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This paper comprehensively investigates the oblique propagation of ion-acoustic solitary waves (IASWs) with arbitrary amplitude in a magnetoplasma consisting of inertialess non-Maxwellian (nonthermal) electrons, inertialess Maxwellian positrons, and inertial adiabatically heated ions. It is postulated that the positive ions demonstrate adiabatic behavior that is distinguished by anisotropic thermal pressure. The study utilizes Sagdeev's pseudopotential theory to analyze the fluid equations of the plasma model and reduce them to the energy equation. Different plasma configuration factors, such as nonthermal parameters, positron concentration, and parallel and perpendicular ion pressure, are being studied to see how they affect the properties of solitary waves with large amplitudes. The findings demonstrate the simultaneous coexistence of compressive and rarefactive IASWs, significantly influenced by positrons and nonthermal electron parameters. The study provides valuable insights into wave phenomena in magnetized plasmas and presents possible applications in both Space and laboratory plasma environments. [ABSTRACT FROM AUTHOR]

In this paper, we present the electron bunches extracted from a dense plasma produced by the interaction of a ns laser at an intensity of 1013W cm-2 with a solid target, when this plasma expands in an electric field. The plasma expansion dynamics measured in the previous works [M. Comet et al., J. Appl. Phys. 119, 013301 (2016)] is used to determine the electron charge density profile at different instants after the laser shot. When applying the electric field, a few 1013 electrons are extracted with continuous energy distributions. Particle-In-Cell simulations are run to understand the extraction process and reproduce the measured energy distributions, with the electron charge density profiles used as inputs. These simulations show that the electron extraction proceeds from the plasma front edge throughout the plasma expansion. [ABSTRACT FROM AUTHOR]

The paper presents results of numerical investigation of a large-volume plasma source based on a DC discharge with gridded anode. Geometry and configuration of the electrodes were chosen so as to ensure the formation of a cathode sheath, which would accelerate electrons up to high energies and inject them into the post-anode space and create plasma. Simulations were carried out using a hybrid model, and distributions of the main discharge parameters were obtained in a wide range of currents. At low currents, cathode sheath occupies whole interelectrode gap while plasma is formed in the post-anode space. It is shown that ions moving through the anode grid into the interelectrode gap cause reduction of discharge voltage when compared to the case of classical obstructed discharge with virtually closed anode grid. At higher currents, however, plasma is formed within the interlectrode gap as well, and ions moving from plasma in the post-anode space become trapped by reversed electric field. This essentially nullifies influence of the post-anode plasma on discharge properties. [ABSTRACT FROM AUTHOR]

Since plasma parameters are altered by dust particles, studying how plasma parameters are related to dust particle growth is an important research issue in dusty plasma. In this paper, the correlation between plasma parameters (electron temperature and ion flux) and particle parameters (particle radius and density) is investigated in silane plasma both experimentally using a floating probe and theoretically by solving balance equations including an additional electron and ion loss to the dust. The results reveal that while the ion flux shows two peak values in the early discharge phase and at the end of coagulation phase, the electron temperature shows a sudden increase in the coagulation step and a gradual decrease in the molecular accretion step. Moreover, the calculated results with the secondary electron emission taken into account produce the best fit with the experimental results. Thus the study confirms that the secondary electron emission plays a crucial role in the coagulation of the dust particles. [ABSTRACT FROM AUTHOR]

The l=3 diocotron mode in an electron plasma confined in a Malmberg–Penning trap has been resonantly excited by means of a rotating electric field applied on an azimuthally four-sectored electrode. The experimental observations are interpreted with a theory based on the linearization of the drift-Poisson equations and by means of two-dimensional particle-in-cell simulations. The experimental technique presented in this paper is able to selectively excite different diocotron perturbations and can be efficiently used for electron or positron plasma control and manipulation. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of air discharges at high pressure which operates in a streamer regime. The aim is to provide a simplified framework to study the chemical kinetics in the discharge gas phase. We have tried to model the characteristics of actually existing devices, such as dc and ac discharges in sharp point-plane, point to point, or coaxial cylinder configuration, dielectric barrier discharges. Within such scheme it is possible to perform numerical simulations in order to predict the evolution of the gas-phase chemical composition. As a first application we have chosen to simulate the chemical kinetics induced in a single isolated streamer and the results of this approach are examined within this paper. In particular, we have studied the detailed temporal evolution of the discharge gas phase using reference values for the simulation free parameters. The interplay between different time scale processes is shown. The effect due to charged as well as metastable species on chemical kinetics has been discussed too. Then we have studied the dependence of chemical species density on discharge parameters such as the electron temperature, the electron density, the transverse radius of the streamer, and the streamer formation time. Many results can be proven relevant for a better understanding of the operating conditions during technological sensible processing based on atmospheric pressure plasma. [ABSTRACT FROM AUTHOR]

This paper presents a non-oscillatory quasineutral plasma model, in which the electric field in the ion momentum equation employs the electron drift-diffusion formulation. Due to the quasineutral assumption, the electron pressure is directly coupled with the ion pressure in the inviscid flux of the ion momentum equation, which is solved using a finite-volume method with nonlinear limiting. The electron-pressure coupled method eliminates the grid-scale numerical oscillations that occur particularly near the stagnation point (the ion bulk velocity is close to zero) of a cross-field discharge plasma when using an uncoupled method. Additionally, such numerical oscillations are also observed when using a quasineutral hybrid model where a grid- or particle-based kinetic approach is employed for ions. The importance of coupling the electron pressure and ion momentum in a quasineutral plasma model is discussed. [ABSTRACT FROM AUTHOR]

