Thin Plasmas Research Articles

Characteristics of Thin Magnetotail Current Sheet Plasmas at Lunar Distances

AbstractThe magnetotail current sheet plays a key role in the dynamics of Earth's magnetosphere. Specifically, the formation and subsequent reconnection of thin (ion‐gyroscale) current sheets are critical components of magnetospheric substorms. However, the precise mechanisms governing the configuration and distribution of current density in these thin current sheets remain elusive. By analyzing a data set consisting of 453 thin current sheet crossings observed by the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission, we explore the statistical properties of the ion and electron pressures and current densities, Ji and Je, in the spacecraft rest frame. Using magnetotail flapping and magnetic field measurements to estimate the total current density, J0, we find that it agrees well with the sum of those from direct ion and electron measurements, Ji + Je, respectively. In 65% of thin current sheets, electrons were found to dominate the contribution to the total current density in the spacecraft frame, with a typical dawnward drift velocity of ≳100 km/s. Diamagnetic drifts of electrons and ions estimated from their respective vertical pressure profiles (along the current sheet normal) reveal that the gradient of electron pressure alone cannot fully account for the observed high values of Je/Ji. Counter‐intuitively, for most (52% of) thin current sheets the electron vertical pressure profile is wider than the ion pressure profile, again suggesting that electron diamagnetism is an insufficient contributor to the current density at such sheets. These findings suggest the presence of a significant E × B dawnward drift that the electrons can fully acquire but ions cannot, being partially unmagnetized. We compare our results with those previously reported for the near‐Earth magnetotail and discuss them in the context of magnetotail current sheet modeling.

An Eigenvalue Analysis of Damping in Optical Thin Plasmas

In this work, the behaviour of magneto-hydrodynamic waves in optically thin plasmas considering dissipative processes, thermal and magnetic diffusion, a given ionization, and the heating and cooling functions are investigated for several particular cases. A numerical eigenvalues analysis of the dimensionless secular equations according to various cases is performed for the entire set of MHD equations.

New radiative loss curve from updates to collisional excitation in the low-density, optically thin plasmas in SPEX

Understanding and modelling astrophysical plasmas on atomic levels while taking into account various assumptions (for example, collisional ionisation equilibrium or photoionisation equilibrium) became essential with the progress of high-resolution X-ray spectroscopy. In order to prepare for the upcoming X-ray spectroscopy missions such as XRISM or Athena, the plasma codes with their models and the atomic databases need to be up to date and accurate. One such update for the plasma code SPEX is presented in this paper where we focus on the radiative loss due to collisional excitation in the low-density, optically thin regime. We also update the atomic data for neutral hydrogen and include the contribution of the dielectronic recombination. With all these updates being implemented in SPEX we finally present the new cooling curve. We include the comparison to other plasma codes (MEKAL, APEC, Cloudy) and other atomic databases (CHIANTI, ADAS). We show how the updated cooling impacts the stability curve for photoionised plasmas and find a new stable branch.

Spectra of Ga-Like to Cu-Like Praseodymium and Neodymium Ions Observed in the Large Helical Device

Extreme ultraviolet (EUV) spectra of highly charged praseodymium (Pr) and neodymium (Nd) ions have been investigated in optically thin high-temperature plasmas produced in the Large Helical Device (LHD), a magnetically confined torus device for fusion research. Discrete spectral lines emitted mainly from highly charged ions having 4s or 4p outermost electrons were observed in plasmas with electron temperatures of 0.8–1.8 keV. Most of the isolated lines of Ga-like to Cu-like Nd ions were identified by a comparison with the recent data recorded in an electron beam ion trap (EBIT). The isolated lines of Pr ions corresponding to the identified lines of Nd ions were easily assigned from a similarity of the spectral feature for these two elements. As a result, some of the lines of Pr ions have been newly identified experimentally for the first time in this study.

