Decrease In Electron Temperature Research Articles

The Influence of Increasing Argon Gas Flow Rate on Plasma Parameters and their Diagnostics by Optical Emission Spectroscopy (OES)

In this research, the plasma parameters were determined each of electron temperature (Te), electron density (ne), plasma frequency (fp), Debye length (λD). The plasma jet operates with argon gas at atmospheric pressure. It is equipped with an alternating AC high voltage power supply of 12 kilovolts and a frequency of 20 kilohertz. Plasma parameters were calculated from the emission spectrum using an optical emission spectrometer (OES) which captured the spectrum resulting from the plasma at various flow rates of argon gas from 0.5 to 5 L/min. Several peaks of argon gas (ArI) and nitrogen gas (N2) were appeared, where the results showed that the highest peak of argon gas was at 763.51 nm for the fifth flow of argon gas. The results indicated the electron temperature (Te) decrease with increased flow rate and ranged (1.296–0.645) ev, while the electron density (ne) increased with flow rate and ranged were (4.054-5.068x1017) cm-3, respectively. In contrast an increase in the intensity of spectral lines was observed.

Numerical simulation of air DBD under standard atmospheric pressure

In order to study the mechanism of dielectric barrier discharge (DBD) producing non-equilibrium plasma, the DBD equipment was simplified into a one-dimensional model. The AC power supply voltage was set as 18 kV, the frequency was set as 10 kHz, and the discharge medium was N2 and O2 with a mass fraction of 4:1 to simulate the air. The impact cross-section, electron mobility and initial electron temperature of the discharge reaction were determined by BOLSIG+. The electric potential field diagram, voltage current relation diagram, electron density diagram, temperature distribution diagram and electron energy distribution function EEDF diagram are obtained. The results show that the electric field reaches the maximum near the gap sheath. Due to the generation of an additional electric field, the current density reaches the maximum at the rising edge of the air gap voltage. The continuous ionization of air particles in the discharge process leads to the accumulation of electrons and the formation of electron collapse, which leads to the increase in electron density and the decrease in electron temperature. As the reduced electric field E/N increases, the proportion of high-energy electrons in the total electron number increases. At this time, the reaction of non-equilibrium plasma intensifies, and more active substances can be produced.

Characterization of the laser-induced breakdown spectroscopy near the gas-liquid two-phase interface.

The characterization of laser-induced breakdown spectroscopy (LIBS) near the gas-liquid two-phase interface was investigated with the laser acting on the sample along the horizontal direction. Simulation of the laser beam focusing process and observation of laser beam spot images show that difference in focusing positions in the air and the solution results from refraction of the laser beam entering the solution from the air and the change of propagation direction on the container lateral. The peak power and mean irradiance of the focused laser beam spot increase with the distance away from the interface, which is attributed to the fact that the loss of laser energy due to the refraction and reflection of light at the interface decreases with the focusing position moving away from the interface. This variation trend of laser irradiance allows for the growth of the spectral line intensity and lifetime with increasing the distance from the interface. The plasma electron density and temperature decrease with the delay time but increase with the distance away from the interface at the same delay time. Our findings help us to gain more insight into the characteristics and evolution mechanisms of LIBS produced near the gas-liquid two-phase interface, which provides theoretical guidance for the correction of LIBS spectra especially in water pollution monitoring.

Physical mechanisms for the transition from type-III to large ELMs induced by impurity injection on EAST

Transition from type-III to large-amplitude ELMs induced by neon injection has been observed in the EAST tokamak at overlapping q95 space between large and small ELMs. With neon injection, pedestal density gradient shows a remarkable increase accompanied by some decrease of pedestal electron temperature, and consequently the pressure gradient increases moderately and edge bootstrap current has minimal change. Further experiment demonstrates that the occurrence of large ELMs after neon injection is highly correlated with the change in edge density. Linear peeling-ballooning stability analysis indicates that the large ELM case is more unstable than the type-III ELM case during the ELM transition. A scan of pedestal density gradient in linear stability analysis shows that the direct destabilizing effect of steep pedestal density gradient on peeling-ballooning instabilities via two-fluid effects could also facilitate the transition to large ELMs. These results could provide more insight into the role of pedestal density gradient on pedestal stability and ELM behavior.