Plasma plumes have found a wide range of applications over the recent decade, stimulating studies of characteristics of plasma plumes generated under different conditions. Regardless of whether they propagate within a dielectric tube or different shielding gases, the behavior of these plumes will be affected by the boundary condition of the plume. Yet, at present, little is known about the behavior of plasma plumes of different polarities, especially negative plasma plumes, when propagating under different boundary conditions. To bridge this gap, in this paper, the characteristics of positive and negative plasma plumes propagating within a quartz tube, ambient Ar, and air are studied. The results reveal that the behavior of the positive plasma plume is similar under three different boundary conditions. However, this is not the case for the negative plasma plume, the behavior of which differs significantly between the three cases. Numerical simulation suggests that electron loss due to the drift in the radial direction impacts significantly the characteristics of the negative plasma plume. [ABSTRACT FROM AUTHOR]

This paper studies the self-consistent interactions between whistler envelope solitons and electron beams in inhomogeneous plasmas, using a Hamiltonian model of wave-particle interaction where nonlinear equations describing the dynamics of whistler and ion acoustic waves and including a beam current term are coupled with Newton equations. It allows describing the parallel propagation of narrowband whistlers interacting with arbitrary particle distributions in irregular plasmas. It is shown that the whistler envelope soliton does not exchange energy with all the resonant electrons as in the case of whistler turbulence but mostly with those moving in its close vicinity (locality condition), even if the downstream particle distribution is perturbed. During these interactions, the soliton can either damp and accelerate particles, or absorb beam energy and cause electron deceleration. If the energy exchanges are significant, the envelope is deformed; its upstream front can steepen, whereas oscillations can appear on its downstream side. Weak density inhomogeneities as the random fluctuations of the solar wind plasma have no strong impact on the interactions of the whistler soliton with the resonant particles. [ABSTRACT FROM AUTHOR]

In this work involving an atmospheric dielectric barrier discharge system, the feasibility of independence control of key plasma parameters through strategic modulation of applied dual-frequency excitation sources is demonstrated. In this paper, a one-dimensional fluid model with semi-kinetic method has been used to investigate the discharge properties of atmospheric dielectric barrier discharge by using 200 kHz/13.56 MHz excitations. Bearing good consistency and coherence with experimental results, the electrical characteristics and typical electron dynamics are studied numerically. It is revealed that the application of the high frequency excitation can restrain the increment of the low frequency current component and is effective in preventing the discharge from transitioning to a filamentary mode. This method then suggests and enables possible approaches to obtain discharges with good stability in described DBD systems. [ABSTRACT FROM AUTHOR]

Experimental observations of a new regime of whistler propagation in the laboratory are reported in this paper. The experiments are carried out in a large laboratory unbound uniform plasma with a density of ne ~ 109-1011 cm-3 and a magnetic field of 1-20G. Studies are performed in the electron magnetohydrodynamic regime, which is governed by electron dynamics with Le⪡L⪡Li and τci⪢τ ⪢ τce, where L and τ are the spatial and temporal scale lengths of the perturbations, Le and Li the electron and ion Larmor radii, respectively, and τci and τce the temporal scales corresponding to the ion gyrofrequency and electron gyrofrequency, respectively. The complete topology of the perturbed wave magnetic field is unravelled by mapping it on a two dimensional grid over repeated plasma shots. It is observed that the excited waves are elongated whistlers in the propagation direction. However, in the perpendicular direction, the extent is limited to scale lengths of the order of natural scale length of plasma, i.e., the skin depth (~c/ωpe), rather than being oblique as predicted by theory and observed in other experiments. The wave shows a feeble resonance cone at an angle of ~10° but no obliqueness. Also, the secondary induction, which helps the wave to propagate further into the plasma, does not manifest within the spatial scale lengths characteristic to this regime. However, the wave is still able to maintain the whistler speed calculated for the given plasma and pulsed current parameters. It is speculated that the natural scale length (skin depth) and in turn the electron inertia may play a significant role, leading to the observed results. A physical model is also presented that could possibly explain the observed phenomena. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) particle-in-cell (PIC; kinetic ions and electrons) and hybrid (kinetic ions; adiabatic and massless fluid electrons) simulations of highly oblique slow shocks (θBn=84° and β=0.1) [Yin et al., J. Geophys. Res., 110, A09217 (2005)] have shown that the dissipation from the ions is too weak to form a shock and that kinetic electron physics is required. The PIC simulations also showed that the downstream electron temperature becomes anisotropic (Te∥>Te⊥), as observed in slow shocks in space. The electron anisotropy results, in part, from the electron acceleration/heating by parallel electric fields of obliquely propagating kinetic Alfvén waves (KAWs) excited by ion-ion streaming, which cannot be modeled accurately in hybrid simulations. In the shock ramp, spiky structures occur in density and electron parallel temperature, where the ion parallel temperature decreases due to the reduction of the ion backstreaming speed. In this paper, KAW and electron physics in oblique slow shocks are further examined under lower electron beta conditions. It is found that as the electron beta is reduced, the resonant interaction between electrons and the wave parallel electric fields shifts to the tail of the electron velocity distribution, providing more efficient parallel heating. As a consequence, for βe=0.02, the electron physics is shown to influence the formation of a θBn=75° shock. Electron effects are further enhanced at a more oblique shock angle (θBn=84°) when both the growth rate and the range of unstable modes on the KAW branch increase. Small-scale electron and ion phase-space vortices in the shock ramp formed by electron-KAW interactions and the reduction of the ion backstreaming speed, respectively, are observed in the simulations and confirmed in homogeneous geometries in one and two spatial dimensions in the accompanying paper [Yin et al., Phys. Plasmas 14, 062104 (2007)]. Results from this study conclude that the inability of the hybrid method to model the Landau resonance of KAW with electrons and the resulting time-evolving electron parallel heating lead to differences between the PIC and hybrid simulations. [ABSTRACT FROM AUTHOR]

