Discharge Tube Radius Research Articles

Creation of stable dust structures in strong magnetic fields

When investigating the dusty plasmas in a magnetic field, there are a number of problems in creating stable dust structures. This is particularly evident in stratified discharge at fields above 1000 G. This report discusses the conditions of the experiment under which for the first time it was possible to create the stable dust structure in the glow discharge with fields up to 10 000 G. The areas of stability and instability of the dust trap have been detected depending on discharge parameters such as a gas sort, a pressure, a current and a radius of discharge tube. The conditions of instability are compared with the literary data on the properties of the glow discharge in plasma. The general recommendations for the creation of the stable dust structure are concluded.

Surface-wave-sustained plasma torch for water treatment

In this study the effects of water treatment by surface-wave-sustained plasma torch at 2.45 GHz are studied. Changes in two directions are obtained: (i) changes of the plasma characteristics during the interaction with the water; (ii) water physical and chemical characteristics modification as a result of the plasma treatment. In addition, deactivation of Gram positive and Gram negative bacteria in suspension are registered. A number of charged and excited particles from the plasma interact with the water. As a result the water chemical and physical characteristics such as the water conductivity, pH, H2O2 concentration are modified. It is observed that the effect depends on the treatment time, wave power, and volume of the treated liquid. At specific discharge conditions determined by the wave power, gas flow, discharge tube radius, thickness and permittivity, the surface-wave-sustained discharge (SWD) operating at atmospheric pressure in argon is strongly non-equilibrium with electron temperature Te much higher than the temperature of the heavy particles (gas temperature Tg). It has been observed that SWD argon plasma with Tg close to the room temperature is able to produce H2O2 in the water with high efficiency at short exposure times (less than 60 sec). The H2O2 decomposition is strongly dependant on the temperature thus the low operating gas temperature is crucial for the H2O2 production efficiency. After scaling up the device, the observed effects can be applied for the waste water treatment in different facilities. The innovation will be useful especially for the treatment of waters and materials for medical application.

Reduced electric field in the positive column of the glow discharge in argon

In this paper we report the measurements we performed of the reduced electric field strength E/p in the positive column in the range of the gas pressure and tube radius product of 0.01 < pR < 30 Torr·cm. We got good agreements with numerical models and experimental data of other authors. We also present two analytical models for the reduced electric field E/p. The first model deals with the ambipolar mode of the positive column of the dc discharge in noble gases. We consider the case of low discharge current values when one observes a balance between the rate of charged particle production due to direct ionization of gas molecules through electron impact and their escape to the discharge tube walls. Simple expressions for the reduced electric field E/p in the positive column in argon are obtained. The second model consists in considering the production and loss of charged particles and metastable atoms and obtaining a simple equation for the reduced electric field E/p depending on the discharge current density, gas pressure and tube radius. These models furnish a good description of experimental data in the whole range of pR values studied.

Analysis on the formation of a FRC by flux measurements and MHD simulations

The evolution of the separatrix and excluded flux radius was investigated in a mirror-less theta pinch with external magnetic and loop probes and by numerical simulation. In the numerical hybrid code the anomalous effect was taken as evolution of lower hybrid drift instability. At mid-plane of the coil the experimental and theoretical values of lift-off time, implosion time and equilibrium plasma radius were very close. The implosion time is 5 ± 1 cm/ps and the separatrix radius at mid-plane is around one half of the discharge tube radius. Within the experimental uncertainty the excluded flux radius has the same value along the coil, suggesting an elongated configuration with closed magnetic field lines.

Theoretical insight into Ar–O2 surface-wave microwave discharges

A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar–O2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surface-wave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar–O2 mixture at 0.5–3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density for surface-wave propagation, ne = 3.74 × 1011 cm−3.It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O2 to pure Ar, and their creation and destruction processes have been identified. The O2 dissociation degree increases with Ar addition into O2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, but decreases with the atomic surface recombination of O-atoms. The density of O− negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions.

A nonlocal model of spatially nonuniform positive column of DC discharge

A self-consistent kinetic model is given of the positive column of low-pressure low-current dc discharge in neon in a tube of circular cross section, which is nonuniform in both the radial and longitudinal directions. The model is based on the solution of kinetic equation for the electron distribution function in nonlocal approximation, of drift equation for ions, of diffusion equation for metastable ions, and of Poisson equation for electric field. Calculations are performed for the case where the longitudinal nonuniformity of discharge is caused by a jumpwise variation of the discharge tube radius.

