Density Of Energetic Electrons Research Articles

Fast Heating of High-Density Plasmas with a Reentrant Cone Concept

A reentrant cone concept for efficient heating of high-density plasmas has been studied as an advanced fast ignition scheme. The roles of the reentrant cone, as indicated by particle-in-cell (PIC) code simulations and confirmed by basic experiments, are reviewed, particularly the efficient collection and guidance of the laser light into the cone tip and the direction of the energetic electrons into the high-density region. It has been shown that the energetic electrons converge to the tip of the cone as a result of the surface electron flow guided by self-generated quasi-static magnetic fields and electrostatic sheath fields. As a result, the energetic electron density at the tip is locally greater than the case of using an open geometry such as a normal flat foil target. Using these advantageous properties of the reentrant cone, efficient fast heating of imploded high-density plasmas has been demonstrated in integrated fast ignition experiments. A hybrid PIC code (LSP) has been used to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas. The simulation results are in reasonable agreement with the integrated experiments. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-electromagnetic force impeding energetic electrons.

Backward wave oscillator regime of the whistler cyclotron instability in an inhomogeneous magnetic field

A linear theory for the backward wave oscillator generation regime of whistler waves in the Earth’s magnetosphere is presented. Using a parabolic profile of the magnetic field and a linear expression for the resonant current in the case of a zeroth order distribution function with a step discontinuity in the velocity component parallel to the magnetic field, the modes of the system are investigated by means of a search procedure. The existence of at least one mode exponentially growing in time is indicative of absolute instability, and such modes have been found. Therefore, an earlier prediction of such a regime, based on the homogeneous magnetic field model, is confirmed. The dependence of growth rates on the frequency mismatch and energetic electron density has been studied. These results yield the characteristic spatial profile and temporal growth rate of small-amplitude whistler-wave disturbances, which are likely to be the seeds for chorus emissions.

Anomalous capacitive sheath with deep radio-frequency electric-field penetration.

A novel nonlinear effect of anomalously deep penetration of an external radio-frequency electric field into a plasma is described. A self-consistent kinetic treatment reveals a transition region between the sheath and the plasma. Because of the electron velocity modulation in the sheath, bunches in the energetic electron density are formed in the transition region adjacent to the sheath. The width of the region is of order V(T)/omega, where V(T) is the electron thermal velocity, and omega is the frequency of the electric field. The presence of the electric field in the transition region results in a collisionless cooling of the energetic electrons and an additional heating of the cold electrons.

Influence of electron injection on electron cyclotron resonance plasma properties and reflected mode electrons (abstract)

The injection of an additional strong focused electron beam from a special designed electron gun into a magnetic electron cyclotron resonance (ECR) confinement field is studied. The electron gun uses a cathode with a long lifetime and resistiveness providing high emission current densities with electron currents up to 50 mA and voltages up to 4 keV. A sequence of aluminum foils is used to investigate the trajectories of the electrons in the magnetic field without plasma. The high density electron beam passes through the foils, welds them, and prints its image into the foils. Details of this technique are described in Ref. 1. Using this technique we see that before the electrons enter the sextupole region the beam moves along the magnetic straight lines preserving its structure. Only a central beam passes through the sextupole region, thereby changing its form due to the interaction with radial components of the magnetic field. A new operation method at our 14.5 GHz ECR ion source is based on so-called reflection mode electrons (RMEs) analogous to a known electron beam ion source operation regime.2 The basic idea is that electrons, which traveling from the cathode in a strong axial field, meet an anticathode potential, are reflected from it, move back to the cathode, and will be reflected again and so on. It can be supposed that the electrons will make reflections up to the moment when the anode aperture of the gun is fulfilled and the electrons will be collected on the anode electrode. Investigations are performed extracting nitrogen ions using the RME beam. As a result we got a clear increase in the beam current of the extracted ions (e.g., at 10 mA electron injection an increase of the current of N5+ ions up to 400%) and a shift of the measured ion charge state distribution to higher mean ionization stages. Measured x-ray spectra from a neon loaded plasma show for the case of RME operation increasing energy shifts to the high energy side of the spectra, i.e., the mean ionization degree of the ions in the plasma increases. They also increase the intensity of the neon K x rays (more than 100% increase for RME injection of Ee=4 keV and Ie=10 mA) indicating that for the same operation parameters the mean density of energetic electrons rises at RME injection, i.e., there are more electrons with energies high enough to ionize K-shell electrons in neon.

