Density Of High-energy Electrons Research Articles

The dynamical behavior of the capacitively coupled argon plasma driven by very high frequency

The dynamical behavior of the capacitively coupled Ar plasma driven by four frequencies (54, 80, 94 and 100[Formula: see text]MHz) at fixed pressure are simulated by the particle-in-cell/Monte-Carlo collisions (PIC/MCC) model based on the electromagnetic field. The magnetic field is generated by plasma current itself. The results show that the electron density, charge density, and electron temperature increase with the frequency increase when radio frequency (RF) power is 40[Formula: see text]W. The electron heating rate and argon ion heating rate near the sheath increase with the frequency increase. The high-energy electron density ([Formula: see text] [Formula: see text]eV) varies nonlinearly with the frequency increase. The high-energy electron density is highest at 80[Formula: see text]MHz among the four frequencies. The electron density, charge density, and high-energy electron density increase with RF power (30, 45, 60, and 75[Formula: see text]W) increase at 80[Formula: see text]MHz. The electron temperature decreases with RF power increase at 80[Formula: see text]MHz. The electron heating rate and argon ion heating rate near the sheath have no change with RF power increase at 80[Formula: see text]MHz. The electron energy probability distributions (EEPF) show that the number of high-energy electrons ([Formula: see text] [Formula: see text]eV) is highest at 80[Formula: see text]MHz. The reason may be the electron resonant heating that occurs in the plasma when the electron cyclotron frequency equals source frequency at 80[Formula: see text]MHz. The electron dynamics of capacitively coupled plasma is important for microelectronics etching and membrane deposition.

Spatial Spectral Characteristics of Partial Discharge with Different Electrode Models

In this paper, the spatial spectral characteristics of partial discharge (PD) under different electrode models are mainly studied. In the initial corona discharge stage, the emission spectrum is mainly emitted by the N2(C3IIu→B3IIg) energy level transition of the N2 second positive band system. The spectrum is in the ultraviolet range of 294–436 nm, and its main peak is at 337 nm. The streamer discharge stage spectrum is mainly emitted by the energy level transition of the second positive band system of N2, N+, NO, and O+ and the first positive band system of N2(B3IIg→A3Σu+). In the gap of different polarity electrodes, the ultraviolet spectrum content near the positive polarity is more abundant. The UV spectra ranges are 202–225 nm and 229–292 nm, respectively. The discharge of the needle–sphere system is more intense in visible light and near-infrared light, with peaks at 500 nm and 777 nm, respectively. In addition, the PD process based on the finite element method is simulated by COMSOL Multiphysics software. The simulation results show that the distribution of high-energy electron density varies with the electrode spacing and discharge model. The influence of particle energy level transition on the spatial spectral characteristics of PD is verified. This work provides important insights and possibilities for future fluorescent fiberoptic sensing and positioning for spatial PD detection and positioning using spectral characteristic peaks as detection quantities or excitations.

Density profile transition and high-energy electron transport in a magnetically expanding radio frequency plasma

The density profile transition and high-energy electron transport in a magnetically expanding radio frequency (RF) plasma were investigated using particle-in-cell and Monte Carlo collision techniques, where both the plasma source and the diffusion region were simulated self-consistently. The simulation results show that the density profile changes from center-peaked to bimodal plasma with increasing magnetic field strength, where bimodal plasma was observed in previous experiments. Then, the density profile transition is discussed with respect to ionization, electron temperature, and high-energy electron density. This indicates that electrons were heated by the RF field and transported radially inward across magnetic field lines. The moving distance of high-energy electrons is explained by an electron-neutral elastic collision. Therefore, the density formation depends on where the electrons are heated and how far the high-energy electrons are transported by an elastic collision, implying the longer existing time of high-energy electrons that move radially inward away from the RF antenna.

Mushroom-instability-driven Magnetic Reconnections in Collisionless Relativistic Jets

Abstract We study the kinetic plasma dynamics in collisionless relativistic jets with velocity shear, by carrying out particle-in-cell simulations in the transverse plane of a jet. It is discovered that intermittent magnetic reconnections (MRs) are driven by mushroom instability (MI), which is an important kinetic-scale plasma instability in the plasma shear flows with relativistic bulk speed. We refer to this sequence of kinetic plasma phenomena as “MI-driven MR.” The MI-driven MRs intermittently occur with moving the location of the reconnection points from the vicinity of the initial velocity-shear surface toward the center of the jet. As a consequence, the number density of high-energy electrons that are accelerated by MI-driven MRs increases with time in the region inside the initial velocity-shear surface with the accompanying generation and subsequent amplification of magnetic fields by MI. The maximum Lorentz factor of electrons increases with initial bulk Lorentz factor of the jet. A possible relation of MI-driven MR to the bright synchrotron emission in the jet spine of an active galactic nucleus jet is also discussed.

