Electron Heating Effect Research Articles

Fluid models calculation of Ar/CF4 radiofrequency capacitively coupled plasmas

A fluid model is used to calculate plasma density in radiofrequency capacitively coupled plasmas (RF-CCPs), assuming geometrically symmetric Ar/CF4. Electrodes are powered by a 200 V sinusoidal wavefront at 13.56 MHz. The gap between the electrodes is 5 cm. The plasma species density is calculated as a function of the gas pressure, electron temperature, and gas composition. As expected from recent experiments, CF3+ and F are dominant for all simulation parameters (Plasma Chemistry and Plasma Processing, 2022; 42: 989-1002, 2013 Pulsed Power Conference (PPC), 19th IEEE, DOI:10.1109/PPC.2013.6627595). The results explain the pathways to perform atomic layer etching and nanolayer deposition processes. In order to reveal the effect of electron heating on the discharge dynamics, the spatiotemporal electron energy equation is coupled with the fluid model. Tailoring the driven potential has been found to control the concentration of some plasma species. When the plasma is driven by the fundamental frequency, ohmic and stochastic heating allow electrons to be heated symmetrically. Higher harmonics give rise to an electrical asymmetry and an electron heating asymmetry between the powered and grounded sheaths. The electron temperature depends on the driven harmonics; it adjusts gain and loss rates and some plasma species densities.

Direct Observation of Low‐Energy Electron Precipitation in the Subauroral Region

AbstractThis study used data collected by the Defense Meteorological Satellite Program (DMSP) and the Radiation Belt Storm Probes (RBSP) to identify a unique subauroral arc in the duskside, which is associated with precipitations of both ions and electrons. The auroral arc was found to be separated from the auroral oval, extending from 14 to 19 magnetic local time hours. Subauroral arcs are usually generated by energetic protons with energies above approximately 10 keV, while enhanced fluxes of low‐energy (<200 eV) electrons were revealed to occur and last for a few hours in this arc region from in situ observations by the DMSP satellite. The observations from RBSP‐B indicate that the low‐energy electron fluxes, with pitch angles near the loss cone, were accompanied by intensified electromagnetic ion cyclotron (EMIC) waves. It is suggested that the enhanced fluxes of cold electrons were heated by the Landau damping of the observed H+ band EMIC waves, which was in the frequency range below the local H+ gyrofrequency and above the local He+ gyrofrequency. Additionally, both electron density and temperature in the ionosphere increased significantly. This study, using conjugate observations of magnetospheric and ionospheric spacecraft, providing evidence of precipitation of low‐energy electrons within the subauroral arc region, thereby offering new insight into understanding the ionospheric effects of cold electron heating by EMIC waves.

Effects of electron heating and surface rippling on Rayleigh–Taylor instability in radiation pressure acceleration

The acceleration of ultrathin targets driven by intense laser pulses induces Rayleigh–Taylor-like instability. Apart from laser and target configurations, we find that electron heating and surface rippling, effects inherent to the interaction process, have an important role in instability evolution and growth. By employing a simple analytical model and two-dimensional particle-in-cell simulations, we show that the onset of electron heating in the early stage of the acceleration suppresses the growth of small-scale modes, but it has little influence on the growth of large-scale modes, which thus become dominant. With the growth of surface ripples, a mechanism that can significantly influence the growth of these large-scale modes is found. The laser field modulation caused by surface rippling generates an oscillatory ponderomotive force, directly modulating transverse electron density at a faster growth rate than that of ions and eventually enhancing instability growth. Our results show that when surface deformation becomes obvious, electron surface oscillation at 2ω0 (where ω0 is the laser frequency) is excited simultaneously, which can be seen as a signature of this mechanism.

Toward streaked collective Thomson scattering measurements on an extreme ultraviolet plasma light source

We show through forward modeling calculations that streaked collective Thomson scattering measurements are feasible on laser-produced tin plasmas generated under conditions relevant for extreme ultraviolet lithography. Using a 532 nm probe laser beam, the feasibility of simultaneous measurements of electron plasma wave (EPW) and ion acoustic wave (IAW) spectra is investigated. Absolute photon counts for laser scattering off both waves are calculated. Probe laser electron heating and bremsstrahlung background radiation effects are accounted for. While a large spatiotemporal region can be successfully probed based on the IAW feature, only one measurement location can be accessed through the EPW as a result of the low signal to noise ratio. A portable/traveling tabletop system is proposed.