Radial Plasma Dynamic in Sequential Pinches

Plasma dynamic and confinement characteristics were investigated with magnetic probes in a theta pinch operating with oscillatory current waveform and hydrogen gas at pressure between 45 and 150 mtorr. Current-sheath implosion was evident after the third half cycle until sixth half cycle when the external current has practically decayed. Each cycle starts with a trapped reversed magnetic field residual from the previous half cycle. Probe-signal fluctuations due to radial hydromagnetic oscillations were also observed. A modified snowplow model including an initial bias field and a flux-loss term gives a reasonable description of the experimental results for plasma radial dynamic and internal trapped field. Typical equilibrium-density profiles are of a hollow type with maximum density around one-third of the discharge-tube radius. Estimations from these profiles show small variation of temperature and density among half cycles in discharges at low pressure. At high-pressure regime, the temperature strongly drops in subsequent half cycles, while the density increases.

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.

Abatement of perfluorinated compounds using microwave plasmas at atmospheric pressure

Microwave plasmas sustained at atmospheric pressure, for instance by electromagnetic surface waves, can be efficiently used to abate greenhouse-effect gases such as perfluorinated compounds. As a working example, we study the destruction and removal efficiency (DRE) of SF6 at concentrations ranging from 0.1% to 2.4% of the total gas flow where N2, utilized as a purge gas, is the carrier gas. O2 is added to the mixture at a fixed ratio of 1.2–1.5 times the concentration of SF6 to ensure full oxidation of the SF6 fragments, providing thereby scrubbable by-products. Fourier-transform infrared spectroscopy has been utilized for identification of the by-products and quantification of the residual concentration of SF6. Optical emission spectroscopy was employed to determine the gas temperature of the nitrogen plasma. In terms of operating parameters, the DRE is found to increase with increasing microwave power and decrease with increasing gas flow rate and discharge tube radius. Increasing the microwave power, in the case of a surface-wave discharge, or decreasing the gas flow rate increases the residence time of the molecules to be processed, hence, the observed DRE increase. In contrast, increasing the tube radius or the gas-flow rate increases the degree of radial contraction of the discharge and, therefore, the plasma-free space close to the tube wall: this comparatively colder region favors the reformation of the fragmented SF6 molecules, and enlarging it lowers the destruction rate. DRE values higher than 95% have been achieved at a microwave power of 6 kW with 2.4% SF6 in N2 flow rates up to 30 standard l/min.

Self-consistent model of a positive column in a glow discharge under free-flight and collisional regimes of charged-particle motion.

We consider the nonlocal theory of a positive column in a glow discharge in two cases, where the mean free path of charged particles is either greater than the discharge tube radius (the free-flight regime) or much less than the radius (the collisional regime). The great bulk of electrons, which determines the density and the discharge current in the axial direction, appears to be trapped by the radial field of a positive column. The electron flux to the wall, which compensates for the ionization in a volume, is determined by fast electrons with energies of the order of wall potential, which are able to leave in a loss cone. The electron kinetic equation, which is solved by averaging it over the radial transits for the two regimes considered, permits us to obtain the electron density and the ionization rate. Thus, we develop the theory of a positive column for the non-Boltzmann electron distribution in the radial field. Under the free-flight regime, this theory is developed by analogy with the Langmuir-Tonks one. Under the collisional regime, the spatial distribution of the potential is obtained from the ion motion equation with the ambipolar diffusion coefficient, which depends on the radial coordinate. The concrete calculations are carried out for the xenon discharge under the free-flight and collisional regimes. The theoretical calculations are compared with the results of experiments on the measurements of the electric field and the densities of metastable and resonance xenon atoms.

A simplified hydrokinetic model for a steady-state microwave discharge sustained by traveling waves at atmospheric pressure conditions

The properties of a microwave-induced argon plasma produced by traveling surface wave at atmospheric pressure are investigated theoretically. A hydrokinetic model is elaborated to obtain the nonequilibrium one-dimensional profiles of electron temperature Te (average electron energy), electron density ne, and the first excited state population density n4s, along the axis of a steady-state discharge. A three-level atomic structure is assumed for the argon atom. A particle balance is included through the continuity equations for ne and n4s. These equations are coupled with an energy balance equation for the electrons. The effects of different parameters on the properties of the argon discharge are investigated: discharge tube radius, gas flow rate, resonant radiation-escape factor, and neutral gas temperature.

