Inhomogeneity Of Plasma Density Research Articles

Simulation of mode conversion from UHR-mode wave to LO-mode wave in an inhomogeneous plasma with different wave normal angles

Abstract We have investigated a linear mode conversion process among UHR-mode, Z-mode, and LO-mode waves by a computer simulation solving Maxwell’s equations and the motion of a cold electron fluid. The characteristics of the wave coupling process occurring in the cold magnetized plasma were examined in detail for the case of an inhomogeneity of plasma density lying perpendicular to the ambient magnetic field. The dependence of the conversion efficiency on the incident wave normal angle, wave frequency, background plasma frequency, and steepness of density gradient was studied. The results show that an efficient mode conversion occurred in the conversion process from Z-mode to LO-mode waves rather than from the coupling between UHR-mode to LO-mode waves. They also show that the highest conversion efficiency was obtained under the specific condition of the wave normal angle for the incident waves. In the specific case of such critical wave normal angles, we found that perpendicular components of refractive indexes became zero at the site of mode conversion, which is consistent with previously published results. We also show that the range of the critical normal angle varied depending on both the plasma frequency and the wave frequency. The simulation results also reveal that, when the steepness of the density gradient was taken into consideration, efficient mode conversion could be expected even in the case of the mismatch of the refractive indexes preventing the close coupling of plasma waves.

Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50–100 g cm−3 in a radius of 15–25 μm. When significant neutron enhancements are achieved by the ultra-intense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15 MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15 MeV.

Detection of dusty plasma near the E-ring of Saturn

We present several independent in-situ measurements, which provide evidence that charged dust in the E-ring interacts collectively with the dense surrounding plasma disk of Saturn, i.e., form a system of dust-plasma interaction. The results are based on data sampled by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini, which allows for interferometry of plasma density inhomogeneities (δ n/ n) with two antenna elements and a Langmuir probe sensor. The interferometer experiment detects two ion populations; one co-rotating with the planetary magnetic field and another moving with near Keplerian speed around Saturn. The full range of RPWS measurements indicates that the Keplerian population consists of colder ions ( T i<few eV), which would interact with the electrical potential cavities associated with the few volts negatively charged E-ring water-rich dust grains. The presence of the charged dust inhibits E× B pick-up of freshly ionized particles by the rotating magnetic field of Saturn, since the local potential gradients near the dust grains are stronger than the large-scale co-rotation electric field. Even so, most ions are eventually energised to energies above the dust potentials of a few volts and become part of a hotter co-rotating ion population. The observed results have direct relevance to the interactions occurring in planet forming accretion disks around young stars, since the physical environment is similar.

Anomalous energy dissipation of electron current pulses propagating through an inhomogeneous collisionless plasma medium

The evolution of fast rising electron current pulses propagating through an inhomogeneous plasma has been studied through electron magnetohydrodynamic fluid simulations. A novel process of anomalous energy dissipation and stopping of the electron pulse in the presence of plasma density inhomogeneity is demonstrated. The electron current essentially dissipates its energy through the process of electromagnetic shock formation in the presence of density inhomogeneity. A direct relevance of this rapid energy dissipation process to the fast ignition concept of laser fusion is shown.

Controlled electron injection into the wake wave using plasma density inhomogeneity

The electron injection, for the laser wake field accelerator, controlled through the plasma density inhomogeneity is studied on a basis of analytical estimates and two- and three-dimensional particle-in-cell simulations. The injection scheme requires a concordance of the density scale length and laser intensity. It is shown that at a sloping inhomogeneity of plasma the wave breaking produces stronger singularity of the electron density than at a density discontinuity, but develops slower. With the help of simulations for a moderate laser intensity, we demonstrate the optimal plasma density gradient, where the electron injection into the wake wave forms the electron beam with low divergence, small energy spread and high energy.

Electron acceleration in a rectangular waveguide filled with unmagnetized inhomogeneous cold plasma

AbstractThis paper deals with the study of propagation of electromagnetic wave in a rectangular waveguide filled with an inhomogeneous plasma in which electron density varies linearly in a transverse direction to the mode propagation. A transcendental equation in ω (microwave frequency) is obtained that governs the mode propagation. In addition, an attempt is made to examine the effect of density inhomogeneity on the energy gain acquired by the electron (electron bunch) when it is injected in the waveguide along the direction of the mode propagation. On the basis of angle of deflection of the electron motion we optimize the microwave parameters so that the electron does not strike with the waveguide walls. Conditions have been discussed for achieving larger energy gain. The plasma density inhomogeneity is found to play a crucial role on the cutoff frequency, fields and dispersion relation of the TE10 mode as well as on the acceleration gradient in the waveguide.

