Positive Column Of Discharge Research Articles

Electric field and plasma emission responses in a low pressure positive column discharge exposed to a low Mach number shock wave

Low Mach number shock waves propagating through a low pressure, nonequilibrium positive column gas discharge have been observed to experience dispersion and velocity changes. It is shown that these effects depend on discharge polarity. Optical and electrical measurements are described which show further polarity-dependent effects in discharge light emission and changes in electrical properties. Using two types of probes, electrical measurements were made of both the global changes in discharge voltage and current and time resolved local electric field changes. The measured behaviors of discharge and shock wave point to very localized triple or quadruple layer electric sheaths connected with the propagating shock wave, which provide local enhanced ionization at the shock front which can sustain the discharge, at least during the short shock propagation time. The postulated density gradient driven large local recirculation current in the potential minimum near this sheath [H. S. Maciel and J. E. Allen, J. Plasma Phys. 42, 321 (1989)] at the shock front will result in large local Joule heating, causing the shock dispersion and shock speed increase, which have been observed in many experiments. Thus, the conclusion is that the effects are due to highly localized gas heating which is facilitated by the response of the positive column plasma to the acoustic shock.

Cathaphoretic confinement of Zn evaporated into helium andneon discharges

Cathaphoresis of Zn evaporated into the positive column of heliumand neon discharges was investigated both experimentally andtheoretically. The side-light intensity of the Zn-I 518.2 nmtransition was recorded along a 4.5 cm long positive columndischarge with an inner diameter of 3 mm. The buffer gas pressure wasvaried between 10 and 25 mbar while discharge currents of 20-60 mAwere applied to the tube. Reasonably good agreement was foundbetween the experimental data and the theoretically predictedaxial distribution of zinc concentration in the tube. The resultscan be useful in the design of the cathaphoretic confinementsection of the different He-Zn laser tubes and a prospectiveNe-Zn laser system, which would operate in the deep ultraviolet region(210 nm).

Measurement of the H 3+ destruction rate due to ambipolar diffusion in an AC positive column discharge

We have developed a method to observe the destruction rate of H 3 + in the positive column, which is limited by ambipolar diffusion. This method is based on the competition between the ambipolar diffusion rate and destruction rate by added impurities. The decrease of a velocity modulated H 3 + absorption signal was studied when small amounts of CH 4, N 2, and CO were added to a pure hydrogen AC positive column discharge. The destruction rate constant of H 3 + due to ambipolar diffusion was then obtained using a steady-state model and previously reported ion-neutral reaction rate constants.

Selection rules for nuclear spin modifications in ion-neutral reactions involving H3+

We present experimental evidence for nuclear spin selection rules in chemical reactions that have been theoretically anticipated by Quack [M. Quack, Mol. Phys. 34, 477 (1997)]. The abundance ratio of ortho-H3+ (I=3/2) and para-H3+ (I=1/2), R=[o-H3+]/[p-H3+], has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas using para-H2 and normal-H2 (75% o-H2 and 25% p-H2). The observed clear differences in the value of R between the p-H2 and n-H2 plasmas demonstrate the spin memory of protons even after ion-neutral reactions, and thus the existence of selection rules for spin modifications. Both positive column discharges and hollow cathode discharges have been used to demonstrate the effect. Experiments using pulsed plasmas have been conducted in the hollow cathode to minimize the uncertainty due to long-term conversion between p-H2 and o-H2 and to study the time dependence of the o-H3+ to p-H3+ ratio. The observed R(t) has been analyzed using simultaneous rate equations assuming the nuclear spin branching ratios calculated from Quack’s theory. In p-H2 plasmas, the electron impact ionization followed by the ion-neutral reaction H2++H2→H3++H produces pure p-H3+, but the subsequent reaction between p-H3+ and p-H2 scrambles protons. While the proton hop reaction (rate constant kH) maintains the purity of p-H3+, the hydrogen exchange reaction (rate constant kE) produces o-H3+ and acts as the gateway for nuclear spin conversion. The value of R(t), therefore, depends critically on the ratio of their reaction rates α=kH/kE. From observed values of R(t), the ratio has been determined to be α=2.4. This is in approximate agreement with the value reported by Gerlich using isotopic species.

