Volume 23, Issue 7, July 2016

When a highcontrast ultrarelativistic (>10^{20} W/cm^{2}) laser beam enters a microsized plasma waveguide, the pulse energy is coupled into waveguide modes, which significantly modifies the interaction between the electrons and electromagnetic wave. Electrons pulled out from the walls of the waveguide form a dense helical bunch inside the channel and are efficiently accelerated by the transverse magnetic modes to hundreds of MeV. The asymmetry in the transverse electric and magnetic fields drives strong oscillations, which lead to the emission of bright, wellcollimated, hard Xrays. In this paper, we present our study on the underlying physics in the aforementioned process using 3D particleincell simulations. The mechanism of electron acceleration and the dependence of radiation properties on different laser plasma parameters are addressed. An analytic model and basic scalings for Xray emission are also presented by considering the lowest optical modes in the waveguide, which is adequate to describe the basic phenomenon. In addition, the effects of highorder modes as well as laser polarization are also qualitatively discussed. The considered Xray source has promising features, potentially making it a competitive candidate for a future tabletop synchrotron source.
 LETTERS


Radiation pressure acceleration of protons to 93 MeV with circularly polarized petawatt laser pulses
View Description Hide DescriptionThe radiation pressure acceleration (RPA) of charged particles has been a challenging task in laserdriven proton/ion acceleration due to its stringent requirements in laser and target conditions. The realization of radiationpressuredriven proton acceleration requires irradiating ultrathin targets with an ultrahigh contrast and ultraintense laser pulses. We report the generation of 93MeV proton beams achieved by applying 800nm 30fs circularly polarized laser pulses with an intensity of to 15nmthick polymer targets. The radiation pressure acceleration was confirmed from the obtained optimal target thickness, quadratic energy scaling, polarization dependence, and threedimensional particleincell simulations. We expect this clear demonstration of RPA to facilitate the realization of laserdriven proton/ion sources delivering energetic and shortpulse particle beams for novel applications.

Resonance in fastwave amplitude in the periphery of cylindrical plasmas and application to edge losses of wave heating power in tokamaks
View Description Hide DescriptionHeating magnetically confined plasmas using waves in the ioncyclotron range of frequencies typically requires coupling these waves over a steep density gradient. This process has produced an unexpected and deleterious phenomenon on the National Spherical Torus eXperiment (NSTX): a prompt loss of wave power along magnetic field lines in front of the antenna to the divertor. Understanding this loss may be key to achieving effective heating and expanding the operational space of NSTXUpgrade. Here, we propose that a new type of mode, which conducts a significant fraction of the total wave power in the lowdensity peripheral plasma, is driving these losses. We demonstrate the existence of such modes, which are distinct from surface modes and coaxial modes, in a cylindrical coldplasma model when a half wavelength structure fits into the region outside the core plasma. The latter condition generalizes the previous hypothesis regarding the occurrence of the edge losses and may explain why fullwave simulations predict these losses in some cases but not others. If valid, this condition implies that outer gap control is a potential strategy for mitigating the losses in NSTXUpgrade in addition to raising the magnetic field or influencing the edge density.

Suppression of phase mixing in driftkinetic plasma turbulence
View Description Hide DescriptionTransfer of free energy from large to small velocityspace scales by phase mixing leads to Landau damping in a linear plasma. In a turbulent driftkinetic plasma, this transfer is statistically nearly canceled by an inverse transfer from small to large velocityspace scales due to “antiphasemixing” modes excited by a stochastic form of plasma echo. Fluid moments (density, velocity, and temperature) are thus approximately energetically isolated from the higher moments of the distribution function, so phase mixing is ineffective as a dissipation mechanism when the plasma collisionality is small.

NonMaxwellian to Maxwellian transitions of atmospheric microplasmas at microwave frequencies
View Description Hide DescriptionParticleincell/Monte Carlo simulations and numerical analysis of a single particle motion are performed for atmospheric He microplasmas at microwave frequencies to determine the characteristics of nonMaxwellian to Maxwellian transition. The left and the right regimes of Paschen curve, divided by this transition, reveal that the transition frequencies depend on the gap of electrodes and the neutral gas pressure to follow scaling laws for a new extended Paschen law. The fluid models are reasonable at the rightside regime of Paschen breakdown areas, but not on the left side, which is highly kinetic for electrons. The plasmas driven by weaker electric fields of high enough frequencies at the rightside Paschen regime breed more energetic electrons.

