Decrease In Electron Temperature Research Articles

Internal energy distributions of BeH, BeD, and BeT molecules created during chemically assisted physical sputtering in JET tokamak plasma

We present here the results of the spectroscopic analysis of the high-resolution visible spectra of beryllium hydride and its isotopologues (BeH, BeD, and BeT), produced during plasma–surface interactions during limiter and divertor JET-ILW (ITER-like Wall) pulses. The hydride production, being an important part of the wall erosion via chemical-assisted physical sputtering, shows visible dependence on plasma and wall conditions, and also on the isotope content of the plasma. This work shows that this dependence is also true for the molecular energy distributions, parameterized by rotational and vibrational temperatures. During the increase in the vessel wall temperature by 350 K, with constant plasma parameters, rotational temperature of the excited BeD molecules increases by 500 K, whereas the vibrational temperature decreases by 400 K. Another experiment was the scan of the integrated edge density, during which electron density increases with the increasing edge density, and electron temperature decreases. During that scan for BeD, Trot decreases by 300 K and Tvib by 400 K over the change in an integrated edge density of ΔNe=+8×1018 m−2. Both trends were compared with the trends in other isotopes, and the qualitative results are similar, but the limited data available restrict the possibility of a full quantitative comparison. To simplify the comparison between the results presented here and other experiments or modeling data, and also to check the comparability of the pulses performed in different isotopes, the Be ion line ratios are used as benchmarks of the edge plasma condition.

Plasma electron characterization in electron chemical vapor deposition

Recently, a novel approach of depositing metallic films with chemical vapor deposition (CVD), using plasma electrons as reducing agents, has been presented and is herein referred to as e-CVD. By applying a positive substrate bias to the substrate holder, plasma electrons are drawn to the surface of the substrate, where the film growth occurs. In this work, we have characterized the electron flux at the substrate position in terms of energy and number density as well as the plasma potential and floating potential when maintaining an unbiased and a positively biased substrate. The measurements were performed using a modified radio frequency Sobolewski probe to overcome issues due to the coating of conventional electrostatic probes. The plasma was generated using a DC hollow cathode plasma discharge at various discharge powers and operated with and without precursor gas. The results show that the electron density is typically around 1016 m−3 and increases with plasma power. With a precursor, an increase in the substrate bias shows a trend of increasing electron density. The electron temperature does not change much without precursor gas and is found in the range of 0.3–1.1 eV. Introducing a precursor gas to the vacuum chamber shows an increase in the electron temperature to a range of 1–5 eV and with a trend of decreasing electron temperature as a function of discharge power. From the values of the plasma potential and the substrate bias potential, we were able to calculate the potential difference between the plasma and the substrate, giving us insight into what charge carriers are expected at the substrate under different process conditions.

A harmonic method for measuring electron temperature and ion density using an asymmetric double probe

The harmonic method using a symmetric double probe was developed for measuring electron temperature and ion density Oh et al (2012 Meas. Sci. Technol. 23 085001). When an alternating voltage is applied to the symmetric double probe where the two areas of the collector for current collection are equal, the fundamental frequency current and third harmonic currents are generated. The electron temperature and ion density are obtained by measuring the fundamental frequency current and the third harmonic current. However, it is observed that the third harmonic current can rapidly decrease to the level of base noise when the ratio of the applied voltage to the electron temperature decreases. Therefore, it is necessary to increase the harmonic currents generated to improve measurement accuracy for electron temperature and ion density. In this paper, a harmonic method using an asymmetric double probe with different collection areas is proposed to measure electron temperature and ion density. By using the double probe with different collector area, the fundamental frequency current and the second harmonic current are generated. In the proposed method, the electron temperature and ion density are obtained by measuring the fundamental frequency current and the second harmonic current. It is found that the accuracy of the electron temperature can be improved by measuring the second harmonic rather than measuring the third harmonic current. For quantitative comparison, the electron temperature and ion density obtained by the proposed method were compared with the electron temperature and electron density obtained by the measurement electron energy probability function, which showed good agreement between them in argon plasma at various conditions. In addition, it was experimentally verified that the electron temperature can be accurately measured even when the chamber is electrically insulated, and a dielectric layer is deposited on the collectors of the double probe, such as in the plasma process.