Combination band spectroscopy of H3+

Thirty rovibrational transitions of H3+ have been observed near 1.25 μm using a tunable diode laser and a positive column discharge. In addition to the H3+ transitions, over 200 transitions between Rydberg states of H2 were observed—these could be discriminated against by using a discharge dominated by He, which apparently collisionally quenches the Rydberg states of H2 without affecting H3+. Twenty-eight of the H3+ transitions have been assigned to the ν1+2ν22←0 band, and provide experimentally determined energy levels for most of the levels up to J=4 in the ν1+2ν22 state. The remaining two H3+ transitions have been assigned to the 2ν1+ν2←0 band. These bands represent a crucial test of ab initio calculations, as they reach higher vibrational levels of H3+ than any yet observed. We have compared our experimental results with recent variational calculations by several groups.

Mechanisms for formation of the electron distribution function in the positive column of discharges under Langmuir-paradox conditions

The form of the electron distribution function in the positive column of low-pressure discharges is examined under conditions such that the electron mean free path exceeds the vessel radius. Its formation is analyzed taking all major factors into account, including elastic and inelastic collisions, radial and axial electric fields, and the loss of fast electrons to the wall. It is shown that the main mechanism controlling the fast part of the distribution function is the loss of electrons to the wall, which is determined by the scattering of electrons into a comparatively small loss cone that depends on the relationship between the axial and radial components of the velocity. Since the elastic collision rate for all elements has a weak dependence on the energy beyond the ionization threshold, ultimately the high-energy part of the electron energy distribution function in the positive column of low-pressure discharges is nearly Maxwellian. The subthreshold portion of the distribution function, in turn, is determined by the energy diffusion, in a comparatively strong field, of Maxwellian electrons which arrive after inelastic collisions. The final electron distribution function is well approximated by an exponential with a single slope over the entire energy range. Only within a narrow range of scattering angles is the electron distribution function strongly depleted by the loss of electrons to the vessel walls. In the end, it is concluded that this phenomenon, like the Langmuir paradox, may be related to aspects of the physics of the formation of the electron distribution function owing to a combination of already known mechanisms, rather than to a hypothetical mechanism for thermalization of the electrons, as assumed up to now in the literature. A comparison of solutions of the model kinetic equation given here with published Monte Carlo calculations and experimental data shows that they are in good agreement.

Energy-resolved electron particle and energy fluxes in positive column plasmas

This paper deals with electron flux densities and electron energy flux densities in positive column discharges. Recent kinetic calculations by Uhrlandt and Winkler have revealed the interesting physical phenomenon of radially inward directed energy flux densities in positive column plasmas. We have used a self-consistent positive column model, based on an accurate and highly detailed Monte Carlo code, to study this effect in more depth. The results of this study show a rather complex physical picture of electron particle and energy flux densities. Electrons with low energies usually exhibit radially outward directed particle and energy flux densities. At energies above the threshold for electronic excitation particle and energy flux densities are usually inward directed. Only close to the wall, at total energies above the wall potential energy, do these flux densities point towards the wall. The thickness of this `wall loss region' scales with the electron mean-free-path for momentum transfer, i.e. with the inverse of the neutral gas density. The results presented can be qualitatively interpreted in terms of the total-energy picture for the electrons.

Modelling discharges in electronegative gases

This paper builds on earlier work to give a consistent treatment of the positive column of discharges in electronegative gases covering the transition from collisionless to collisional. In particular it seeks to elucidate the conditions under which there is an ion-ion plasma core surrounded by an electron-ion plasma, and when there is not. The parameters which describe the processes of ionization, attachment, detachment and recombination are related to the central negative ion density relative to the electron density and, where appropriate, the size of the core. The use, by earlier workers, of the Boltzmann approximation to describe the negative ion distribution and to obtain ambipolar diffusion coefficients at higher pressures is shown not to be justified. This leads to the clarification of an inconsistency in the literature. Where possible, the work is related to other recent treatments of the same problem in order to begin to build a comprehensive picture of such discharges. The need to have results which combine both detachment and recombination as the negative ion loss processes is identified as outstanding. This, when rectified, should lead to a fully comprehensive treatment.

Infrared Spectroscopy of H3O+: The ν1 Fundamental Band

The infrared spectrum of H3O+ in positive column discharges of H2/O2 gas mixtures has been studied by a difference frequency laser spectrometer. The ν1 fundamental band of H3O+ was identified in the region of the strong . Molecular constants were obtained by the least-squares fitting of the observed frequencies, and band origins of the and subbands were determined to be 3389.656(2) and 3491.170(2) cm−1, respectively. During this study, assignment of the ν3 fundamental band was extended to higher J, K transitions, which do not fit to the calculated pattern well, but have definitely been assigned by using the ground state combination differences and relative intensities. Vibration–rotation interactions between the ν1 and ν3 states have been considered, which explained some large discrepancies between observed and calculated frequencies and led to the identification of forbidden transitions. Energy differences in the ground state between the ΔK = 3 rotational levels were obtained from the combination differences of the forbidden and allowed transitions, which led to an accurate determination of C and DK. Equilibrium structure of H3O+ has been derived to be re = 0.974(1) Å and αe = 113.6(1)°.

