Ahmed El-Habachi

High-pressure hollow cathode discharges

Reducing the diameter of the cathode hole in a plane anode - hollow cathode geometry to m has allowed us to generate direct current discharges in argon at atmospheric pressure. Up to pressure times cathode hole diameter (pD) values of approximately 5 Torr cm, and at sub-mA currents, glow discharges (predischarges) are observed with a shape which is determined by the vacuum electric field. In the same pD range, but at higher currents of up to approximately 4 mA, the discharges are of the hollow cathode discharge type. At pD values exceeding 5 Torr cm the predischarges turn into surface discharges along the mica spacer between the electrodes. At currents > 4 mA filamentary, pulsed discharges are observed. Qualitative information on the electron energy distribution in the microdischarges has been obtained by studying the VUV emission from ionized argon atoms and the argon excimer radiation at 130 nm. The results of the spectral measurements indicate the presence of a relatively large concentration of electrons with energies > 15 eV over the entire pressure range. The fact that the current - voltage characteristic of the microdischarges has a positive slope over much of the current range where excimer radiation is emitted indicates the possibility of forming arrays of these discharges and using them in flat panel excimer lamps.

Emission of excimer radiation from direct current, high-pressure hollow cathode discharges

A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathode discharge, has been found to be an intense source of xenon and argon excimer radiation peaking at wavelengths of 170 and 130 nm, respectively. In argon discharges with a 100 μm diam hollow cathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure range from 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders of magnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 V for argon and 400 V for xenon discharges, at current levels on the order of mA. The resistive current–voltage characteristics of the microdischarges indicate the possibility to form arrays for direct current, flat panel excimer lamps.

Microhollow cathode discharge excimer lamps

Microhollow cathode discharges are high-pressure, nonequilibrium gas discharges between a hollow cathode and a planar or hollow anode with electrode dimensions in the 100 μm range. The large concentration of high-energy electrons, in combination with the high-gas density favors excimer formation. Excimer emission was observed in xenon and argon, at wavelengths of 128 and 172 nm, respectively, and in argon fluoride and xenon chloride, at 193 and 308 nm. The radiant emittance of the excimer radiation was found to increase monotonically with pressure. However, due to the decrease in source size with pressure, the efficiency (ratio of excimer radiant power to input electrical power), has for xenon and argon fluoride a maximum at ∼400 Torr. The maximum efficiency is between 6% and 9% for xenon, and ∼2% for argon fluoride.

Electron density measurements in an atmospheric pressure air plasma by means of infrared heterodyne interferometry

An infrared heterodyne interferometer has been used to measure the spatial distribution of the electron density in direct current, atmospheric pressure discharges in air. Spatial resolution of the electron density in the high-pressure glow discharge with characteristic dimensions on the order of 100 µm required the use of a CO2 laser at a wavelength of 10.6 µm. For this wavelength and electron densities greater than 1011 cm-3 the index of refraction of the atmospheric air plasma is mainly determined by heavy particles rather than electrons. The electron contribution to the refractive index was separated from that of the heavy particles by taking the different relaxation times of the two particle species into account. With the discharge operated in a repetitive pulsed mode, the initial rapid change of the refractive index was assumed to be due to the increase in electron density, whereas the following slower rise is due to the decrease in gas density caused by gas heating. By reducing the time between pulses, direct current conditions were approached, and the electron density as well as the gas density, and gas temperature, respectively, were obtained through extrapolation. A computation inversion method was used to determine the radial distribution of the plasma parameters in the cylindrical discharge. For a direct-current filamentary discharge in air, at a current of 10 mA, the electron density was found to be 1013 cm-3 in the centre, decreasing to half of this value at a radial distance of 0.21 mm. Gaussian temperature profiles with σ = 1.1 mm and maximum values of 1000-2000 K in the centre were also obtained with, however, larger error margins than for electron densities.

Generation of intense excimer radiation from high-pressure hollow cathode discharges

By reducing the diameter of the cathode opening in a hollow cathode discharge geometry to values on the order of 100 μm, we were able to operate these discharges in noble gases in a direct current mode up to atmospheric pressure. High-pressure discharges in xenon were found to be strong sources of excimer radiation. Highest intensities at a wavelength of 172 nm were obtained at a pressure of 400 Torr. At this pressure, the vacuum ultraviolet (VUV) radiant power of a single discharge operating at a forward voltage of 220 V and currents exceeding 2 mA reaches values between 6% and 9% of the input electrical power. The possibility to form arrays of these discharges allows the generation of flat panel VUV lamps with radiant emittances exceeding 50 W/cm2.

