Density Of Metastable Atoms Research Articles

Influence of metastable atoms in the simulation of hollow cathode discharge

The characteristics of hollow cathode discharge are investigated by using two-dimensional fluid model combined with a transport model for metastable atoms (F-M model) in argon. It shows that the stepwise ionization is one of main important mechanism for electrons production. The distribution of electric potential, density of electrons, ions, and metastable atoms are calculated with a pressure of 10 Torr and a voltage of 250 V. The peak density of electron and ion is 1.2×1013 cm−3, and the peak density of metastable atoms is 3.5×1013 cm−3. The results obtained in F-M model are compared with that in fluid model (without metastable atoms involved). Metastable atoms are found to play an important role in the discharge. In addition, with the increase of pressure and voltage, the percentage of stepwise ionization in the total ionization increase, and the difference of discharge characteristics simulated by these two kinds of models rises.

Determination of Ar metastable atom densities in Ar and Ar/H2 inductively coupled low-temperature plasmas

Ar metastable atoms are important energy carriers and surface interacting species in low-temperature plasmas that are difficult to quantify. Ar metastable atom densities (NAr,m) in inductively coupled Ar and Ar/H2 plasmas were obtained using a model combining electrical probe measurements of electron density (Ne) and temperature (Te), with analysis of spectrally resolved Ar plasma optical emission based on 3p → 1s optical emission ratios of the 419.8 nm line to the 420.1 nm line. We present the variation of NAr,m as the Ar pressure and the addition of H2 to Ar are changed comparatively to recent adsorption spectroscopy measurements.

The influence of the spatial nonuniformity on the measurement of Ar*(1s5) density by the self-absorption technique

The self-absorption technique is a simple method to determine the line integrated density of metastable atoms in low-pressure plasmas. In order to employ this technique, it is necessary to make an assumption on the spatial profiles of both the emission intensity and the absorbing species, which usually are unknown and can be highly nonuniform. Taking Ar*(1s5) atoms in a capacitively coupled plasma as an example, the influence of nonuniformity on the measurement of the line integrated density of the absorbing species is investigated analytically. It is proved that when the two spatial profiles are the same, the obtained line integrated density is independent of the functional form of this profile. This is also true if the density of the absorbing species is small (weak absorption), even though the emission profile is different from that of the absorbing species. However, if the density is high (strong absorption), the choice of the profiles has a significant influence on the deduced line integrated density. In the experiment, it is found that in argon–oxygen mixture discharges, in which Ar*(1s5) density is low (weak absorption), the measured densities by self-absorption are in very good agreement with the results obtained from laser absorption. However, in pure argon discharge, when the density is high (strong absorption), the measured densities by self-absorption are significantly smaller than that by laser absorption. Both phenomena have been predicted by the model results. The smaller densities obtained by self-absorption in the pure argon discharge are attributed to the assumption of the same spatial profile used in the model.

Argon metastable and resonant densities in a low-pressure Ar–N2 inductively coupled plasma

Argon metastable (3P2 and 3P0) and resonant (3P1 and 1P1) atom densities have been measured in a low-pressure (1–20 mTorr) inductively coupled plasma generated in argon–nitrogen gas mixtures with nitrogen contents ranging from 0 to 90% using optical absorption spectroscopy techniques. A significant decrease of the metastable density is found for increasing nitrogen contents, more pronounced as the pressure is increased. As for the resonant atoms, their behaviour differs from that of metastable atoms, which results from the growing influence of radiation trapping with pressure. A simplified model accounting for several Ar metastable population/depopulation mechanisms (electron excitation/ionization, diffusion and excitation transfer from Ar metastable to nitrogen molecules) has been used to interpret these results. It shows that excitation transfer reactions from Ar metastable atoms to nitrogen become the main depopulation process, especially for large nitrogen contents and pressures above 10 mTorr.

Simulation of hollow cathode discharge by combining the fluid model with a transport model for metastable Ar atoms

The characteristics of rectangular hollow cathode discharge are studied based on a fluid model combined with a transport model for metastable Ar atoms in argon. The distribution of potential, density of electrons and ions, and the density of metastable atoms are calculated at a pressure of 10 Torr. The peak density of electron and ion is 4.7×1012 cm-3, and the peak density of metastable atoms is 2.1×1013 cm-3. Results obtained in terms of fluid-metastable hybrid model are compared with that in terms of the fluid model, which show that the electron produced by stepwise ionization is one of the important source of new electrons, and the metastable atoms have an obvious effect on the hollow cathode discharge. Compared with the results calculated in terms of fluid model, the density of electrons obtained in terms of hybrid model increases, and the depth of cathode sheath and the averaged electron energy decrease.