A comparison of kinetic and fluid models of the positive column of discharges in inert gases

The models of a positive column of an inert gas discharge at low pressures and small currents are developed in the frameworks of nonlocal kinetic and local fluid theories. Both models are based on the joint solution of the corresponding kinetic equation, ion-motion equation and Poisson equation that permit one to describe bulk plasma and near-wall sheaths of space charge. The theories differ principally in the descriptions of the electron component of a plasma. In the fluid theory the radial flux of electrons is a combination of field and diffusive fluxes, but in the kinetic model a similar representation is impossible. A comparison of the distribution functions and macroscopic properties calculated by kinetic and fluid theories for the discharge under the same conditions is performed. This comparison is carried out for the models of the positive column developed under the assumption of plasma quasi-neutrality and with regard to the deviations from quasi-neutrality. The use of the fluid theory beyond its application range (low pressures and small currents) does not reflect the physical picture of discharge sustenance and leads to incorrect results with respect to several parameters.

New efficient low-pressure gas-discharge source of optical radiation using hydroxyl OH

An experimental investigation was made of the possibility of developing a new efficient gas-discharge source of optical radiation based on an OH hydroxyl molecular additive. It is shown that under certain discharge conditions, the luminous efficiency of a positive discharge column in a rare gas-hydroxyl mixture is similar to that of a mercury discharge. The results suggest that it may be possible to develop a new ecologically innocuous source of optical radiation which may in future replace mercury luminescence light sources.

Radiometric characterization of xenon positive column discharges

We use both experimental diagnostics and computational modelling to characterize the electrical properties and vacuum ultraviolet emission of positive column discharges in both pure xenon and mixtures of xenon with other rare gases. Discharge conditions include xenon pressures of 10-100 mTorr at room temperature, discharge currents of 100-500 mA and positive column diameters of 1.1-5 cm. Our primary interest is the efficient generation of large quantities of ultraviolet radiation for use in fluorescent light sources. We therefore focus on the total output of atomic xenon resonance radiation at 147 nm, reported in units of watts of 147 nm radiation per unit length of positive column, and also the efficiency by which electrical input power is converted into 147 nm radiation. Over the range of discharge conditions considered, we observe outputs in excess of of discharge length and efficiencies in excess of 80%, but we also find that there is a strong tradeoff between efficiency and output, and that these two figures of merit cannot be simultaneously maximized. A change in the behaviour of the discharge is observed to occur near 25 mTorr in 2.2 cm diameter tubes and is attributed to the transition from a discharge sustained by multistep electron-impact ionization to one sustained by single-step electron-impact ionization.

Experimental comparison of rotational and gas kinetic temperatures in and He- discharges

We report an experimental comparison of the rotational temperature and the gas kinetic temperature of and in a positive column discharge. The discharge is operated in an oven. The oven temperature, determined with a thermocouple, provides a lower bound for the gas kinetic temperature. An upper bound on the temperature is obtained by adding to the oven temperature a calculated increase in the gas temperature due to the discharge. The contribution of the discharge to the gas temperature is calculated under the assumption of complete conversion of input power into gas heating so that the estimate provides an upper bound. The rotational temperature of is determined directly with a Boltzmann plot of the first negative system. For , the ratio of the first minimum to the secondary maximum for the band profile of the second positive system is used to obtain the rotational temperature. We compare experimental spectra with simulated spectra generated under the assumption that the rotational temperature is equal to the minimum and maximum gas kinetic temperatures. The reliability of the rotational temperature as a gas temperature diagnostic under our experimental conditions is discussed for all bands investigated.

Density Distributions in a Rectangular Positive Column at Intermediate Pressures

Spatial profiles of the plasma density and ion drift velocity in an infinite rectangular dc discharge positive column at intermediate pressures are obtained based on the quasi-neutral transition theory for two-dimensional geometry. Assuming that the ionization by electron impacts occurs at a constant ratio in the two directions, relevant differential equations for plasma parameters are solved by means of variable separation and the plasma profiles are determined as explicit functions of actual distances in the column. The results derived are an extension of the high-pressure Schottky theory to the low-pressure region.