Two-dimensional particle-in-cell simulation of electron heating effect in multiple hollow cathode trenches with curved boundary

A particle-in-cell simulation is suitable for simulating capacitively coupled plasma devices because it calculates the trajectory of particles with arbitrary energy distributions and has the advantage of analyzing sheath dynamics precisely. Implementing the ghost grid method in the Poisson solver makes it possible to simulate a curved structure in the structured grids. The electron transport and heating mechanism are determined according to the two-dimensional (2D) sheath dynamics. The hollow cathode effect on electron transport was analyzed by varying the hole shape and gas pressure. The spatial distributions of electron density and temperature are influenced by the energy relaxation length of electrons, which is affected by the energy distribution function and gas pressure. As a result, a new electron heating mode appears in the 2D structure.

Enhanced heating in plasma bulk due to electron cyclotron resonance in weakly magnetized capacitively coupled plasmas

An enhanced electron heating mechanism based on a resonance between the cyclotron motion of electrons and radio frequency (rf) electric field in the plasma bulk is reported in weakly magnetized capacitively coupled argon plasmas at low pressure. When the electron cyclotron frequency coincides with the applied power source frequency, the bulk electrons can continuously acquire energy from the background electric field within certain rf periods during the cyclotron motion, inducing overall distinct increase of excitation rate and electron temperature in the plasma bulk. This enhanced electron heating effect has been examined by a combination of kinetic particle simulations, experimental measurements, and an analytical model, and the dynamics of electrons are revealed at resonant conditions.

Precipitating Electron Energy Spectra and Auroral Power Estimation by Incoherent Scatter Radar With High Temporal Resolution

AbstractThis study presents an improved method to estimate differential energy flux, auroral power and field‐aligned current of electron precipitation from incoherent scatter radar data. The method is based on a newly developed data analysis technique that uses Bayesian filtering to fit altitude profiles of electron density, electron temperature, and ion temperature to observed incoherent scatter spectra with high time and range resolutions. The electron energy spectra are inverted from the electron density profiles. Previous high‐time resolution fits have relied on the raw electron density, which is calculated from the backscattered power assuming that the ion and electron temperatures are equal. The improved technique is applied to one auroral event measured by the EISCAT UHF radar and it is demonstrated that the effect of electron heating on electron energy spectra, auroral power, and upward field‐aligned current can be significant at times. Using the fitted electron densities instead of the raw ones may lead to wider electron energy spectra and auroral power up to 75% larger. The largest differences take place for precipitation that produces enhanced electron heating in the upper E region, and in this study correspond to fluxes of electrons with peak energies from 3 to 5 keV. Finally, the auroral power estimates are verified by comparison to the 427.8 nm auroral emission intensity, which shows good correlation. The improved method makes it possible to calculate unbiased estimates of electron energy spectra with high time resolution and thereby to study rapidly varying aurora.

Application of Nanosilicon to the Sintering of Mg-Mg2Si Interpenetrating Phases Composite.

The new in situ fabrication process for Mg-Mg2Si composites composed of interpenetrating metal/intermetallic phases via powder metallurgy was characterized. To obtain the designed composite microstructure, variable nanosilicon ((n)Si) (i.e., 2, 4, and 6 vol.% (n)Si) concentrations were mixed with magnesium powders. The mixture was ordered using a sonic method. The powder mixture morphologies were characterized using scanning electron microscopy (SEM), and heating and cooling-induced thermal effects were characterized using differential scanning calorimetry (DSC). Composite sinters were fabricated by hot-pressing the powders under a vacuum of 2.8 Pa. Shifts in the sintering temperature resulted in two observable microstructures: (1) the presence of Mg2Si and MgO intermetallic phases in α-Mg (580 °C); and (2) Mg2Si intermetallic phases in the α-Mg matrix enriched with bands of refined MgO (640 °C). Materials were characterized by light microscopy (LM) with quantitative metallography, X-ray diffraction (XRD), open porosity measurements, hardness testing, microhardness testing, and nanoindentation. The results revealed that (n)Si in applied sintering conditions ensured the formation of globular and very fine Mg2Si particles. The particles bonded with each other to form an intermetallic network. The volume fraction of this network increased with (n)Si concentration but was dependent on sintering temperature. Increasing sintering temperature intensified magnesium vaporization, affecting the composite formation mechanism and increasing the volume fraction of silicide.