The power loss requirement in the non-thermal ³He-³He fusion plasma

The control of power loss is the most important issue in the plasma and it should be minimised by suitable selection of design parameters. This research is done with the aim of determining power loss and the parameters affecting the ³He-³He plasma with isotropic non-Maxwellian electron velocity distribution function (EVDF). In order to investigate the power loss requirement, bi-Maxwellian EVDF is considered. The results show that the rate of entropy production decreases as the electron density decreases and collision time increases. The ratio of the minimum power loss to the fusion power decreases as the electron temperature, and electron and helium-3 densities decrease, and the ion temperature increases. It is favourable to decrease the electron temperature lower than the thermal limit. Moreover, decreasing the electron temperature increases the fusion power.

The discharge characteristics of low-pressure capacitively coupled argon plasma with Langmuir probe

The discharge characteristics and electronic behavior in the low pressure capacitively coupled argon plasma is studied by the radio frequency (RF) compensated Langmuir probe based on the Druyvesteyn method. The effects of excitation frequencies (15, 50, 81.36, 100 MHz) and power (10∼80 W) at electrode on the electron energy probability function (EEPF), electron density (ne), electron temperature (Te) and plasma potential (Vp) are discussed. The experimental results indicate as the frequency increases, the EEPF evolves from a single Maxwellian distribution to a convex distribution and finally turns into a Druyvesteyn distribution. Simultaneously, the EEPF curve shows an upward trend with power increase and the low-energy electron group temperature and the high-energy electron group temperature of 81.36 MHz both are highest. The low-energy electron group temperature of 50 MHz is lowest. The electron density first decreases and then increases rapidly as the excitation frequency increases, whereas increases with power increasing. The electron temperature decrease with frequencies increase (after 50 MHz). When the frequency is 15 and 50 MHz, the effective electron temperature drops with the increase of power, and when the frequency is 81.36 and 100 MHz, it rises with the increase of power, which reflects the discharge parameters have complex influence on the electronic behavior. Additionally, the plasma potential is also greatly affected by excitation frequency and power. It rises significantly with the increase of power and first drops and then rises with the increase of frequency. The heating mechanism is discussed by the ratio of stochastic heating to ohmic heating with frequency increase.

Spectroscopy of the Mirror Instability Growth Rate in Hollow Electrodes Discharge (HED) Plasma

In this work, an experimental study was conducted about the effect of gas pressure on the growth rate of the mirror instability produced in hollow electrodes discharge (HED) plasma in two regions: inter-electrodes gap and internal cathode cavity, by optical emission spectroscopy. Optical emission spectroscopy measurements, at different gas pressures in two regions under study, show that the electron number density (ne) increase with increasing gas pressure from 0.04 to 0.2 Torr. While the electron temperature (Te) decrease with increased gas pressure. In addition, the growth rate increase with increasing electron temperature anisotropy in both regions.

Gas-puff induced cold pulse propagation in ADITYA-U tokamak

Short bursts (∼1 ms) of gas, injecting ∼1017–1018 molecules of hydrogen and/or deuterium, lead to the observation of cold pulse propagation phenomenon in hydrogen plasmas of the ADITYA-U tokamak. After every injection, a sharp increase in the chord-averaged density is observed followed by an increase in the core electron temperature. Simultaneously, the electron density and temperature decrease at the edge. All these observations are characteristics of cold pulse propagation due to the pulsed gas application. The increase in the core temperature is observed to depend on the values of both the chord-averaged plasma density at the instant of gas-injection and the amount of gas injected below a threshold value. Increasing the amount of gas-puff leads to higher increments in the core-density and the core-temperature. Interestingly, the rates of rise of density and temperature remain the same. The gas-puff also leads to a fast decrease in the radially outward electric field together with a rapid increase in the loop-voltage suggesting a reduction in the ion-orbit loss and an increase in Ware-pinch. This may explain the sharp density rise, which remains mostly independent of the toroidal magnetic field and plasma current in the experiment. Application of a subsequent gas-puff before the effect of the previous gas-pulse dies down, leads to an increase in the overall electron density and consequently the energy confinement time.

