Evolution Of Electron Density Research Articles

Propagation of nanosecond discharge in an air gap containing a water droplet: modelling and comparison with time-resolved images

The plasma-water interface is a complex medium characterized by interesting physical and chemical phenomena useful for many applications such as water processing or material synthesis. In this context, optimizing the transport of reactive species from plasma to water is crucial, and it may be achieved by increasing the surface-to-volume ratio of the processed object. Herein, we study the characteristics of a streamer produced by nanosecond discharge in air gap with a droplet of deionized water. The discharge is characterized experimentally by electrical measurements as well as by 1 ns-intergated ICCD images. To report plasma properties that are not accessible through experiment, such as the spatio-temporal evolution of electron density, electric field, and space charge density, a 2D fluid model is developed and adapted to the experimental geometry. Due to the fast propagation of the ionization front, the droplet is considered as a solid dielectric. The model solves Poisson’s equation as well as the drift-diffusion equation for electrons, positive ions, and negative ions. The utilized transport coefficients are tabulated as a function of the reduced electric field. Helmholtz equations are also included in the model to account for photoionization. The electron impact ionization source obtained from the model is compared to experimental 1 ns-integrated ICCD images, and a good agreement is observed. Finally, the model is used to investigate the influence of droplet dielectric permittivity and wetting angle (the angle between a liquid surface and a solid surface) on the properties of the discharge. Overall, the data reported herein demonstrate that the model can be used to investigate plasma properties under different conditions.

Mixed, quantum-classical description of electron density transfer in the collision process

In this work, we investigate an ion-atom model describing the time-dependent evolution of electron density during the collision. For a S 3 + − H system, numerical simulations are based on classical trajectory calculations, and the electron density behaviour is described with the time-dependent Schrödinger equation. We apply the finite difference method to obtain quantitative insights into the charge transfer dynamics, providing detailed information about the spatial and temporal evolution of the collision process. The results are given for representative examples of the collision, from eV to keV range of energies, in head-on collision as well as for different values of impact parameter. A validity and precision of the proposed model and interpretation of the particle collision in terms of eigenstates are also discussed.

Comparative study of the decay process of LIP and gas arc in SF6/N2 gas mixture by Thomson scattering

The SF6/N2 gas mixture mitigates the challenges associated with the use and liquefaction problems of pure SF6. It has great significance to analyze the decay characteristics of the plasma in it. Laser-induced plasma (LIP) and gas arc represent two distinct types of plasma, differing significantly in energy density and duration. This study utilized collective Thomson scattering diagnostics to investigate the temporal and spatial evolution of electron density of these two plasmas, with varying SF6/N2 mix ratios as the gas medium. Our findings indicate that initially, the electron density in gas arc is lower, and its decay rate is generally slower compared to LIP. However, as the SF6 concentration increases, the decay process accelerates for both LIP and gas arcs. It is worth noting that when the SF6 volume fraction exceeds 70%, the decay rate of electron density approaches that of pure SF6 in both plasma types, suggesting a saturation effect near a 70% SF6/N2 mix ratio in terms of electron density decay.

On learning latent dynamics of the AUG plasma state

In this work, we demonstrate the utility of state representation learning applied to modeling the time evolution of electron density and temperature profiles at ASDEX-Upgrade (AUG). The proposed model is a deep neural network, which learns to map the high dimensional profile observations to a lower dimensional state. The mapped states, alongside the original profile's corresponding machine parameters, are used to learn a forward model to propagate the state in time. We show that this approach is able to predict AUG discharges using only a selected set of machine parameters. The state is then further conditioned to encode information about the confinement regime, which yields a simple baseline linear classifier, while still retaining the information needed to predict the evolution of profiles. We, then, discuss the potential use cases and limitations of state representation learning algorithms applied to fusion devices.

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

Investigations of Flaring Plasma Parameters during an M-class Flare Using the Differential Evolution Method and XSM/Chandrayaan-2 Observations

We employ the differential evolution (DE) method to analyze observations from the Solar X-Ray Monitor on board the Chandrayaan-2 spacecraft. DE belongs to the family of evolutionary algorithms that find solutions using mechanisms inspired by biological processes. This approach enables us to simultaneously calculate the distribution of the differential emission measure and elemental abundances through an iterative process. We establish a model for the emission sources of flaring plasma, incorporating temperature, emission measure, and abundances of eight elements: Mg, Al, Si, S, Ar, Ca, Fe, and Ni, for an M3.9 GOES-class solar flare that occurred on 2021 May 7 (SOL2021-05-07T19:04). Our analysis covers various phases of the flare, determining the evolution of temperature, emission measure, and elemental abundances. Additionally, utilizing data from the Spectrometer Telescope for Imaging X-rays on board the Solar Orbiter, we investigate the evolution of hard X-ray source morphology, source volume, electron density, and thermal behavior of the flaring plasma throughout the event. The results reveal notable variations in elemental abundances between photospheric and coronal values during different flare phases. This emphasizes the significance of elemental abundance information in comprehending X-ray emissions during solar flares.