This paper investigates the physical mechanisms that cause beneficial and detrimental performance effect observed to date in Hall effect thrusters with wall electrodes. It is determined that the wall electrode sheath can reduce ion losses to the wall if positioned near the anode (outside the dense region of the plasma) such that an ion-repelling sheath is able to form. The ability of the wall electrode to form an ion-repelling sheath is inversely proportional to the current drawn--if the wall electrode becomes the dominant sink for the thruster discharge current, increases in wall electrode bias result in increased local plasma potential rather than an ion-repelling sheath. A single-fluid electron flow model gives results that mimic the observed potential structures and the currentsharing fractions between the anode and wall electrodes, showing that potential gradients in the presheath and bulk plasma come at the expense of current draw to the wall electrodes. Secondary electron emission from the wall electrodes (or lack thereof) is inferred to have a larger effect if the electrodes are positioned near the exit plane than if positioned near the anode, due to the difference in energy deposition from the plasma. [ABSTRACT FROM AUTHOR]

In this paper, we qualitatively analyzed peculiarities of laser phase behavior associated with the accelerated electron in a chirped laser pulse. We unveiled the relationship between the changes in the orientation of the electron trajectory and the cusps in magnitude of the phase velocity of the optical field along the electron trajectory in a chirped laser pulse. We also explained how the chirp effect induced the singular point of the phase velocity. Finally, we discussed the phase velocity and phase witnessed by the electron in the particle's moving instantaneous frame. [ABSTRACT FROM AUTHOR]

The paper presents the results of studies of a coaxial gun for a plasma-filled electron diode. Effects of the discharge channel diameter and gun current on characteristics of the plasma and pulse generated in the diode were investigated. The electron beam with maximum energy of ≥1 MeV at the current of ≈100 kA was obtained in the experiments with a plasma-filled diode. The energy of ≈5 kJ with the peak power of ≥100 GW dissipated in the diode. [ABSTRACT FROM AUTHOR]

The spectrum of Lyapunov exponents is powerful tool for the analysis of the complex system dynamics. In the general framework of nonlinear dynamics, a number of the numerical techniques have been developed to obtain the spectrum of Lyapunov exponents for the complex temporal behavior of the systems with a few degree of freedom. Unfortunately, these methods cannot be applied directly to analysis of complex spatio-temporal dynamics of plasma devices which are characterized by the infinite phase space, since they are the spatially extended active media. In the present paper, we propose the method for the calculation of the spectrum of the spatial Lyapunov exponents (SLEs) for the spatially extended beam-plasma systems. The calculation technique is applied to the analysis of chaotic spatio-temporal oscillations in three different beam-plasma model: (1) simple plasma Pierce diode, (2) coupled Pierce diodes, and (3) electron-wave system with backward electromagnetic wave. We find an excellent agreement between the system dynamics and the behavior of the spectrum of the spatial Lyapunov exponents. Along with the proposed method, the possible problems of SLEs calculation are also discussed. It is shown that for the wide class of the spatially extended systems, the set of quantities included in the system state for SLEs calculation can be reduced using the appropriate feature of the plasma systems. [ABSTRACT FROM AUTHOR]

Precaution to remove the serious effect of electrode contamination in Langmuir probe experiments has not been taken in many space measurements because the effect is either not understood or ignored. We stress here that one should pay extra attention to the electrode contamination effect to get accurate and reliable plasma measurements so that the long time effort for sounding rocket/satellite missions does not end in vain or becomes less fruitful. In this paper, we describe two main features of voltage-current characteristic curves associated with the contaminated Langmuir probe, which are predicted from the equivalent circuit model, which we proposed in 1970's. We then show that fast sweeping dc Langmuir probes can give reliable results in the steady state regime. The fast sweeping probe can also give reliable results in transient situations such as satellite moves through plasma bubble in the ionosphere where the electron density drastically changes. This fact was first confirmed in our laboratory experiment. [ABSTRACT FROM AUTHOR]

In this paper, we consider an unmagnetized plasma consisting of warm adiabatic ions, superthermal electrons, and thermal positrons. Nonlinear cylindrical and spherical modified Korteweg-de Vries (KdV) equations are derived for ion acoustic waves by using reductive perturbation technique. It is observed that an increasing positron concentration decreases the amplitude of the waves. Furthermore, the effects of the superthermal parameter (k) on the ion acoustic waves are found. [ABSTRACT FROM AUTHOR]

The expressions for ion and electron thermal forces and electron friction force acting on a spherical dust grain from plasma embedded into a magnetic field are found. The electron and ion distribution functions were considered by using 21 moments Grad approximation. Dust grain with a radius much smaller than ion and electron gyroradii was assumed to be either negatively or relatively weakly positively charged. The forces found in this paper can be important, particularly for dust dynamics in fusion plasmas with stagnated plasma flow. [ABSTRACT FROM AUTHOR]

Dielectric window breakdown, whose mechanism is not thoroughly understood, is a major factor of limiting the transmission and radiation of high-power microwave on the order of 1 GW. In this paper, a one-dimensional fluid-like sheath model is developed to investigate the sheath structures formed at different gas pressures. The dominant processes during the surface flashover are isolated by this model. In vacuum, electron multipactor is self-sustained by secondary electron emission, a positive space-charge potential is formed on the dielectric surface. With increasing gas pressure, electron-neutral ionization prevails against secondary electron emission. The multipactor effect is suppressed by the shielding of plasma electrons. This leads to the sheath potential changing gradually from a positive space-charge potential to a negative space-charge potential. For argon gas pressure lower than 14 Torr, the sheath is space charge limited. A potential minimum could be formed in front of the dielectric which traps secondary electrons emitted from the wall. With the higher argon gas pressure, the number density of ions becomes comparable to that of electrons, all surface produced electrons are accelerated toward the presheath region. Therefore, the normal sheath is formed and the resulting surface flashover on the dielectric surface becomes rf-driven volumetric breakdown. [ABSTRACT FROM AUTHOR]

