Concentration Of Metastable Atoms Research Articles

Study of axial double layer in helicon plasma by optical emission spectroscopy and simple probe

In this work we used a passive measurement method based on a high-impedance electrostatic probe and an optical emission spectroscope (OES) to investigate the characteristics of the double layer (DL) in an argon helicon plasma. The DL can be confirmed by a rapid change in the plasma potential along the axis. The axial potential variation of the passive measurement shows that the DL forms near a region of strong magnetic field gradient when the plasma is operated in wave-coupled mode, and the DL strength increases at higher powers in this experiment. The emission intensity of the argon atom line, which is strongly dependent on the metastable atom concentration, shows a similar spatial distribution to the plasma potential along the axis. The emission intensity of the argon atom line and the argon ion line in the DL suggests the existence of an energetic electron population upstream of the DL. The electron density upstream is much higher than that downstream, which is mainly caused by these energetic electrons.

Influence of dust void on neon DC discharge

The diffusion/drift model of the positive column of glow discharge in neon with fine dust particles was used to study the role of a dust cloud with a void in the interaction between plasma and dust particles in the range of neon pressure and discharge current where dust particles may form structures with cavities. The results represent the nonlocal effect of void size on plasma composition, configuration of electric field and on distributions of plasma components in discharge with voids in dust structures. Simulations show that the electric field strength and the metastable atom concentration inside the void are higher than in the discharge without dust particles, while electron concentration may be either higher or lower.

Effect of metastable neon atoms in a positive column of glow discharge with dust particles

The diffusion/drift model of the positive column of glow discharge in neon was used for the analysis of the role of neon metastable atoms in the interaction between neon plasma and dust particles. The radial profiles of electrons and metastable atoms were simulated in the typical range of neon pressure and discharge current where dust particles may form dense dust structures changing the plasma properties. The results showed the nonlocal effect of dust particles on the plasma composition. Within the dust structure, the metastable atom concentration was shown to be higher than in the discharge without dust particles at the same discharge parameters; the ratio of concentrations of metastable atoms and electrons may increase with increasing dust particle concentration in a certain range of discharge parameters. The partial contribution of metastable atoms to the ionization was found to be higher than their losses on the surface of dust particles and increased when the gas pressure was increased.

Transitions Between and Control of Guided and Branching Streamers in DC Nanosecond Pulsed Excited Plasma Jets

Plasma bullets are ionization fronts created in atmospheric-pressure plasma jets. The propagation behavior of those bullets is, in the literature, explained by the formation of an interface between the inert gas and the ambient air created by the gas flow of the plasma jet, which guides these discharges in the formed gas channel. In this paper, we examine this ionization phenomenon in uniform gases at atmospheric pressure where this interface between two gases is not present. By changing electrical parameters and adding admixtures such as oxygen, nitrogen, and air to the gas flow, the conditions for which plasma bullets are present are investigated. Nanosecond time-resolved images have been taken with an ICCD camera to observe the propagation behavior of these discharges. It is argued that the inhomogeneous spatial concentration of metastable atoms and ions, due to the laminar gas flow and the operation frequency of the discharge in the range of a few kilohertz, is responsible for the guidance of the ionization fronts. Furthermore, conditions have been observed at where the branching of the discharge is stable and reproducible over time in the case of a helium plasma by adding admixtures of oxygen. Possible mechanisms for this phenomenon are discussed.

Light Source with a Thermoionic Cathode

A light source with a thermoionic cathode has been investigated in helium at atmospheric pressure; the spectral radiance of the light source in the range 200–800 nm and the cathode temperature (3510 K) have been determined. Its employment as a light source in absorption spectroscopy for determination of the concentration of triplet and singlet metastable helium atoms in an atmospheric-pressure glow discharge has been demonstrated. The total concentration of metastable atoms was ∼3.5·1013 cm−3.