Redshift Evolution of Electron Density in the Interstellar Medium at z ∼ 0–9 Uncovered with JWST/NIRSpec Spectra and Line-spread Function Determinations

We present electron densities n e in the interstellar medium (ISM) of star-forming galaxies at z = 4–9 observed by the JWST/NIRSpec GLASS, Early Release Observations, and CEERS programs. We carefully evaluate the line-spread functions of the NIRSpec instrument as a function of wavelength with the calibration data of a planetary nebula taken on board, and obtain secure [O ii] λ λ3726, 3729 doublet fluxes for 14 galaxies at z = 4.02–8.68 falling on the star formation main sequence with the NIRSpec high- and medium-resolution spectra. We thus derive the electron densities of singly ionized oxygen nebulae with the standard n e indicator of the [O ii] doublet, and find that the electron densities of the z = 4–9 galaxies are n e ≳ 300 cm−3 significantly higher than those of low-z galaxies at a given stellar mass, star formation rate (SFR), and specific SFR. Interestingly, the typical electron densities of the singly ionized nebulae increase from z = 0 to z = 1−3 and z = 4–9, which is approximated by the evolutionary relation of n e ∝ (1 + z) p with p ∼ 1–2. Although it is not obvious that the ISM property of n e is influenced by global galaxy properties, these results may suggest that the nebula densities of high-z galaxies are generally high due to the compact morphologies of high-z galaxies evolving by (r vir ∝ (1 + z)−1) for a given stellar (halo) mass whose inverse square corresponds to the p ∼ 2 evolutionary relation. The p ∼ 1−2 evolutionary relation can be explained by a combination of the compact morphology and the reduction of n e due to the high electron temperature of high-z metal-poor nebulae.

Evolution of valence electron density of compressed Na–Au alloys

Evolution of valence electron density of compressed Na–Au alloys

TEMPORAL EVOLUTION OF THE PLASMA DENSITY CAVITY CAUSED BY INHOMOGENEOUS STOCHASTIC ELECTRIC FIELDS

The time evolution of the formation of a plasma density cavity caused by inhomogeneous stochastic electric fields is investigated. The Fokker-Planck equation, which governs the temporal evolution of the plasma electron density due to localized stochastic inhomogeneous electric fields in the frequency range of lower hybrid oscillations, is solved numerically. The spatial dependence of the plasma electron density for various times is obtained.

Investigations on 355 nm picosecond laser machining of carbon fiber reinforced polymer composites

Carbon fiber reinforced polymer (CFRP) composites have emerged as excellent performance structural materials with a wide range of applications in the aerospace industry. Ultrafast laser is a promising and versatile tool for manufacturing these composites due to its advantages of high peak intensity and ultrashort pulse duration. Herein, a theoretical model based on the Fokker–Planck law and the two-temperature equation, i.e. the ultrafast responses of a two-dimensional heterogeneous material assembly of resin matrix padded with carbon fiber reinforced phase irradiation by 355 nm picosecond laser is established. The evolution of laser-excited free electron density is analyzed, and the relationship between the optical properties of substrate and electron density is revealed. The spatial and temporal distribution of electron and lattice temperatures are thus characterized. The ablation thresholds of resin and carbon fiber are determined, the accuracy of which are verified by experiments. On this basis, orthogonal experiments are conducted to analyze the influence law of process parameters on the ablation depth and heat affected zone (HAZ) width. Optimized parameter groups are discovered to realize the high-quality machining of CFRP composite with considerable depth-to-width ratio and limited HAZ width. This work offers new insight into the fundamental regimes and high-quality machining for picosecond laser manufacturing of CFRP composites.

