Influence Of Metastable Atoms Research Articles

Influence of Metastable Atoms on the Heating of Microparticles in the Plasma of a Gas Discharge in Neon

Influence of Metastable Atoms on the Heating of Microparticles in the Plasma of a Gas Discharge in Neon

Influence of metastable atoms on the formation of nonlocal EDF, electron reaction rates, and transport coefficients in argon plasma

The influence of metastable atoms on formation of the nonlocal electron distribution function (EDF) and other characteristics of gas-discharge plasma in argon is investigated in this paper. Superelastic collisions of electrons with metastable atoms strongly affect the shape of the EDF, the behavior of reaction rate constants, and other plasma characteristics. An interesting phenomenon of EDF replication emerges with a period of excitation threshold; this effect can increase the rate constants of excitation and ionization processes by several orders of magnitude, especially at the periphery of the plasma volume. Thus, collisions between electrons and metastable atoms are important in the formation of a nonlocal EDF, and neglecting them can lead to large errors in calculations of plasma characteristics.

Dynamics of hollow cathode discharge using fluid models with and without metastable atom involvement in helium

The characteristics of hollow cathode discharge are simulated by two types of fluid models in helium, one with metastable atom involvement and another without. Discharge current, particle density, and ionization rate are simulated. Results show that metastable atoms exert an important influence on the steady state discharge and the temporal characteristics of this discharge. Discharge is maintained at a lower sustain voltage in the model with metastable atoms than in that without metastable atoms. At the same low sustain voltage, marked differences in quantitative and qualitative characteristics are observed between the two models. With increasing sustain voltage, the discharge parameters simulated by the two models become qualitatively similar, but their quantitative differences increase. The discharge current and electron density in the model with metastable atoms are significantly higher than that in the model without metastable atom involvement at the same sustain voltage. Furthermore, the discharge current and electron density gap simulated by the two models increase as the sustain voltage increases. The difference in characteristics between the two models originate from variation of the contribution of different ionization types to the generation of new electrons at different sustain voltages. With increasing sustain voltage, stepwise ionization originating from metastable atoms becomes increasingly crucial to the discharge. Results further show that the temporal characteristics of the two models are similar when the metastable atom density is low. However, in the model with metastable atoms, discharge current and particle density continue to rise until a steady discharge is obtained after a quasi-steady stage. The different temporal characteristics observed between the models appear to originate from the influence of metastable atoms. The percentage of stepwise ionization relative to the total ionization increases with time.

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.

The influence of metastable atoms and the effect of the nonlocal character of the electron distribution on the characteristics of the positive column in an argon discharge

Commercial CFDRC software (http://www.cfdrc.com/˜cfdplasma) is used to self-consistently simulate the plasma of the positive column (PC) of a dc discharge in argon. The software allows simulations in an arbitrary 3D geometry by using Poisson’s equation for the electric potential and fluid equations for the heavy components and by solving a nonlocal kinetic equation for electrons. It is shown that, in calculating the electron distribution function, the local approximation is almost always inapplicable under real conditions of a diffuse PC usually met in practice (pR<(5–10) cm Torr). The influence of metastable atoms, which can substantially affect the parameters of the PC plasma, is considered. It is shown that superelasic collisions play an important role in enriching the fast component of the electron distribution function and that the Penning ionization can result in an ascending volt-ampere characteristic of the positive column.

Codimension-two bifurcations and ionization instability in a glow discharge

We investigate the bifurcations of ionization waves from the homogeneous stationary state of the positive column in a neon glow discharge. The underlying physical model is based on a hydrodynamic description which takes into account the external current circuit and the influence of metastable atoms. Periodic boundary conditions are chosen and by performing a linear stability analysis, we identify the instability boundaries in parameter space and compare the properties of the critical modes with corresponding experimental data. The normal form constants of a codimension-two bifurcation are determined and their dependence upon the plasma parameters is discussed. The calculated bifurcation phenomena show that the nonlinear properties of ionization waves depend not only on the plasma of the positive column but also on the external circuit.

Positive Charge Generation at a SiO2/Si Interface due to Bombardment with Metastable Atoms

The influence of metastable atoms on a SiO2/Si structure is examined to determine the source of damage in ULSI devices during plasma enhanced processes. Holes were generated at the SiO2 surface by the impact of metastable atoms of rare gases. Holes trapped at the interface formed positive charges, and the density of these positive charges increased with the increasing energy of the metastable atoms. The yields of the positive charge generation were between 0.01 and 0.1, which are on the same order as those caused by vacuum ultraviolet photons, and these values are not negligible. Thus, the influence of metastable atoms must be taken into consideration to control the damage that occurs during plasma enhanced processes.