High-spatial-resolution image reconstruction-based method for measuring electron temperature and density of the very near field of an applied-field magnetoplasmadynamic thruster

A method based on image reconstruction is proposed for measuring the electron temperature () and density () distribution of the axisymmetric low-temperature argon plasma plume of applied-field magnetoplasmadynamic thrusters (AF-MPDTs). The method uses a complementary metal–oxide–semiconductor camera with narrowband filters of 460 and 500 nm to obtain the raw projection images in specific wavelength ranges. An image reconstruction method is developed to construct relative emission intensity profiles. The electron temperature distribution can be obtained by combining the relative emission intensity ratio of two profiles with a simple argon kinetic model. The electron density distribution can be calculated by constructing a function of relative emission intensity containing and . To verify the feasibility of this method, we diagnosed a very near field plume of a 15 kW AF-MPDT and obtained the distribution of parameters with a spatial resolution of 0.23 mm. The results show that the maximum and are located on the centerline near the exit plane of the thruster and decrease with both radial and axial distances. The second peak appears in the radial direction of both the and distributions, while their zones do not overlap. A comparative analysis of measurement results obtained by this method and the Langmuir probe method is performed to prove its accuracy. Moreover, this measurement method has the advantages of high spatial resolution and the ability to diagnose high-density and magnetized plasma. This method can be also applied to other optically thin axisymmetric low-temperature argon plasmas.

Two-lines method for estimation of plasma temperature and characterization of plasma parameters in optically thick plasma conditions.

In this paper, the characteristics of laser-induced plasmas are studied by investigation of the spectral line features in laser-induced breakdown spectroscopy (LIBS) experiments. The plasma is produced by focusing of a Nd:YAG laser on standard Al-alloy samples at 30 mJ energy. Here, with the assumption of having a homogenous plasma and by using a semiemperical technical method, the plasma temperature is calculated by the proposal of a new two-lines method. Moreover, by utilizing some theoretical equations, the plasma parameters and the self-absorption magnitude are evaluated according to the radiative transfer equations in local thermodynamic equilibrium (LTE) conditions. The main advantages of this method are that without discrimination between thin plasmas and thick ones, and as well as without straight quantification of the degree of self-absorption, the plasma temperature can be calculated. The results showed that determination of the intensities of the spectral lines, transition parameters, and Stark broadening parameter is adequate for plasma characterization in a typical LIBS experiment.

Self-consistent electron energy distribution functions, vibrational distributions, electronic excited state kinetics in reacting microwave CO2 plasma: An advanced model

An advanced model for the calculation of electron energy distribution functions (eedfs), vibrational distributions, and electronic excited state densities of reacting CO2 in microwave (MW) discharges has been developed for clarifying: (1) the role of electronic states of the relevant neutral species in affecting the eedf and (2) the contribution to the CO2 dissociation of the electron impact and heavy particle dissociation mechanisms. To model the discharge, the power density typical of MW discharges is used as a parameter. Different case studies including optically thick and thin plasmas and the dependence of the CO2 dissociation rates on the gas temperature are investigated. The results show that at a low gas temperature, i.e., 300 K, the heavy-particle dissociation mechanism, also called the pure vibrational mechanism, prevails on the electron impact dissociation one, while at a high gas temperature, i.e., 2000 K, the two mechanisms become competitive and the global behavior strongly depends on the choice of electron impact dissociation cross sections. Large differences appear in the eedf, especially in the post-discharge regime, when considering thick and thin plasmas. In the thick case, a well-structured eedf appears as a result of superelastic collisions mainly involving the electronic states of the relevant neutral species. In the thin plasma, many peaks disappear because the concentration of the excited states strongly decreases. Finally, our model gives the results of conversion and energy efficiency as well as vibrational distributions in satisfactory agreement with the corresponding results calculated by the Antwerp group.