Rapid Variability of Sgr A* across the Electromagnetic Spectrum

Abstract Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We have analyzed a comprehensive submillimeter (including new observations simultaneous with NIR monitoring), NIR, and 2–8 keV data set. Submillimeter variations tend to lag those in the NIR by ∼30 minutes. An approximate Bayesian computation fit to the X-ray first-order structure function shows significantly less power at short timescales in the X-rays than in the NIR. Less X-ray variability at short timescales, combined with the observed NIR–X-ray correlations, means the variability can be described as the result of two strictly correlated stochastic processes, the X-ray process being the low-pass-filtered version of the NIR process. The NIR–X-ray linkage suggests a simple radiative model: a compact, self-absorbed synchrotron sphere with high-frequency cutoff close to NIR frequencies plus a synchrotron self-Compton scattering component at higher frequencies. This model, with parameters fit to the submillimeter, NIR, and X-ray structure functions, reproduces the observed flux densities at all wavelengths, the statistical properties of all light curves, and the time lags between bands. The fit also gives reasonable values for physical parameters such as magnetic flux density B ≈ 13 G, source size L ≈ 2.2R S , and high-energy electron density n e ≈ 4 × 107 cm−3. An animation illustrates typical light curves, and we make public the parameter chain of our Bayesian analysis, the model implementation, and the visualization code.

E-H mode transitions and high-energy electron characteristics of helical antenna coupled plasma

Metastable and high-energy electron characteristics obtained from optical emission spectroscopy are used to analyze the dependence of the H mode on the magnetic field strength and discharge pressure. The results show that the H-mode characteristics gradually appears as the magnetic field strength is increased, the reason being that electrons undergo multiple acceleration-collision cycles at high magnetic field strength, thereby the metastable ionization will be increased. This improves energy utilization and making the H mode appearing. The variation in the density of metastable states and the Langmuir probe data shows that the electron energy distribution function evolves from non-Maxwellian to Maxwellian. The radial constraint of the magnetic field to the electrons and thus reduces the electron heating efficiency. Moreover, the increase in electric field strength with magnetic field leads to an increase in energy obtained by the electrons per unit distance. The competition between the two makes the number of high-energy electrons decrease rapidly first, and then increase slowly with magnetic field strength increasing. The turning point increases with the increase of discharge pressure and radio-frequency (RF) power. And the higher the pressure the lower the high-energy electron. For fields between 105.5 G and 212.7 G. In the H-mode regime, and with increasing RF power, the number of high-energy electrons will be sudden rise after experiencing a steady increase. The sudden rise RF power increase with magnetic field and decrease with discharge pressure increase. However, at high magnetic fields (>265 G) and high power (>450 W), the high-energy electron density decreases with power increasing.

Measurement of electronegativity during the E to H mode transition in a radio frequency inductively coupled Ar/O2 plasma* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11675039, 11875101, and 11935005) and the Fundamental Research Founds for the Central Universities, China (Grant Nos. DUT18TD06 and DUT20LAB201).

This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency (rf) inductively coupled plasma (ICP) in a mixture of Ar and O2. The densities of the negative ion and the electron, as well as their ratio, i.e., the electronegativity, are measured as a function of the applied power by laser photo-detachment combined with a microwave resonance probe, under different pressures and O2 contents. Meanwhile, the optical emission intensities at Ar 750.4 nm and O 844.6 nm are monitored via a spectrograph. It was found that by increasing the applied power, the electron density and the optical emission intensity show a similar trench, i.e., they increase abruptly at a threshold power, suggesting that the E to H mode transition occurs. With the increase of the pressure, the negative ion density presents opposite trends in the E-mode and the H-mode, which is related to the difference of the electron density and energy for the two modes. The emission intensities of Ar 750.4 nm and O 844.6 nm monotonously decrease with increasing the pressure or the O2 content, indicating that the density of high-energy electrons, which can excite atoms, is monotonically decreased. This leads to an increase of the negative ion density in the H-mode with increasing the pressure. Besides, as the applied power is increased, the electronegativity shows an abrupt drop during the E- to H-mode transition.