Ar-Hg放電における253.7nm光発光効率の管径依存性

The dependence of 253.7 nm emission efficiency on the discharge tube radius in argon-mercury-mixture gases is investigated using the simulation model for a positive column. The calculated waveforms of electric field, 253.7nm emission, electron energy and electron density show good agreement with the measured waveforms, respectively, for 50 Hz and 50 kHz operations. The obtained results are as follows: (1) The 253.7 nm emission efficiency takes a maximum with the tube radius under the constant discharge power and the constant mercury atom density condition, even though the dependence of emission efficiency on the discharge tube radius is rather weak except for the small radius region.(2) At the large radius region, the effective radiative decay-time increases with the radius. The density at radiative excitation level also decreases with the radius due to the decrement of electron energy and electron density. Thus, 253.7 nm emission decreases even if the discharge volume is increased.(3) At the small radius region, the increasing rate of the density at radiative excitation level begins to decrease with the radius, since the excited atoms at the triplet levels are excited to higher levels. Thus, 253.7 nm emission decreases with a decreasing discharge volume.

Experimental and theoretical study of the CF4 DC glow discharge positive column

An experimental and theoretical study of the positive column of the CF4 DC glow discharge is presented. It includes the experimental determination of the reduced electric field Ez /N and the electron impact excitation rate constant KAr of the line ( lambda =750.3 nm, epsilon th=13.5 eV) of the actinometric addition (Ar) in CF4 plasma for a wide range of parameters (pressure p=0.2-2.3 Torr, current density j=4-35 mA cm-2). All experimental values of the reduced electric field can be described within the limits of experimental error by the universal dependence Ez/N on r0N (r0=0.9 cm, the discharge tube radius). Also the experimental dependence of kAr on Ez/N has been obtained. These results have been used to test different theoretical models of the CF4 discharge positive column. Comparison of the experimental and calculated values of Ez /N and KAr has shown the validity of the theoretical model developed in this work. This model takes into account the large negative ion concentration in the positive column and predicts its division into regions of ion-ion plasma (in the centre) and electro-ion plasma (near the tube wall). The model also accounts for the effect of non-locality of the electron energy distribution function. It has been shown that this effect can be accounted for by solving the Boltzmann equation for the EEDF in the spatially non-uniform case. The incorrectness of the notion of ambipolar diffusion in the plasma of a strongly electronegative gas with high negative ion concentrations has been reaffirmed.

Computer simulation of microwave and DC plasmas: comparative characterisation of plasmas

Self-consistent calculations of the parameters of self-sustained MIP and DCP in argon and in model gases in the diffusion-controlled regime have been carried out. The calculated parameters of argon microwave produced plasmas (MIP) and direct current plasmas (DCP) are in satisfactory agreement with experimental data. The use of the proposed invariant group of parameters, namely specific absorbed power Pd, plasma gas pressure p, discharge tube radius R and tube wall temperature TW, for the comparisons of different types of electrical discharges have been grounded. The results of calculations in argon using different models of discharge and the results of calculations in model gas plasmas are given.

Ambipolar diffusion theory of the hot-cathode negative glow.

Ambipolar diffusion theory of the negative glow (NG) and Faraday dark space (FDS) of a cylindrical gas discharge maintained by thermionic emission from a hot cathod is discussed. Electrons are divided into two groups: thermionic beam electrons and plasma electrons. Radial diffusion losses are handled by introducing the usual diffusion length. Inputs to the model include probabilities of excitation and ionization by thermionic beam electrons and by plasma electrons, electron and ion temperatures and mobilities, and discharge tube radius

Radiation Efficiency and Kinetic Calculations for the Positive Column in Hg/Ar Mixtures II. Impact of Tube Radius and Current Density

