Low-pressure Dc Glow Discharge Research Articles

Electric discharge fluid modelling with the contribution of convective and drift energy effects

A fluid model has been used in this work to analyze the electric and energetic behavior of a low-pressure DC glow discharge in Ar chosen as a gas test. The governing equations are the first three moments of the Boltzmann transport equations under their complete form without using the classical-drift-diffusion approximation for the momentum transfer equation while the energy conservation equation involves both thermal and drift energies. In the framework of the local energy approximation, the basic data needed more particularly in the collision source terms for both momentum transfer and energy equations are determined from a multi term solution of Boltzmann equation. Due to the strong coupling with electric field obtained from Poisson equation and the high sheath gradients, the transport equations are numerically solved using a powerful Galerkin finite elements method. This model, after a validation from comparison with literature results, is then used to analyze the convective and drift energy effects on the electric discharge characteristics. Present results show a large influence of the convective term in comparison to the drift-diffusion approximation, mainly on the electric field and charged density profiles due to the antagonist effect induced by this term on the electron and ion motion which reinforces the charge space. Present results show also the discharge characteristic changes mainly in the sheath due to the drift energy consideration.

Experimental Study of the Stability of the Interface between a Liquid Electrolyte and the Glow Discharge Plasma

The stability of the interface between a liquid electrolyte and the plasma of a contracted low-pressure dc glow discharge in air is investigated by means of digital photography. Water solutions of potassium permanganate and copper sulfate were used as electrolytes. It is found that, in the case of potassium permanganate, the instability of the interface leads to ejection of the electrolyte into the plasma and extinction of the discharge. Discharge modes with different types of quasi-steady interface are observed for copper sulfate at different values of the discharge current: a smooth interface, a solitary wave perturbation, regular ripples, and a churning foamed turbulent mixing zone.

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.

Microprocessing by intense pulsed electron beam

Generation of intense electron beams by superposing two discharges, namely, a low-pressure DC glow discharge and a high-current pulsed discharge at pressures and voltages very similar to that of the pseudospark gap devices, has been studied. The small beam diameter, high peak current, and short pulse length are the characteristics of the electron beam generated using this technique. This technique does not require high vacuum facilities, and applications such as microprocessing, X-ray generation, and preionization for high-power lasers are feasible. The electron beam current is in the range of 10-30 A. Microprocessing as drilling tenths of microns diameter holes in different materials such as copper, titanium, and tantalum has been performed. The microstructure of the irradiated materials has been investigated by means of scanning electron microscope (SEM) and atomic force microscope (AFM). Currently, the possibility of processing a matrix of holes is being considered. In this paper, preliminary study regarding the interaction of intense short electron beams with different metallic targets is presented. The electron generator consists of a fast filamentary discharge in low-pressure gases, and the filling gases such as helium, argon, or nitrogen at approximately 0.1 torr pressure are used. Some of the important characteristics of the device for drilling and micromachining are that the targets can be easily changed, and the beam can be deflected by means of a magnetic field. After the initial device characterization, several holes at different experimental conditions were drilled onto the target materials, and the interaction of the beam with these different targets was investigated by means of a scanning electron microscope.

Study of the influence of the discharge tube material on the dc glow discharge in CO2 laser mixture

We report on investigations of the low-pressure DC glow discharge in CO2 laser mixture (6% CO2: 24% N2: 70% He) by means of optical emission spectroscopy and Langmuir probes measurements. The investigation of the main spectral systems has been performed in two discharge tubes of the same geometry but different material in order to study possible influences of the tube material on the discharge properties. One discharge tube has been made of Silica, the other one of Simax (composition of Simax is the same as Pyrex glass). Different pressure dependencies of CO spectral bands and the electric field strengths in these tubes have been observed. This result indicates distinctions in heterogeneous plasma-chemical interactions on the discharge tube walls.