Magnetohydrodynamics for collisionless plasmas from the gyrokinetic perspective
View Description Hide DescriptionThe effort to obtain a set of MagnetoHydroDynamic (MHD) equations for a magnetized collisionless plasma was started nearly 60 years ago by Chew et al. [Proc. R. Soc. London, Ser. A 236(1204), 112–118 (1956)]. Many attempts have been made ever since. Here, we will show the derivation of a set of these equations from the gyrokinetic perspective, which we call it gyrokinetic MHD, and it is different from the conventional ideal MHD. However, this new set of equations still has conservation properties and, in the absence of fluctuations, recovers the usual MHD equilibrium. Furthermore, the resulting equations allow for the plasma pressure balance to be further modified by finiteLarmorradius effects in regions with steep pressure gradients. The present work is an outgrowth of the paper on “Alfven Waves in Gyrokinetic Plasmas” by Lee and Qin [Phys. Plasmas 10, 3196 (2003)].
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

On relativistic space charge limited current in planar, cylindrical, and spherical diodes
View Description Hide DescriptionThis paper revisits the relativistic limiting current in planar, cylindrical, and spherical diodes, with alternative analytic and numerical treatments which are easy to implement. Convenient, approximate expressions for the limited current are presented for gap voltages up to 10 MV. They are accurate to within 1% for planar diode, and to within 4% for both cylindrical and spherical diode in the range , where ra and rc are, respectively, the anode and cathode radius.

Oscillating plasma bubble and its associated nonlinear studies in presence of low magnetic field
View Description Hide DescriptionOscillating plasma bubbles have been created around a cylindrical mesh grid of 75% optical transparency in a DC plasma system with a low magnetic field. Plasma bubbles are created by developing ion density gradient around a cylindrical grid of 20 cm in diameter and 25 cm in height, inserted into the plasma. Relaxation and contraction of the plasma bubbles in the presence of external conditions, such as magnetic field and pressure, have been studied. A Langmuir probe has been used to detect the plasma floating potential fluctuations at different imposed experimental conditions. Nonlinear behavior of the system has been characterized by adopting nonlinear techniques such as Fast Fourier Transform, Phase Space Plot, and Recurrence Plot. It shows that the system creates highly nonlinear phenomena associated with the plasma bubble under the imposed experimental conditions. A theoretical and numerical model has also been developed to satisfy the observed experimental analysis. Moreover, observations are extended further to study the growth of instability associated with the plasma bubbles. The intention of the present work is to correlate the findings about plasma bubbles and their related instability with the one existing in the equatorial Fregion of the ionosphere.

Optimization of interaction conditions for efficient short laser pulse amplification by stimulated Brillouin scattering in the strongly coupled regime
View Description Hide DescriptionPlasma amplification of low energy, a short (∼100–500 fs) laser pulse by an energetic long (∼10 ps) pulse via strong coupling Stimulated Brillouin Backscattering is investigated with an extensive analysis of onedimensional particleincell simulations. Parameters relevant to nowadays experimental conditions are investigated. The obtained seed pulse spectra are analyzed as a function of the interaction conditions such as plasma profile, pulses delay, and seed or pulse duration. The factors affecting the amount of energy transferred are determined, and the competition between Brillouinbased amplification and parasitic Raman backscattering is analyzed, leading to the optimization of the interaction conditions.

3D electrostatic gyrokinetic electron and fully kinetic ion simulation of lowerhybrid drift instability of Harris current sheet
View Description Hide DescriptionThe eigenmode stability properties of threedimensional lowerhybriddriftinstabilities (LHDI) in a Harris current sheet with a small but finite guide magnetic field have been systematically studied by employing the gyrokinetic electron and fully kinetic ion (GeFi) particleincell (PIC) simulation model with a realistic iontoelectron mass ratio . In contrast to the fully kinetic PIC simulation scheme, the fast electron cyclotron motion and plasma oscillations are systematically removed in the GeFi model, and hence one can employ the realistic . The GeFi simulations are benchmarked against and show excellent agreement with both the fully kinetic PIC simulation and the analytical eigenmode theory. Our studies indicate that, for small wavenumbers, ky, along the current direction, the most unstable eigenmodes are peaked at the location where , consistent with previous analytical and simulation studies. Here, is the equilibrium magnetic field and is the wavevector perpendicular to the nonuniformity direction. As ky increases, however, the most unstable eigenmodes are found to be peaked at . In addition, the simulation results indicate that varying , the current sheet width, and the guide magnetic field can affect the stability of LHDI. Simulations with the varying mass ratio confirm the lower hybrid frequency and wave number scalings.

Elliptically polarized electromagnetic waves in a magnetized quantum electronpositron plasma with effects of exchangecorrelation
View Description Hide DescriptionThe dispersion properties of elliptically polarized electromagnetic waves in a magnetized electronpositronpair (EPpair) plasma are studied with the effects of particle dispersion associated with the Bohm potential, the Fermi degenerate pressure, and the exchangecorrelation force. Two possible modes of the extraordinary or X wave, modified by these quantum effects, are identified and their propagation characteristics are investigated numerically. It is shown that the upperhybrid frequency and the cutoff and resonance frequencies are no longer constants but are dispersive due to these quantum effects. It is found that the particle dispersion and the exchangecorrelation force can have different dominating roles on each other depending on whether the X waves are of short or long wavelengths (in comparison with the Fermi Debye length). The present investigation should be useful for understanding the collective behaviors of EP plasma oscillations and the propagation of extraordinary waves in magnetized dense EPpair plasmas.