Opacity of Krypton X-ray lines in high-temperature plasma implosion

For accurate electron temperature and density diagnostics in laser high-temperature plasma implosion, opacity of Krypton (Kr) helium-[Formula: see text] and Balmer-[Formula: see text] lines are discussed, and ratio of escape factor of the two X-rays are calculated, both for Gaussian and Holtzmarkian profiles. Finally, the line ratios of the two X-ray lines are analyzed for optically thin and thick plasmas. Results indicate that for the Gaussian profile, the line ratio varies greatly with the opacity of the Kr helium-[Formula: see text] line, which provides excellent source for temperature and electron density diagnostics; while for the Holtzmarkian profile, the line ratio varies less with the opacity of the Kr helium-[Formula: see text] line, which can be used for escape factor diagnostics. This method is significant in accurate plasma diagnostics using X-rays under the condition of optically thick.

Эвристическая модель для восстановления рентгеновской части солнечного спектра по спутниковым данным в интересах геофизических приложений

Model and computational algorithm for recovering the X-ray component of the solar spectrum based on GOES XL (0.1–0.8 nm), XS (0.05–0.4 nm or 0.05–0.3 nm) and SDO QD (0.1–7 nm) channels data are presented. The model based the Mewe approximation of emission from optically thin plasmas that presents a temperature-like spectrum. The possibility to present the result spectrum as a superposition of Mewe spectrums placed in the solar absorbing atmosphere at one optical depth for its temperature parameter energy is suggested in this paper. So the model is a variation of the multi-temperature approximation. Spectrum parameters are determined on the basis of support functions, approximation expressions for which are given in appendix.

Heuristic model of solar X-ray spectrum according to satellite data for geophysical applications

Model and computational algorithm for recovering the X-ray component of the solar spectrum based on GOES XL (0.1–0.8 nm), XS (0.05–0.4 nm or 0.05–0.3 nm) and SDO QD (0.1–7 nm) channels data are presented. The model based the Mewe approximation of emission from optically thin plasmas that presents a temperature-like spectrum. The possibility to present the result spectrum as a superposition of Mewe spectrums placed in the solar absorbing atmosphere at one optical depth for its temperature parameter energy is suggested in this paper. So the model is a variation of the multi-temperature approximation. Spectrum parameters are determined on the basis of support functions, approximation expressions for which are given in appendix.

Analysis of radiative opacities for optically thin and thick astrophysical plasmas

The resolution of the radiative transfer equation in radiation-hydrodynamic simulations of astrophysical plasmas require radiative opacities. In this work, an analysis of the monochromatic and multigroup opacities of an astrophysical plasma mixture has been carried out. The study has been made in ranges of electron temperatures and densities of 1−1000 eV and 1011−1020 cm−3, respectively, a wide range of plasma conditions that can be found in several astrophysical scenarios where local and non-local thermodynamic regimes are attained. Collisional-radiative calculations were performed to obtain the plasma level populations and the monochromatic opacities in that range of plasma conditions, covering both thermodynamic regimes. Since the astrophysical mixture includes chemical elements from hydrogen to iron, their contribution to the total opacity will depend on the plasma conditions and we have made a characterization of their contribution as a function of the electron density and temperature and also of the photon frequency. Multigroup and gray approaches are commonly used in the radiative transfer equation in radiation-hydrodynamic calculations. We have analyzed the influence of the number of the groups in the accuracy of the multigroup opacities and we have showed that, for a given plasma condition, the opacity of the multicomponent plasma in the gray approach may be considerably influenced by only some of the contributing elements, due to the influence of the weighting function in the mean, which can lead to great differences with respect to the monochromatic opacity. Finally, since there are situations in which the self-absorption of the plasma radiation becomes relevant due to the dimensions of the plasma, we have performed an analysis of the influence of the radiation trapping in the monochromatic and multigroup opacities in terms of the plasma conditions and the width of the plasma slab, assuming the plasma with planar geometry and we have also studied the departures of the local thermodynamic regime.

