Positive Column Region Research Articles

Two-dimensional axisymmetric simulations and the heating effects associated with DC atmospheric pressure discharges during the post-streamer stage

Numerical results are presented for the development of the post-streamer discharge stage in atmospheric pressure air. The model used comprises Poisson, charged-particle continuity and Navier-Stokes equations developed in two-dimensional cylindrical axisymmetric co-ordinates. Applied direct current voltage of 20% above the breakdown threshold is applied in a 1 mm gap between two parallel plate electrodes. Starting from a single electron as the initial condition, the transitory regime from the streamer to the glow discharge is analysed, and the glow discharge is shown to consist of the cathode fall, negative glow, positive column and anode regions. The positive column is shown to propagate in the form of a return wave towards the anode. The very fast redistribution of the electric field just after the streamer hits the cathode is presented. Furthermore, the current density at the cathode fall and anode regions is shown to increase and extend radially outwards, justifying the inclusion of a two-dimensional axisymmetric model to study the radial effects in the discharge. Neutral gas heating starts to occur with the initiation of the post-streamer discharge stage, and the neutral gas temperature increases at the cathode by approximately 180 K.

Spectroscopic Study of Argon DC Glow Discharge

In this paper, emission spectra of both positive column (PC) and negative glow (NG) regions of the dc glow discharge have been measured at different pressures and currents. The intensity of the lines in the NG is in the order of five times of that corresponding lines' intensity in the PC region. It is found that the line intensity increases linearly with the discharge current, while it increases as Palpha with the gas pressure. The electron temperature Te has been estimated using the line-to-line-intensity-ratio technique. It is found that Te derived by this technique generally decreases with the pressure. Also, Te in NG region is about 3/2 of that in the PC

Improvement of luminous efficacy in plasma display panels by a counter-type electrode configuration with a large gap

The discharge characteristics of plasma display panel with coplanar and counter-type electrode configurations are compared using the numerical modeling and experiment with respect to real and macrocells, respectively. Numerical analysis shows that the ultraviolet (UV) efficiency and driving voltage of counter type at a gap distance of 230μm are located at similar levels to those of coplanar type at a gap distance of 60μm. The UV efficiency for counter type is enhanced with the rise of xenon fraction and gap distance, between which the large gap operation is more advantageous to high UV efficiency. The measured temporal evolution of infrared emission reveals that the cathode layer plays an important role in forming the discharge current after the gas breakdown. It is found from the time-averaged visible and infrared emissions for the counter type that as the gap distance becomes larger, the positive column region increases but the sheath regime remains almost unchanged. On the other hand, the variation of gap distance gives a little influence on the average discharge current at the same applied voltage. The UV efficiency is thus greatly improved with the gap distance. When the gap becomes double, the UV efficiency is improved by 75%, which is well agreed with the results predicted in the numerical modeling.

Characterization of a dc atmospheric pressure normal glow discharge

Atmospheric pressure dc glow discharges were generated between a thin cylindrical anode and a flat cathode. Voltage–current characteristics, visualization of the discharge and estimations of the current density indicate that the discharge is operating in the normal glow regime. Emission spectroscopy and gas temperature measurements using the 2nd positive band of N2 indicate that the discharge forms a non-equilibirum plasma. Rotational temperatures are 700 K and 1550 K and vibrational temperatures are 5000 K and 4500 K for a 0.4 mA and 10 mA discharge, respectively. The discharge was studied for inter-electrode gap spacing in the range of 20 µm–1.5 cm. It is possible to distinguish a negative glow, Faraday dark space and positive column regions of the discharge. The radius of the primary column is about 50 µm and is relatively constant with changes in electrode spacing and discharge current. Estimations show that this radial size is important in balancing heat generation and diffusion and in preventing thermal instabilities and the transition to an arc.

グロー放電トレーサー法による極超音速流中の境界層内速度分布測定

Spark-tracer technique using glow discharge is presented to measure velocity profiles across boundary layers around models in hypersonic flows. At first, a high voltage and high frequency discharge circuit and a discharge electrodes system used in the measurement are described. Next, to demonstrate the capability of the technique, the measurement of a velocity profile across a flat plate boundary layer is performed. The experimental result agrees well with the computational one based on the Navier-Stokes equations. Moreover, to clarify characteristics of the high frequency glow discharge generated in hypersonic flows, discharge gap voltage and current measurement is carried out and the follows are concluded. A positive column region is used as a tracer in the technique. Its characteristics are affected by retained ions in the column and also by the column length. Finally, from the fact that the experimental and numerical velocity profiles agree well, we may conclude that 1mJ of electric energy which is released in an impulsive discharge does not disturb the flow.

Electron distribution function in the positive column and anode region of the glow discharge in neon

The results of probe measurements of the electron distribution function (EDF) in the homogeneous and striated positive column and the anode region of the glow discharge in neon are presented in this work. A model of the formation of the EDF based on the concepts of nonlocal kinetics of electrons, which takes in to account the non-elastic energy balance of electrons, is presented in this report. It is shown that the proposed model permits to calculate the EDF in the investigated regions of the glow discharge.