Temporal evolution of plasma parameters in a pulse-modulated capacitively coupled Ar/O2 mixture discharge

The time-resolved measurements of the plasma parameters are carried out in a pulse-modulated low pressure capacitively coupled discharge operated in argon and argon–oxygen mixtures. Measurements are performed at a constant radio-frequency (RF) power of 130 W at 13.56 MHz, with the pulse repetition frequency of 1 kHz and 50% duty cycle. A single RF compensated Langmuir probe, synchronized with the modulated signal, is used to investigate the effects of filling gas pressure and O2 content in the discharge on the temporal evolution of electron density ne, electron temperature Te, plasma potential Vp, and electron energy probability function (EEPF). ne shows a rapid increase with time during the start of the pulse-on phase, followed by a gradual increase to attain a steady state value. During the pulse-off phase, ne decreases gradually. ne drops at a higher pressure and with increasing O2 content in the discharge. The temporal variations in Te and Vp, on the other hand, are more abrupt than the temporal variation in ne during the rise and the fall of the pulse. Both Te and Vp show an increase with rising pressure and with increasing O2 content in the discharge. The investigation of the temporal evolution of the EEPF reveals that the discharge is in a highly non-equilibrium state at the start of the pulse and evolves from bi-Maxwellian to Maxwellian distribution with the passage of time. The density of low energy electrons decreases, while the density of high energy electrons increases with increasing pressure and O2 content in the discharge. Moreover, several peaks and broadening in the EEPF profile are also noted with increasing O2 content.

Enhanced removal of toluene by pulse discharge plasma coupled with MgO cathode and graphene Mn–Ce bimetallic oxide

Herein, MgO cathode and graphene Mn–Ce bimetallic oxide were utilized to jointly enhance the removal of toluene in pulsed discharge plasma (PDP). Compared to the common cathode, the MgO cathode enhanced the density of high energy electrons, and then induced to higher removal of toluene. However, the removal of toluene by PDP/MgO system was still insufficient, and there was a large amount of underutilized O3 in the products. Based on this, Mn–Ce/graphene catalysts were introduced into PDP/MgO system. The Mn–Ce (8:1)/graphene catalyst had the highest catalytic activity. Under the discharge power of 2.1 W, toluene degradation rate and CO2 selectivity increased by 27.5% and 22.0%, respectively. This was ascribed to the synergistic effect of the solid solution formed between MnOx and CeOx, increasing the proportion of Oads on the surface of the catalyst. The higher Oads/Olatt ratio lead to the better catalytic activity, which was conducive to the complete transformation of the intermediate products to CO2 and H2O. According to the detected products, the degradation pathway and the mechanism of toluene degradation were proposed finally. The PDP itself, field emission effect of MgO cathode and catalytic effect of Mn–Ce/graphene for jointly improve the toluene removal and CO2 selectivity.

A high-energy electron density modulator driven by an intense laser standing wave

AbstractA high energy electron density modulator from a high-intensity laser standing wave field is studied herein by investigating the ultrafast motion of electrons in the field. Electrons converge at the electric field antinodes, and the discrete electron density peaks modulated by the field located at the corresponding laser phases of kx = nπ, (n = 0, 1, 2, …), that is, the modulation period is 1/2 the wavelength of the individual laser. We also discussed the influence of the laser parameters such as laser intensity and waist size on the beam modulator. It is shown that a long interaction length (waist) or sufficiently high field intensity is essential for relativistic electron density modulation.

Effects of Ar Addition to O2 Plasma on Plasma-Enhanced Atomic Layer Deposition of Oxide Thin Films

A method for significantly increasing the growth rates (GRs) of high- k oxide thin films grown via plasma-enhanced atomic layer deposition (PE-ALD) by enhancing the plasma density through the addition of Ar gas to the O2 plasma oxidant was developed. This approach led to improvements of ∼60% in the saturation GRs of PE-ALD ZrO2, HfO2, and SiO2. Furthermore, despite the significantly higher GR enabled by PE-ALD, the mechanical and dielectric properties of the PE-ALD oxide films were similar or even superior to those of films grown via the conventional O2 plasma process. Optical emission spectroscopy analyses in conjunction with theoretical calculation of the electron energy distribution function revealed that adding Ar gas to the O2 plasma increased the density of high-energy electrons, thereby generating more O2 plasma species, such as ions and radicals, which played a key role in improving the GRs and the properties of the films. This promising approach is expected to facilitate the high-volume manufacturing of films via PE-ALD, especially for use as gate insulators in thin-film transistor-based devices in the display industry.