Generation of Intermediate Drift Bursts by Magnetohydrodynamic Waves in the Solar Corona

The plasma mechanism of radio emission generation in an inhomogeneous medium is investigated. In the model under study, the electron beam with loss-cone distribution generates upper-hybrid waves that, in turn, are transformed into radio emission. It is shown that the influence of the plasma density inhomogeneity limits the plasma waves’ intensity considerably due to variation in their wave vector. The results are used to interpret the intermediate drift (IMD) bursts. A model is proposed in which these bursts are reflections of propagating small-scale (with amplitudes of about 1% and sizes of hundreds of kilometers) magnetohydrodynamic (MHD) disturbances of magnetic tubes. It is shown that this model allows us to explain the spectral parameters of the bursts in question. At present, the lack of precise and independent data about the magnetic field does not allow us to decide definitively between the existing models (whistler or MHD waves) of the IMD bursts; nevertheless, if the proposed model is correct, it can be used to determine the characteristics of the coronal MHD waves.

Electromagnetic solitary waves in the saturation regime of stimulated Brillouin backscattering

Recent particle-in-cell simulations of the stimulated Brillouin backscattering (SBBS) of electromagnetic radiation have shown that even at sub-relativistic intensities (Iλ2 = 1016 Wμm2/cm2) non-drifting solitary waves, “solitons” for short, are easily produced, and remain almost unchanged all along the simulation time, typically for several thousands of optical cycles. They appear in the form of stable local concentrations of electromagnetic radiation trapped inside quasi-neutral density holes. The plasma density inhomogeneity associated with their presence disrupts the resonant SBBS amplification. The cavitation process is accompanied by strong electron and ion heating. The physical characteristics of such solitons are discussed and they are compared with the theoretical predictions of an analytical model for localized solution of the Maxwell equations in warm plasma.

Resonant transition radiation in plamsa with magnetic inhomogeneities

We calculate the spectrum of the resonant transition radiation generated by a fast charged particle moving in plasma with small-scale random magnetic inhomogeneities. We determine the conditions under which this type of transition radiation dominates over the transition radiation on plasma density inhomogeneities. We discuss possible applications of the resonant transition radiation in astrophysical and geophysical conditions.

Particle-In-Cell simulations of circularly polarised Alfvén wave phase mixing: A new mechanism for electron acceleration in collisionless plasmas

In this work we used Particle-In-Cell simulations to study the interaction of circularly polarised Alfv\'en waves with one dimensional plasma density inhomogeneities transverse to the uniform magnetic field (phase mixing) in collisionless plasmas. In our preliminary work we reported discovery of a new electron acceleration mechanism, in which progressive distortion of the Alfv\'en wave front, due to the differences in local Alfv\'en speed, generates an oblique (nearly parallel to the magnetic field) electrostatic field. The latter accelerates electrons through the Landau resonance. Here we report a detailed study of this novel mechanism, including: (i) analysis of broadening of the ion distribution function due to the presence of Alfv\'en waves and (ii) the generation of compressive perturbations due to both weak non-linearity and plasma density inhomogeneity. The amplitude decay law in the inhomogeneous regions, in the kinetic regime, is demonstrated to be the same as in the MHD approximation described by Heyvaerts and Priest (1983).

Diagnostics of dynamic phenomena in thermal plasma jets by a CCD camera

A series of thermal plasma jet images taken by a charge coupled device (CCD) camera were used for the diagnostics of dynamic patterns generated in turbulent flow conditions. These patterns generated in turbulent thermal plasma jets manifest themselves by inhomogeneities of plasma density, temperature, and different dynamical behavior that make these structures visible. The resulting considerable modulation of light emission is analyzed either directly by comparing the successive images or by means of the correlation analysis.

Kinetic Alfvén waves in preflare plasma

Low-frequency instabilities of plasma waves in the arch structures in solar active regions have been investigated before a flare. In the framework of mechanism of “direct initiation” of instability by slowly increasing (quasi-static) large-scale electric field in a loop the dispersion relation has been studied for the perturbations which propagate almost perpendicularly to the magnetic field of the loop. The case has been considered, when amplitude of weak (“subdreicer”) electric field sharply increases before a flare, low-frequency instability develops on the background of ion-acoustic turbulence and thickness of this turbulent plasma layer plays the role of mean characteristic scale of inhomogeneity of plasma density. If the values of the main plasma parameters, i.e. temperature, density, magnetic field amplitude allow to neglect the influence of the shear of magnetic strength lines on the instability development, then two types of the waves can be generated in preflare plasma: the kinetic Alfvén waves and some new kind of the waves from the range of slowly magneto-acoustic ones. Instability of kinetic Alfvén waves has clearly expressed threshold character with respect to the amplitude of “subdreicer” electric field. This fact seems to be useful for the short-time prediction of a flare in arch structure. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-velocity structure observed in the inner solar wind