Commissioning and initial heating results of the neutral beam injection system in Versatile Experiment Spherical Torus

The initial neutral beam heating experiment in Versatile Experiment Spherical Torus (VEST) has been conducted via neutral beam characterizations such as neutralization efficiency, beam fraction, and shine-through loss. For the neutralization efficiency and shine-through loss estimation, a secondary electron emission (SEE) detector, which can be used in rapidly changing plasma, has been developed. The measurement results of the monatomic fraction by Doppler shift spectroscopy and the neutralization efficiency by SEE detector are 65%, and 70%, respectively. The shine-through loss measurement shows that the injection of a 12 keV neutral beam into a typical ohmic discharge of VEST results in a large amount of shine-through loss. To reduce the shine-through loss, a new target plasma discharge (0.17 T, 120 kA, 13 ms) with increased plasma density of ~2×1019m−3 is developed by providing additional gas injection. The electron-heating effect has been successfully observed for this target plasma with a 200 kW neutral beam injection at the beam energy of 12 keV. The electron temperature increases from 20 to 68 eV without significant density change when the neutral beam is injected.

The effect of electron heating on plasma decay in H2:O2 mixture excited by a repetitively pulsed nanosecond discharge

Plasma decay was experimentally studied in the afterglow of a repetitively pulsed nanosecond discharge in a stoichiometric H2:O2 mixture. An additional bias DC electric field was applied to heat electrons during the discharge afterglow between the high-voltage pulses. The energy input per pulse was so small that the changes in both chemical composition and gas temperature were negligible. Using the microwave interferometer, the temporal evolution of the electron density in the discharge afterglow was measured when it decreased from 3 × 1012 to 5 × 1010 cm−3. Measurements were performed for various numbers of discharge pulses (various degrees of fuel oxidation) at room gas temperature and pressures from 1 to 2 Torr. The effect of electron heating on the rate of plasma decay was most profound for low oxidation degrees and practically disappeared after complete fuel oxidation. A kinetic model was suggested for describing ion-molecule processes and plasma decay in the discharge afterglow at various H2 oxidation degrees. The analysis of calculated results showed that plasma decay was governed by dissociative recombination of electrons with simple molecular ions at low oxidation degrees and with hydrated H3O+(H2O) k ions at sufficiently high oxidation degrees. An increase of H2O fraction due to H2 oxidation in the mixture led to a less efficient electron heating during plasma decay in the additional electric field. This was associated with extremely high cross-sections for the elastic and inelastic scattering of low-energy electrons by H2O molecules. As a result, the rate of electron-ion recombination increased with increasing number of discharge pulses, whereas the effect of electric field became less profound, in agreement with the observations.

Comparison of modulated UHF PMSE characteristics at different altitude ranges

Heating of electrons in PMSE region with powerful high frequency (HF) radio waves is called artificial electron heating. This is used as a diagnostic tool to study the interesting radar phenomenon of Polar Mesosphere Summer Echoes (PMSE). Using the PMSE observations of EISCAT UHF (930 MHz) radar from 2003 to 2015, the effect of artificial electron heating on PMSE is studied for the first time in terms of modulated PMSE characteristics. The modulated PMSE characteristics including the PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot, are analyzed and compared between different height ranges. Our results indicate variability in these characteristics at different height ranges, however, statistics reveal that unlike VHF PMSE these characteristics show a clear difference between the comparatively lower (below 84.0 km) and upper (above 84.0 km) height ranges. It is found that in below 84.0 km height ranges a greater number of heating cycles show PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot as compared to above 84.0 km height ranges. All characteristics found are in good agreement with literature especially at just after the heater-on time where no opposite behavior of PMSE intensity reduction (onset overshoot) is found, and at the heater-off time where the unheated PMSE intensity never found smaller than the heated PMSE intensity. The observed response of the PMSE to heating by a powerful radio wave is used to estimate the associated electron temperature enhancement and PMSE modulation index. PMSE modulation index is the ratio of heated PMSE to unheated PMSE, which describes that how much the heated PMSE varies around its unheated PMSE. In below 84 km height ranges, the PMSE modulation index and enhanced electron temperatures are greater than that in above 84 km height ranges. The difference in modulated properties of PMSE below and above 84 km is due to the difference in electron temperature enhancement, which depends on the electron density below the PMSE altitudes.