A model for DC magnetron sputtering of bi-component target in Ar ambient: case study of Ti–Cu target

A simple model for magnetron sputtering of bi-component target was developed in Ar ambient at steady state with plasma chemistry consideration. Dependence of quantities such as Ar ion current, electron density, ground and metastable excited Ar atom densities, total pressure and mean deposition rate on total discharge current have been investigated. Area ratio is independent of total discharge current and only relates to alloy composition. Ar ion current linearly increases trend with total discharge current. The values of electron (ion) density, ground and metastable excited state Ar atom densities are of the orders of 109, 1014 and 1010 cm−3, respectively. Mean deposition rate is calculated with scattering (collisions) consideration.

Density of metastable atoms in the plume of a low-pressure argon microplasma

Spatially resolved measurements of the density of metastable excited atoms in the plume of an argon microplasma are presented. The microplasma device is operated at a relatively low pressure, on the order of 1 Torr, and is exhausted into a vacuum. Line-integrated densities of excited argon neutrals in the exhaust plume are measured using tunable diode laser absorption spectroscopy. The densities of argon metastables in both 1s5 and 1s3 states are measured. These line-integrated density measurements are converted to three-dimensional density maps using the Abel inversion. The density of 1s5 argon peaks at a value of approximately 1018 m−3 near the outlet orifice, while the 1s3 density is roughly five times lower everywhere. It is found that, far from the face of the microplasma outlet orifice, metastable density follows axial and angular distributions consistent with that expected of vacuum gas expansion as predicted by classic rarified flow theory. Integrated metastable density is found to be conserved as the plume expands through 4 mm, suggesting a net production of excited species in the first millimetre and a constant population further downstream.

Measurement of xenon plasma properties in an ion thruster using laser Thomson scattering technique

This paper reports on the development of a method for measuring xenon plasma properties using the laser Thomson scattering technique, for application to ion engine system design. The thresholds of photo-ionization of xenon plasma were investigated and the number density of metastable atoms, which are photo-ionized by a probe laser, was measured using laser absorption spectroscopy, for several conditions. The measured threshold energy of the probe laser using a plano-convex lens with a focal length of 200 mm was 150 mJ for a xenon mass flow rate of 20 μg/s and incident microwave power of 6 W; the probe laser energy was therefore set as 80 mJ. Electron number density was found to be (6.2 ± 0.4) × 10(17) m(-3) and electron temperature was found to be 2.2 ± 0.4 eV at a xenon mass flow rate of 20 μg/s and incident microwave power of 6 W. The threshold of the probe laser intensity against photo-ionization in a miniature xenon ion thruster is almost constant for various mass flow rates, since the ratio of population of the metastable atoms to the electron number density is little changed.

On the high argon metastable atom density measured near the cathode surface of a hollow cathode discharge

We report laser absorption measurements of spatial density distributions of Ar metastable (Arm) and resonant state atoms in a cylindrical hollow-cathode discharge (HCD). The evaluation of the measured data by standard analysis indicates high Arm densities near the walls of the HCD. To obtain convincing evidence for these high densities, we critically examine the effect of diffraction phenomena on the measured distributions. This analysis confirms that—while diffraction has a significant effect—the Arm densities near the cathode wall are in the 1016 m−3 range. The measured values suggest that, in addition to the discharge volume, metastable atoms are likely to be created at the surface of the cathode. Further investigation is needed to clarify the processes responsible for the observed behavior.

Metastable atom and electron density diagnostic in the initial stage of a pulsed discharge in Ar and other rare gases by emission spectroscopy

Temporal measurements of the emission intensities of the Ar 419.8 and 420.1 nm spectral lines combined with Ar plasma modeling were used to examine the metastable atom and electron density behavior in the initial stage of a pulsed dc discharge. The emission intensity measurements of these spectral lines near the start of a pulsed dc discharge in Ar demonstrated a sharp growth of metastable atom and electron densities which was dependent on the applied reduced electric fields. For lower electric fields, the sharp growth of metastable atom density started earlier than the sharp electron density growth. The reverse situation was observed for larger electric fields. This presents the possibility for controlling plasma properties which may be useful for technological applications. Similar measurements with spectral lines of corresponding transitions in other rare gases are examined.

Populations of metastable and resonant argon atoms in radio frequency magnetron plasmas used for deposition of indium-zinc-oxide films

This work reports optical absorption spectroscopy measurements of the number density of Ar atoms in resonant (3P1, 1P1) and metastable (3P2, 3P0) states in rf magnetron sputtering plasmas used for the deposition of ZnO-based thin films. While the density of Ar 3P2 and 3P0 was fairly independent of pressure in the range of experimental conditions investigated, the density of Ar 3P1 and 1P1 first sharply increased with pressure and then reached a plateau at values close to those of the 3P2 and 3P0 levels at pressures above about 50 mTorr. At such pressures, ultraviolet radiation from resonant states becomes trapped such that these levels behave as metastable states. For a self-bias voltage of −115 V and pressures in the 5–100 mTorr range, similar number densities of Ar resonant and metastable atoms were obtained for Zn, ZnO, and In2O3 targets, suggesting that, over the range of experimental conditions investigated, collisions between these excited species and sputtered Zn, In, and O atoms played only a minor role on the discharge kinetics. The metastable-to-ground state number density ratios were also fitted to the predictions of a global model using the average electron temperature, Te, as the only adjustable parameter. For all targets examined, the values of Te deduced from this method were in excellent agreement with those obtained from Langmuir probe measurements.

