Positive Column Plasma Research Articles

Plasma wave propagation with a plasma density gradient

Plasma waves with the plasma diffusion velocity un due to a plasma density gradient are described in a positive column plasma. The ion wave is generated by the perturbation of the operating frequency 106 s−1 and it propagates with the group velocity ug∼cs2/un∼(105–106) m/s, where cs is the acoustic velocity in a fine tube fluorescent lamp, while the electron wave cannot be generated with a turbulence of low frequency less than the electron oscillation frequency ωpe. The propagation of the lighting signal observed in long tube fluorescent lamps is well understood with the propagation of ion waves occurring along the plasma density gradient.

Radiative characteristics and electronic transport coefficients plasma-based water vapor

The results of studies of the positive column plasma radiation of a longitudinal glow discharge in water vapor and mixtures He- H 2 O with partial pressures p (H 2 O) = 50-200 Pa; 2.0-2.5 kPa and p (He) = 0,1-1,5 kPa are presented. Analysis of radiation in the spectral range 130-350 nm showed that at low water vapor pressure the main in the emission plasma spectra were bands of hydroxyl OH (X-A) (Δλ = 280-320 nm) and OH (A-C) (Δλ = 140 -200 nm). The optimization of the hydroxyl emission bands OH intensity, depending on the water vapor pressure and partial composition of the He - H 2 O mixture, as well as the discharge current is conducted. Optimum conditions of the radiator at the bands OH (X-A) and OH (A-C) are identified. Based on the effective interaction cross sections database of low energy electrons with water molecules, for a discharge in water vapor and mixtures of p (Re) - p (H2O) = 9.1 compiled by us the electron energy distribution functions and electron kinetic coefficients in the range of the parameter E/N = 1 - 300 Tg are calculated. Analysis of helium influence on the positive column plasma parameters in a strong dilution of water vapor with helium is conducted. The results are of interest for the optimization and modeling of spontaneous UV-VUV radiation gas-discharge sources on the basis of the decay products of water molecules in the low density plasma.

Dynamics of plasma–electrode coupling in fluorescent lamp discharges

A time dependent model of a low pressure, mercury–rare gas discharge with thermionic electrode is presented. The model is applicable to ac-operated fluorescent lamps, which is the focus of this work. The model describes a one-dimensional negative glow plasma that is bounded on one side by a thermionic electrode and a sheath, and on the other by a positive column plasma. The electrode/sheath component of the model, together with the mutually interacting negative glow plasma, allows for self-consistent calculation of the electrode sheath potential. The model describes a smooth transition in the plasma parameters from electrode to positive column and thus reveals the spatial extent of the influence of the electrode and sheath processes. A detailed description of the model is presented, as well as results of calculations pertaining to a standard fluorescent lamp. Also shown are measurements from a 2 mm interferometer and an internal floating probe, both of which compare favourably with the calculations.

On the structure of the two-dimensional spatially periodic inner transition layers in a gas-discharge plasma

We investigate the structure of the spatially periodic inner boundary layers in the plasma of a positive glow-discharge column produced in a long cylindrical tube with an electropositive gas inside. Asymptotic methods, namely, the method of boundary functions, are used to analyze the initial mathematical model. We consider the formation of contrast burst-type structures. We have found all principal terms of the boundary-layer asymptotics of the solution. The results obtained are compared with the available probe measurements of basic physical parameters of ionization waves (strata) in neon at low pressures.

Magnetic field effects on electrical parameters of rf excited CO 2 lasers

In the present Letter a rf excited CO 2 laser embedded in an external, constant, and homogeneous magnetic field is considered. The magnetic field effects on some discharge parameters such as V–I characteristics, impedance of sheaths and positive column of plasma, intensity of visible emission from plasma and thickness of positive column are investigated. There is an increase in thickness of positive column and output power in presence of magnetic field. Magnetic field leads to an increase in the discharge voltage and impedance for lower current densities and a decrease for higher ones. There is a current density in which the magnetic field has no effects on discharge voltage and impedance. There are two peaks on intensity of visible emission from the discharge which at higher magnetic field are pushed out toward the electrodes.