The rotational spectrum of rhomboidal SiC3

Rhomboidal SiC3, a planar ring with C2v symmetry and a transannular C–C bond, was detected at centimeter wavelengths in a pulsed supersonic molecular beam with a Fourier transform microwave (FTM) spectrometer, and was subsequently observed in a low-pressure dc glow discharge with a free-space millimeter-wave absorption spectrometer. The rotational spectrum of SiC3 is characterized by large harmonic defects and large splitting of the K-type doublets. Lines in the centimeter-wave band were very strong, allowing the singly substituted isotopic species to be observed in natural abundance. Measurements of the normal and five isotopically substituted species with the FTM spectrometer provided conclusive evidence for the identification and yielded an experimental zero-point (r0) structure. Forty-six transitions between 11 and 286 GHz with Ka⩽6 were measured in the main isotopic species. Three rotational and nine centrifugal distortion constants in Watson’s A-reduced Hamiltonian reproduce the observed spectrum to within a few parts in 107 and allow the most intense transitions up to 300 GHz to be calculated with high accuracy. The spectroscopic constants confirm that SiC3 is a fairly rigid molecule: the inertial defect is comparable to those of well-known planar rings and the centrifugal distortion constants are comparable to molecules of similar size. The number of SiC3 molecules in our supersonic molecular beam in each gas pulse is at least 3×1011, so large that electronic transitions may be readily detectable by laser spectroscopy.

UV Radiation of Low Pressure XeCl* and KrCl* Glow Discharges

AbstractThe positive column of low pressure DC glow discharges in Xe/Cl2 and Kr/Cl2 gas mixtures has been investigated with respect to its UV radiation power and radiative efficiency for a wide range of parameters (total gas pressure: 1–30 mbar, current 10–30 mA, partial pressure of Cl2: 1–33%). Also the radial distribution of the particle number densities of XeCl* (B) and KrCl* (B) has been determined by absolutely calibrated emission spectroscopy. Optimum UV (190–350 nm) output and efficiency has been found in gas mixtures with a relative Cl2 partial pressure of 2–3% and total gas pressure of 12–18 mbar. At these parameters, the UV radiation power per column length is greater than 40 Watt/m with a radiative efficiency of 15–18%. These discharges could be used for UV induced photochemical processes.

Dissociative excitation of hydrogen in rf and dc glow discharges through H 2

The spectral and spatial profile of H β from the low pressure rf and dc glow discharges in hydrogen is studied in order to reveal the excitation mechanism of the fast excited H fragments. Measurements were performed both for the normal and abnormal dc glow discharges. Spatial distributions of the Balmer β radiation reflect the local plasma conditions in the discharge, especially the excitation efficiency which is used to determine the excitation kinetics in hydrogen discharges. Spectral H β profiles were measured and used to determine the kinetic energy of excited H atoms and to check which of the mechanisms describes best the results observed in our experiment. We have also calculated the number densities of vibrationally excited levels by solving a set of vibrational master equations for the conditions similar to those of our experiments, as excitation from the vibrationally excited ground-state hydrogen molecules may be used to explain the changes in the intermediate wing component of the line profile with the changing current.

Hydrogenated amorphous silicon films obtained by a low pressure dc glow discharge

Films of a-Si: H have been deposited by means of a dc hot cathode discharge of SiH4 with electrostatic confinement at a pressure as low as 0.4 Pa. The plasma used is quite quiescent as has been observed by means of reproducible Langmuir probe measurements. Substrates have been placed at different locations in between the electrodes, some of them facing the anode and the others facing the cathode. Films deposited on substrates facing the cathode present a granular, non-columnar, structure, an IR spectrum with only SiH absorption peaks, and a very low photoresponse. Films deposited on substrates facing the anode have a similar IR spectrum but are homogeneous, have lower hydrogen content, and present a high photoresponse. The optical absorption coefficientα shows in all samples theαnE=C(E−E0) x behaviour, but with exponentx=3 and notx=2 as is usually considered in a-Si∶H.

Vibrational excitation in H2 and D2 electric discharges

Spontaneous Raman scattering has been used to measure the effective vibrational temperature of the v=1 level as a function of input power in a self-sustaining low pressure dc glow discharge in pure H2 and D2 gases. A nonmonotonic dependence of vibrational temperature on input specific power, yielding Tv=1440–1960 °K in H2, has been measured downstream of the discharge. Results of numerical calculations modeling the discharge/afterglow are presented. The agreement between theory and experiment is good and suggest that the behavior observed is caused by the effect of the temperature dependent vibrational–translation energy transfer rate.