Determination of metastable atom concentration by use of electrostatic probe technique

A method for determination of the metastable atom concentration in high pressure (>100 Torr) high density (> 1012cm23); helium plasma from current–voltage characteristics of a single electrostatic probe is described. It is shown, that the flux of metastable atoms to the probe is controlled by ion sheath thickness and consequently by probe bias. The method for calculation of metastables concentration from the negative part of the current–voltage probe characteristics is proposed. The metastables concentrations measured in pulsed microwave discharge are in agreement with values calculated from the metastable balance equation.

Translational nonequilibrium of the shock wave front in an argon-helium gas mixture

It is experimentally established that the concentration of metastable atoms appearing in the front of a shock wave propagating in a gas phase is greater by one order of magnitude for an argon-helium 1: 1 mixture than for pure argon with the same Mach number and initial pressure. The average “longitudinal” temperature of the shock wave front is determined. An expression for the concentration of excited particles is derived that takes into account the “ longitudinal” character of the gas temperature in the shock wave front.

The Influence of the Gas‐Temperature on the Behaviour of the Positive Column of a Neon Glow Discharge and its Description by a Nonlinear Equivalent Circuit

AbstractThe concentration of metastable atoms can significantly be influenced by changing the gas temperature [1]. Starting from a set of balance equations, consisting of the balance equation for the electrons, the balance equation for excited atoms and the current balance equation the impedance behaviour is calculated in dependence on the gas temperature. Based on a theoretical description of the discharge plasma a nonlinear equivalent circuit representing the behaviour of the positive column at higher gas‐temperatures (250 K – 500 K) is developed. The circuit posseses only positive parameters for the reactances which can be calculated by means of the elementary processes occuring in the discharge plasma. Experimental investigations of the static current/voltage characteristic and the impedance behaviour support the proposed circuit model.

A method for measuring cross sections for electron-impact excitation out of metastable levels of atoms

A method for measuring cross sections for electron-impact excitation out of the metastable levels of the He atom is described. A hollow cathode discharge is used to produce an atomic beam consisting of ground-level He atoms and the He(21S) and He(23S) metastable atoms. An electron beam of energy below 20 eV crosses the atomic beam exciting the metastable atoms to higher levels, and the intensity of the radiation emitted by atoms in these higher levels is utilized to determine the cross sections. Because of the very low concentration of metastable atoms in the atomic beam, the emission signal is extremely weak. A number of special techniques have been developed to detect these very low-level signals. Absolute calibration of the cross section is accomplished by referencing the emission signal that resulted from electron excitation out of the metastable level to the emission signal that resulted from the 23S→33P or 21S→31P laser optical absorption.

Plasmadiagnostik mit Emittierenden Sonden Durch Ausnutzung der Örtlichen Störung des Ionisationsgleichgewichts

The emissive probe is used generally to determine exclusively the plasma potential. The emissive probes have distinct advantages over collecting probes, because several conditions which restrict the spectrum of application fall away. In this paper the methods for the determination of plasma potentials are discussed and completed by new techniques using the local perturbation of the ionization balance. On the other hand it is shown that further plasma data can be obtained exploiting the plasma response on the emitted electrons accelerated by the probe bias voltage. In this way the total ionization rate, the portion of direct ionization, the concentration of metastable atoms and the life time of metastables are determined for selected conditions.

Theoretical description of gas discharges containing excited atoms at low pressures

For low-pressure gas discharges a theory is established taking into account excited atoms, in particular metastable atoms, and stepwise ionisation processes. A macroscopic boundary condition for the outward-directed drift velocity of the excited atoms is derived. Using this condition the concentration of metastable atoms can be calculated. At a low degree of ionisation and low gas density the drift considerably reduces the density of the metastables. Saturation of the concentration of the metastables occurs as the electron density is increased. For argon the spatial distributions of the particle densities and of the drift velocities of the metastable atoms and of the charge carriers are calculated. The influence of the metastable atoms and of the stepwise ionisation on the electron temperature is discussed.