Divergent relativistic electrons are produced during the intense laser interacting with overdense or solid targets. When these energetic electrons propagate through a structured two-layer target, a strong interface magnetic field will be generated due to the plasma density difference near the interface. This field will affect the motions of the electrons and can be utilized to control the beam divergence. In this paper, the effect of the target density profile on beam control is explored via a physical model as well as two-dimensional particle-in-cell simulations. It is shown that the fast electron number and forward current can be greatly concentrated and enhanced if the plasma density of the inner layer is suitably lower than that of the outer layer. [ABSTRACT FROM AUTHOR]

Measurements of the image charge induced on electrodes provide the primary means of diagnosing plasmas in the Lawrence Non-neutral Torus II (LNT II) [Phys. Rev. Lett. 100, 155001 (2008)]. Therefore, it is necessary to develop techniques that determine characteristics of the electron plasma from features of the induced image charge signal. This paper presents a numerical study which finds that the frequency of the image charge signal due to the toroidal version of the m=1 diocotron mode is proportional to the total trapped charge and inversely proportional to magnetic field strength, as in the cylindrical case. In the toroidal case, additional information about the m=1 motion of the plasma can be obtained by analysis of the image charge signal amplitude and shape. Finally, results from the numerical simulations are compared to experimental data from the LNT II and plasma characteristics are reported. [ABSTRACT FROM AUTHOR]

Ion acoustic solitons may be associated with the electrostatic solitary structures observed in the space plasma environment. Earlier models describing these solutions assume Maxwellian distribution for electrons imbedded in the fluid ions and predict enhanced density that have difficulty in explaining some of the observed features of depleted density associated with electrostatic solitons. Improved models incorporating nonthermal electrons are shown to be able to produce solitary structures with depleted density [R. A. Cairns et al., Geophys. Res. Lett. 22, 2709 (1995)]. In this paper general solutions of ion acoustic solitons are derived for which the electrons are modeled by the kappa distribution characterized by the κ parameter that has been found to well represent the particle distribution observed in space plasmas. It is shown that solitary structures with depleted density are unlikely to exist in non-Maxwellian plasmas with general κ distribution. Detailed comparisons are made of the solutions based on Cairns et al. and κ velocity distributions. [ABSTRACT FROM AUTHOR]

Gas, fluid, or solid Cherenkov-type detectors have been widely used in high-energy physics for determination of parameters of charged particles, which are moving with relativistic velocities. This paper presents experimental results on the detection of runaway electrons using Cherenkov-type detectors in the ISTTOK tokamak discharges. Such detectors have been specially designed for measurements of energetic electrons in tokamak plasma. The technique based on the use of the Cherenkov-type detectors has enabled the detection of energetic electrons (energies higher than 80 keV) and determination of their spatial and temporal parameters in the ISTTOK discharges. Obtained experimental data were found in adequate agreement to the results of numerical modeling of the runaway electron generation in ISTTOK. [ABSTRACT FROM AUTHOR]

This paper presents two-fluid simulations of forced magnetic reconnection with finite electron inertia in a collisionless three-dimensional (3D) cube with periodic boundaries in all three directions. Comparisons are made to analogous two-dimensional (2D) simulations. Reconnection in this system is driven by a spatially localized forcing function that is added to the ion momentum equation inside the computational domain. Consistent with previous results in similar, but larger forced 2D simulations [B. Sullivan, B. N. Rogers, and M. A. Shay, Phys. Plasmas 12, 122312 (2005)], for sufficiently strong forcing the reconnection process is found to become Alfvénic in both 2D and 3D, i.e., the inflow velocity scales roughly like some fraction of the Alfvén speed based on the upstream reconnecting magnetic field, and the system takes on a stable configuration with a dissipation region aspect ratio on the order of 0.15. [ABSTRACT FROM AUTHOR]

When a weak probe laser pulse is injected into a wakefield excited by a short high-intensity pump laser pulse, the probe pulse will be Raman scattered by the wakefield. It is possible to determine the density profile from the spectrum of this forward Raman scattered probe laser [R. E. Slusher and C. M. Surko, Phys. Fluids 23, 472 (1980)]. In this paper, an analytical solution for the multiple sidebands of the forward Raman scattering of the probe laser pulse is presented. These multiple sidebands are connected with the steepening of density perturbation of the wakefield. More detailed information of the laser wakefield can be obtained from these multiple sidebands. The propagation of the probe pulse in wakefields is studied with one-dimensional particle-in-cell (PIC) simulations. The analytical solution and the results of PIC simulations are consistent with each other and other experiments. [ABSTRACT FROM AUTHOR]

Superthermal particle distributions well-described by the family of κ-distributions have been observed in various astrophysical plasmas. In this paper, the drift kink instability in the current sheet with a κ-distribution is investigated in the framework of linear kinetic theory. The orbit integrals are treated numerically using the exact unperturbed particle orbits, and the resulting eigenvalue problem of the integro-differential equations is solved using the spectral method. The growth rate, eigenmode structure, and parametric dependencies of the kink mode are examined and compared with the case of the standard Harris current sheet. The results show that the drift kink instability in the κ-distribution current sheet resembles its counterpart in the standard Harris sheet, but has a smaller growth rate and real frequency for small value of κ. It is also demonstrated that a background population can enhance the growth rate of the kink mode, making the growth rate significant at the physical value of the ion-electron mass ratio. [ABSTRACT FROM AUTHOR]

