Theory of harmonic generation in a plasma produced by the photoionization of gas atoms with electrons in the <Emphasis Type="Italic">np</Emphasis> states

For a plasma produced by the photoionization of hydrogen-like atoms with electrons in the np states, a theory is developed that describes the nonlinear plasma polarizability due to electron-ion collisions, which governs the bremsstrahlung-induced coherent harmonic generation. The effective partial collision frequencies are obtained as functions of the pump field intensity for the first four p states of hydrogen-like atoms and for the third, fifth, seventh, ninth, and eleventh harmonics. These analytic results make it possible to establish the scalings of the collision frequencies with pump field intensity, the principal quantum number, and the number of the generated harmonic. In the case of pump fields of comparatively low intensities, some qualitative differences are revealed between these scalings and the corresponding scalings obtained for the Bethe regime of suppression of the photoionization barrier in a gas of hydrogen-like atoms with electrons in the ns states.     

Theory of the polarization bremsstrahlung from thermal electrons scattered by the Debye sphere of an ion in a plasma

The polarization bremsstrahlung from thermal electrons scattered by the Debye sphere of an ion in a plasma is studied in the quasiclassical approximation. The model of the local plasma frequency is used to check the validity of the asymptotic expression for the polarizability of the electron cloud of an ion in the high-frequency range. This asymptotic expression is then used to derive a formula for the intensity of the total effective polarization bremsstrahlung. The R factor (the ratio of the contribution from the polarization bremsstrahlung to the contribution from conventional static bremsstrahlung) is obtained as a function of the plasma coupling parameter and electron density in order to analyze the role of the polarization bremsstrahlung in the total bremsstrahlung of the thermal plasma electrons. The spectral intensity of the effective polarization bremsstrahlung is calculated in the rotational approximation, which was previously employed in the theory of conventional static bremsstrahlung. It is shown that the spectral intensity of the polarization bremsstrahlung from thermal electrons scattered by the Debye sphere around an ion, as compared with the polarization bremsstrahlung by fast superthermal electrons, decreases more gradually with increasing frequency.     

Investigation of the electron energy distribution function in hollow-cathode glow discharges in nitrogen and oxygen

The mechanism for the formation of the inverse electron distribution function is proposed and realized experimentally in a nitrogen plasma of a hollow-cathode glow discharge. It is shown theoretically and experimentally that, for a broad range of the parameters of an N₂ discharge, it is possible to form a significant dip in the profile of the electron distribution function in the energy range ε=2–4 eV and, accordingly, to produce the inverse distribution with df(ε)/d?>0. The formation of a dip is associated with both the vibrational excitation of N₂ molecules and the characteristic features of a hollow-cathode glow discharge. In such a discharge, the applied voltage drops preferentially across a narrow cathode sheath. In the main discharge region, the electric field E is weak (E<0.1 V/cm at a pressure of about p～0.1 torr) and does not heat the discharge plasma. The gas is ionized and the ionization-produced electrons are heated by a beam of fast electrons (with an energy of about 400 eV) emitted from the cathode. A high-energy electron beam plays an important role in the formation of a dip in the profile of the electron distribution function in the energy range in which the cross section for the vibrational excitation of nitrogen molecules is maximum. A plasma with an inverted electron distribution function can be used to create a population inversion in which more impurity molecules and atoms will exist in electronically excited states.     

Analysis of ionization and recombination processes under the coronal equilibrium conditions by using a statistical atomic model

Ionization and recombination processes accompanying collisions of free electrons with plasma ions are considered using a statistical atomic model in which ionization and recombination are regarded as the processes of pair electron collisions in the electron gas of an atom. An expression for the ionization rate as a function of the ionization energy I and temperature T is derived. According to this expression, the ionization rate at I ? T is proportional to exp(?I/T). The statistical atomic model provides an estimate of the recombination rate for an ion with an arbitrary nuclear charge number Z, whereas more exact calculations of the recombination rate can be performed only for large Z. The model explains relatively low values of I/T (as compared to those given by the Saha equation) under the coronal equilibrium conditions and predicts a reduction in I/T with increasing Z. The values of I/T and the average ion charge number obtained from the balance equation for multielectron ions with the use of one fitting coefficient agree with the tabulated data calculated in the multilevel coronal model.     