Population inversion mechanism of optically pumped metastable rare gas based on laser-induced preionization

A 532 nm laser was used to induce ionization of Ar–He mixture, and metastable 4s [3/2]2 level Ar atoms were prepared. The interaction mechanism between laser, Ar atoms, He atoms and free electrons in laser-induced plasma (LIP) was revealed. Through time-resolved atomic emission spectroscopy, two regimes separated by a few microseconds were found during temporal evolution of Ar I 811.5 nm (4p [5/2]3→4s [3/2]2) line. It was demonstrated that the first regime was generated by the “photoexcitation + radiation + collisional relaxation” process, while the second regime was generated by the “ion-electron recombination” process. The above conclusion was confirmed again by the temporal evolution of electron temperature and electron density, and it was also demonstrated that the ionization of Ar atoms induces the ionization of He atoms. A 811.5 nm laser was utilized to excite the metastable 4s [3/2]2 level Ar atoms to the 4p [5/2]3 level, when changing the delay time of the 811.5 nm laser lagging behind the 532 nm laser, the generated 912.3 nm (4p [1/2]1→4s [3/2]2) fluorescence intensity also showed different pattern of variations. The analysis indicated that “photoexcitation + radiation + collisional relaxation”, “ion-electron recombination” and quenching processes will affect the density of metastable 4s [3/2]2 level Ar atoms. Amplified spontaneous emission (ASE) signal at 912.3 nm was obtained along the direction of pump laser. This work verifies the feasibility of optically pumped metastable rare gas laser (OPRGL) based on laser-induced preionization, and provides an alternative scheme for the future development of OPRGL.

Electron transport properties and free electron density evolution during the phase change in VO2 thin films

The experimental results on the phase change behavior of vanadium dioxide (VO2) as a function of temperature are invariably examined in the light of the strong electron correlation-based Mott-Hubbard model or the periodic lattice distortion based Peierls model with limited success. Further, the experimental conditions-based nanostructure imparted to the VO2 thin films also plays an important role. Hence, it has been suggested that the theoretical model that best suits the experimental conditions needs to be applied. In this work, we have examined the electron transport properties of VO2 thin films of various thicknesses all along the reversible phase change cycle. These results have led us to carefully examine the free electron density (ne) evolution all along the phase transition cycle and to establish for the first time the reversible hysteresis cycles of this important parameter. The intimate correlation between the degree of change in ne and the resistance seen in our samples leads us to believe that our results may be in favor of the Mott-Hubbard model of phase transition.

Impact of self-sputtering in high power impulse magnetron sputtering (HiPIMS) with helium

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal.

Static and transient optical properties of thin film indium tin oxide during laser excitation

The mechanisms of ultrafast laser-induced free electron generation and the effect of the free electrons on the transient optical properties in indium tin oxide is investigated by means of numerically solving rate equations and ultrafast transient experiments. A dynamical model for the transient optical properties of indium tin oxide thin films during excitation with an ultrashort laser pulse with fluences between 0.4 J cm−2 to 2.3 J cm−2 is developed and compared to pump probe microscopy during and after excitation with laser pulses. In the non-excited case, thin film interferences dominate the shape of the reflection spectrum. During laser excitation, (quasi-)free electron dynamics drastically alter the optical properties. The temporal evolution of the (quasi-)free electron density is numerically computed using rate equations for irradiation with laser pulses with various peak fluences for a pulse duration of 700 fs and a central wavelength of 1056 nm. We found that a thermal ionization term that takes electronic states within the band gap into account has to be considered to reproduce the reflectivities obtained by pump probe measurements. Additionally, we found that cascade ionization dominates the multiphoton ionization for indium tin oxide.

Effects of pulse rise time on electron dynamics properties in nitrogen–oxygen mixture under repetitive nanosecond pulses

The effects of pulse rise time on the temporal evolution of electron energy and density under repetitive nanosecond pulses in atmospheric nitrogen with 100 ppm oxygen impurities are investigated in this paper by a two-dimensional particle-in-cell/Monte Carlo collision model. It is found that the peak value of mean electron energy increases with decreasing pulse rise time in the single pulsed discharge. However, in the repetitive pulsed discharge approximated by pre-ionization, the peak value of mean electron energy no longer varies with the pulse rise time, showing a saturation trend with decreasing pulse rise time. Whether or not pre-ionization is present, the time required for the mean electron energy to reach its peak is approximately equal to the pulse rise time. It is worth noting that the presence of pre-ionization enhances the tracking ability of the mean electron energy to the pulse waveform during the pulse rise edge. Although after the peak of the pulse, the mean electron energy terminates the tracking process to pulse waveform due to the formation of high-density avalanches and even streamers, its energy decay rate gradually decreases with the increase in the pre-ionization density. Therefore, when the pulse repetitive frequency is greatly increased or the pre-ionization density is increased by other means, it is possible to achieve the complete control of the mean electron energy by pulse waveform modulation.

Estimation of electron temperature dynamics in warm dense gold using measured and DFT-calculated plasma frequency

In this study, we report the temperature evolution of electron density of states (DOS) along with the thermophysical and optical properties of gold calculated by means of density functional theory (DFT). To corroborate the effects due to temperature-dependent electron density of states, we performed ellipsometric pump-probe experiments with broadband white-light continuum probe pulse that covered both the intraband and the interband region of the dielectric permittivity of gold. The intraband part was analyzed to give plasma frequency and collision rate as a function of the pump-probe delay. The electron temperature evolution of warm dense gold is determined by comparing the experimental plasma frequency dynamics with the DFT-calculations. The study also emphasizes the essential role of temperature-dependent modifications to the electron DOS and the thermophysical properties in order to model the non-equilibrium states of matter.