The possibility of the nonlinear decay of a localized perturbation into the ion-acoustic solitons is studied. The present paper is a theoretical attempt to model the experimental soliton excitation [S. Yi et al., Phys. Plasmas 4, 2436 (1997)] in which solitons are excited by a grid modulating near the ion plasma frequency. It has been reported that a localized wave was created near the grid that would decay nonlinearly into the ion-acoustic soliton and a wave packet [T. E. Sheridan et al., Phys. Plasmas 5, 3165 (1998)]. For this purpose, an unbounded plasma composed of the cold positively charged ions and hot electrons is considered. Electron trapping is included in the model as the result of their nonlinear resonant interaction with the localized potential well. A Gaussian initial perturbation is used to model the localized perturbation. The numerical calculation through a modified Korteweg–de Vries equation simulates the decay of the initial Gaussian perturbation into several ion-acoustic solitons and a wave packet. It is shown that the electron trapping has an essential influence on the decay process. [ABSTRACT FROM AUTHOR]

This study examines the properties and evolution of strong, laminar double layers in the downward current region of the aurora and present several previously unpublished events from the Fast Auroral SnapshoT (FAST) satellite. Analysis of an event is presented for which the FAST satellite appears to have dwelled on the high-potential side (high-altitude side) of a double layer for an extended period, suggesting that a small (few percent of density) background of suprathermal electrons can strongly influence the structure and stability of double layers. From these observations, it is inferred that the accelerated electrons can be accelerated by either of two classes of double layers: (1) self-regulated double layers (varying field-aligned potential, with weak or no suprathermal electron background) or (2) externally regulated double layers (less variable field-aligned potential, with significant suprathermal electron background). Elements of the interpretation presented here are supported by numerical simulations, as reported in two accompanying papers. [ABSTRACT FROM AUTHOR]

For many applications of gyrodevices, it is extremely important to increase their overall efficiency and reduce the power consumption. Therefore, at present, there are many gyrotrons operating with depressed collectors. These gyrotrons and their depressed collectors are typically designed with the use of available numerical codes. This paper is devoted to the analysis of the energy recuperation in gyrotrons, which is performed with the use of the Hamiltonian formalism. Such consideration gives some insight into the physics of gyrodevices and can be useful for estimating the benefits from utilizing depressed collectors. Both single-cavity gyromonotrons and multicavity gyrodevices in which the last cavity is excited by a prebunched beam are analyzed. Special attention is paid to a three-cavity, frequency-quadrupling gyrodevice. [ABSTRACT FROM AUTHOR]

The next step toward fusion as a practical energy source is the design and construction of ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)], a device capable of producing and controlling the high-performance plasma required for self-sustaining fusion reactions, i.e., “burning plasma.” ITER relies in part on ion-cyclotron radio frequency power to heat the deuterium and tritium fuel to fusion temperatures. In order to heat effectively, the radio frequency wave fields must couple efficiently to the dense core plasma. Calculations in this paper support the argument that this will be the case. Three-dimensional full-wave simulations show that fast magnetosonic waves in ITER propagate radially inward with strong central focusing and little toroidal spreading. Energy deposition, current drive, and plasma flow are all highly localized near the plasma center. Very high resolution, two-dimensional calculations reveal the presence of mode conversion layers, where fast waves can be converted to slow ion cyclotron waves. When minority ions such as deuterium or helium-3 are used to damp the launched waves, these ions can be accelerated to high energies, forming suprathermal tails that significantly affect the wave propagation and absorption. By neglecting the toroidal localization of the waves and the finite radial excursion of the energetic particle orbits, the quasilinear evolution of these suprathermal ion tails can be simulated self-consistently in one spatial dimension and two velocity dimensions. [ABSTRACT FROM AUTHOR]

Streaming instabilities of intense charged particle beams propagating along a solenoidal magnetic field in a background plasma are studied analytically and numerically. It is shown that the growth rate of the electromagnetic Weibel instability is modified by a relatively weak solenoidal magnetic field such that ωce>βbωpe, where ωce is the electron gyrofrequency, ωpe is the electron plasma frequency, and βb is the ion-beam velocity relative to the speed of light. Moreover, the Weibel instability is limited to very small propagation angles and long longitudinal wavelengths satisfying k∥2≲k⊥2 and c2k∥2≲ωpb2ωpi2/(ωpb2+ωpi2), where ωpb and ωpi are the plasma frequencies of the beam ions and the background plasma ions, respectively. For shorter longitudinal wavelengths, the electrostatic lower-hybrid instability becomes dominant. In this paper, the growth rates of various electrostatic beam-plasma instabilities and the electromagnetic Weibel instability are compared, and the space-time development of the modified two-stream instability is studied in detail and compared with numerical simulations. [ABSTRACT FROM AUTHOR]

This paper discusses the existence of ion-acoustic solitons in a plasma consisting of hot and cool ions, and hot electrons, where the words “hot” and “cool” reflect the ratio of thermal effects to inertial effects. For the hot species general polytropic pressure relations are considered, as well as the isothermal limit, γ=1 (Boltzmann distribution). It is shown that negative potential solitons are not supported. Existence domains in parameter space (effective soliton Mach number and fractional cool ion density) for positive potential solitons are found, the limits being imposed by physical effects such as vanishing densities. Numerical results are given for a typical value of γ>1 and for γ=1, for three representative values of the hot ion to electron temperature ratio. [ABSTRACT FROM AUTHOR]