極超音速流中における衝撃波可視化のための放電特性について

An impulsive glow discharge technique is proposed to visualize shock waves around models in short duration hypersonic facilities. To clarify the characteristics of the discharge generated across a shock wave in hypersonic flows, gap voltage and current measurements are carried out. It shows a constant voltage characteristic of a typical normal glow discharge, that is, when the gap current increases, the gap voltage is kept constant. A shock wave is visualized as a nonluminous layer in its uniform positive column region. The shock wave inclination angle generated around a flat plate agreed well with that predicted by oblique shock wave theory, namely, the glow discharge did not distort the shock shape. The ratio of the electric energy discharged into the flow to the total enthalpy is found to be at most 0.1.

PLASMAKIN: A chemical kinetics library for plasma physics modeling

PLASMAKIN is a software library to handle physical and chemical data used in plasma physics modeling and to compute kinetics data from the reactions taking place in the gas or at the surfaces — particle generation and loss rates, photon emission rates and energy exchange rates. PLASMAKIN supports any number of chemical species and reactions, is independent of problem dimensions and can be used both in stationary and time-dependent problems. PLASMAKIN supports a large number of species properties and reaction types. Namely gas or electron temperature dependent collision rate coefficients, vibrational and cascade levels, evaluation of branching ratios, superelastic and other reverse processes, three-body collisions, radiation imprisonment and photoelectric emission. Non-standard rate coefficient functions can be handled by a user supplied routine. The interaction of the user with the library is limited to the preparation of an input file characterizing the interacting species and reactions, accessing the data or the chemical kinetics results through a reduced number of procedure calls with similar interfaces. PLASMAKIN allows the user to concentrate on the physical nature of the problems independently of the gas mixture properties and reactions model. As an example a program has been prepared to evaluate the maintenance field, equilibrium populations, relative contribution of each reaction, photon emission spectra and the energy losses rates on the positive column region of a discharge in electropositive gases. The program is applied to a discharge in neon. The library is written in Fortran 95 and has error support.

Improving the luminance and luminous efficacy of miniature fluorescent lamps for liquid crystal display backlighting by using a double frequency drive

A new method of driving miniature fluorescent tubes for liquid crystal display backlighting, with diameters as small as 2 mm, has been devised with the objective of improving the luminance and luminous efficacies of such lamps. The driver voltage wave form consists of high frequency oscillations at the leading and trailing edges of conventional square-wave form voltage pulses of relatively low frequency. The wave form is produced simply by inserting coils between low frequency square-wave pulse generators and the lamp electrodes. A lamp driven in this way operates in a different discharge mode from the conventional glow discharge obtained in the absence of the coils. The twofold intensity increase of visible emission from the positive column region indicates that the vacuum ultraviolet emission increases, enhancing the excitation of the phosphor in the lamp. There is evidence that the luminous efficacy is also increased by a factor of 2 as a result of more efficient use of the input energy. The minimum voltage needed to maintain a discharge is reduced.

Novel plasma polymerization system using a d.c. glow discharge

Plasma polymerization using a direct current (d.c.) glow discharge was carried out using several organic compounds. We have developed a plasma-polymerization instrument capable of using not only gaseous monomers but also liquids and solids as the starting materials. The characteristics of films prepared by d.c. glow discharge were examined by Fourier transform infrared spectroscopy and Raman spectroscopies, electron spectroscopy for chemical analysis, and contact angle measurements. The chemical structures of the deposited films were greatly influenced by the applied d.c. voltages during the discharge. Hydrogen-free carbon films were prepared from plasma polymerization of both naphthalene and benzene; the structure of these films, evaluated by Raman spectra, were quite different. Comparison of films deposited in different regions of the plasma, such as in the negative glow-phase region and in the positive column region, were performed. From these experiments similar film compositions were obtained.

Two-dimensional, hybrid model of low-pressure glow discharges.

A self-consistent, two-dimensional hybrid fluid-particle model is presented and used to describe the electrical behavior of dc low-pressure discharges in the current range ${10}^{\mathrm{\ensuremath{-}}7}$--${10}^{\mathrm{\ensuremath{-}}2}$ A in argon for products of the gas pressure and the gap spacing (pd) from 1 to 3.3 cm torr. The two-dimensional distributions of the potential, charged particle densities and ionization source term at steady state are shown to illustrate the discharge behavior during the transition from the normal to the abnormal regimes. For the larger values of pd, a positive column region as well as the cathode regions are clearly apparent. The model used here consists of Poisson's equation for the electric field coupled to the continuity equations for the electrons and ions with the important feature that the ionization source term appearing in the continuity equations is nonlocal and determined from a Monte Carlo simulation. This description yields a unified physical picture of discharge behavior in the cathode fall, negative glow, and positive column regions over a wide range of discharge currents.