Nonlocal particle loss effects on the electron kinetics in a direct current helium diffusion-controlled positive column

The electron energy distribution function (EEDF) in a positive column of low-pressure and discharge current is determined not only by the local collision processes and the axial electric field action, but also by the transport phenomenon, the radial ambipolar diffusion due to the gradient of plasma density. Thus, to completely determine the EEDF, the Boltzmann equation including radial inhomogeneity terms has to be solved. The present work proposes a simplified method to account for the radial inhomogeneity, when the electron kinetics in the central part of the positive column can be reduced to be one energy-dimensional. The radial diffusion of electrons is taken into account via a wall loss term. A greatly simplified kinetic equation is obtained and its numerical solutions agree well with the EEDF determined from Langmuir probe measurements in a helium dc discharge positive column. Also, a comparison of the present method with local and nonlocal approach theories is made. A discrepancy is observed, especially at high energies, where either local or nonlocal approach theories predict too-large values of EEDF.

An Explanation for Positive Differential dc Voltage-Current Characteristics in Weakly Ionized, Low-Pressure Positive Column Gas Discharge Plasmas.

A physical explanation is given for the positive differential voltage-current characteristic seen at low current in weakly ionized, low-pressure positive column discharges in argon/mercury gas mixtures.

An experimental comparison of rotational temperature and gas kinetic temperature in a discharge

An experimental comparison of rotational temperatures determined from optical emission spectroscopy (OES) of with the gas kinetic temperature is reported. A positive column discharge in a high-temperature oven is used. The oven temperature, determined with a thermocouple, provides a lower bound for the gas kinetic temperature. A thermal conduction calculation using the total power input to the discharge provides an upper bound for the gas kinetic temperature. A range of gas temperatures and pressures is explored. Two bands of molecular hydrogen were examined: the R branch of the G (0,0) band and the R branch of the (0,0) Fulcher band. Measurements of the rotational temperature using emission from the band fall within the gas kinetic temperature bounds for temperatures above . Data from the Fulcher band do not show a correlation with the gas temperature and give a constant rotational temperature independent of oven temperature.

Model of a weakly ionized, low-pressure xenon dc positive column discharge plasma

A model of a weakly ionized, low-pressure positive column discharge is described. Of particular interest is the generation of atomic resonance radiation. The model computes species densities as a function of radial position using fluid continuity equations. Electron impact collision rates are found from the zero-dimensional Boltzmann equation. The appropriate electron impact cross sections, heavy particle collision rate coefficients, and radiative decay rates for xenon have been assembled from the literature, and are summarized. Model predictions compare favourably with measured axial electric field values in a xenon positive column discharge, while the model is found to overpredict the electron temperature at low xenon pressure.

Reaction Kinetics and Chemical Quasi‐Equilibria of the Ozone Synthesis in Oxygen DC Discharges

AbstractThe spatial distribution of ozone and of oxygen atoms was studied along the active and the passive zone of a dc discharge (positive column, pressure: p = (4 … 10) · 102 Pa, current: I = 2 … 50 mA, flow rate: F = 5 … 100 sccm) in flowing oxygen. The composition of the final output O2/O3‐mixture is controlled by relaxation processes in the passive reactor zone. It is affected sensitively by the total number density and the gas temperature in the afterglow. Steady states meaning reversible chemical quasi‐equilibria were observed and analysed extensively. Within a detailed kinetic model the formation of these equilibria can be explained quantitatively. The synthesis to ozone is controlled above all by the metastable O2 (a1Δg) species, which modify drastically the results for the basic mechanism, considering the O atoms in the 3P and 1D states.

Observation of characteristic, polarity-dependent, Doppler shifts from neutral species in the positive column of a discharge plasma

Doppler shifts of emissions from neutral species have been observed for the first time in a positive column of a dc glow discharge. For a detailed study, we have chosen emissions of H2 and its isotopomers in a hydrogen discharge using a high resolution Fourier-transform spectrometer. Experimetal evidence supports the conclusion that the observed shifts derive from momentum transfer during electron-molecule collisions that produce the emissive excited states. A theoretical model has been formulated to relate Doppler shifts to the drift velocity of electrons in the discharge as well as to the momentum transfer of electron-molecule collisions. Predictions based on the model are consistent with the experimental observations.