Impurity effects on trapped electron mode in tokamak plasmas
View Description Hide DescriptionThe effects of impurity ions on the trapped electron mode (TEM) in tokamak plasmas are numerically investigated with the gyrokinetic integral eigenmode equation. It is shown that in the case of large electron temperature gradient (), the impurity ions have stabilizing effects on the TEM, regardless of peaking directions of their density profiles for all normalized electron density gradient . Here, is the major radius and is the electron density gradient scale length. In the case of intermediate and/or small , the light impurity ions with conventional inwardly (outwardly) peaked density profiles have stabilizing effects on the TEM for large (small) , while the light impurity ions with steep inwardly (outwardly) peaked density profiles can destabilize the TEM for small (large) . Besides, the TEM driven by density gradient is stabilized (destabilized) by the light carbon or oxygen ions with inwardly (outwardly) peaked density profiles. In particular, for flat and/or moderate , two independent unstable modes, corresponding respectively to the TEM and impurity mode, are found to coexist in plasmas with impurity ions of outwardly peaked density profiles. The high Z tungsten impurity ions play a stronger stabilizing role in the TEM than the low Z impurity ions (such as carbon and oxygen) do. In addition, the effects of magnetic shear and collision on the TEM instability are analyzed. It is shown that the collisionality considered in this work weakens the trapped electron response, leading to a more stable TEM instability, and that the stabilizing effects of the negative magnetic shear on the TEM are more significant when the impurity ions with outwardly peaked density profile are taken into account.

Effect of turbulence on the dissipation of the spacecharge wave in a bounded turbulent plasma column
View Description Hide DescriptionThe dispersion relation and the dissipation process of the spacecharge wave propagating in a bounded plasma such as a cylindrical waveguide are investigated by employing the longitudinal dielectric permittivity that contains the diffusivity based on the Dupree theory of turbulent plasma. We derived the dispersion relation for spacecharge wave in terms of the radius of cylindrical waveguide and the roots of the Bessel function of the first kind which appears as the boundary condition. We find that the wave frequency for a lowerorder root of the Bessel function is higher than that of a higherorder root. We also find that the dissipation is greatest for the lowestorder root, but it is suppressed significantly as the order of the root increases. The wave frequency and the dissipation process are enhanced as the radius of cylindrical waveguide increases. However, they are always smaller than the case of bulk plasma. We find that the diffusivity of turbulent plasma would enhance the damping of spacecharge waves, especially, in the range of small wave number. For a large wave number, the diffusivity has little effect on the damping.

A field theory approach to the evolution of canonical helicity and energy
View Description Hide DescriptionA redefinition of the Lagrangian of a multiparticle system in fields reformulates the singleparticle, kinetic, and fluid equations governing fluid and plasma dynamics as a single set of generalized Maxwell's equations and Ohm's law for canonical forcefields. The Lagrangian includes new terms representing the coupling between the motion of particle distributions, between distributions and electromagnetic fields, with relativistic contributions. The formulation shows that the concepts of selforganization and canonical helicity transport are applicable across singleparticle, kinetic, and fluid regimes, at classical and relativistic scales. The theory gives the basis for comparing canonical helicity change to energy change in general systems. For example, in a fixed, isolated system subject to nonconservative forces, a species' canonical helicity changes less than total energy only if gradients in density or distribution function are shallow.

Design of geometric phase measurement in EAST Tokamak
View Description Hide DescriptionThe optimum scheme for geometric phase measurement in EAST Tokamak is proposed in this paper. The theoretical values of geometric phase for the probe beams of EAST PolarimeterInterferometer (POINT) system are calculated by path integration in parameter space. Meanwhile, the influences of some controllable parameters on geometric phase are evaluated. The feasibility and challenge of distinguishing geometric effect in the POINT signal are also assessed in detail.

A study on the highorder mode oscillation in a fourcavity intense relativistic klystron amplifier
View Description Hide DescriptionThe highorder mode oscillation is studied in designing a fourcavity intense relativistic klystron amplifier. The reason for the oscillation caused by highorder modes and a method to suppress these kinds of spurious modes are found through theoretical analyses and the study on the influence of major parameters of a high frequency structure (such as the oscillation frequency of cavities, the cavity Q value, the length of drift tube section, and the characteristic impedance). Based on much simulation, a fourcavity intense relativistic klystron amplifier with a superior performance has been designed, built, and tested. An output power of 2.22 GW corresponding to 27.4% efficiency and 61 dB gain has been obtained. Moreover, the highorder mode oscillation is suppressed effectively, and an output power of 1.95 GW corresponding to 26% efficiency and 62 dB gain has been obtained in our laboratory.