Magnetic-field modulation of topological electronic state and emergent magneto-transport in a magnetic Weyl semimetal

The modulation of topological electronic state by an external magnetic field is highly desired for condensed-matter physics. Schemes to achieve this have been proposed theoretically, but few can be realized experimentally. Here, combining transverse transport, theoretical calculations, and scanning tunneling microscopy/spectroscopy (STM/S) investigations, we provide an observation that the topological electronic state, accompanied by an emergent magneto-transport phenomenon, was modulated by applying magnetic field through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6. A giant unconventional anomalous Hall effect (UAHE) is found during the magnetization re-orientation from easy axes to hard ones in magnetic field, with a UAHE peak around the low field of 5 kOe. Under the reasonable spin-canting effect, the folding of the topological anti-crossing bands occurs, generating a strong Berry curvature that accounts for the observed UAHE. Field-dependent STM/S reveals a highly synchronous evolution of electronic density of states, with a dI/dV peak around the same field of 5 kOe, which provides evidence to the folded bands and excited UAHE by external magnetic fields. This finding elucidates the connection between the real-space non-collinear magnetism and the k-space topological electronic state and establishes a novel manner to engineer the magneto-transport behaviors of correlated electrons for future topological spintronics.

Dynamic features of the electron drift and electron properties in a HiPIMS discharge

Information on the evolution of electron properties during high-power impulse magnetron sputtering (HiPIMS) operation of planar magnetrons enables the study of fundamental physical processes. In this work, incoherent Thomson scattering is implemented for the non-invasive, spatiotemporally-resolved characterization of electron properties and drifts in the HiPIMS regime of a planar magnetron. In the ionization region of argon and helium plasmas, the azimuthal electron drifts are directly measured perpendicular to the magnetic field and are found to evolve according to a changing balance of E × B and diamagnetic electron drifts, while radial electron drifts, measured parallel to the magnetic field, can be attributed to plasma expansion/contraction and centrifugal forces. The evolutions of electron density and temperature in the afterglow plasma phase show the existence of two time scales for the variation of plasma properties. These characterizations provide detailed information on electron properties and dynamics in regions of the magnetic trap ordinarily inaccessible to invasive diagnostics.

Application of Recurrent Neural Network to Modeling Earth's Global Electron Density

AbstractThe total electron density is a fundamental quantity in the Earth's magnetosphere and plays an important role in a number of physical processes, but its dynamic global evolution is not fully quantified yet. We present an implementation of a specific type of recurrent neural network (encoder‐decoder), which is distinct from previous models, to construct global electron density based on the multiyear data from Van Allen Probes. The history of geomagnetic indices is first encoded into a hidden state H, then together with auxiliary information (satellite location), they are decoded into the quantity of interest (total electron density in this study). In this process the input of historical geomagnetic indices is detangled from the satellite location and is processed chronologically by the encoder. As a result, time evolution of geomagnetic indices is explicitly embedded in the structure and the encoded hidden state H can be viewed as the representation of the inner magnetospheric state. The magnetospheric state is then decoded to predict global electron density evolution. Our results show that the model can capture the dynamical evolution of total electron density with the formation and evolution of stable and evident plume configurations that roughly agree with global observations. Our findings demonstrate the importance of applying recurrent neural networks to specify the inner magnetospheric state in a novel way, which will potentially improve our fundamental understanding of wave and particle dynamics in the Earth's magnetosphere.

Theoretical and experimental study of the overdense plasma generation in a miniaturized microwave ion source

To understand the plasma evolution mechanism of microwave ion source (MIS), a hybrid discharge heating (HDH) mode is proposed. That mode contains two parts: ignition discharge by surface wave plasma (SWP) and ionization by electron cyclotron resonance. Compared with the traditional electron cyclotron heating (ECH) mode, the HDH mode has a wider scope of application for MIS with a chamber diameter smaller than the cutoff size. The spatio-temporal evolution of electric field, power deposition, electron temperature, and electron density of a miniaturized microwave ion source (MMIS) at Peking University is investigated based on the HDH mode. In addition, the MMIS is optimized based on the theoretical results of the HDH mechanism. Preliminary experiments show that a mixed hydrogen continuous wave beam of up to 25 mA at 30 keV can be extracted with a power efficiency of 25 mA/100 W.