The conceptual development of stationary plasma thrusters

The history of the development of the concept of the stationary plasma thruster is described. The data obtained indicate the possibility of creating extended (over a distance substantially longer than the Debye radius) electric fields in a fully ionized plasma with a relatively high electron temperature (T_e>10 eV) and a conductivity close to the classical one. Based on these results, a number of fundamentally new plasma-dynamic systems were proposed; in particular, the principles of plasma optics were formulated and verified experimentally. In the course of these investigations, new physical processes, such as the formation of the distribution function of the electrons in their collisions with the wall and the effect of the near-wall conductivity, were discovered. The structure of the Debye layer for the case in which the coefficient of the secondary electron emission of a dielectric wall is larger than unity was investigated.     

Studies of the structure of C pellet ablation clouds in W7-AS

The structure of the ablation clouds surrounding carbon pellets injected into the ECR-heated Wendelstein 7-AS plasma has been studied. Snapshot and integrated photographs obtained in the spectral ranges containing the CII (720 ± 5 nm and 723 ± 1 nm) and CIII (770 ± 5 nm) spectral lines were analyzed over a wide range of the bulk plasma parameters. It is found that the cloud luminosity profile along the magnetic field is exponential with either one or two characteristic decay lengths of about a few millimeters and a few centimeters. The smaller length corresponds to the zone closer to the pellet. There is good agreement between the characteristic decay lengths deduced from snapshot and integrated photographs. The characteristic decay lengths were obtained along the entire pellet trajectory and were found to change weakly in the central region and to grow at the plasma periphery (generally, in inverse proportion to the plasma electron density). In the central region, the characteristic decay lengths are about a few millimeters and 1 cm. They depend weakly on the bulk plasma temperature and decrease with increasing bulk plasma density. These lengths agree fairly well with estimates of the ionization length of carbon ions into the C²⁺, C³⁺, and C⁴⁺ charge states, respectively, assuming that ionization is provided by the hot electrons of the bulk plasma and that the cloud expands with the ion-acoustic velocity at a temperature of ～1 eV. The results obtained prove that the cloud structure in the vicinity of the pellet is mainly determined by the bulk plasma electrons.     

Current scaling for the radiative characteristics of a micropinch discharge

The absolute VUV and soft X-ray (hν > 100 eV) yield from a micropinch discharge is measured for a fixed current of 150 kA. The current scaling in the range of 30–250 kA is found for a number of the discharge parameters: the VUV and soft X-ray yield, the electron temperature, the effective temperature of suprathermal electrons, and the energy of bremsstrahlung emission from thermal electrons. The experimental data are in good agreement with the simulations performed by using the model of radiative collapse in fast Z-pinches in plasmas of high-Zelements.     

Mechanisms on electrical breakdown strength increment of polyethylene by aromatic carbonyl compounds addition: a theoretical study

A theoretical investigation is accomplished on the mechanisms of electrical breakdown strength increment of polyethylene at the atomic and molecular levels. It is found that the addition of aromatic carbonyl compounds as voltage stabilizers is one of the important factors for increasing electrical breakdown strength of polyethylene, as the additives can trap hot electrons, obtain energy of hot electrons, and transform the aliphatic cation to relatively stable aromatic cation to prevent the degradation of the polyethylene matrix. The HOMO-LUMO energy gaps (E _g), the ionization potentials (IPs), and electron affinities (EAs) at the ground states of a series of aromatic carbonyl compounds are obtained at the B3LYP/6-311+G(d,p) level. The theoretical results are in good agreement with the available experimental findings, show that 2,4-dioctyloxybenzophenone (Bzo) and 4,4'-didodecyloxybenzil (Bd) molecules can effectively increase the electrical breakdown strength when they are doped into polyethylene because of their much smaller E _g values than all the other studied aromatic carbonyl molecules and excellent compatibility with polymers matrix.     