In a preceding part of the paper, based on experimental methods and on a corresponding kinetic model, especially the impact of the effective life times of the 63P1 and 61P1 Hg resonance levels on the main properties of the low pressure Hg-–Ar positive column has been studied. In the presented second part these investigations, which have been performed under the aspect of their application in fluorescent lamps, will be continued. They are related to the effects of tube radius and discharge current density. New fluorescent lamps have reduced tube radii from R = 18 to 13 mm and compact lamps down to R = 5 mm. Thus it is of interest to study the electron distribution function and main macroscopic properties of such low pressure discharge plasmas, where the latter properties are obtained by adequate energy space averaging. At first, results are reported and discussed which are related to the dependence of main plasma parameters on the Hg partial pressure when reducing the discharge tube radius for constant discharge current i and argon pressure pAr. Starting from the electron distribution function the particle and energy budget will be studied in detail, especially of course the change of the ultraviolet radiation output from both Hg resonance levels 63P1 and 61P1 and, in addition, of visible radiation. Then, to understand the effect of the discharge current density, as a representative example two cases with different values of R, i and PAr but with equal current density have been investigated and discussed. Altogether the investigations made will shed light upon the complex relations in the mentioned mixture plasma and give hints to select suitable parameter values which can be useful to improve light sources.

Tube diameter and wave frequency limitations when using the electro magnetic surface wave in the m=1 (dipolar) mode to sustain a plasma column

An exhaustive experimental investigation of the conditions required to sustain a plasma column through the propagation of the m=1 mode surface wave has been conducted. It reveals that, given a discharge tube radius a, there corresponds a minimum frequency value fm below which the discharge cannot be achieved; conversely, for a given operating frequency f, the tube radius must exceed some minimum value am for the plasma to be sustained. These minimum conditions required to obtain the discharge are observed to obey a scaling law of the form (fa)m≂const., where the constant is independent of the gas nature and pressure. Theoretically, the dispersion equation of the m=1 mode wave shows no low-frequency cutoff. However, it is found that the specific dependence of the wave attenuation coefficient on the frequency and on the tube diameter can ultimately account for the observed limitations when the wave is used to sustain a plasma. A discharge stability criterion is proposed that recovers the observed scaling law determining the minimum tube radius and wave frequency values.

A collisional-radiative model applicable to argon discharges over a wide range of conditions. I. Formulation and basic data

A collisional-radiative model with an extended region of applicability is developed for an argon atom plasma. Atom-atom inelastic collisions and diffusion losses of the metastable states along with the electron-atom inelastic collisions and radiative processes are considered in this model, taking into account 65 effective levels. Among the analytical expressions used for the corresponding cross sections, special attention is paid to those determining the set of cross sections for excitation by electrons from the ground state, owing to the possibility of utilising the formulae recommended in kinetic modelling studies of discharges in argon or in mixtures including argon atoms. The numerical method developed makes it possible to investigate the mechanisms by which the excited levels are populated in a non-equilibrium argon plasma characterised (even in the case of a non-Maxwellian electron distribution) by a set of parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground state atom population n1, the discharge tube (or the plasma column) radius R and the optical escape factors Lambda mn and Lambda m, which are dependent only on the quantities Ta, n1 and R in many cases of practical interest.

Relaxation of metastable atoms during the afterglow in a copper vapor laser

A study was made of the spatial kinetics of the decay of metastable atoms, resolved over the radius of a copper vapor gas discharge tube, under conditions typical of the self-heating regime. The investigations were carried out for a time interval of 1–100μsec after the excitation pulse, with a radial resolution of 2–3 mm, in the entire lasing temperature range. A profile of the population of the metastable states having a minimum on the axis transformed into a profile with a maximum on the discharge axis during the interpulse time interval. An analysis was made of the possible processes during the afterglow of the discharge and estimates were obtained of the electron density and temperature in the laser plasma.

Theoretical description of discharge plasma and calculation of maximum output intensity of He-Ne waveguide lasers as a function of discharge tube radius

This paper deals with the properties of He-Ne waveguide lasers. An improved theory of the positive column is presented which assumes that at low currents, wall charges restrict the column to a narrow plasma channel and cause the electron temperature to rise. This theory is tested by comparison of measured and calculated current-voltage characteristics and shows good qualitative agreement. Taking into consideration the wall sheath at small currents, the output intensity is calculated. It turns out that an optimum tube radius, R \approx 0.1 mm, exists at which the output intensity has its maximum.