The effect of matrix composition on radially resolved argon metastable atom populations in an emission ICP

We report radially resolved relative number densities for argon metastable atoms recorded by bleaching absorbers in a short segment of the absorption path of a continuous wave diode laser with a pulsed dye laser. The densities were measured in the presence and in the absence of high concentrations of metallic species. Mg, Ca, Sr, Ba, Y, Cu, La, Nd, Sm, and Gd were introduced into the plasma to model a variety of matrix species. At a height of 12 mm above the load coil, all of the introduced matrix species with the exception of copper caused significant increases in the steady-state argon metastable atom density. To examine the relationship between the argon metastable atoms and analyte or matrix ions, we recorded transient changes in argon metastable density in response to pulsed laser radiation tuned to a resonance ionic line of a selected metal in the plasma. Saturation of the ionic resonance lines caused transient decreases in the argon metastable atom density on a nanosecond time scale. The response of the argon metastable atoms to excitation of metal species is too fast to be due to collisions between excited metal ions and argon metastable atoms. We attribute the observed responses to heating of the electrons by suprathermal inelastic collisions between electrons and excited metal ions, followed by enhanced collisional excitation by electrons of argon atoms in the 4s levels.

Measurement of Helium Metastable Atom Densities in a Plasma-Based Ambient Ionization Source

We describe direct imaging of the densities of helium metastable atoms in the afterglow of a helium dielectric-barrier discharge (He-DBD) using collisionally assisted laser-induced fluorescence (LIF). For the conditions tested, comparison of fluorescence images of a He-DBD with analogous maps of emission from highly excited helium atoms revealed that helium metastable atom densities did not correlate well with emission from the plasma. Fluorescence images also showed that helium metastable atom densities increased substantially when a glass slide was placed 10.0 mm from the discharge capillary in a geometry typical for desorption-ionization experiments. We also studied the effect hydrogen has on the helium metastable atom densities. The hydrogen severely quenched the metastable state leaving it virtually undetectable. Emission was quenched as well, but to a lesser extent. The addition of 1% H(2) to the helium in the source provided nearly a factor of 2 improvement in the sensitivity of the signal for coumarin 47 when the plasma was used to ionize the dye under ambient conditions, despite the quenching of the helium metastable atom population.

Interaction between Dielectric Barrier Discharge and Positive Streamer in Helium Plasma Jet at Atmospheric Pressure

We have investigated the discharge mechanisms in a helium plasma jet with a coaxial dielectric barrier discharge (DBD) configuration, which can generate low-temperature plasma plume mainly by positive corona streamer propagation. In this study, we focused on the relationships between the coaxial DBD inside a glass tube and the positive streamer propagating in the plasma plume, using novel types of electrode configurations composed of insulated electric wires. The discharge characteristics were investigated by measuring plume length, discharge currents of the DBD, and the line-integrated density of helium metastable atoms in the plume. The experimental results indicate that the excited-species density in the plume is largely dependent on surface-charge density accumulated by the DBD. Moreover, we analyzed the spatiotemporal distribution of electrical potential around the tube exit by the finite element method, and discussed the mechanism determining plume length.

Fluid simulation for influence of metastable atoms on the characteristics of capacitively coupled argon plasmas

One-dimensional self-consistent fluid model is used to simulate the capacitively coupled argon plasma, in which the metastable effect on the plasma parameters at different discharge conditions is investigated. The results show that due to the metastable atom existence, the bulk plasma density drops significantly, especially at high pressures, high voltages, and high frequencies, accompanied by the decrease in electron temperature in the bulk. When the pressure and voltage are high, the metastable atom density is characterized by a saddle distribution in the axis direction. However, with the decrease in voltage and pressure, the metastable atom density becomes a parabolic distribution. Besides, the curve of plasma density with frequency has a minimum, and so is the profile of metastable atom density.