An Occam’s razor approach to chemical hardness: <Emphasis Type="Italic">lex parsimoniae</Emphasis>

The term “chemical hardness” refers to the resistance to deformation of the electronic density of a system; the greater this resistance, the “harder” the system. Polarizability, a physical property, is an inverse measure of resistance to deformation and thus should be inversely related to hardness. This is indeed generally accepted. Hardness has been postulated to be the second derivative of a system’s energy with respect to its number of electrons, despite the fact that this involves the differentiation of a noncontinuous function. This second derivative is typically approximated as the difference between the ionization energy I and the electron affinity A of the ground-state system, which results in ambiguity in that many molecules do not form stable negative ions. For atoms, the quantity I ? A does vary approximately inversely with polarizability, but this is only because the electron affinity is usually relatively low and ionization energy is known to be inversely related to polarizability for atoms. However, molecular polarizability depends primarily upon volume, and so does not show an acceptable inverse correlation with I ? A. Since both hardness and polarizability refer to the same property of a system—its resistance to deformation of the electronic density, we propose that the reciprocal of polarizability be taken to be a measure of hardness. We show that polarizabilities that are not known can be estimated quite accurately in terms of the average local ionization energies on the atomic or molecular surfaces and, for molecules, their volumes.     

Measurements of the electron density and degree of plasma ionization in a plasma target based on a linear electric discharge in hydrogen

Laser interferometry methods were used to measure the density of free electrons and degree of plasma ionization in a hydrogen target intended for experiments on determining energy losses of heavy ion beams in an ionized matter. It is shown that the linear electron density can be varied in the range from 3.3 × 10¹⁷ to 1.3 × 10¹⁸ cm^?2 by varying the initial plasma parameters (the hydrogen pressure in the target and the discharge current). The error in measuring the linear electron density in the entire range of the varied plasma parameters was less than 1%. The maximum degree of plasma ionization achieved at the initial gas pressure of 1 mbar was 0.62 ± 0.05.     

Ponderomotive Self-focusing of Surface Plasma Wave

A large amplitude surface plasma wave (SPW) propagating over a conductor–vaccum interface with Gaussian intensity profile transverse to the direction of propagation ( $ \widehat{z} $ ) and surface normal ( $ \widehat{x} $ ) is shown to undergo periodic self-focusing due to ponderomotive nonlinearity. The ponderomotive force on electrons arises due to the rapid decline in surface wave amplitude with the depth inside the conductor. In case of plasma, this leads to ambipolar diffusion of plasma, whereas in metals, only electron displacement occurs until the space charge balances the ponderomotive force on electrons. For a surface plasma wave, having Gaussian amplitude profile in y, the maximum electron density depression occurs on the axis (y?=?0) and the effect weakens as |y| increases. The axial portion of SPW thus travels with slower phase velocity than the nonaxial portion leading to self-focusing.     

Stability of hybrid modes of a single-component electron plasma containing an admixture of background gas ions

The spectrum of eigenmodes of a waveguide completely filled with a cold electron plasma containing a small admixture of ions produced due to electron-impact ionization of background gas atoms is calculated numerically. The calculations were performed within the entire range of allowable values of the radial electric and longitudinal magnetic fields for both magnetized and unmagnetized ions by using the earlier derived nonlocal dispersion relation [Plasma Phys. Rep. 36, 563 (2010)]. The spectrum consists of three families of electron modes with frequencies equal to the Doppler-shifted upper and lower hybrid frequencies and modified ion cyclotron (MIC) modes. When the Doppler shift caused by electron rotation in the crossed electric and magnetic fields compensates for the hybrid frequency, the electron modes become low-frequency modes and interact with the ion modes. For m = 1, only the lower hybrid modes can be low-frequency ones, whereas at m ≥ 2, both lower and upper hybrid modes can be low-frequency ones. The spectrum of modes having the azimuthal number m = 2 is thoroughly analyzed. It is shown that, in this case, the lower hybrid modes behave similar to the m = 1 modes. The dispersion curves of the upper hybrid modes intersect with all harmonics of the MIC frequency (positive, negative, and zero) and are unstable in the vicinities of the intersections. The maximum value of the instability growth rate is several times higher than the ion plasma frequency. The MIC modes are unstable within a wide range of the field strengths, and their growth rates are two orders of magnitude slower. Instabilities are caused by the relative motion of electrons and ions (the transverse current) and the anisotropy of the ion distribution function.     

Bremsstrahlung radiation detection for small animal imaging using a CCD detector

The use of optical methods for the detection of radionuclides is becoming an established tool for preclinical molecular imaging experiments. In this paper we present a set of proof of principle experiments showing that planar bremsstrahlung radiation images can be detected with an intensifying screen using a small animal optical imager based on charge coupled device detector.We develop a bremsstrahlung source using a ³²P-ATP vial placed in a Plexiglas box, the source with an intensifying screen on top was placed inside a small animal optical imaging system. Bremsstrahlung radiation images were produced with the ³²P-ATP source only and also with a pair of pliers placed between the source and the screen. We found that the pair of pliers absorption image matches the shape of the object.Spatial resolution measurements were not performed however, the bremsstrahlung image of the pliers show that the resolution is relatively poor due to a large penumbra effect.We conclude that it is possible to produce planar bremsstrahlung images using optical imaging devices.     

