationary multifluid equations are used to study the formation and properties of the magnetized sheath near the boundary of a dusty plasma. The impacts of the strength of the magnetic field, the dust and plasma number densities, and the electron temperature on the sheath structure and spatial distributions of various quantities are investigated. It is shown that for a given angle of incidence of the magnetic field, there is a threshold magnetic field intensity above which some kind of large regular inhomogeneities develop on the spatial profile of the dust particles. The sheath thickness, the electron and ion number densities, and the absolute dust charge are strongly affected by the variation in the dust number density. The sheath demonstrates a nonlinear dependence on the electron temperature; as the electron temperature rises, the sheath first is broadened and the absolute wall potential decreases but then at higher temperatures the sheath becomes narrower and the absolute wall potential increases.
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52.65.-y, 52.55.-s, 52.40.Kh, 52.27.Lw

Dust kinetic Alfvén and acoustic waves in a Lorentzian plasma

N. Rubab, N. V. Erkaev, and H. K. Biernat

Phys. Plasmas 16, 103704 (2009) (6 pages)

Online Publication Date: 15 October 2009

Full Text: PDF (174 kB)

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Dust kinetic Alfvén waves (DKAWs) with finite Larmor radius effects have been examined rigorously in a uniform dusty plasma in the presence of an external magnetic field. A dispersion relation of low-frequency DKAW on the dust acoustic velocity branch is obtained in a low-beta Lorentzian plasma. It is found that the influence of the Lorentzian distribution function is more effective for perpendicular component of group velocity as compared with parallel one. Lorentzian-type charging currents are obtained with the aid of Vlasov theory. Damping/instability due to dust charge fluctuation is found to be insensitive with the form of distribution function for DKAW. The possible applications to dusty space plasmas are pointed out.
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52.27.Lw, 52.35.Bj, 52.35.Dm, 52.25.Fi, 52.25.Dg, 52.35.Py

Electrostatic drift modes in quantum dusty plasmas with Jeans terms

Haijun Ren, Zhengwei Wu, Jintao Cao, and Paul K. Chu

Phys. Plasmas 16, 103705 (2009) (7 pages)

Online Publication Date: 30 October 2009

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Electrostatic drift waves (EDWs) are investigated in nonuniform quantum magnetized dusty plasmas by taking into account dust gravitational effects with the help of the quantum hydrodynamic model. Ions and electrons are viewed as low-temperature Fermi gases, whereas quantum effects are neglected for the dust grains. The analytical dispersion relationship of the quantum EDWs is derived. Quantum effects are shown to affect the dispersion of EDW significantly. The Jeans terms induce a driftlike instability, which does not exist with the absence of gravitational effects. The criteria and growth rate of the kind of instability are presented. Our results are relevant to dense astrophysical objects such as the interiors of astrophysical compact objects (e.g., white dwarfs and neutron stars).
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03.65.-w, 52.35.Kt, 52.27.Lw, 52.25.Xz
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Effect of energetic electrons on near-wall sheath voltage in the cathode region of a cold cathode direct current discharge

J. Blessington, S. F. Adams, V. I. Demidov, and J. M. Williamson

Phys. Plasmas 16, 104501 (2009) (4 pages)

Online Publication Date: 28 October 2009

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It is experimentally demonstrated that energetic electrons originating from the cathode or produced in volumetric processes in the cathode region of a cold cathode direct current discharge can create a large potential drop in the near-wall sheath. This voltage drop may be much greater than kTe/e (where k is the Boltzmann constant, Te is the electron temperature, and e is the electron charge). Due to the large near-wall sheath voltage, slow and moderately energetic electrons cannot reach the wall and move toward the anode. Application of additional potentials to the wall can change the amount of energetic electrons reaching the wall. This effect can be used for regulation of the near-cathode plasma and near-wall sheath properties and thus may be useful in technical applications.
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52.80.-s, 52.40.Kh

Random walk of electrons in a gas in the presence of polarized electromagnetic waves: Genesis of a wave induced discharge

Sudeep Bhattacharjee and Samit Paul

Phys. Plasmas 16, 104502 (2009) (4 pages)

Online Publication Date: 30 October 2009

Full Text: PDF (478 kB)

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The average number of collisions N of seed electrons with neutral gas atoms during random walk in escaping from a given volume, in the presence of polarized electromagnetic waves, is found to vary as N=B(Lambda/lambda)2/[1+C(Lambda/lambda)]2, indicating a modification to the conventional field free square law N=A(Lambda/lambda)2, where Lambda is the characteristic diffusion length and lambda the mean free path. It is found that for the field free case A=1.5 if all the electrons originate at the center and is 1.25 if they are allowed to originate at any random point in the given volume. The B and C coefficients depend on the wave electric field and frequency. Predictions of true discharge initiation time tauc can be made from the temporal evolution of seed electrons over a wide range of collision frequencies. For linearly polarized waves of 2.45 GHz and electric field in the range (0.6–1.0)×105  V/m, tauc=5.5–1.6  ns for an unmagnetized microwave driven discharge at 1 Torr argon.
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52.80.Pi, 52.40.Db, 52.25.Fi, 52.20.Fs

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