The purpose of the present paper is to describe the effects of target electron collisions on proton energy loss straggling in plasmas at any degeneracy. Targets are considered fully ionized so electronic energy loss is only due to the free electrons. The analysis is focused on targets with electronic density around solid values ne≃1023 cm-3 and with temperature around T≃10 eV; these targets are in the limit of weakly coupled electron gases. These types of plasma targets have not been studied extensively, though they are very important for inertial confinement fusion. The energy loss straggling is obtained from an exact quantum-mechanical evaluation, which takes into account the degeneracy of the target plasma, and later it is compared with common classical and degenerate approximations. Then electron collisions in the exact quantum-mechanical straggling calculation are considered. Now the energy loss straggling is enhanced for energies smaller than the energy before the maximum, then decreases around this maximum, and finally tends to the same values with respect to noncollisional calculation. Differences with the same results but not taking into account these collisions are as far as 17% in the cases analyzed. As an example, proton range distributions have been calculated to show the importance of an accurate energy straggling calculation. [ABSTRACT FROM AUTHOR]

Full characterization of nighttime ionospheric plasma requires access to both the ion and electron thermal core populations. Efforts to measure particle distributions with rocket and satellite detectors designed to study low energy particles are hindered because the magnitude of spacecraft charging is on the order of or greater than the energy of the bulk of ionospheric particles. This paper presents initial laboratory investigations exploring the formation of plasma sheaths with ionospheric electron energies, densities, and Debye lengths. The goal is to identify the difficulties and solutions to obtaining both thermal electron and ion velocity distributions on one payload. Sheaths around a long cylinder biased with respect to the vacuum vessel wall are studied to verify measurement procedures. An experimental setup in which two conducting spheres, with area ratios in excess of 100, are biased with respect to one another and not referenced to the wall simulates a payload-detector system. Data are compared with simple planar, cylindrical, and spherical sheath models. Ion-rich sheaths conformed with expectations. The criterion for the formation of electron-rich sheaths was found not only to depend on the ratio of ion collector area to electron collector area but also the ratio of the collector area to sheath area. Nonmonotonic electron sheaths obtained by embedding a positively biased electrode within the sheath of a more negative conductor are also explored. It was found that the superposition of two different potential geometries led to the formation of the nonmonotonic potential structures. These initial plasma ion and electron sheath investigations both clarify the behavior of a thermal electron detector previously flown in the ionosphere and explore a low energy parameter regime that is understudied in the laboratory. [ABSTRACT FROM AUTHOR]

The scope of this paper is to present and benchmark the first version of a quasilinear calculation, QuaLiKiz, based on a fast linear gyrokinetic code, Kinezero [C. Bourdelle, X. Garbet, G. T. Hoang, J. Ongena, and R. V. Budny, Nucl. Fusion 42, 892 (2002)] accounting for all unstable modes and summing over a wave-number spectrum. The fluctuating electrostatic potential frequency and wave-number spectra are chosen based on turbulence measurements and nonlinear simulations results. A peculiar focus on particle transport is developed. The directions of compressibility and thermodiffusion convections of ions and electrons are analytically derived for passing and trapped particles in both ion and electron turbulence. Also, the charge and mass dependence of trace heavy impurity convection is analytically estimated. These results are compared with quasilinear simulations done by QuaLiKiz. Finally, the impact of accounting for all unstable modes and of summing over the wave-number spectrum is shown to reverse in some cases the direction of particle fluxes. [ABSTRACT FROM AUTHOR]

This paper presents an analysis and subsequent discussion of the phenomena of self-focusing of single electromagnetic Gaussian beams and cross-focusing of multiple coaxial beams in fully ionized magnetoplasma, taking into account the Ohmic heating of the electrons by the beams and loss of energy by electrons due to collision with the ions and electronic thermal conduction; the energy gained by ions in collision with the electrons has been equated to the energy lost on account of ionic thermal conduction. It is seen that the inclusion of the ionic thermal conduction reduces self/cross-focusing for high values of the magnetic field (νe≪Ωc) and enhances the same for low values of the magnetic field (νe>ωc); here νe is the electron collision frequency and ωc,Ωc are the cyclotron frequencies of electrons and ions, respectively. The wave frequency is assumed to be much higher than the cyclotron frequency of the electrons. The results lead to the conclusion that considerable error occurs by neglecting ionic thermal conduction in a collisional plasma. [ABSTRACT FROM AUTHOR]

The linear stability of dissipative drift electrostatic modes in tokamak plasmas is studied numerically with an extended version of the gyrokinetic code Kinezero [C. Bourdelle et al., Nucl. Fusion 42, 892 (2002)] including a modified Krook collision operator to account for collisional effects on the trapped electron response. This new version of Kinezero has been successfully tested and benchmarked against the results of a more sophisticated and complete gyrokinetic solver, GS2 [M. Romanelli, C. Bourdelle, and W. Dorland, Phys. Plasmas 11, 3845 (2004)]. The critical density and temperature gradients for ion gradient driven modes (ITG) and trapped electron modes (TEM) have been computed for different values of collisionality. The threshold for TEM disappears as the collisionality increases. A detailed study of the dependence of the growth rate of the dissipative TEM on collisionality and density gradient is presented in the paper. The range of parameters where the dissipative trapped electron modes are destabilized by increasing collisionality corresponds to very steep density gradients that can hardly be achieved in today's large tokamaks. [ABSTRACT FROM AUTHOR]

External injection of an electron bunch in the laser wakefield can result in femtosecond accelerated bunches with relatively low energy spread. In this paper it is shown that the density transition from vacuum to plasma can play an important role in the trapping process. The plasma wavelength in this transition region changes continuously, which means that the injected electrons see an altering wakefield. This can result in strong defocusing of the injected bunch. It is found that the effect becomes stronger for stronger wakefields, longer transition lengths, and lower injection energies. The transition region can be avoided if the bunch is injected into the wakefield at an angle. Injecting the bunch at an angle allows the bunch to be wider and results in more charge being trapped. The dynamics of the bunch in this case are similar to the dynamics of a bunch injected in front of the laser pulse. [ABSTRACT FROM AUTHOR]