Simulation of lithium transport using the BOUT++ framework

A numerical model that calculates the collisional interactions between the lithium atoms from a lithium pellet and the background plasmas has been upgraded. The ion density (Ni), electron temperature (Te), ion temperature (Ti) and parallel ion velocity (V∥,i) are used to characterize the background plasmas. The lithium atom density (NLia) and parallel velocity (V∥,a) of lithium atoms evolve with time. For each lithium ion, the density (NLin+), temperature (TLin+) and parallel velocity (V∥,Lin+) are self-consistently calculated.A C-mod lower single null equilibrium is used to generate the grid for the BOUT++ simulation. The lithium atoms can be fully ionized to Li3+ in ∼2 μs. The rapid radial and poloidal expansion of the lithium ions are found in the simulation. After the collision interaction process, the electron temperature rapidly decreases at the pellet location; then, it rapidly poloidally expands, and the temperature at the pellet location starts to recover. The electron pressure increases at the pellet location despite the decrease in electron temperature because of the extra electrons from the lithium ionization. The ion pressure profile decreases in the pellet location due to the decrease in ion temperature.

Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe

Aims.We studied the properties and occurrence of narrowband whistler waves and their interaction with strahl electrons observed between 0.17 and 0.26 au during the first encounter of Parker Solar Probe.Methods.We used Digital Fields Board band-pass filtered (BPF) data from FIELDS to detect the signatures of whistler waves. Additionally parameters derived from the particle distribution functions measured by the Solar Wind Electrons Alphas and Protons (SWEAP) instrument suite were used to investigate the plasma properties, and FIELDS suite measurements were used to investigate the electromagnetic (EM) fields properties corresponding to the observed whistler signatures.Results.We observe that the occurrence of whistler waves is low, nearly ~1.5% and less than 0.5% in the analyzed peak and average BPF data, respectively. Whistlers occur highly intermittently and 80% of the whistlers appear continuously for less than 3 s. The spacecraft frequencies of the analyzed waves are less than 0.2 electron cyclotron frequency (fce). The occurrence rate of whistler waves was found to be anticorrelated with the solar wind bulk velocity. The study of the duration of the whistler intervals revealed an anticorrelation between the duration and the solar wind velocity, as well as between the duration and the normalized amplitude of magnetic field variations. The pitch-angle widths (PAWs) of the field-aligned electron population referred to as the strahl are broader by at least 12 degrees during the presence of large amplitude narrowband whistler waves. This observation points toward an EM wave electron interaction, resulting in pitch-angle scattering. PAWs of strahl electrons corresponding to the short duration whistlers are higher compared to the long duration whistlers, indicating short duration whistlers scatter the strahl electrons better than the long duration ones. Parallel cuts through the strahl electron velocity distribution function (VDF) observed during the whistler intervals appear to depart from the Maxwellian shape typically found in the near-Sun strahl VDFs. The relative decrease in the parallel electron temperature and the increase in PAW for the electrons in the strahl energy range suggests that the interaction with whistler waves results in a transfer of electron momentum from the parallel to the perpendicular direction.