Near-infrared electronic spectrum of CH2+

Four vibronic bands of the A 2B1(Piu)<--X 2A1 electronic transition of CH2+ have been observed in the near infrared from approximately 11,000-13,000 cm(-1) using a highly sensitive spectrometer based on a Ti:sapphire laser and a positive column plasma. The two states are derived from the lowest 2Piu state of the linear conformation as a result of a strong Renner-Teller effect. As expected, the rovibronic structure of the CH2+ spectrum is very similar to that of the isoelectronic BH2 radical reported by Herzberg and Johns. Results of a preliminary analysis are given in which the rotational structures of the two states are treated independently, the ground state as a near-prolate asymmetric rotor and the excited state as a linear molecule.

Experimental and theoretical investigations of a low-pressure He–Xe discharge for lighting purpose

Low-pressure cylindrical dc glow discharges in a mixture of helium and 2% xenon are studied by experiment and self-consistent modeling. They can be used for the design of mercury-free vacuum ultraviolet sources and fluorescent lamps for publicity lighting. Experimental diagnostics of the column plasma includes measurements of the axial electric field strength and of the axis densities of the four lowest excited states of xenon. The electric field is determined from probe measurements. The particle densities are derived from the results of tunable diode laser absorption spectroscopy. Experimental investigations are assisted by a self-consistent analysis of the dc positive column plasma. A comparison between calculated and measured values of the axial electric field strength and the densities of excited xenon atoms is presented and discussed. The validated model is used for optimization of the discharge conditions by variation of the discharge current, gas pressure, and tube radius with respect to the radiation power and efficiency of the 147 nm resonance line of xenon. The discussion includes an analysis of the power budget of the column plasma.

Oscillations of the positive column plasma due to ionization wave propagation and two-dimensional structure of striations

Time and space resolved measurements of the plasma potential in axial and radial directions in S- and P-striations in neon are performed. The measurements in different radial positions were carried out with high spatial resolution by means of simultaneous displacement of electrodes relative to the stationary probe. The plasma potential was found to be a superposition of the potentials of ionization wave and plasma oscillations relative to the electrodes. A method of decomposition of the measured spatio-temporal structure of the potential in components associated with the plasma oscillations and ionization wave propagation is proposed. A biorthogonal decomposition of the spatio-temporal structure of the potential is performed. A comparison of the decomposition results obtained by the two methods is made. The experiments revealed a two-dimensional structure of the potential field in an ionization wave. Qualitative discussions of the reasons for the occurrence of this two-dimensional structure are presented based on the analysis of the kinetic equation and the equation for the potential.

Scaling laws for the spatial distributions of the plasma parameters in the positive column of a dc oxygen discharge

Comprehensive self-consistent simulations of the positive column plasma of a dc oxygen discharge are performed with the help of commercial CFDRC software (http://www.cfdrc.com/~cfdplasma), which enables one to carry out computations in an arbitrary 3D geometry using fluid equations for heavy components and a kinetic equation for electrons. The main scaling laws for the spatial distributions of charged particles are determined. These scaling laws are found to be quite different in the parameter ranges that are dominated by different physical processes. At low pressures, both the electrons and negative ions in the inner discharge region obey a Boltzmann distribution; as a result, a flat profile of the electron density and a parabolic profile of the ion density are established there. In the ion balance, transport processes prevail, so that ion heating in an electric field dramatically affects the spatial distribution of the charged particles. At elevated pressures, the volume pro- cesses prevail in the balance of negative ions and the profiles of the charged particle densities in the inner region turn out to be similar to each other. © 2003 MAIK "Nauka/Interperiodica".

Imaging the Spatial Distribution of Microwave Intensity Using the Recombination Continuum Emitted by the Positive Column of the Cs–Xe DC Discharge

We study experimentally the possibility of using the recombination continuum (RC) emitted by a nonequilibrium plasma of the positive column (PC) of a Cs–Xe discharge for real-time imaging of the spatial distribution of microwave intensity. A uniform plane slab of dense plasma of the PC of a Cs–Xe discharge with aperture 10×8 cm2 was used for the imaging. A continuous-wave (CW) magnetron was the microwave source with frequency 35.4 GHz and power up to 20 W. We measured the dependence of the RC intensity on the intensity of the microwaves incident on the plasma. Spatial distributions of the microwave intensity for a microwave beam and the H01 mode of a circular waveguide were imaged. The results of these experiments show that spatial distributions of microwave intensity measured using RC agree well with the distributions obtained using other methods. A temporal resolution of 10 μs was demonstrated and an energy-flux sensitivity of about 4·10-5 J/cm2 was achieved in the microwave-imaging experiments.