Influence of electrode temperature on the metastable atom concentration in an argon-filled diode

The influence of electrode temperature on the time delay of electrical breakdown in an argon-filled (13.3 mbar) diode at electrode temperatures of 300, 523 and 673K has been investigated by measuring the electrical breakdown time delay (td) dependence on both the time interval between two successive measurements ( tau ) ('memory curves') and the applied voltages (Uw). The results obtained show that values of td at 673K are about one order of magnitude smaller than those at 300K, whether cathode or anode was heated. It was shown that the complete curve td=f(Uw) moved towards smaller values of td with increasing temperature. Conclusions on the change with temperature of the temperature equilibrium concentration of metastables are drawn.

Opto-galvanic spectroscopy of a neon discharge: Mechanism studies

Irradiation of gaseous discharges with a tunable laser produces easily observed voltage changes at wavelengths which correspond to electronic transitions for species in the discharge. This work reports observations made in the 572–654 nm region on a neon glow discharge, which is operated at 5 torr (670 Pa) pressure and 2–25 mA current. A wide variety of absorption transitions have been detected which originate in the 1s n ( 3P 2,1,0, 1P 1) and 2p n excited states. The resulting voltage signals are both positive and negative, and some transitions exhibit sign changes as a function of discharge current. Some of the results can be interpreted in terms of processes which directly affect the neon metastable atom concentration ( 3P 2, 3P 0) and states also produce voltage changes which can be explained in terms of processes which either enhance or decrease ionization.

Changes in the concentration of metastable atoms in pulsed discharges

In [i, 2] the change in the concentration of metastable helium atoms in the afterglow of a pulsed direct-flow discharge was determined. In [3] the behavior of the concentration of metastable helium atoms in the plasma of a hollow-cathode pulse discharge was investigated. It was shown that the concentration of metastable atoms in the flow of the current pulse depends strongly on its amplitude and duration. Therefore, measuring the concentration of metastable atoms in a pulsed direct-flow discharge should facilitate a clearer understanding of their role both in the course of the current pulse and in the afterglow.

Effects of Added O2 upon Argon Emission from an rf Glow Discharge

Emission and absorption measurements have been made in rf glow discharges through pure argon and through argon with 1% O2 added. The results indicate that added O2 reduces the amount of excitation of neutral argon, particularly the concentration of metastable atoms. At the same time, increased emission is observed from argon ions when O2 is added. This is attributed to an increase in the argon ion concentration. The results of these experiments have important implications regarding the mechanisms operative in rf sputtering processes. Namely, under the conditions studied (rf power density of 10 W/cm2 and argon pressure 4×10−2 Torr) direct electron impact ionization appears to be the predominant maintenance mechanism of the glow rather than ionization resulting from metastable atom collisions. In addition, it is proposed that addition of oxygen to the argon discharge atmosphere serves to increase the secondary electron emission coefficient of the fused silica target.

Measurement of Time Varying Optical Absorption

Time sampling techniques have been applied to the study of optical absorption transients. The method is described in terms of its use for the study of the decay of the concentration of metastable atoms following a pulsed discharge. Light from a capillary source is passed through the discharge tube. The transmitted intensity of one of the spectral lines which is absorbed by the metastable atoms is measured using an interference filter and a gated photomultiplier. The photomultiplier gate occurs at twice the discharge frequency so that alternate pulses of the photomultiplier output are reduced by absorption. The component of the photomultiplier output at the discharge frequency is proportional to the absorption and can be measured with a narrow band amplifier and synchronous detector. Since the time resolution of the system is determined by the width of the photomultiplier gate, the response of the synchronous detector can be made slow enough to average the absorption signal over many decay periods. Fluctuations in the number of electrons leaving the cathode of the photomultiplier limit the useful sensitivity of the present system to approximately two parts in 104. This represents a hundredfold improvement in sensitivity over that available with previous techniques. The results of some studies of helium metastables are presented in order to illustrate the detail which can be obtained with this technique.