Opacity effects on laser-produced plasma radiation sources

The escape of spectral line radiation from laser-produced plasma radiation sources with moderate opacity is examined using a simple model of emission with a planar geometry, constant source function, and an empirically determined optical depth. The model is applied to determine the radiation produced by laser irradiation of tin targets used as the source of radiation in extreme ultraviolet (EUV) lithography. Variations in emission relative to optically thin plasmas in agreement with previous experimental measurements of both the angular variation of the emission of EUV light at 13.5 nm and the effect of plasma opacity in reducing EUV emission at laser intensities above 1011 W cm−2 are found. The model is extended to predict optimum conditions for future lithography radiation sources at ≈6.7 nm.

Nuclear γ-ray emission from very hot accretion flows

Optically thin accretion plasmas can reach ion temperatures Ti ≥ 1010 K and thus trigger nuclear reactions. Using a large nuclear interactions network, we studied the radial evolution of the chemical composition of the accretion flow toward the black hole and computed the emissivity in nuclear γ-ray lines. In the advection dominated accretion flow (ADAF) regime, CNO and heavier nuclei are destroyed before reaching the last stable orbit. The overall luminosity in the de-excitation lines for a solar composition of plasma can be as high as few times 10−5 the accretion luminosity (Ṁc2) and can be increased for heavier compositions up to 10−3. The efficiency of transformation of the kinetic energy of the outflow into high energy (≥100 MeV) γ-rays through the production and decay of π0-mesons can be higher, up to 10−2 of the accretion luminosity. We show that in the ADAF model up to 15% of the mass of accretion matter can “evaporate” in the form of neutrons.

Radiative properties for astrophysical plasma mixtures in nonlocal thermodynamic equilibrium

Radiative properties play a pivotal role in astrophysical plasma flows and are needed in radiation-hydrodynamic simulations in order to understand their behavior and also to interpret the plasma emission spectra, which are valuable diagnostic tools. Radiative properties of astrophysical plasma mixtures have been commonly calculated for low-density optically thin plasmas assuming coronal equilibrium and for high density assuming local thermodynamic equilibrium. However, there are wide ranges of conditions in which these thermodynamic regimes are not achieved and the plasma is in the nonlocal thermodynamic equilibrium regime. In the present work, a study of the plasma radiative properties of oxygen and iron and an astrophysical plasma mixture in nonlocal thermodynamic steady-state equilibrium is carried out. The ranges of electron temperatures and densities considered are 1--1000 eV and ${10}^{11}\text{--}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, respectively. In the study, departures from coronal and local thermodynamic equilibria in terms of the density and temperature are also analyzed. Large differences in the radiative properties that can reach two orders of magnitude when the plasma is far from these thermodynamic regimes are obtained. These analyses are done assuming the plasma to be optically thin. A brief study of the influence of the plasma self-absorption in the radiative properties of oxygen and iron is made. For that purpose, the plasma is assumed with planar geometry and the study is performed in terms of the width of the plasma slab and electron temperature and density.

Systematic Observation of EUV Spectra from Highly Charged Lanthanide Ions in the Large Helical Device

We have systematically observed extreme ultraviolet (EUV) spectra from highly charged ions of nine lanthanide elements with atomic numbers from 60–70 in optically thin plasmas produced in the Large Helical Device (LHD). Discrete spectral features with isolated lines from relatively higher charge states around Cu-like ions are observed under high temperature conditions around 2 keV, while narrowed quasicontinuum features from charge states around Ag-like ions are observed under low temperature conditions below 1 keV. The positions of the lines and the quasicontinuum features systematically move to shorter wavelengths as the atomic number increases. The wavelengths of the main peaks in the quasicontinuum features agree well with those of singlet transitions of Pd-like ions reported previously. We have easily identified discrete spectral lines from Cu-like and Ag-like ions, some of which are experimentally identified for the first time in the LHD. Their wavelengths are compared with theoretical calculations using a GRASP family of atomic codes. The theoretical values are synthesized to the LHD experimental data for the cases of Ag- and Pd-like ions.