The relationship between pulsating auroras observed from the ground and energetic electrons and plasma density measured at geosynchronous orbit

We present comparisons of ground‐based all‐sky video recordings of pulsating auroral events and magnetically conjugate geosynchronous orbit measurements of high‐energy electrons and low‐energy plasma. The large‐scale pulsation events were found to be closely tied to substorm electron injections. For most events, only upper limits were available for the plasma density, which was ≤1–2 cm−3. We were unable to find any clear dependence between the plasma density and the pulsations. There were, however, small, rapid fluctuations in the density which had apparent spatial scales similar to those of individual pulsating auroral patches. These simultaneous measurements of the electron flux, plasma density, and pulsation period should be of use in constraining theoretical treatments of the pulsating aurora problem. We find that the conditions for minimum plasma density and plasma density gradient predicted by Demekhov and Trakhtengerts (1994) may not be supported by the geosynchronous data but that their prediction for minimum energetic electron density is supported. Predictions by Davidson and Chiu (1987) of the minimum change in plasma density and electron flux also seem to be supported. The data indicate that the principal factor controlling the onset of pulsations over a particular area may be the arrival of substorm‐injected electrons at the conjugate equator. This conclusion could provide the basis for a visible magnetic mapping of the substorm injection region through observations of the pulsating aurora.

Ultrafast carrier-carrier scattering among photoexcited nonequilibrium carriers in GaAs.

We have studied carrier-carrier scattering in photoexcited electron-hole plasmas in GaAs at plasma densities from ${10}^{15}$ to ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, using numerical solution of the dynamically screened Boltzmann equation and classical molecular dynamics. The solution of the dynamically screened Boltzmann equation indicates that for excited electrons, scattering among the injected carriers is as important a scattering process as LO-phonon emission at densities greater than about 8\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, and at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ the photoexcited electrons are nearly thermalized in 150 fsec. As a result of weaker screening, the interaction between carriers has a stronger effect in this case than when a low density of energetic electrons is immersed in a cool background plasma, where previous work has shown that carrier-carrier scattering becomes as significant as LO-phonon emission at a density of about 8\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. We also find that classical molecular-dynamics calculations are dominated by nonphysical effects at short times, arising from the pointlike nature of the simulated carriers.

Auroral kilometric radiation sources: In situ and remote observations from Viking

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generation of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low‐amplitude Z and O modes are generated at the same time as a larger‐amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest‐frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak ‐ fce)/fce ≈ 0.025 in AKR sources. Similarly, the low‐frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E ≥ 1 keV). The total density inside AKR sources is of the order of 1 cm−3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of ≥1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a ≈ 90°. The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding ∂f/∂v⊥ > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser.

Generation-annealing of oxide and interface traps at 150 and 298 K in oxidized silicon stressed by Fowler-Nordheim electron tunneling

Generation and thermal annealing of oxide and interface traps at 150 and 298 K are investigated by Fowler-Norheim electron tunneling stress. Experiments indicate that the generation rate of positive oxide charges is higher at 150 than 298 K. This could be accounted for by the higher density of energetic electrons in the oxide and at gate/oxide interface at lower temperatures due to the larger electron-phonon scattering mean free path. The annealing rates of interface traps and negative oxide traps are higher at 298 than 150 K. The higher annealing rate of the interface traps by hydrogen at higher temperatures could be interpreted by the larger hydrogen arrival rate at the interface due to the larger hydrogen diffusivity in the silicon dioxide at higher temperatures. The higher annealing rate of negative oxide traps at higher temperatures is attributed to a higher emission rate of electrons trapped at the shallow neutral electron traps at higher temperatures.