First Experiments on Reduction the Heat Load on the Divertor Plates
 of the Globus-M2 Tokamak Using Nitrogen Seeding
 and Their Comparison with Simulation Results

At the compact spherical Globus-M2 tokamak, a series of experiments was conducted to study theeffect of the injection of nitrogen on the discharge parameters. The experiments were carried out in dischargesin deuterium in the divertor configuration, and the auxiliary heating was performed by deuteriumneutral beam injection. During the nitrogen seeding, a substantial decrease in electron temperature near thedivertor was recorded as well as a sharp decrease of the heat flux onto the divertor plate, while the density andtemperature of the main plasma changed insignificantly. Simulations by the SOLPS-ITER showed a satisfactoryagreement with the experiment.

Numerical Study of Ar/O2 Discharge Properties Influenced by Small Secondary Electron Emission in Dual-Frequency Atmospheric Pressure Discharge

A one-dimensional fluid model was employed to investigate the influence of the small secondary electron emission (SEE) coefficient on the characteristic properties of an Ar/O2 gas discharge at atmospheric pressure driven by a dual frequency source. The study includes basic physical quantities such as particle density, electron temperature, the electron heating mechanism, and energy loss. The research results illustrate that with an increase in the secondary electron emission coefficient, the electron density increases, while electron temperature and electric field decrease. The densities of various particles increase to different degrees, except for the metastable O2* molecule and the O atom. The density of the metastable O2* molecule and the density of the O atom are hardly affected by the SEE coefficient; however, the time required for both to reach steady state decreases. The time required for the electron density to reach steady state increases. Electron heating and energy loss increase to varying degrees when the SEE coefficient changes from 0.001 to 0.01.

Influence of plasma parameters on low-k SiCOH film grown by plasma-enhanced chemical vapor deposition using dimethyldimethoxysilane

Here, we report the influence of the electron density and the electron temperature on the reduction of dielectric constant in the SiCOH thin film using dimethyldimethoxysilane (DMDMS) as a function of pressure. The measured electron density and electron temperature decreased as increasing the pressure. The decrease in the measured electron density with increasing pressure can be attributed to local electron kinetics. Moreover, the decreasing electron temperature is caused by increased inelastic collisions between electrons and neutral species in the chamber. It is identified that CH3 radicals are less dissociated from DMDMS molecular at higher pressure by a quadrupole mass spectroscopy. As increasing pressure, the increase of the Si–CH3 bonds in SiCOH thin films is confirmed by Fourier transform infrared spectroscopy. As a result, the dielectric constant and refractive index of the SiCOH films are reduced at low electron density and electron temperature.

Study of the evolution of pulsed plasma under an external longitudinal magnetic field

An experimental study on the role of an external longitudinal magnetic field on the characteristics changes of pulsed plasma stream, produced in argon medium, is carried out at different time spans of its evolution. The spectroscopic observations are time integrated and are carried out at different times of plasma formation for pulsed discharge. This study gives insights into the recombination and diffusion phase of the plasma species in the presence of the magnetic field. The transition of plasma species from a dominant recombination phase to a diffusional phase is well revealed by the density profile during the time evolution. Moreover, the decrease in electron temperature and the increase in electron excitation temperature explain the energy transfer to electrons due to metastable quenching, and the system gradually approaches equilibrium. The magnetic field also affects the transitions of the ionized argon population between different energy levels. It is found that faster decay occurs for transitions of different plasma species to non-metastable states, while the populations of metastable states exist for a longer time. In addition, the time-resolved morphology changes of the plasma stream are also observed by high speed imaging, which shows the flow structure of the plasma stream at different time frames. The imaging of the plasma stream evolution shows the initial ejection of the plasma sheet from the electrode assembly, its detachment, the steady flow, and gradually its nature of dying out.

High-speed plasma diagnostics based on the floating harmonic method in inductively coupled plasma and pulsed plasma

Abstract The floating harmonic method is a diagnostic technique for obtaining plasma parameters, such as ion density and electron temperature, by applying a sinusoidal voltage to a floating probe. The typically applied frequency is in the kilohertz range. This method has been widely used in plasma diagnostics of semiconductor processes due to its robustness to RF fluctuations and fast measurement speed. However, recently, pulsed plasma has become common in semiconductor processes. As the plasma sheath is analyzed with a high-time-resolution diagnostic method such as phase-resolved optical emission spectroscopy, the development of high-speed plasma diagnostic techniques has become increasingly important. In this study, we investigated high-speed plasma diagnostic measurements based on the floating harmonic method. When the frequency of the voltage applied to the floating probe increases up to 1 MHz, the electron temperature can be underestimated due to the currents flowing through the capacitive sheath and the ceramic sleeve of the probe. We found that the displacement current of the probe sheath increases rapidly compared to the conduction current as the plasma density and electron temperature decrease. We also removed the additional harmonic currents flowing through the ceramic sleeve via two approaches. The plasma parameters obtained using the proposed method are in good agreement with the measurements performed using the floating harmonic method in the kilohertz range. Moreover, the electron temperature of the pulsed plasma was measured.