Maxwell-fluid simulations on a flat-topped moderately overdense plasma slab (typically n0/nc=1–2) by Berezhiani et al. [Phys. Plasmas 66, 062308 (2005)] {see also the previous work of Tushentsov et al. [Phys. Rev. Lett. 87, 275002 (2001)]} were seen to lead to dynamic penetration of an ultrahigh intensity laser pulse into an overdense plasma. Two qualitatively different scenarios for the penetration of laser pulse into the overdense plasma were presented depending on the background density. In the first one, the penetration of laser energy occurs by soliton-like structures moving into the plasma. In the last one, electron cavitation occurs and the penetration is possible over a finite length only. A kinetic extension is made in this paper using Vlasov-Maxwell simulations. Vlasov simulations revealed a rich variety of new phenomena associated with the trapped particle dynamics, which cannot be described in fluid models. Most notably is the observation, during the penetration phase of the pump electromagnetic wave, of a beat-wave heating scenario induced by the Doppler shift on the reflected wave at the (moving) wave front. This beat-wave generates low-frequency acoustic-like electron modes characterized by coherent trapping-type structures in phase space leading to an efficient (nonstochastic) heating process. [ABSTRACT FROM AUTHOR]

An electron injector concept for a laser-plasma accelerator was developed by E. Esarey et al. [Phys. Rev. Lett. 79, 2682 (1997)] and G. Fubiani et al. [Phys. Rev. E 70, 016402 (2004)]; it relies on the use of counterpropagating ultrashort laser pulses. In the latter work, the scheme is as follows: the pump laser pulse generates a large-amplitude laser wakefield (plasma wave). The counterpropagating injection pulse interferes with the pump laser pulse to generate a beatwave pattern. The ponderomotive force of the beatwave is able to inject plasma electrons into the wakefield. In this paper, this injection scheme is studied using one-dimensional Particle-in-Cell simulations. The simulations reveal phenomena and important physical processes that were not taken into account in previous models. In particular, at the collision of the laser pulses, most plasma electrons are trapped in the beatwave pattern and cannot contribute to the collective oscillation supporting the plasma wave. At this point, the fluid approximation fails and the plasma wake is strongly inhibited. Consequently, the injected charge is reduced by one order of magnitude compared to the predictions from previous models. [ABSTRACT FROM AUTHOR]

New direct-drive laser target designs with KrF laser light take advantage of the shorter wavelength to lower the laser energy required for substantial gain (>30×) to sub-MJ level. These low laser-energy pellets are useful in systems that could form an intermediate step towards fusion energy, such as the proposed Fusion Test Facility [S. P. Obenschain et al., Phys. Plasmas 13, 056320 (2006)]. The short wavelength laser should allow higher intensity (and higher pressure) without increasing the risk of laser-plasma instabilities. The higher pressure in turn allows higher velocities to be achieved while keeping the low aspect ratios required for hydrodynamic stability. The canonical laser energy has been chosen to be 500 kJ. A target design is presented with various laser pulse shapes and both 1D and 2D simulation results are shown. The sensitivity of these targets to both low-mode and high-mode perturbations is examined. The analysis and simulations in this paper indicate that significant gain (G=57) can be achieved for these targets even in the presence of hydrodynamic instabilities. [ABSTRACT FROM AUTHOR]

In this paper, algorithms for the solution of two-fluid plasma equations are presented and applied to the study of field-reversed configurations (FRCs). The two-fluid model is more general than the often used magnetohydrodynamic (MHD) model. The model takes into account electron inertia, charge separation, and the full electromagnetic field equations, and it allows for separate electron and ion motion. The algorithm presented is the high-resolution wave propagation scheme. The wave propagation method is based on solutions to the Riemann problem at cell interfaces. Operator splitting is used to incorporate the Lorentz and electromagnetic source terms. The algorithms are benchmarked against the Geospace Environmental Modeling Reconnection Challenge problem. Equilibrium of FRC is studied. It is shown that starting from a MHD equilibrium produces a relaxed two-fluid equilibrium with strong flows at the FRC edges due to diamagnetic drift. The azimuthal electron flow causes lower-hybrid drift instabilities (LHDI), which can be captured if the ion gyroradius is well resolved. The LHDI is known to be a possible source of anomalous resistivity in many plasma configurations. LHDI simulations are performed in slab geometries and are compared to recent experimental results. [ABSTRACT FROM AUTHOR]

Electron emission from discharge chamber walls is important for plasma maintenance in many low-pressure discharges. The electrons emitted from the walls are accelerated by the sheath electric field and are injected into the plasma as an electron beam. Penetration of this beam through the plasma is subject to the two-stream instability, which tends to slow down the beam electrons and heat the plasma electrons. In the present paper, a one-dimensional particle-in-cell code is used to simulate these effects both in a collisionless plasma slab with immobile ions and in a cross-field discharge of a Hall thruster. The two-stream instability occurs if the total electron velocity distribution function of the plasma-beam system is a nonmonotonic function of electron speed. Low-pressure plasmas can be depleted of electrons with energy above the plasma potential. This study reveals that under such conditions the two-stream instability depends crucially on the velocity distribution function of electron emission. It is shown that propagation of the secondary electron beams in Hall thrusters may be free of the two-stream instability if the velocity distribution of secondary electron emission is a monotonically decaying function of speed. In this case, the beams propagate between the walls with minimal loss of the beam current and the secondary electron emission does not affect the thruster plasma properties. [ABSTRACT FROM AUTHOR]