Nonthermal electron velocity distribution measured by electron cyclotron emission in Alcator C tokamak.

The nonthermal electron velocity distribution of lower-hybrid-rf--heated plasmas in the Alcator C tokamak is deduced from measurements of electron cyclotron emission with a specialized vertical viewing optics arrangement. At a density of n${\ifmmode\bar\else\textasciimacron\fi{}}_{e}$=0.7\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$, the reconstructed distribution function in the range 50 keV\ensuremath{\le}E\ensuremath{\le}230 keV shows enhancement in the parallel direction with a perpendicular spread equivalent to ${T}_{\ensuremath{\perp}}$\ensuremath{\approxeq}30 keV. The line-average energetic electron density in this energy range is approximately ${10}^{17}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ during the rf.

Direct generation of the auroral kilometric radiation by the Maser synchrotron Instability: Physical mechanism and parametric study

Recent observations at low altitudes made in the source region of the auroral kilometric radiation (AKR hereinafter) strongly support the maser synchrotron instability (MSI) as the relevant generation process for the AKR. This was first realized by Wu and Lee (1979). In a recent work (Le Quéau et al. (1984), hereinafter called reference 1) we have proposed an analytical treatment of the MSI. The present work is a continuation of this analytical study. First we investigate the physical process of the MSI, and simple analytical expressions, valid for any distribution functions of nonthermal electrons are given. It is shown that supraluminous X mode waves (ω/k∥c ≫ 1) can resonate with relatively low energy electrons provided that the cold plasma parameter, εc = ωpc²/ωc², is much smaller than unity. The proper frame of reference for studying the resonant coupling between the X mode and nonthermal electrons moves at a parallel velocity k∥c²/ω. Resonance curves are then circles centered around this value and electrons diffuse with almost constant parallel velocities. The difference between the MSI and the standard cyclotron theory (where resonance curves are straight lines while diffusion curves are circles centered around ω/k∥) is stressed. A parametric study of the instability conditions for such a supraluminous wave is conducted. We investigate the role played by θ, the propagation angle εb, the normalized density of energetic electrons δ, the energy width of the part of the distribution function where an inversion of population (∂f/∂υ⊥ > 0) occurs, and finally u, the parallel velocity which characterizes the maximum of ∂f/∂υ⊥ integrated along a resonance circle. In the case of the shifted loss cone distribution used here, u is simply its parallel bulk velocity. Largest growth rates are obtained for k∥c²/ω = u.

Radio emission from AM Herculis - The quiescent component and an outburst

The VLA has been used to search for radio emission from the AM Her-type binaries VV Pup, EF Eri, PG 1550 + 191, CW 1103 + 354, and AN UMa, at 4.9 GHz. A remarkable 10-min outburst was detected from AM Her at 4.9 GHz, which was about 20 times more intense than the quiescent emission and was essentially 100 percent circularly polarized. It is suggested that the quiescent emission of AM Her can be accounted for by 500-keV electrons trapped in the magnetosphere of the white dwarf, provided that the electron energy spectrum is quite hard and that the spectral hardness or number density of energetic electrons increases with radius, while the outburst is probably due to an electron-cyclotron maser operating near the surface of the red dwarf companion. The implied existence of a 1000-gauss localized magnetic field and a corona on the red dwarf has consequences for mass transfer, field line interactions, and variable activity.

Measurements of energetic electrons in a magnetized hot filament discharge

Measurements of the energy distribution function of primary electrons in a hot filament discharge in a magnetic field have been made. These results show a smooth distribution function which decreases monotonically with increasing energy up to a value approximately equal to the discharge voltage; the shape of the distribution is independent of other discharge parameters. An ionization kinetics model is used to estimate the energetic electron density, which is shown to vary linearly with plasma density, inversely with fill pressure, and decrease with increasing discharge voltage. At the highest plasma densities bulk heating, possibly due to coulomb drag on primary electrons, is observed.