Propagation of a de-excitation wavefront in the positive column of a pulsed Xe discharge

When a fast rise pulse voltage is applied to a low pressure, 2-10 mm diameter Xe discharge tube, emission covers the entire positive column region with the breakdown. Within a microsecond, however, the emission intensity starts to decrease from the cathode end of the column and the dark region spreads towards the anode. The speed of this 'de-excitation wavefront' varies from 0.5*103 to 2.5*103 m s-1, depending on the applied voltage, and also on the capacitance which is in parallel with the cathode resistor. The propagation of the de-excitation wave is considered to be due to the migration of excess electrons from the cathode towards the anode, followed by the reduction of impedance, and hence the electric field, of the positive column. In a 1 mm diameter tube, however, the de-excitation wave is not observed, and the emission extinguishes simultaneously over the entire positive column 0.4 mu s after the breakdown. The difference between the two extinction modes is attributed to the discharge current density. It is also shown that, in the positive column, excited species which emit vacuum ultraviolet photons are generated mainly during the initiation stage of the breakdown period. The VUV radiation efficiency can therefore be increased by narrowing the width of the pulse voltage.

Electron-concentration measurement in the anode region of a glow discharge at atmospheric pressure in flowing helium

The authors measured the electron concentr tions in anode layers at atmospheric pressure in flowing helium from the broadening of the H /SUB beta/ hydrogen line, which gives the highest accuracy on account of Stark-effect broadening. The spectra were photographed on RF-3 film with a DFS-8 spectrograph with a linear dispersion of 0.623 nm/mm. The H /SUB beta/ line was photometered with an IFO-451 microphotometer along the transverse coordinate with 100-fold magnification intervals of 0.015. Measurements on a helium plasma with amalogous parameters have shown that the translational temperature as measured from the rotational structure of molecular CN spectra is close to the population temperature for helium levels as determined from the relative intensities of He lines. In conclusion, the considerable changes in electron distribution on contraction do not have any appreciable effect on the parameters of the positive column and cathode region. Consequently, the contraction may be due to an instability occurring in the anode region and producing the observed falling branch of the voltage-current characteristic

Dynamics of magnetically stabilized laser discharge

The dynamics of a rotating plasma in a transverse electrode geometry appropriate to high power CO2 laser discharge pumping is examined. The magneto-gas-dynamic fluid model used reveals that the spatially nonuniform electric and magnetic fields within the discharge cause the plasma and neutral gas rotational velocities to be strongly sheared within the cathode fall and the positive column regions. This sheared rotation serves to both stabilize the discharge against glow to arc transitions and to drive a centrifugal neutral gas flow within the volume. The concomitant bulk gas cooling effect so produced is particularly beneficial in a high powered CO2 electrical discharge laser as it inhibits thermal bottle necking. An order of magnitude increase in discharge power loading has been achieved utilizing a simple ’’magnetic electrode’’ discharge configuration.

Negative glow and positive column discharges for helium/metal-vapour lasers-a comparison

The comparative abilities of the negative glow and positive column regions of one and the same helium/zinc discharge to tolerate an increasing density of the metal vapour have been investigated. Spatially resolved profiles of sidelight emission from particular upper laser levels of Zn II were recorded over a wide range of discharge conditions. Evidence that the helium ion production rate can be maintained in the negative glow at pressures of zinc vapour as high as 0.05-0.5 Torr is the undiminished intensity of Zn II levels known to be populated by heavy-body collision processes. The same transitions, excited in the positive column, show a rapid decline in their intensity at zinc vapour pressures above 0.005 Torr. The difference in behaviour is ascribed to the existence, independent of metal vapour density, of a significant tail of high energy electrons in the negative glow electron energy distribution. The factors which determine the optimum partial pressure in the two cases are compared.

The Mutual Interaction of Plasma Electrons

This paper considers some of the processes taking place in a simplified model of the plasma or positive column region of an electrical discharge through gas at low pressure. Particular attention is given the direct mutual interaction of the electrons, and the effect of this on the distribution of velocities.The first part deals with the question in a general way. To determine the velocity distribution function, two balance equations are set up. One expresses the conservation of energy, and the second the conservation of momentum parallel to the field. From these is derived one non‐linear integral equation.The geometry of the mutual scattering process is examined, and the necessary integrations are performed insofar as these depend only on this geometry. As an example the case of elastic sphere collisions is carried through to a point where integrations over the distribution function, f, become necessary.In the second part the cross section function σ is derived for electrone‐lectron scattering. As an interaction potential, the shielded Coulomb value was used.In the third part an attempt is made to find functions which are solutions of the problem. Under the assumptions used, it is found that the function urn:x-wiley:00971421:media:sapm1939181174:sapm1939181174-math-0002 is a fairly good approximate solution.It is found that experimental conditions can exist for which the theory predicts a value of the parameter g nearly unity. This corresponds to a nearly maxwellian distribution.