Role of secondary electrons and metastable atoms in the electron-beam activation of argon-silane mixtures

The energy and spatial degradation of the primary beam electrons and the production of high-energy secondary electrons in ionizing collisions are analyzed by solving the Boltzmann integral equation for the electron distribution function. The effect of the primary and secondary electrons on the direct ionization of an Ar-SiH₄ mixture, the production of metastable argon atoms, and the dissociation of monosilane molecules is investigated over a wide range of the beam electron energies, argon pressures, and monosilane concentrations. The influence of metastable Ar* atoms on the dissociation of SiH₄ is studied by using the balance equation for metastable argon atoms and the equation for the ambipolar diffusion of ions and low-energy secondary (plasma) electrons in the beam plasma. It is shown that the main contribution to the activation of an Ar-SiH₄ mixture in an electron-beam plasma is provided by secondary electrons with energies higher than the excitation threshold for argon and the dissociation threshold for monosilane, whereas the contribution from metastable argon atoms, though potentially being comparable with that from secondary electrons, is less than in gas-discharge plasmas.     

A Relativistic Neutron Fireball from a Supernova Explosion as a Possible Source of Chiral Influence

We elaborate on a previously proposed idea that polarized electrons produced from neutrons, released in a supernova (SN) explosion, can cause chiral dissymmetry of molecules in interstellar gas-dust clouds. A specific physical mechanism of a relativistic neutron fireball with Lorentz factor of the order of 100 is assumed for propelling a great number of free neutrons outside the dense SN shell. A relativistic chiral electron–proton plasma, produced from neutron decays, is slowed down owing to collective effects in the interstellar plasma. As collective effects do not involve the particle spin, the electrons can carry their helicities to the cloud. The estimates show high chiral efficiency of such electrons. In addition to this mechanism, production of circularly polarized ultraviolet photons through polarized-electron bremsstrahlung at an early stage of the fireball evolution is considered. It is shown that these photons can escape from the fireball plasma. However, for an average density of neutrals in the interstellar medium of the order of 0.2 cm⁻³ and at distances of the order of 10 pc from the SN, these photons will be absorbed with a factor of about 10⁻⁷ due to the photoeffect. In this case, their chiral efficiency will be about five orders of magnitude less than that for polarized electrons.     

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m³ as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current I _b , energy E _b , and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures T _e ∼ 0.4 eV and T _e ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I ₀ ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I ₀ specifies the number of electrons produced inside the chamber per second, and it grows up with E _b , I _b , and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.     

Polarization bremsstrahlung from a fast structural ion in a plasma

Polarization bremsstrahlung from a fast hydrogen-like ion in a plasma is calculated and analyzed in the first Born approximation with allowance for the contribution from two radiation mechanisms (channels): (i) radiation from the conversion of the electromagnetic field of the ion into a real photon by plasma electrons and (ii) radiation from the virtual excitation of the bound electron of the ion. It is shown that the intensity of the polarization bremsstrahlung generated via the second channel is sharply peaked in narrow spectral-angular ranges around the eigenfrequencies of the electron core of the fast ion and, moreover, the spectral-angular intensity distribution depends strongly on the velocity of the incident particle. The dependence of both the polarization bremsstrahlung mechanisms on the plasma parameters is investigated.     

Microwave and Optical Gas Discharges in Superstrong Pulsed Fields

A review is given of theoretical papers on gas breakdown in high-power pulsed microwave and optical fields under conditions such that the electron oscillatory energy in the wave field is much higher than the ionization energy of gas atoms. In microwave fields, which are much weaker than the atomic field, the ionization mechanism for gas atoms is governed by electron-impact avalanche ionization. In high-power optical fields that are comparable in strength to the atomic field, the gas atoms are ionized via the tunneling of the bound electrons. It is shown that, in both cases, the electrons obey similar, highly anisotropic distributions, thereby strongly affecting the stability of the discharge plasma.