Structure of DC magnetron sputtering discharge at various gas pressures: a two-dimensional particle-in-cell Monte Carlo collision study

We report the results of a particle-in-cell Monte-Carlo collision simulation of an axially symmetric DC magnetron discharge with a 5 cm diameter flat cathode in argon at pressures from 1 to 10 mTorr at a constant discharge current of about 0.5 A. Calculations show that the cathode region, where almost the entire discharge voltage drops, consists of a cathode sheath 0.1–0.2 mm wide and a presheath about 2 cm wide, where most of the ionizations occur, separated by a region 0.25–0.35 mm wide, where the plasma potential remains almost unchanged and the plasma density reaches its maximum value. Most of the discharge voltage drops in the presheath at low gas pressure, and in cathode sheath at high pressure. The ratio of sheath to presheath voltages increases linearly with pressure. The distribution of the ionization rate has two maxima: near the cathode sheath and in the presheath. The fraction of ionizations near the cathode sheath increases with pressure. The electron energy distribution function (EEDF) is generally a two-temperature function. At low pressures at a distance of less than 1 cm from the cathode, the EEDF becomes one-temperature. A high-energy tail is observed on the EEDF near the cathode; the fraction of electrons in the tail (in the order of tenths of a percent at 10 mTorr) and their energy, determined by the sheath voltage, increase with pressure. The electron temperature decreases with pressure due to a decrease of the electric field in the presheath, which leads to a decrease of energetically accessible regions of collisionless electron motion and to a corresponding decrease in the energy that electrons can obtain in these regions. The dependence of the discharge voltage on the gas pressure has a minimum at about 3 mTorr, which occurs due to the competition of two processes on pressure increase: a decrease in the electron temperature and a decrease in the fraction of electrons returning back to the cathode. Plasma density waves are observed in the presheath region at pressures of 1–3 mTorr.

Impact of divertor closure on edge plasma behavior in EAST H-mode plasmas

A series of H-mode discharges have been conducted in the EAST tokamak to study the effect of divertor closure on divertor neutral distribution and edge plasmas by scanning the location of the outer strike point. When the outer strike point moves close to the corner within ∼, the gauge in upper sub-divertor shows that the neutral pressure increases, and the reciprocating probe at outer mid-plane indicates that electron temperature decreases and electron density increases in the scrape-off layer (SOL). In the meantime, the tungsten impurity concentration and radiation power in the main plasma decrease remarkably, while the main plasma density and stored energy increase slightly. SOLPS-ITER simulation without particle drifts is performed to better understand the impact of divertor closure. The simulation clearly demonstrates that a more closed divertor produces more neutrals around the corner with the increase of electron density and the decrease of electron temperature in the SOL, consistent with experimental results. Furthermore, more neutrals escape the divertor region and penetrate into the pedestal region, which induces more pedestal fuelling, increasing pedestal electron density. EAST will upgrade the lower divertor with a more closed geometry. These results provide important guidance for the upcoming upgrade.

Particle-in-cell simulations of the alpha and gamma modes in collisional nitrogen capacitive discharges

We perform one-dimensional (1D) particle-in-cell (PIC) simulations to study the α and γ modes in an intermediate pressure 1.6 Torr, 2.5 cm gap capacitive nitrogen discharge driven at 13.56 MHz with current density amplitudes J 0 = 7 to 150 A m−2. In the α-mode, ion-induced secondary electron emission has minimal effects on the discharge equilibrium, while in the γ-mode, it is essential for sustaining the discharge. As in a previous study of a comparable argon discharge, [Kawamura et al 2020, J. Vac. Sci. Technol. A 38, 023003], the nitrogen discharge in the α-mode can be described by a ‘passive bulk’ model in which the ionization is negligible in the central bulk region and is due solely to electron sheath heating. The nitrogen discharge undergoes an α to γ transition in the applied J 0 range, characterized by an increase in density and a decrease in electron temperature, sheath widths and sheath voltages. For the γ-mode, we calculate the scalings of the maximum sheath width s m and the rf sheath voltage amplitude V 1 with J 0. We compare the PIC results in the α-mode to the passive bulk model (modified for nitrogen), and, in the γ-mode with the expected J 0-V 1 and s m-V 1 curves. We find good agreement with the simulations in both the α and γ regimes, and the α to γ transition is well predicted by the models.