Abstract Measurements of the motion of plasma density inhomogeneities in the inner solar wind are presented. The speeds were estimated using a cross-correlation analysis of radio frequency fluctuations of the Galileo spacecraft measured simultaneously at widely spaced ground stations. The radial projections of the correlation baselines on the pattern plane were of the order of several thousand kilometers. For cross-correlation functions calculated with comparatively short averaging times, we find that a pronounced two-velocity configuration is occasionally observed over the range of heliocentric distances 20 R ⊙ R ⊙ . The typical mean speed for such observations is about 300–400 km/s and the difference between the two predominant speeds is about 150–200 km/s. These results may indicate that the density fluctuations are associated with slow magnetosonic waves propagating in opposite directions at the local speed of sound in the reference frame moving with the mean solar wind speed. Quite reasonable estimates of the solar wind speed and speed of sound are obtained from this model. Another possible explanation of the two-velocity structures is that two independent solar wind streams are present simultaneously along different segments of the radio ray path.

The jump of reverse drift rate of V-shaped structure and the large-scale inhomogeneity of plasma density in the solar atmosphere

A rare solar radio V-shaped structure was observed on August 25, 1999 by the 4.5-7.5 GHz spectrograph of the Purple Mountain Observatory, China. It consists of a reverse drift component followed by a normal drift one, which seems to be an unusual frequency drift event because of the jump of reverse drift rate from 3.22 to 1.53 GHz/s and the fastest drift rate of -3.53 GHz/s. According to their different drift rates, the V-shaped structure can be divided into three parts. By the consideration of plasma emission of an electron beam, a three-component solar atmospheric model with large and small scale lengths lambda, lambda(1) and lambda(2) to describe the equilibrium solar atmosphere and local inhomogeneity as well as the energy loss of the electron beam due to electron-electron collision is proposed. The local scale lengths lambda(1) = 5.5 x 10(9), lambda(1) = 7.7 x 10(9) cm and (υ) over bar (1) = 0.41 c, (υ) over bar (2) = 0.25 c at Parts I and 2 of the reverse drift component are obtained. The variations of scale length of lambda(1) (1) > (υ) over bar (2), especially (υ) over bar (1) > (υ) over bar (2), could be responsible for the jump of drift rate. The normal drift rate would be interpreted as the increase of average energy E' of the electron beam at the mirror due to the decrease in electrons of lower energy via energy loss of collision. The three-component solar atmospheric model and the energy loss of the electron beam are discussed.

A role of magnetic advection mechanisms in the formation of a sunspot belt

We examined two “magnetic antibuoyancy” effects: i) turbulent diamagnetism and ii) magnetic advection caused by vertical inhomogeneity of plasma density in the SCZ (the $\nabla \rho$ effect). The Sun's rotation which yields the $\nabla \rho$ effect with new properties was taken into account. It is shown that at high latitudes antibuoyancy effects block the magnetic fields in the deep layers of the SCZ. However, in the region located near-equator the $\nabla \rho$ effect, modified by rotation, causes the upward magnetic advection. So it can facilitate penetration of strong magnetic fields to solar surface where they then arise in the “royal zone” as the sunspots. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Terrestrial low-frequency bursts: Escape paths of radio waves through the bow shock

From the analysis of 119 low-frequency (LF) burst spectra observed onboard the Wind spacecraft, we propose an interpretation of the frequency–time characteristics including the low frequency cutoff of the LF burst spectra, and we use these characteristics to sound the bow shock structure at large tailward distances from Earth. When observed from within the solar wind, LF bursts appear to be made of two spectral components. The high frequency one is bursty and observed above twice the solar wind plasma frequency f psw. The low frequency one is diffuse (ITKR) and its spectrum extends from about 2 f psw to a cutoff frequency f c not much higher than f psw; its onset time δt( f) increases as the frequency f decreases. For each of the 119 events observed from near the Lagrange point L1, the solar wind density variations were measured and the variations of the density jump across the shock calculated from plasma data all along a shock model over more than 2000 R E. But, except for a few events, neither the solar wind overdensities nor the shock density barrier can prevent waves with frequencies below f c from reaching the spacecraft. Scattering on plasma density inhomogeneities was then introduced to account for the propagation of the LF burst waves in the magnetosheath, from near Earth to their escape point through the bow shock at a frequency-dependent distance | X esc( f)| (GSE), and then in the solar wind to the spacecraft. In such media, at frequencies between 2 f psw and f psw, the bulk speed of the scattered waves decreases rapidly as f decreases, and this accounts for the observed variations of the onset time δt( f). Angular scattering can also account for the observed cutoff at f c if the distance | X esc( f)| increases exponentially when f/ f psw decreases. As the shock model we used meets that requirement, we consider that this model is valid, which implies that the bow shock still exists beyond 1000 R E from the Earth. The observed decrease of the average spectral intensity of the LF burst between about 1.5 f psw and 2 f psw can also be explained by the scattering in the solar wind if we take into account the angular distribution of the rays when they leave the bow shock.