Substantiation of the two-temperature kinetic model by comparing calculations within the kinetic and fluid models of the positive column plasma of a dc oxygen discharge

Results from kinetic and fluid simulations of the positive column plasma of a dc oxygen discharge are compared using commercial CFDRC software ( http://www.cfdrc.com/˜cfdplasma ), which enables one to perform numerical simulations in an arbitrary 3D geometry with the use of both the fluid equations for all the components (fluid model) and the kinetic equation for the electron energy distribution function (kinetic model). It is shown that, for both the local and nonlocal regimes of the formation of the electron energy distribution function (EEDF), the non-Maxwellian EEDF can satisfactorily be approximated by two groups of electrons. This allows one to take into account kinetic effects within the conventional fluid model in the simplest way by using the proposed two-temperature approximation of the nonequilibrium and nonlocal EEDF (2T fluid model).

Positive-column plasma studied by fast-flow glow discharge mass spectrometry: could it be a "Rydberg gas?".

Ions created from the fast-flowing positive column plasma of a glow discharge were monitored using a high voltage magnetic sector mass spectrometer. Since the field gradient and sheath potentials created by the plasma inside the source opposed cation transfer, it is inferred that the ions detected were the field-ionized Rydberg species. This is supported by the mass spectral changes which occurred when a negative bias was applied to the sampling aperture and by the contrasting behavior when attached to a quadrupole analyzer. Reaction with H2 (titrated into the flowing plasma) quenched not only the ionization of discharge gas Rydberg atoms but also the passage of electric current through the plasma, without significant changes to the field and sheath potentials. Few "free" ions were present and the lifetimes of the Rydberg atoms detected were much longer than seen in lower pressure experiments, indicating additional stabilization in the plasma environment. The observations support the model of the flowing plasma, given previously [R. S. Mason, P. D. Miller, and I. P. Mortimer, Phys. Rev. E 55, 7462 (1997)] as mainly a neutral Rydberg atom gas, rather than a conventional ion-electron plasma.

Model of a He–Xe low-pressure dc positive column plasma

The positive column plasma of dc glow discharges at low pressure in a mixture of helium and 2% xenon is studied. Such glow discharges are favored candidates for the design of mercury-free light sources. A self-consistent model of the column plasma is presented based on strict radially resolved treatment of the nonlocal non-equilibrium kinetics of the electron component, the space-charge potential and the densities of ions and excited atom states. A detailed reaction kinetic scheme of the low-lying xenon excited states has been developed and is included in the model. The model is validated by a comparison with measurements of the axial electric field and the densities of the lowest metastable and resonant xenon levels. The latter was obtained by tunable diode laser absorption and probe diagnostics. The initial results of the model show a pronounced radial structure of the rare-gas column plasma and nonlocal properties of the electron power budget caused by space-charge confinement. The impact of the uncertainty of atomic data used in the model on the quantitative results is investigated in detail. A large impact of the cross sections of electron-impact excitation of the xenon ground state and the excitation and ionization of the xenon excited states, which are known with insufficient reliability only, has been observed.

Strong double-layer formation by shock waves in nonequilibrium plasmas

Strong double-layer formation by acoustic shock wave (<M=2) propagation in a low-pressure, N2 positive column plasma has been quantified, near the shock front, by measurements of the voltage jump and the enhancements of both the B (3)Pi(g)-A (3)Sigma(+)(u) plasma emission and the electron density. The large polarity dependence (or shock direction) of these effects and potential jumps of more than 20 kT(e) across the shock-created interface indicate that the measured shock-induced electron heating effects (localized excitation and ionization rate) are caused by the formation of strong double layers maintained by the traveling shock front.