Hopf bifurcations in balance equations of glow discharges

Starting from the hydrodynamic equations describing the positive column of glow discharges in inert gases, the instability of the axially homogeneous state is investigated. Dirichlet boundary conditions at the ends of the positive column are chosen. Stimulated by experiments, the influence of metastable atoms and of the outer circuit is taken into consideration by additional equations. Center manifold and normal form theories are used to characterize the codimension-one bifurcations. Depending on the current, the length of the positive column and the resistance of the outer circuit supercritical and subcritical Hopf bifurcations are found. The importance of the results with respect to the experiments on the ionization instability in a neon discharge is discussed. {copyright} {ital 1997} {ital The American Physical Society}

Cross sections for HeII( n = 4 → n = 3) and CVI( n = 7 → n = 6) emission for charge exchange diagnostics at tokamaks

Charge exchange spectroscopy of fusion plasmas relies on an accurate knowledge of state selective electron capture cross sections. Here we discuss the gathering and evaluation of such cross sections, with emphasis on electron capture by He 2+ and C 6+. The influence of metastable atoms on the plasma diagnostics by means of visible light charge exchange spectroscopy is recognized and treated.

On the influence of metastable atoms on surface-wave produced helium plasmas

The influence of excited (metastable) states of helium on the ionization and excitation processes in surface-wave sustained plasmas is studied experimentally as well as theoretically. The experimental studies are performed employing emission and absorption spectroscopy methods. The theoretical investigations are based on a collisional-radiative model coupled to self-consistent electron-energy distribution functions. The influence of the stepwise ionization on the axial distribution of the electric-field strength is discussed. Also the impact of the radial distribution of excited atoms is taken into account. Reasonable agreement between measured and calculated population densities of the 23S level is found. The axial changes of the spectral line intensity ratios of a helium singlet line (41S-21P) to a triplet line (43S-23P) are found experimentally. By comparison with the theoretical model it is shown that these modifications can (to a great extent) be attributed to the changing contribution of stepwise excitation of the triplet level.

Investigation of the influence of metastable atoms upon the state of the plasma near the cathode

Investigation of the influence of metastable atoms upon the state of the plasma near the cathode

Excitation of the helium autoionising states in He (11S, 21,3S) on He (11S) collisions, between 5 and 140 keV

The autoionisation of the helium atom has been experimentally studied, by electron spectrometry, in collisions between neutral helium atoms in the energy range 5-140 keV. The fast neutral beam is formed by charge exchange of a fast helium ion beam in helium gas, and its composition is discussed. Above 10 keV the shapes and intensities of the 2s2 1S, 2s2p 3P, 2p2 1D, 2s2p 1P and He-(1s2s2)2S lines are determined by a numerical fitting procedure. Excitation of these autoionising states at low energy is discussed within the quasimolecular model. The experimental results are analysed in terms of the collisional system symmetry. The comparison of slow and fast atom excitations is used to determine the energy range within which the quasimolecular model holds. The influence of metastable atoms present in the incident beam is shown to explain the 2s2p 3P line excitation below 30 keV, within the quasimolecular model. Discrepancies between shapes and intensities of lines emitted (at given collision energy and emission angle) by the two atoms are interpreted as being due to an excitation by fast metastable atoms, within an atomic model.

Distorted quasi-equilibrium distribution functions in isotropic plasmas containing metastable atoms. Application to helium

The influence of metastable atoms on the equilibrium distribution function of an isotropic plasma is studied. It is shown that there is a quasi-stationary solution to this problem resulting from the balance between electron-electron interactions, inelastic and super-elastic collisions. The distribution function is composed of thermal Maxwellian electrons and of a bump at the excitation energy potential of the metastables. Evaluation of the electronic density in this bump is made for the case of helium.

The influence of metastable atoms and molecules on the re-ignition of high-frequency discharges

The high-frequency, electrodeless, breakdown voltage of a gas is reduced during the period immediately following a discharge. The variation of this reduction with afterglow duration has been measured. Discharges were produced with a capacitatively coupled 15 MHz transmitter, in hydrogen, neon and nitrogen gases in the pressure range 0·8-6 mm Hg, and with afterglow durations in the range 2-140 ms.The results in Ne and N2 are shown to be consistent with the slow reduction in the densities of metastable molecules in the afterglow, brought about by their diffusion to the walls of the discharge tube and subsequent deactivation. In N2, it is further argued that metastable molecules are created in the afterglow by nitrogen atom recombination. This view is supported by the observation of the decay of the intensity of visible radiation in the afterglow.From the time and pressure dependence of the reductions in breakdown field, the diffusion coefficients of metastable Ne atoms in Ne and metastable N2 molecules in N2 were found to be 159±13 cm2 s−1 and 71·3±5·9 cm2 s−1 at 1 mm Hg respectively.No reductions in breakdown field beyond those expected from variations in electron diffusion coefficients were observed in hydrogen, owing to the absence of any sufficiently long-lived metastable hydrogen molecules.Finally, the ratio of the rate of ionization per electron for electron ionizing collisions with excited atoms and molecules to that with atoms and molecules in their ground state has been deduced, and is shown to be greater than unity for short afterglow times.

Excitation in fast He-He collisions: the influence of metastable atoms

Excitation in collisions involving the passage of 70 keV He atoms through thin targets of helium have been studied by a spectroscopic analysis of the spontaneous decay of the 41S, 41D, 43S and 43D excited products of single collisions. A Doppler shift separation of target and projectile emission was possible. The 43S and 43D projectile and target emissions are shown to be strongly dependent on the metastable population of the beam.

Influence of metastable atoms on the floating potential of a langmuir probe

Abstract The quantitative effect of bombardment of a probe surface by metastable atoms on the floating-probe potential is discussed for the case of the positive column in a dc discharge in helium.