Generation of Electromagnetic Waves by Ion Beams at the Magnetopause

We present a theory for the generation of both long and short wavelength (in comparison with the ion gyroradius) electromagnetic waves that are triggered by ion beams at the Earth's magnetopause. We use a fluid model for long wavelength limit, and a hybrid approach (a kinetic theory for ion response and a fluid model for electron response) for short wavelength limit, and derive the linear dispersion relations for both cases. The latter are analyzed numerically for parameters corresponding to the Earth's magnetopause. It is shown that the ion beam with a speed greater than the ion thermal speed and the plasma density inhomogeneity with scalelength less than the electron skin-depth destabilize the electromagnetic waves of lower-hybrid frequencies. The present study may be useful for understanding the origin of numerous scale-sized fluctuations that are observed at the Earth's magnetopause.

Surface-wave produced discharges in hydrogen: II. Modifications of the discharge structure for varying gas-discharge conditions

The self-consistent structure of diffusion-controlled surface-wave-sustained discharges (SWSDs) in hydrogen is presented in terms of its dependence on the varying gas-discharge conditions (wave-frequency and gas-pressure). First, results for the interrelated variations of the plasma characteristics (concentrations of the ionic (H+, H2+ and H3+) and neutral-gas (H and H2) species, electron and gas temperatures, power Θ absorbed on average by an electron) over a wide range of variation of the electron concentration n̄e (averaged over the discharge cross section) are discussed for different gas-pressures. This exhibits not only the influence of changing gas-pressure but also the influence of varying wave-frequency, since, as is known, the range of the n̄e variation along the plasma columns in SWSDs shifts towards higher n̄e with increase in the wave-frequency. The necessity of using effective ion mobilities for lower pressure is demonstrated. The redistribution—with variation in the electron concentration—of the main processes which contribute to Θ is shown. Next, the complete structure of microwave and rf-discharges is shown by also including the resulting axial profiles of electron concentrations and wave characteristics (wavenumber and space damping rate). The wave behaviour is described in radially inhomogeneous plasmas with arbitrary collisions. With the choice of rf- and microwave-discharges, the results obtained cover regimes of weak and strong collisions. The type of electron heating in the wave field is discussed. The influence of changing the wall temperature as well as of varying the wall temperature along the discharge length is described. Finally, effects related to the radial plasma-density inhomogeneity and changing radius of the discharge tube are presented.

Random density inhomogeneities and focusability of the output pulses for plasma-based powerful backward Raman amplifiers

Random plasma density inhomogeneities may defocus the output pulses of powerful backward Raman amplifiers (BRA). Because of ultrahigh intensities of even nonfocused BRA outputs, such distortions, if occur, are then difficult to correct. A simple expression is derived for the largest BRA length for which the output pulse focusability is not yet spoiled. Interestingly, this limitation does not depend on the pump laser intensity. A useful effect of plasma inhomogeneities that might help to suppress premature pump backscattering by thermal noise is also pointed out.

Global analysis of ICRF wave coupling on Tore Supra

The Tore Supra tokamak is equipped with a multi-megawatt ion cyclotron range of frequency (ICRF) system for heating and current drive. The coupling of the fast wave to the plasma, characterized by the distributed coupling resistance along the radiating straps, is a crucial issue in order to launch large RF powers.Many factors can have an effect on ICRF wave coupling. Quantitative prediction from theoretical modelling requires the knowledge of the local inhomogeneous plasma density profile in front of the antenna for running sophisticated antenna codes. In this work, we have rather followed a `global' approach, based on Tore Supra experimental results, for the parametric study of the coupling resistance. From a large data base covering seven experimental campaigns (∼2250 shots), a scaling law of the coupling resistance including the main parameters of the plasma and of the antenna configuration is established. This approach is found to be reliable for the analysis of coupling in the different scenarios: He/D2 gas filling, gas/pellets for plasma fuelling, plasma leaning on inner wall/low field side limiter, limiter/ergodic divertor configuration, minority heating/direct electron heating.From one scenario to another, a significant variation of the coefficients of the scaling law is found. The study of these variations allows to get some insight on the main physical mechanisms which influence the ICRF wave coupling in a tokamak operation, such as the wall conditioning and recycling conditions, RF sheaths or frequency.