Investigation of Spatially Resolved Spectrum of N2 (C3<inline-formula> <tex-math notation="LaTeX">$\Pi_{\rm{u}} \rightarrow$ </tex-math> </inline-formula> B3<inline-formula> <tex-math notation="LaTeX">$\Pi_{\rm{g}}$ </tex-math> </inline-formula>) in Positive Nanosecond Pulsed Streamer Corona Discharge

In this paper, the spatially resolved spectrum of the nitrogen second positive system ( $\text{C}^{3}\Pi _{\mathrm {u}} \to \text {B}^{3}\Pi _{\mathrm {g}}$ ) has been measured in a nanosecond pulsed streamer corona N2 discharge with a wire-plate electrode structure. The spatial distributions of emission intensity of N2 ( $\text{C}^{3}\Pi _{\mathrm {u}} \to \text {B}^{3}\Pi _{\mathrm {g}}$ , 0–0) at different pulse peak voltages, pulse repetition rates, and gas components are used to explore the distribution and variation of the high electron density (higher than 11.03 eV) with the reaction radiation rate analysis. It is found that the high electron density increases with increasing both the peak voltage and pulsed repetition rate but decreases with increasing the oxygen contents. The spatial distribution of the emission intensity of N2 ( $\text{C}^{3}\Pi _{\mathrm {u}} \to \text {B}^{3}\Pi _{\mathrm {g}}$ , 0–0) is normalized to the value of the emission intensity at the wire electrode at different experimental conditions. The results show that the normalized distribution of the spatially resolved spectral intensity of N2 ( $\text{C}^{3}\Pi _{\mathrm {u}} \to \text {B}^{3}\Pi _{\mathrm {g}}$ , 0–0) is independent of pulsed peak voltage, pulsed repetition rate, and gas component, which indicate that the normalized distributions of the high-energy electron density in the gap space at different experimental conditions are similar. These experimental results are significant to understand the discharge characters of pulsed streamer corona discharge and establish the molecule reaction dynamics model of pulsed streamer corona discharge.

Plasma states and carbon film deposition in glow discharge connected to dielectric barrier discharge

We developed a plasma source by connecting a direct current glow discharge (GD) with a dielectric barrier discharge (DBD) via a ground mesh (DB-GD). Based on the Langmuir probe measurements, the electron density of the DB-GD increased and the electron temperature decreased compared with those of the GD. Calculations of the electron energy distribution functions revealed that the DB-GD possessed a lower density of high-energy electrons and higher density of low-energy electrons than the GD did. We also confirmed that the DB-GD generated with methane gas was capable of producing an amorphous carbon film.

Emission spectroscopy of Ar + H2 + C7H8 plasmas: C7H8 flow rate dependence and pressure dependence

We have studied toluene flow rate dependence and pressure dependence of Ar emission intensities at 425.9 nm, 750.4 nm and 811.5 nm for Ar + H2 + C7H8 plasmas. Electron density and the electron impact excitation rate coefficients are nearly the same for Ar(100%) and Ar+H2(33%) plasmas. Toluene addition leads to a decrease in the high energy electron density, whereas the effective electron temperature and low energy electron density remain almost the same. The effective electron temperature, and Ar metastable density and/or low energy electron density decrease significantly with increasing the total pressure from 0.1 to 5 Torr.

Study of Microhollow Cathode Glow Discharge to Improve the Wettability on Surface of Polypropylene Film

The ion density distribution has been made and the ionization rate profile has been studied at moderate to atmospheric pressure inside the cavity of microhollow cathode glow discharge to better understand the microplasma behavior, aiming thin film deposition. This methodology sounds as a quite straightforward approach for very investigated, but not well understood, glow discharge at atmospheric pressure. An analytical solution for the continuity equation was proposed, which permits to obtain the ion density distribution and ionization rate profile inside the microplasma. The radial ionization rate has a maximum at the edge of the negative glow where the density of high-energy electrons is higher increasing the number of ionization collisions. The solution of continuity equation shows that microhollow cathode glow discharge has elevated ion density as would be for high pressure discharges. The results were compared with spectroscopic measurements employing argon and Balmer lines. This microplasma was used as glow plasma based aiming the improvement of the wettability on polypropylene film.