Target normal sheath acceleration with a large laser focal diameter

The dependence of the laser-driven ion acceleration from thin titanium foils in the Target Normal Sheath Acceleration (TNSA) regime on target and laser parameters is explored using two dimensional particle-in-cell simulations. The oblique incidence (θL=45°) and large focal spot size (w0=40μm) are chosen to take an advantage of quasi one-dimensional geometry of sheath fields and effective electron heating. This interaction setup also reveals low and achromatic angular divergence of a proton beam. It is shown that the hot electron temperature deviates from the ponderomotive scaling for short laser pulses and small pre-plasmas. This deviation is mainly due to the laser sweeping, as the short duration laser pulse each moment in time effectively heats only a fraction of a focal spot on the foil. This instantaneous partial heating results in an electron temperature deviation from the ponderomotive scaling and, thus, lower maximum proton energies than it could have been expected from the TNSA theory.

Numerical Analysis of Artificial Electron Heating Effects on Polar Mesospheric Winter Echoes

In this paper, an analytical model is used to analyze the modulated polar mesospheric winter echoes (PMWE). The winter parameters were introduced to simulate the effects of different parameters during the artificial electron heating of PMWE. The important role of the charged dust particle in the creation of PMWE is confirmed again. It is found that during the heating of PMWE, the increases of the dust size, dust charge, electron temperature, initial electron density, and ion-neutral collision frequency cause the increase of the electron density irregularity, and hence the PMWE strength. However, with increasing the dust density, the electron density irregularity and the PMWE strength decrease.

Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave.

The effect of electron heating on hydrocarbon plasma decay after high-voltage nanosecond discharge

An experimental study of plasma decay in the afterglow of a high-voltage nanosecond discharge in ethane:argon mixtures was performed under conditions when the mean electron energy significantly exceeded the mean energy of neutral particles at room temperature and pressures from 2 to 10 Torr. Electrons were heated during the plasma decay (i) by retarding electron thermalization through a strong dilution of the hydrocarbon with argon and (ii) by the application of an additional external electric field. Using the microwave interferometer, the temporal evolution of the electron density during plasma decay was studied in the range from 5 × 1010 to 3 × 1012 cm−3. The analysis of the experimental data allowed us to obtain the dependence of the effective recombination coefficients of electrons with hydrocarbon ions on the electron temperature. A simplified kinetic scheme was proposed for the description of electron and ion reactions during the plasma decay for our experimental conditions. Calculations on the basis of this scheme showed that the plasma decay is controlled by the dissociative recombination of molecular and cluster hydrocarbon ions.

Effect of current-induced electron heating on irradiated magnetoresistance in high-mobility 2D electron systems

We present a theoretical work to study the influence of an external DC-current on the irradiated magnetoresistance through a very high-mobility 2D electron system. Recent experiments report that an increasing DC-current through the system under radiation, dramatically reduces magnetoresistance, including background and irradiated contributions. On the one hand, the former ends up progressively negative (negative giant magnetoresistance) and on the other hand, the effect of radiation, i.e., the radiation-induced resistance oscillations, tends to vanish. In our theoretical model, the DC-current–induced hot electrons when scattered by charged impurities end up, to an increasing extent, between Landau levels with a very low density of states to get to. This makes the scattering process much less effective, decreasing both the magnetoresistance itself and the radiation-induced resistance oscillations. We conclude that both effects are not separable, being tightly linked by the scattering process.

Impact of additional plasma heating on detached plasma formation in divertor simulation experiments using the GAMMA 10/PDX tandem mirror

The transition of a detached to attached plasma experiment has been performed in GAMMA 10/PDX by applying an additional plasma heating pulse of electron cyclotron heating (ECH). In a plasma detachment experiment, a short pulse (25 ms) of ECH was applied at the east plug-cell to examine the effects of electron heating on the plasma parameters in the D-module. It was found that the ion flux increased significantly during ECH injection. In the ECH injection period, the ion flux increased with the increasing impurity injection, which indicates the impurity ion components are enhanced by ionization in the D-module due to the application of ECH. During ECH injection period, the increase of the ion flux near the corner of the target plate shows a clear dependence on the gas species. The 2D visible emission has been captured by the high-speed camera and a bright emission near the corner of the target has been observed. The spectroscopic measurement in the D-module shows that the impurity ion emission increases remarkably during ECH injection. These results indicate the detached plasma change to the attached state.