Spatio-temporal measurements of overshoot phenomenon in pulsed inductively coupled discharge* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11675039, 11875101, 11935005, and 12075049) and the Fundamental Research Funds for the Central Universities of China (Grant Nos. DUT18TD06 and DUT20LAB201).

Pulse inductively coupled plasma has been widely used in the microelectronics industry, but the existence of overshoot phenomenon may affect the uniformity of plasma and generate high-energy ions, which could damage the chip. The overshoot phenomenon at various spatial locations in pulsed inductively coupled Ar and Ar/CF4 discharges is studied in this work. The electron density, effective electron temperature, relative light intensity, and electron energy probability function (EEPF) are measured by using a time-resolved Langmuir probe and an optical probe, as a function of axial and radial locations. At the initial stage of pulse, both electron density and relative light intensity exhibit overshoot phenomenon, i.e., they first increase to a peak value and then decrease to a convergent value. The overshoot phenomenon gradually decays, when the probe moves away from the coils. Meanwhile, a delay appears in the variation of the electron densities, and the effective electron temperature decreases, which may be related to the reduced strength of electric field at a distance, and the consequent fewer high-energy electrons, inducing limited ionization and excitation rate. The overshoot phenomenon gradually disappears and the electron density decreases, when the probe moves away from reactor centre. In Ar/CF4 discharge, the overshoot phenomenon of electron density is weaker than that in the Ar discharge, and the plasma reaches a steady density within a much shorter time, which is probably due to the more ionization channels and lower ionization thresholds in the Ar/CF4 plasma.

Effects of the 21 June 2020 Solar Eclipse on Conjugate Hemispheres: A Modeling Study

AbstractAn annual solar eclipse occurred on 21 June 2020. The maximum eclipse path of this solar eclipse covers the geomagnetic latitude 10–20°N in East Asia. The geomagnetic conjugate region of the eclipse region covers most of the land in Australia. It provides a valuable opportunity to study the ionospheric responses not only in the solar eclipse region but also in the conjugate region. Utilizing a coupled ionosphere and thermosphere model, we modeled global ionospheric responses and identified significant variations in electron density and electron temperature in both the eclipse region and its conjugate region. The simulated results show a significant enhancement in electron density and large decrease in electron temperature at high altitudes in the conjugate region. Two control simulations were carried out to study the main reason for the ionospheric disturbances in the conjugate hemisphere by cutting off transhemisphere photoelectron transport and thermal conduction process along field lines. The simulated results suggest that the ionospheric disturbances in the conjugate hemisphere are caused by the combined effects of transhemisphere photoelectron transport, thermal conduction, and thermospheric wind changes. Furthermore, the thermal conduction process along filed lines should be the main reason.

Keeper Ignition and Discharge Characteristics of Hollow Cathode Center-Mounted on Hall Thruster

This Paper explores keeper ignition and discharge characteristic of a hollow cathode center mounted on a Hall thruster to discuss the optimum startup sequence. A strong magnetic field along the hollow cathode in the center-mount configuration is considered to play an important role in the ignition and discharge; however, the characteristics in this configuration has never been reported. The effect of cathode mass flow rate on the ignition and discharge with 3 A keeper ignition is examined both with and without magnetic field. The influence of cathode heater power and flow from the anode is also investigated. The lower flow rate causes strong ionization instability, while the higher flow rate seems to induce ion acoustic turbulence. Applied magnetic field improves the ionization instability due to the decrease in electron temperature through plasma production. These results indicate ignition sequences over flow rate with magnetic field appear promising. On the other hand, the confined electrons around the magnetic field line prevent the initiation of the discharge. The ignition and instability are improved with high heat-up power because more thermionic emission is obtained. The anode flow provides slight improvement to the characteristics. The Fourier transform also demonstrates the presence of ionization instability and ion acoustic turbulence. The behavior of peak frequencies of ionization instability can be explained based on the predator–prey theory.