Influence of Irradiation on the Characteristic of a Glow Discharge in Pure Rare Gases

Irradiation of the positive column of a glow discharge in pure rare gases by the radiation from a second discharge tube filled with the same gas influences the $V\ensuremath{-}I$ characteristic of the irradiated glow discharge, shifting it to a higher voltage for the same current. The effect arises from the reduction of the concentration of metastable atoms by absorption. In the case of helium the effect could be studied separately for the two mainly absorbed lines 10 830A and 20 581A. Practically the whole effect is caused by the latter. This result can be understood by considering the possible transitions involved.

AN ATTEMPT TO DETECT THE PRESENCE OF METASTABLE ATOMS IN ACTIVE NITROGEN BY LIGHT ABSORPTION

In order to explain the nitrogen afterglow, Cario and Kaplan have assumed the presence of metastable 2D and 2P atoms in active nitrogen. The nitrogen emission lines at 1492 Å and 1742 Å have these metastable states as lower states, and should therefore be absorbed by active nitrogen. However, no absorption has been found by the writers. This leads to 1/6000% as an upper limit for the stationary concentration of metastable atoms in active nitrogen, which is compatible with the experiments. In order to account for the observed intensity of the afterglow, on the basis of Cario-Kaplan's theory, a lifetime of the metastable 2P atoms [Formula: see text] has to be assumed and a concentration of metastable molecules greater than 1/330%.

A METHOD OF DETERMINING THE IMPEDANCE OF HOT CATHODE DISCHARGE TUBES

By means of a superposed alternating current the negative resistance of hot cathode neon and mercury direct current arcs was measured and found to follow closely the slope of the static characteristic of the neon arc and to be widely different in the case of the mercury arc. At the same time the reactance for the ripple current in the arc was determined. This reactance is explained on the basis of a time lag in the concentration of metastable atoms; from the phase shift the order of magnitude of the lifetime of the metastable atoms is computed and the result is in agreement with calculations made by others.

Collisions of the Second Kind and their Effect on the Field in the Positive Column of a Glow Discharge in Mixtures of the Rare Gases

A study was made of the collision processes in the uniform positive column of a glow discharge in mixtures composed of two of the rare gases, helium, neon and argon and a mixture of each of these gases with mercury vapor, by means of measurements of the electric field and by spectroscopic observations. The discharge tube was of cylindrical form, 4.4 cm in diameter, with plane parallel electrodes The electrode distance could be varied from zero to 32 cm by means of an external electromagnet. The phenomena were studied with currents from 20 to 40 m.a. and pressures from 5 to 30 mm of mercury. The mixtures were circulated through the discharge tube and a purifying system during the entire time that observations were being made. It was very important to maintain extreme purity of the gases. The results show that the electrical and spectral characteristics of the uniform positive column in mixtures of monatomic gases can be explained principally in terms of collisions of the second kind between the ions or metastable atoms of one gas and the neutral atoms of the other. The effect of limitation of electron velocities is insufficient to explain the results, and its effect is shown to be negligible as compared with collisions of the second kind when the concentration of one gas is very small. The necessary condition for a large effect to be produced in the electrical and spectral characteristics of the positive column by a small percentage of one gas added to another is that a close resonance exists between the metastable states of the main gas and the ionization potential or excited states of the added gas atom or ion. By the introduction of as little as 0.4 percent neon or argon into helium, a marked increase in the electric field is produced, and the spectrum emitted is changed almost completely from the arc spectrum of helium to that of neon or argon respectively. The addition of mercury vapor at room temperature to neon caused a decrease of 45 percent in the field while the addition of mercury to either argon or helium produced a small increase. In the cases where the excitation and ionization potentials of the added gas were above the metastable states of the main gas, no reactions between the two gases could occur to affect the concentration of metastable atoms. In these cases practically no change in either the spectral or electrical characteristics were observed upon adding a small amount of one gas to another. However, in larger proportions the change produced in the field was proportional to the amounts of the two gases and the difference between the field in each of the pure gases. This result is accounted for mainly by the difference in the energy of the ionization processes in the two gases by electron impact.