Investigation of resonance and metastable atoms in the afterglow of aHe-Xe positive column plasma

The metastable Xe(6 3P2) and resonance Xe(6 3P1) particle number densities have been studied in the positive column of apulsed glow discharge in a mixture composed of He at pressure pHe = 2.5 Torr and Xe at pxe = 0.05 Torr. Time-resolved measurementswere performed in the active discharge phase and in the afterglow with a laserabsorption technique. The non-stationary balance equation system forpopulations of excited states, electron density and temperature has beensolved to describe the decay of the Xe(6 3P2) state density during1000 µs and that of Xe(6 3P1) state during 40 µs in theafterglow. If only cooling of electrons in elastic collisions is consideredthen a fast decrease of the electron temperature to that of the neutral gaswithin 10 µs and a slow decay of the excited atoms, which does notcorrespond to the experimentally observed decay curves, are obtained. Thepopulation and decay of excited states are to a large extend influenced by theintermixing process due to electron collisions, with a rate coefficientexponentially depending on the electron temperature in the range from 300 to1000 K. Additional heating of slow electrons in collisions with fast onesappearing in chemo-ionization and superelastic collisions maintains theelectron temperature at a level of about 1000 K over hundreds of microseconds.This leads to a faster decay of the metastable atoms. The temporal course ofelectron temperature in the afterglow was analysed taking the heatingmechanism of slow electrons into account. The experimental curves of the decayof metastable and resonance atoms at discharge currents of 20-200 mA wereinterpreted.

Excitation of RF Oscillations in a Discharge with Negative Differential Conductivity

The excitation of oscillations in a discharge with negative differential conductivity is studied experimentally. The possibility is demonstrated of amplifying oscillations in the cathode dark space at frequencies close to the electron plasma frequency of the positive-column plasma. The phase velocities of waves at these frequencies are determined. When the waves pass from the cathode dark space to the discharge positive column, their phase velocities decrease; the closer the frequency is to the electron plasma frequency, the more pronounced the decrease in the phase velocity. As the intensity of oscillations increases, the discharge becomes non-steady-state. This is confirmed by the time evolution of the current-voltage characteristic. The shape of the current-voltage characteristic, its splitting, and the rate at which it varies depend on the input RF power. The decrease in the cathode dark space indicates that the ionization processes in the discharge are strongly influenced by electron plasma oscillations excited due to the collective interaction of the electron beam formed at the cathode with the discharge plasma. It is these processes that determine the maximum values of both the frequency of the excited oscillations and the power that can be withdrawn from the discharge.

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.

Theoretical modelling of CW perturbations of excited states, axialfield and electron density for laser diagnostics of positive columnplasmas

We present a theoretical treatment of the fundamental perturbationscaused when continuous wave tunable laser radiation is absorbed by thelow-temperature plasma of a noble gas positive column. For the first time wepresent a full theoretical description of the contributions due to the inducedchanges in the electric field, electron density and radiation trapping, aswell as the primary laser perturbations. Comparison of our theory withexperimental observation shows that the primary perturbations are generallydominant but in some instances it is the field perturbation which has thegreatest effect.

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.

A method to solve the nonlinear kinetic equation of the “nonlocal approach” including Coulomb interaction of electrons

The detailed study of the radially dependent kinetics of the electrons in the space charge confined, cylindrically symmetric positive column plasma of low current dc discharges requires the solution of a complicated elliptic boundary value problem for the isotropic part of the electron velocity distribution function. To avoid this laborious and time consuming task, in the limit of weak energy dissipation in electron—atom collisions the elliptic kinetic equation has been replaced by the so-called “nonlocal approach”. In this approximation a much simpler, spatially integrated kinetic equation is finally deduced which has to be solved numerically. When increasing the discharge current the nonlinear Coulomb interaction between the electrons becomes more and more important and has, in addition to the electron—atom collisions, to be included in the kinetic treatment. The paper reports on basic aspects of the numerical solution method, developed for the nonlinear kinetic equation of the nonlocal approach when additionally including the electron—electron interaction, and on several applications of the method. In particular, the impact of increasing electron density and thus of the COULOMB interaction between the electrons on their kinetics in the column plasma is investigated and discussed in the frame of the nonlocal approach.