The transport of polarized synchrotron radiation in a turbulent medium

We have developed a model to describe the transport of polarized synchrotron radiation in an optically thin source possessing small amplitude fluctuations in thermal electron density, energetic electron density, and vector magnetic field. The analysis employs an approximation termed the small phase approximation, which is similar to the first order Rytov approximation in the theory of wave propagation in a random medium. Expressions are derived relating the (observable) autocorreltions and cross-correlations of the Stokes parameters Q and U to the correlation functions of the aforementioned physical quantities. The last section of the paper is intended for observers, and describes how data from polarization maps may be used to determine the intensity and spectral characteristics of turbulence in a synchrotron source, such as a radio galaxy.

Generation of the auroral kilometric radiation

In two preceding papers (Wu and Lee, 1979, Lee et al., 1980) an effective loss‐cone distribution function is used in the stability analysis relevant to the theory of auroral kilometric radiation. Furthermore, in these discussions it is assumed that the population of low‐energy background electrons prevails over that of the energetic electrons. Obviously, this assumption may not be justifiable at high altitudes. The present work considers a loss‐cone distribution close to that observed and, moreover, includes the effect of a parallel electric field on the loss‐cone distribution function. It is found that this effect tends to enhance the growth rate. We have also removed the assumption concerning the high population of background electrons. We find that at high altitudes, where the density of energetic electrons may dominate and the usual cold electron dispersion relation is invalid, the cutoff frequency of the X mode can be significantly affected by the energetic electrons and thereby the growth rate is indirectly altered. In the present discussion, the emission of the auroral kilometric radiation is studied over a range of altitudes. We have considered three density models and two electric field models. In addition to the temporal growth rates, the corresponding e folding distances of spatial amplification are also computed and discussed.

Diagnostic methods for sputtering plasmas

This paper reviews the parameters that can be used to characterize glow discharge plasmas of the type used in many sputtering devices, and the methods available to measure those parameters. The review is also generally applicable to the plasmas used in ion plating and plasma-activated reactive evaporation. The objective is to provide a brief overview and a guide to the literature for thin-film specialists. The emphasis is on negative glow-type plasmas, and the diagnostic techniques of optical spectroscopy and electrostatic probes, since these methods are particularly effective for such plasmas and are relatively easy to implement in the laboratory. Guidelines are presented for a corona interpretation of absolute intensity line emission measurements in terms of the density of energetic electrons in an Ar-negative glow. Cases are described in which electrostatic probe measurements were used to investigate performance characteristics of cylindrical-post and cylindrical-hollow magnetron sputtering sources.

Nonlinear Flute Instabilities Inherent in Two-Component Plasmas

It is shown that a cold plasma in a weak magnetic field coexisting with a low density of energetic electrons is linearly and nonlinearly unstable if the distribution of energetic electrons is of the loss cone type, or is spherically symmetric in velocity space but nearly monoenergetic. Growth rates for linear and nonlinear instabilities are calculated and compared. It is shown that nonlinear growth rates may dominate for very low wave energy.

Inverse Compton Effect as a Possible Cause of the X-Ray Radiation of Solar Flares

SHKLOVSKY1 has suggested the inverse Compton effect as a possible cause of the solar flare-burst emission of hard X-ray photons. This process, as its name implies, involves a scattering interaction between a relativistic electron and a low-energy photon. The interaction produces an X-ray photon and the electron loses a corresponding amount of its energy. The X-ray photon production rate from this process is directly proportional to the density of ambient photons, the density of energetic electrons and the square of the electron energy. Using a radiation density which is twice the photospheric value, he estimates that it is possible to account for the observed flare-burst X-ray emission in the 1–100 keV region if the total energy of 10–100 MeV electrons in a flare is of the order of 1030 ergs.