TURBULENT, EXTREME MULTI-ZONE MODEL FOR SIMULATING FLUX AND POLARIZATION VARIABILITY IN BLAZARS

The author presents a model for variability of the flux and polarization of blazars in which turbulent plasma flowing at a relativistic speed down a jet crosses a standing conical shock. The shock compresses the plasma and accelerates electrons to energies up to gamma(max) > 1E4 times their rest-mass energy, with the value of gamma(max) determined by the direction of the magnetic field relative to the shock front. The turbulence is approximated in a computer code as many cells, each with a uniform magnetic field whose direction is selected randomly. The density of high-energy electrons in the plasma changes randomly with time in a manner consistent with the power spectral density of flux variations derived from observations of blazars. The variations in flux and polarization are therefore caused by continuous noise processes rather than by singular events such as explosive injection of energy at the base of the jet. Sample simulations illustrate the behavior of flux and linear polarization versus time that such a model produces. The variations in gamma-ray flux generated by the code are often, but not always, correlated with those at lower frequencies, and many of the flares are sharply peaked. The mean degree of polarization of synchrotron radiation is higher and its time-scale of variability shorter toward higher frequencies, while the polarization electric vector sometimes randomly executes apparent rotations. The slope of the spectral energy distribution exhibits sharper breaks than can arise solely from energy losses. All of these results correspond to properties observed in blazars.

Modulation of electron energy distributions and discharge parameters in a dual frequency ICP discharge

Using a radio frequency (RF) compensated Langmuir probe, modulations in electron energy distribution (EED) and plasma potential are investigated in a discharge produced by a large-area dual frequency/dual antenna inductively coupled plasma source. The discharge is ignited using two frequencies (2 and 13.56 MHz). It is observed that the EEDs can be tailored by varying the power ratio of the two frequencies. Increasing the power level of the low frequency (P2 MHz) enhances the population density of high-energy electrons; however, increasing the high-frequency power (P13.56 MHz) increases the low-energy electron population density. At a fixed total power (P2 MHz + P13.56 MHz), the higher the low-frequency power (P2 MHz) content, the higher the population density of high-energy electrons; however, this trend reverses with high-frequency power (P13.56 MHz). The influence of power ratio on plasma density (ne), plasma temperature (Te) and plasma potential (Vp) has also been studied. It is found out that the plasma parameters have similar trends with RF power irrespective of its frequency. The value of ne increases, and Te and Vp decrease with increasing power. At a fixed P2 MHz, Vp increases with increasing P13.56 MHz. However, Vp decreases with increasing P2 MHz at a fixed value of P13.56 MHz.

Quasi-equilibrium electron density along a magnetic field line

A methodology is developed to determine the density of high-energy electrons along a magnetic field line for a low-β plasma. This method avoids the expense and statistical noise of traditional particle tracking techniques commonly used for high-energy electrons in bombardment plasma generators. By preserving the magnetic mirror and assuming a mixing timescale, typically the elastic collision frequency with neutrals, a quasi-equilibrium electron distribution can be calculated. Following the transient decay, the analysis shows that both the normalized density and the reduction fraction due to collision converge to a single quasi-equilibrium solution.

Current-voltage Characteristics of Graphene Nanoribbon Schottky Diodes

Theoretical investigation demonstrates that the current-voltage characteristics of a metal/graphene nanoribbon with donor impurities contact under forward bias condition has typical properties of a metal/silicon contact. Assuming that the work function of graphene nanoribbon will not change with its width, the results demonstrate that under a given biasing voltage, the current density of the contact decreases with the decrement of the nanoribbon width. Such a current-density decrement is especially large when the graphene-nanoribbon width drops to the few nanometers range. Furthermore, the high-energy electron density increases with the decrement of the nanoribbon width, which also impacts the current density significantly. Therefore, the relationship between the decrement of the current density and the nanoribbon width is non-exponential. The results in this paper also illustrate that the current density of metal/graphene nanoribbon contact at a given biasing voltage increases slowly with the increment of the donor density.

Time-resolved investigation of an asymmetric bipolar pulsed magnetron discharge: Influence of pressure

A bipolar pulsed magnetron deposition discharge has been studied with pulse frequencies of 100 and 150 kHz, respectively. The discharge was operated in an argon/oxygen mixture at different total pressures with a circular magnesium target as cathode. Time-resolved Langmuir double probe measurements and time-resolved optical emission spectroscopy have been used to analyse the temporal behaviour of the pulsed discharge in the substrate region. It is found that typical structures in the time dependence of both charge carrier density and optical emission which are observed in the “on”-phase of the discharge are significantly influenced by the pressure. If the pressure is increased, the discharge is built up faster with all the structures appearing earlier. Additionally, the intensity of the structures is changed. We demonstrate that this on the one hand due to enhanced collisions of the electrons with neutral gas particles. On the other hand, the wave form of the target voltage is altered in connection with the increased pressure which subsequently feeds back to the discharge. A study of the temporal behaviour of the argon emission in the “off”-phase reveals an approximately exponential decay which is also faster when the pressure is increased. By comparison of argon lines which are either predominantly excited by direct transition or via cascades from upper levels we found that at higher pressure cascading plays a less important role as the density of high energy electrons is reduced.