Ionization asymmetry effects on the properties modulation of atmospheric pressure dielectric barrier discharge sustained by tailored voltage waveforms

The dielectric barrier discharge (DBD) is a promising technology to generate high density and uniform cold plasmas in atmospheric pressure gases. The effective independent tuning of key plasma parameters is quite important for both application-focused and fundamental studies. In this paper, based on a one-dimensional fluid model with semi-kinetics treatment, numerical studies of ionization asymmetry effects on the properties modulation of atmospheric DBD sustained by tailored voltage waveforms are reported. The driving voltage waveform is characterized by an asymmetric-slope fundamental sinusoidal radio frequency signal superimposing one or more harmonics, and the effects of the number of harmonics, phase shift, as well as the fluctuation of harmonics on the sheath dynamics, impact ionization of electrons and key plasma parameters are investigated. The results have shown that the electron density can exhibit a substantial increase due to the effective electron heating by a spatially asymmetric sheath structure. The strategic modulation of harmonics number and phase shift is capable of raising the electron density significantly (e.g., nearly three times in this case), but without a significant increase in the gas temperature. Moreover, by tailoring the fluctuation of harmonics with a steeper slope, a more profound efficiency in electron impact ionization can be achieved, and thus enhancing the electron density effectively. This method then enables a novel alternative approach to realize the independent control of the key plasma parameters under atmospheric pressure.

Electron heating and thermal relaxation of gold nanorods revealed by two-dimensional electronic spectroscopy

To elucidate the complex interplay between the size and shape of gold nanorods and their electronic, photothermal, and optical properties for molecular imaging, photothermal therapy, and optoelectronic devices, it is a prerequisite to characterize ultrafast electron dynamics in gold nanorods. Time-resolved transient absorption (TA) studies of plasmonic electrons in various nanostructures have revealed the time scales for electron heating, lattice vibrational excitation, and phonon relaxation processes in condensed phases. However, because linear spectroscopic and time-resolved TA signals are vulnerable to inhomogeneous line-broadening, pure dephasing and direct electron heating effects are difficult to observe. Here we show that femtosecond two-dimensional electronic spectroscopy, with its unprecedented time resolution and phase sensitivity, can be used to collect direct experimental evidence for ultrafast electron heating, anomalously strong coherent and transient electronic plasmonic responses, and homogenous dephasing processes resulting from electron-vibration couplings even for polydisperse gold nanorods.

Effect of driving frequency on electron heating in capacitively coupled RF argon glow discharges at low pressure**Project supported by the National Natural Science Foundation of China (Grant No. 51172101).

A one-dimensional (1D) fluid model on capacitively coupled radio frequency (RF) argon glow discharge between parallel-plates electrodes at low pressure is established to test the effect of the driving frequency on electron heating. The model is solved numerically by a finite difference method. The numerical results show that the discharge process may be divided into three stages: the growing rapidly stage, the growing slowly stage, and the steady stage. In the steady stage, the maximal electron density increases as the driving frequency increases. The results show that the discharge region has three parts: the powered electrode sheath region, the bulk plasma region and the grounded electrode sheath region. In the growing rapidly stage (at 18 μs), the results of the cycle-averaged electric field, electron temperature, electron density, and electric potentials for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are compared, respectively. Furthermore, the results of cycle-averaged electron pressure cooling, electron ohmic heating, electron heating, and electron energy loss for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are discussed, respectively. It is also found that the effect of the cycle-averaged electron pressure cooling on the electrons is to “cool” the electrons; the effect of the electron ohmic heating on the electrons is always to “heat” the electrons; the effect of the cycle-averaged electron ohmic heating on the electrons is stronger than the effect of the cycle-averaged electron pressure cooling on the electrons in the discharge region except in the regions near the electrodes. Therefore, the effect of the cycle-averaged electron heating on the electrons is to “heat” the electrons in the discharge region except in the regions near the electrodes. However, in the regions near the electrodes, the effect of the cycle-averaged electron heating on the electron is to “cool” the electrons. Finally, the space distributions of the electron pressure cooling the electron ohmic heating and the electron heating at 1/4T, 2/4T, 3/4T, and 4/4T in one RF-cycle are presented and compared.