Comparison of the laser spot movement inside cylindrical and spherical hohlraums

Compared with cylindrical hohlraums, the octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the whole capsule implosion process in indirect drive inertial confinement fusion. However, the narrow space between laser beams and the hohlraum wall may disturb laser propagation inside the spherical hohlraum. In this work, the laser propagation inside the spherical hohlraum and cylindrical hohlraum is investigated experimentally by measuring laser spot movement at the SGIII-prototype laser facility. The experimental results show that the laser propagations inside the spherical hohlraum and the cylindrical hohlraum are totally different from each other due to different hohlraum structures. For the spherical hohlraum, although the laser energy is mainly deposited in the initial position of the laser spot during the whole laser pulse, some laser energies are absorbed by the ablated plasmas from the hohlraum wall. Because the laser beam is refracted by the thin plasmas near the laser entrance hole (LEH) region, the laser spot in the spherical hohlraum moves toward the opposite LEH. In contrast, the laser spot in the cylindrical hohlraum moves toward the LEH along the laser path due to the plasma expansion. When the laser is to be turned off, the accumulated plasmas near the LEH region in the cylindrical hohlraum absorb a majority of laser energy and hinder the laser arriving at the appointed position on the hohlraum wall.

Characterization of electrostatic shock in laser-produced optically-thin plasma flows using optical diagnostics

We present a method for evaluating the properties of electrostatic shock in laser-produced plasmas by using optical diagnostics. A shock is formed by a collimated jet in counter-streaming plasmas in nearly collisionless condition, showing the steepening of the transition width in time. In the present experiment, a streaked optical pyrometry was applied to evaluate the electron density and temperatures in the upstream and downstream regions of the shock so that the shock conditions are satisfied, by assuming thermal bremsstrahlung emission in optically thin plasmas. The derived electron densities are nearly consistent with those estimated from interferometry.

X-ray emission from thin plasmas

Every observation of astrophysical objects involving a spectrum requires atomic data for the interpretation of line fluxes, line ratios and ionization state of the emitting plasma. One of the processes which determines it is collisional ionization. In this study an update of the direct ionization (DI) and excitation-autoionization (EA) processes is discussed for the H to Zn-like isoelectronic sequences. In the last years new laboratory measurements and theoretical calculations of ionization cross sections have become available. We provide an extension and update of previous published reviews in the literature. We include the most recent experimental measurements and fit the cross sections of all individual shells of all ions from H to Zn. These data are described using an extension of Younger's and Mewe's formula, suitable for integration over a Maxwellian velocity distribution to derive the subshell ionization rate coefficients. These ionization rate coefficients are incorporated in the high-resolution plasma code and spectral fitting tool SPEX V3.0.

Coupling of Plasma Chemistry, Vibrational Kinetics, Collisional‐Radiative Models and Electron Energy Distribution Function Under Non‐Equilibrium Conditions

The coupling between the electron energy distribution function (eedf) and the kinetics of vibrationally and electronically excited states is discussed. The first case study concerns the role of heterogeneous recombination in affecting the concentration of atomic hydrogen in RF plasmas, with consequences on eedf and negative ion formation. The coupling of eedf and a collisional‐radiative model for atomic hydrogen is discussed for optically thin and thick plasmas, finding large differences in the creation of eedf plateaux. The last case study deals with a self‐consistent coupling of eedf and kinetics of vibrationally and electronically excited states in CO2 cold plasmas emphasizing the role of pure vibrational mechanisms in the dissociation of CO2.

Forward Modeling of EUV and Gyrosynchrotron Emission from Coronal Plasmas with FoMo

The FoMo code was developed to calculate the EUV emission from optically thin coronal plasmas. The input data for FoMo consists of the coronal density, temperature and velocity on a 3D grid. This is translated to emissivity on the 3D grid, using CHIANTI data. Then, the emissivity is integrated along the line-of-sight to calculate the emergent spectral line that could be observed by a spectrometer. Moreover, the code has been extended to model also the radio emission from plasmas with a population of non-thermal particles. In this case, also optically thick plasmas may be modelled. The radio spectrum is calculated over a large wavelength range, allowing for the comparison with data from a wide range of radio telescopes.