Study on characteristics of electron parameters on inert gas addition in a capacitively coupled SF6/O2 plasma

In this paper, characteristics of electron parameters of SF6/O2 and inert gas mixture in a capacitively coupled plasma were studied. Here, gases such as He, Ar, and Xe were added to SF6/O2 mixture and electron energy probability functions (EEPFs) were measured. The electron parameters were acquired, which agreed well with EEPF behaviors. Normally, the inert gas functioned as an electron source and the electron density tended to increase. When the inert gas ratio exceeded other gases, the effect of the mixture varied on each gas. He showed its unique behaviors with the increase in electron temperature. Ar and Xe showed consistent behaviors with increasing electron density and decreasing electron temperature as the inert gas proportion increases. Different behaviors of the electron parameters in inert gases can be explained by the complex contribution of electron attachment of SF6 and the ionization rate of each inert gas.

The Influence of the Magnetic Field on the Plasma Characteristics in Hollow Electrodes Discharge System

This work is an experimental study about the effects of gas pressure and magnetic field on plasma characteristics produced in an internal hollow electrodes discharge (HED) system. The results show that the breakdown voltage values increase with increasing the working pressure (especially with the presence of a magnetic field). The breakdown voltage depends on the p.d. product, where p is the gas pressure and d is the distance between the electrodes. While the values of current discharge decrease with the increase of the working pressure. The temperature of electron and the number density of electron are calculated from the Boltzmann method and the broadening of Stark, respectively. The results showed that the electron number density ( ) and plasma frequency ( ) increase with increasing the gas pressure, especially with the presence of a magnetic field, i.e. the plasma is more stable with the presence of magnetic field. While the electron temperature ( ) and Debye length ( ) decrease with increasing the gas pressure.

Influence of Cylindrical Electrode Configuration on Plasma Parameters in a Sputtering System

In the present work, the effect of the cylindrical configurations of the sputtering device electrodes on the plasma parameters (Debye length, electron temperature, electron density, plasma frequency) is studied. Also, the effect of the argon gas pressure on the discharge properties is examined with gas pressures of (0.08, 0.2, 0.4 and 0.6) Torr. The properties of the plasma are diagnosed by optical emission spectrometry. The spectroscopic method is adopted for examining the atomic spectra of argon emission. The electron temperature is determined by the Boltzmann method. While, the Stark-widening method was employed for calculating the electron number density. The voltage against current curves of the cylindrical sprayer discharges shows the voltage to be nearly constant with slight increase in current. An increase in pressure causes the cathode cascades to compress and the negative glow to become thinner and more luminous. Plasma properties such as electron temperature and Debye length decrease with increasing pressure, while electron number density and plasma frequency increase.

Performance Analysis of an Electron Cyclotron Resonance Thruster with Various Propellants

The performance of an electron cyclotron resonance thruster is analyzed for operation with argon, krypton, xenon, and water vapor to understand how mass utilization efficiency and frozen flow losses vary across propellant type. The device was operated from 20 to 220 W for flow rates of 0.06–0.4 mg/s. Thrust performance is measured using an inverted pendulum thrust stand, and plasma characteristics are measured using a retarding potential analyzer, ExB probe, Langmuir probe, and emissive probe. Specific impulse is generally found to decrease as the ratio of mass flow rate over power () increases. Probe measurements indicate that this trend is attributed to a combination of lower propellant mass utilization and decreased electron temperatures. The mass utilization efficiency is found to increase with the ratio of thruster length to ionization mean free path for all propellants; however, the data do not support existing theoretical scaling laws, and a more accurate empirical scaling law is proposed. The decrease in electron temperature with also drives an increase in the relative frozen flow losses, which are found to be about a factor of two higher for water propellant. Multiply charged ions did not constitute a significant fraction of the plume for the atomic species. Mass utilization efficiency and frozen flow losses describe the trends in the thrust efficiency but not the overall magnitude, which suggests that microwave coupling and diffusion to the walls are dominant loss processes.

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.