From the molten bridge rupture to the local thermal equilibrium arc: optical emission spectroscopy of electric arcs during contacts opening

We investigate by time-resolved optical emission spectroscopy the properties of electric arcs forming at the opening of Cu contacts and with a current of a few kA. The systematic observation of well-resolved transitions demonstrates that the dense Cu vapor that results from the rupture of the molten metal bridge between the contacts expands rapidly. Solving the radiative transfer equation through the line-of-sight to fit the lineshape of the detected spectra, we obtain that both the pressure and the temperature of the Cu-dominated plasma increase during the first 100 µs of arcing. At the same time, the peak wavelength and broadening of Cu I transitions exhibit a non-monotonic time-dependence. In particular, both the blueshift and narrowing of these transitions by Stark effect during the first 40 µs of arcing demonstrate a decrease in electron temperature. This result leads us to the conclusion that it takes at least 40 µs for the arc to be at local thermal equilibrium after the rupture of the molten metal bridge.

Conversion of CO2 in a low-powered atmospheric microwave plasma: In-depth study on the trade-off between CO2 conversion and energy efficiency

The influence of microwave input power (P) and gas flow rate (F) was compared by analyzing the change of CO2 conversion and energy efficiency with the specific energy input. It was found that the influence of input power and flow rate on CO2 conversion (P > F) is opposite to that on energy efficiency (P < F). When the input power and flow rate were increased in the same proportion from (60 W, 600 mL/min) to (150 W, 1500 mL/min), CO2 conversion and energy efficiency could be both improved by a factor of 1.43. The emission spectroscopy diagnostic suggested that this improvement could be attributed to more electron-impact vibrational excitation, which was proved by the increase in vibrational temperature and electron density associated with the decrease in electron temperature.

Simulation of Discharge Characteristics for the Plasma Etching of Large Area SiO2 Substrates

The electron density distribution and average electron temperature distribution in reactive ion etching (RIE) chamber are of great significance for the ionization and excitation reaction rate. The uniformity of radial distributions of the electron density and average electron temperature in the discharge chamber greatly affects the uniformity of the etching, especially the plasma etching of large area SiO2 substrates. We study the effects of discharge conditions (including power and pressure) on the electron density and average electron temperature of plasma-etched 400 mm SiO2 substrates in the reactive ion etching chamber; the simulation results show that the allowable major discharge conditions strongly affect the characteristics of plasma in the large area reactive ion etching chamber. Specifically, with increase in the power both the electron density and average electron temperature increase, being accompanied by deteriorating uniformity of their radial distributions. As the pressure increases, the electron density increases but average electron temperature decreases, being accompanied by deteriorating uniformity of their radial distributions. These methods and conclusions can provide reference for the improvement of cavity structure and large area RIE equipment designing and selection of the process parameters.

Effect of low frequency power on the electron energy distribution function in argon inductively coupled plasmas

In plasma processing and application, the electron energy distribution function (EEDF) is of fundamental interest because the ion and radical densities related to physical and chemical reactions on the substrate are predominantly governed by the EEDF or electron temperature. In this paper, the effect of low frequency power on the EEDF is investigated when 2 MHz power is added to the plasma originally driven at 13.56 MHz. In a 13.56 MHz operation, the EEDF shows a Maxwellian-like distribution, and as the RF power increases, the electron density increases and the electron temperature decreases. However, when a small amount of 2 MHz power is applied to the 13.56 MHz discharge, the electron density slightly increases and the electron temperature significantly increases. In dual-frequency operation, EEDFs have a low slope of low-energy region and evolve into a Druyvesteyn-like distribution. It turns out that the dual-frequency operation can significantly change the electron temperature. This is consistent with the results calculated using the analytical electron heating model, and the relevant heating mechanism is also presented.