Investigation of discharge mechanisms in helium plasma jet at atmospheric pressure by laser spectroscopic measurements

We have measured spatiotemporal structures of excited species by laser spectroscopic methods in a plasma jet, which was driven by a bipolar impulse voltage pulse train of the order of kilohertz repetition rate applied across a pair of electrodes wrapped around a glass tube with a helium gas flow. We noticed the differences between the positive and the negative phases of the voltage applied to the front-side electrode placed closer to the tube exit while the back-side electrode was grounded. The experimental results showed that the radial distribution of the excited species had a hollow shape at the centre in the positive voltage phase, while it had a more uniform shape in the negative phase. The peak density of the helium metastable atom in the positive phase was almost constant irrespective of the peak applied voltage. However, it increased with the increase in the peak applied voltage in the negative phase. The mechanism causing these differences was argued from the respects of positive and negative corona discharges. We have also investigated the property of the plasma plume under conditions similar to material processing with a conductive substrate placed in front of the plasma jet. In this case, the plasma production by electron impact ionization became dominant near the substrate as was revealed from the spatiotemporal distributions of helium metastable atom and nitrogen ion densities.

Absorption spectroscopy measurements of resonant and metastable atom densities in atmospheric-pressure discharges using a low-pressure lamp as a spectral-line source and comparison with a collisional-radiative model

The well-known absorption method for measuring metastable and resonant atom densities in reduced-pressure discharges using a low-pressure lamp as a spectral-line source is extended to plasmas sustained at atmospheric pressure where the line profiles are of the Voigt type rather than Gaussian. This technique is further applied to determine the axial distribution of metastable and resonant atom densities in a tubular argon surface-wave (microwave) discharge. At atmospheric pressure, the discharge is radially contracted and determination of the absorption length through lateral probing of the tube becomes a critical issue. Good agreement of the metastable and resonant atom densities calculated from a collisional-radiative model with the measured values supports the validity of the method developed. The model underlines the influence of the molecular ion kinetics on the Ar I 4s level population and reveals an efficient 3-step ionization process involving the 4s and 4p levels for atomic ion formation.

Metastable helium density probe for remote plasmas

Helium metastable atom density was spatially determined by a modified electrostatic probe in a remote plasma. The probe structure was similar to that of a guard ring probe. Opposite polarity voltages were applied to the inner probe and the guard ring to shield both electrons and ions from the vicinity of the inner probe. Therefore, the inner probe current is due to secondary electrons generated by the de-exciting helium metastable atom flux. The photoelectron current was removed by shielding and orienting the probe 90 degrees to the direction of the plasma-generated photon flux. Helium metastable atom density on the order of 10(7) cm(-3) was measured. Limitations on the use of this technique are revealed by comparisons with simulated metastable distributions.

Positive column of HF discharge in He/N2 mixture: excitation and ionization mechanisms

High frequency (HF) stationary capillary discharge in He/N2 mixtures at low (⩽0.5%) fractions of N2 was studied at medium pressures. In the experimental part, both electrical and spectral characteristics were recorded. On the basis of experimental results the densities of electrons and helium metastable atoms were determined and densities of and ions were estimated. The analysis showed that in the positive column the He atom is excited by electron impact with the metastable He atom. The main route of production of ions is via the Penning reaction. The proposed mechanisms explain the experimental findings.

Absolute intensity calibration of emission spectra: application to the forbidden 346 nm nitrogen line for N(2P°) metastable atoms density measurement in flowing afterglow

A novel method, based on emission signal from the excitation transfer reaction: Ar*( 3 P 2 ) + N 2 ( X ) N 2 *( C 3 ) + Ar, is proposed for the intensity calibration of the spectroscopic optical detection system in absolute scale. It is applied for the measurement of N( 2 P°) metastable atoms density in the Short Lived Afterglow (SLA) of a 440 Pa nitrogen discharge produced by a 433 MHz resonant cavity. This density is deduced from the absolute intensity of the forbidden N( 2 P°– 4 S°) line at 346.65 nm, whose transition probability is only 0.005 s −1 . The N( 2 P°) density variation in the SLA resembles those of N 2 ( A 3 ) metastable molecules and electrons or the emission intensities of first positive (1 + ), second positive (2 + ) and first negative (1 − ) systems of N 2 . It first decays after the discharge zone up to a minimum and hence increases by almost a factor of thirty to reach a maximum value of 6 × 10 17 m −3 at the maximum of the SLA. It is proposed that N( 2 P°) density results from a local equilibrium between its production: N 2 ( A 3 ) + N( 4 S) N( 2 P) + N 2 ( X 1 , v ) and loss: N 2 ( X 1 , v ≥ 10)+ N( 2 P) N 2 ( A 3 ) + N( 4 S) reactions, which strongly couple the atomic and molecular metastable states and hence recycle N 2 ( A 3 ) metastable molecules produced in the SLA. The balance equation of N( 2 P°) density provides a N 2 ( X 1 ; v ≥ 10) density of 6.5 × 10 20 m −3 at the maximum of the SLA. This corresponds to 1% of the total N 2 molecules in vibrationally excited levels v ≥ 10.