On the structure of the twostream instability–complex GHamiltonian structure and Krein collisions between positive and negativeaction modes
View Description Hide DescriptionThe twostream instability is probably the most important elementary example of collective instabilities in plasma physics and beamplasma systems. For a warm plasma with two charged particle species, the instability diagram of the twostream instability based on a 1D warmfluid model exhibits an interesting band structure that has not been explained. We show that the band structure for this instability is the consequence of the Hamiltonian nature of the warm twofluid system. Interestingly, the Hamiltonian nature manifests as a complex GHamiltonian structure in wavenumber space, which directly determines the instability diagram. Specifically, it is shown that the boundaries between the stable and unstable regions are locations for Krein collisions between eigenmodes with different Krein signatures. In terms of physics, this rigorously implies that the system is destabilized when a positiveaction mode resonates with a negativeaction mode, and that this is the only mechanism by which the system can be destabilized. It is anticipated that this physical mechanism of destabilization is valid for other collective instabilities in conservative systems in plasma physics, accelerator physics, and fluid dynamics systems, which admit infinitedimensional Hamiltonian structures.

The impact of positrons beam on the propagation of super freak waves in electronpositronion plasmas
View Description Hide DescriptionIn this work, we examine the nonlinear propagation of planar ionacoustic freak waves in an unmagnetized plasma consisting of cold positive ions and superthermal electrons subjected to cold positrons beam. For this purpose, the reductive perturbation method is used to derive a nonlinear Schrödinger equation (NLSE) for the evolution of electrostatic potential wave. We determine the domain of the plasma parameters where the rogue waves exist. The effect of the positron beam on the modulational instability of the ionacoustic rogue waves is discussed. It is found that the region of the modulational stability is enhanced with the increase of positron beam speed and positron population. Second as positrons beam increases the nonlinearities of the plasma system, large amplitude ion acoustic rogue waves are pointed out. The present results will be helpful in providing a good fit between the theoretical analysis and real applications in future laboratory plasma experiments.

Fluxdriven algebraic damping of m = 1 diocotron mode
View Description Hide DescriptionRecent experiments with pure electron plasmas in a Malmberg–Penning trap have observed the algebraic damping of m = 1 diocotron modes. Transport due to small field asymmetries produces a low density halo of electrons moving radially outward from the plasma core, and the mode damping begins when the halo reaches the resonant radius r = Rw at the wall of the trap. The damping rate is proportional to the flux of halo particles through the resonant layer. The damping is related to, but distinct from, spatial Landau damping, in which a linear waveparticle resonance produces exponential damping. This paper explains with analytic theory the new algebraic damping due to particle transport by both mobility and diffusion. As electrons are swept around the “cat's eye” orbits of the resonant waveparticle interaction, they form a dipole (m = 1) density distribution. From this distribution, the electric field component perpendicular to the core displacement produces E × Bdrift of the core back to the axis, that is, damps the m = 1 mode. The parallel component produces drift in the azimuthal direction, that is, causes a shift in the mode frequency.

Collisional relaxation of biMaxwellian plasma temperatures in magnetized plasmas
View Description Hide DescriptionIn the literature, collisional processes are customarily discussed within the context of the BoltzmannBalescuLenardLandau type of collision integral, but such an equation is strictly valid for unmagnetized plasmas. For plasmas immersed in the ambient magnetic field, the foundational equation that describes binary collisions must be generalized to include the effects of magnetic field. The present paper makes use of such an equation in order to describe the collisional relaxation of temperatures under the assumption of biMaxwellian velocity distribution function. The formalism derived in the present paper may be useful for studying the effects of binary collisions on the isotropization of temperatures in the solar wind plasma, among possible applications.

A theoretical study for parallel electric field in nonlinear magnetosonic waves in threecomponent plasmas
View Description Hide DescriptionThe electric field parallel to the magnetic field in nonlinear magnetosonic waves in three component plasmas (twoionspecies plasma and electronpositronion plasma) is theoretically studied based on a threefluid model. In a twoionspecies plasma, a magnetosonic mode has two branches, highfrequency mode and lowfrequency mode. The parallel electric field and its integral along the magnetic field, , in the two modes propagating quasiperpendicular to the magnetic field are derived as functions of the wave amplitude ϵ and the density ratio and cyclotron frequency ratio of the two ion species. The theory shows that the magnitude of F in the highfrequencymode pulse is much greater than that in the lowfrequencymode pulse. Theoretical expressions for and F in nonlinear magnetosonic pulses in an electronpositronion plasma are also obtained under the assumption that the wave amplitudes are in the range of , where is the electron to ion mass ratio.