This paper reviews the concepts, recent history, and current challenges for realizing ultracompact particle accelerators based on surfing on plasma waves. Ideas that seemed fanciful when first proposed by John Dawson in the late 1970s have now come to fruition as a result of the development of ultrashort high-power laser and particle beam drivers, tunnel-ionized plasma sources, and one-to-one particle simulation capability. Breakthroughs in the past 2 years have dramatically advanced the field. Laser-driven gas jets now produce 100 MeV beams of electrons that are monoenergetic and that have transverse beam quality and brightness exceeding that in conventional accelerators. Electron-beam driven plasma waves driven by the 28 GeV electron beam of the Stanford Linear Accelerator Center have been used to accelerate electrons in the tail of the beam by more than 10 GeV in less than 30 cm. The prospects for extending both of these schemes to the energy frontier (currently 50 GeV) and beyond are discussed. [ABSTRACT FROM AUTHOR]

This paper describes the behavior of nickel in low confinement (L-mode) and high confinement (H-mode) Joint European Torus (JET) discharges [P. J. Lomas, Plasma Phys. Control. Fusion 31, 1481 (1989)] characterized by the application of radio-frequency (rf) power heating and featuring ITER (International Thermonuclear Experimental Reactor) relevant collisionality. The impurity transport is analyzed on the basis of perturbative experiments (laser blow off injection) and is compared with electron heat and deuterium transport. In the JET plasmas analyzed here, ion cyclotron resonance heating (ICRH) is applied either in mode conversion (MC) to heat the electrons or in minority heating (MH) to heat the ions. The two heating schemes have systematically different effects on nickel transport, yielding flat or slightly hollow nickel density profiles in the case of ICRH in MC and peaked nickel density profiles in the case of rf applied in MH. Accordingly, both diffusion coefficients and pinch velocities of nickel are found to be systematically different. Linear gyrokinetic calculations by means of the code GS2 [M. Kotschenreuther, G. Rewoldt, and W.M. Tang, Comput. Phys. Commun. 88, 128 (1995)] provide a possible explanation of such different behavior by exploring the effects produced by the different microinstabilities present in these plasmas. In particular, trapped electron modes driven by the stronger electron temperature gradients measured in the MC cases, although subdominant, produce a contribution to the impurity pinch directed outwards that is qualitatively in agreement with the pinch reversal found in the experiment. Particle and heat diffusivities appear to be decoupled in MH shots, with χe and DD>DNi, and are instead quite similar in the MC ones. In the latter case, nickel transport appears to be driven by the same turbulence that drives the electron heat transport and is sensitive to the value of the electron temperature gradient length. These findings give ground to the idea that in ITER it should be possible to find conditions in which the risk of accumulation of metals such as nickel can be contained. [ABSTRACT FROM AUTHOR]

This paper presents a derivation of the phase-matching conditions for generation of the third-harmonic (3ω1) and combination frequencies (ω1+2ω2), by two fundamental electromagnetic waves of frequencies ω1 and ω2, propagating in the extraordinary mode along a magnetic field in a collisional plasma. Expressions for the corresponding optimum distance of propagation for maximum output and the associated values of the phase functions have been derived. In the case of the third-harmonic generation phase matching occurs when the gyrofrequency of the electrons is twice the wave frequency (for all values of plasma frequency). In the case of the combination frequencies, the phase-matching condition is represented by an expression for plasma frequency ωp in terms of the wave frequencies ω1, ω2 and the gyrofrequency ωc of electrons. Furthermore, it is seen that in the (ωc/ω1)-(ω2/ω1) plane, phase matching occurs in a narrow region; for every point in this region, there is a corresponding value of (ωp/ω1). The dependence of optimum distance of propagation and the corresponding value of the phase function on electron collision frequency νav/ω1 have been graphically illustrated for a specific case. An estimate of the maximum value of the third-harmonic conversion efficiency has also been made. [ABSTRACT FROM AUTHOR]

This paper reports on a study of electron runaway for a nonideal plasma in an external electric field. Based on pseudopotential models of nonideal fully and partially ionized plasmas, the friction force was derived as a function of electron velocities. Dependences of the electron free path on plasma density and nonideality parameters were obtained. The impact of the relative number of runaway electrons on their velocity and temperature was considered for classical and semiclassical models of a nonideal plasma. It has been shown that for the defined intervals of the coupled plasma parameter, the difference between the relative numbers of runaway electron values is essential for various plasma models. [ABSTRACT FROM AUTHOR]

This paper studies the effect of nonlinear left-hand polarized waves supported by a proton beam on the linear circularly polarized instabilities driven by the same beam. It shows that the nonlinear wave can either stabilize or destabilize the linear instabilities. The effects depend on the amplitude of the nonlinear wave and on the temperature of the system. It also shows that purely electrostatic ion-acoustic-like waves, can be destabilized by the large amplitude wave. The latter instabilities do not occur in the absence of the nonlinear waves. [ABSTRACT FROM AUTHOR]

Radial electron heat transport in a low-collisionality, current-carrying resistive plasma confined in an axisymmetric toroidal magnetic field is hypothesized to be caused by the paleoclassical collisional processes of parallel electron heat conduction and radial magnetic-field diffusion. The electron distribution is Maxwellianized and the electron temperature equilibrated over a long length L (> the poloidal periodicity half-length πR0q) along helical magnetic-field lines that are diffusing radially with the resistivity-induced magnetic-field diffusivity Dη≡η∥nc/μ0≃νe(c/ωp)2. This produces a paleoclassical radial electron heat diffusivity χepc that is a multiple M≃L/(πR0q)∼10>1 of the magnetic-field diffusivity: χepc≃(3/2)MDη . New paleoclassical model developments in this paper include full axisymmetric toroidal magnetic-field geometry, evolution of toroidal, poloidal, and helical magnetic fluxes, effects of temporally varying magnetic fluxes, introduction of electron guiding center radial diffusion effects induced by poloidal magnetic-flux diffusion into electron drift kinetics via a Fokker-Planck procedure, and determination of both axisymmetric and the usually dominant helically resonant paleoclassical radial electron heat transport. [ABSTRACT FROM AUTHOR]