Series operation of direct current xenon chloride excimer sources

Stable, direct current microhollow cathode discharges in mixtures of hydrochloric acid, hydrogen, xenon, and neon have been generated in a pressure range of 200–1150 Torr. The cathode hole diameter was 250 μm. Sustaining voltages range from 180 to 250 V at current levels of up to 5 mA. The discharges are strong sources of xenon chloride excimer emission at a wavelength of 308 nm. Internal efficiencies of approximately 3% have been reached at a pressure of 1050 Torr. The spectral radiant power at this pressure was measured as 5 mW/nm at 308 nm for a 3 mA discharge. By using a sandwich electrode configuration, consisting of five perforated, alternate layers of metal and dielectric, a tandem discharge—two discharges in series—could be generated. For an anode–cathode–anode configuration the excimer irradiance, recorded on the axis of the discharge, was twice as large as that of a single discharge. The extension of this basic tandem electrode structure to a multiple electrode configuration allows the generatio...

Parallel operation of microhollow cathode discharges

Summary form only given. Microhollow cathode discharges (MHCDs) are high pressure gas discharges between a cathode, which contains a circular opening and an arbitrarily shaped anode. The diameter of the cathode hole as well as the electrode gap is approximately 100 /spl mu/m. Operation on such a small spatial scale enables stable direct current glow discharge operation even at high pressure. Microhollow cathode discharges have been operated at atmospheric pressure in rare gases (e.g., argon, xenon), rare gas halogen mixtures (e.g., argon fluoride, xenon chloride) and in air. Stable dc high pressure glow discharge operation is of interest in lighting, plasma processing, and as plasma cathodes for air plasma ramparts. The required plasma size for these applications exceeds that of a single microhollow cathode discharge and therefore requires their arrangement in arrays. Parallel operation of up to sixteen micro discharges has been reported using distributed ballast. A simpler way to generate arrays is to operate the glow discharges in a range where the voltage current characteristic has a positive slope, e.g. In the abnormal glow region. This can be achieved by limiting the cathode surface to a small value such that even at low currents it is completely covered by the plasma. In order to reduce the cathode area, the cathode surface was covered with a dielectric, such that only the cylindrical surface area of the cathode opening was available as cathode.

Excimer emission from microhollow cathode discharges

Summary form only given. Microhollow cathode discharges (MHCDs) combine the possibility for direct current, high-pressure operation with non-equilibrium plasma conditions necessary for efficient excimer formation. When operated in rare gases (Xe, Ar, Ne) or rare gas halides (ArF, XeCl) these discharges were found to be intense sources of excimer radiation. Conversion efficiencies (from input electrical power to output optical power) of several percent were achieved. Although modeling results predict a monotonous increase of radiant power with pressure, in MHCDs it has a maximum at 400 Torr. The observed maximum of the radiant power at constant current was found to be due to the nonlinear reduction of the excimer source area with increasing pressure. The excimer source is located in the cathode opening only at high pressures and low currents. Otherwise, the source extends over the cathode surface outside of the hole. The emitting area decreases by a factor of four over the pressure range from 200 Torr to 760 Torr, whereas the radiant emittance increases monotonically with pressure up to 10 W/cm/sup 2/ at atmospheric pressure. For DC operation, the current was limited to 8 mA to avoid thermal damage.

High pressure hollow electrode discharges

Summary form only given. Reduction of the cathode hole diameter into the submillimeter range has allowed us to extend the pressure range for hollow electrode discharge operation to values on the order of 50 Torr. In recent experiments with cathode holes of 0.2 mm diameter we obtained stable glow discharge operation up to approximately 900 Torr in argon. The current-voltage (I-V) characteristics of these discharges (with currents ranging from the tens of /spl mu/A to ten mA) show three distinct discharge modes: at low current, a discharge with positive differential resistivity, followed by a range with strong increase in current and reduction in voltage, and, at high current, again a resistive discharge mode. The resistive characteristic of high pressure hollow electrode discharges over a large range of current allows us to generate arrays of these discharges for use as flat panel, direct current, excimer lamps.

Electron density measurements in pulsed atmospheric pressure air plasmas by IR heterodyne interferometry

Summary form only given. Microhollow cathode discharges have been shown to serve as plasma cathodes for atmospheric pressure air discharges. The high pressure discharges are operated dc at currents from 10 mA up to 30 mA and at average electric fields of 1.25 kV/cm. The electron density in the dc discharge was estimated to be in the range of 10/sup 12/ to 10/sup 13/ cm/sup -3/. In order to determine this value more accurately an interferometric technique has been used. Since sufficient spatial resolution requires the use of light sources with wavelengths on the order and preferable less than the characteristic dimension of the micro plasma (100 /spl mu/m) a CO/sub 2/ laser was used. At this wavelength (10.6 /spl mu/m) the index of refraction of atmospheric air plasmas with electron densities of 10/sup 13/ cm/sup -3/ is mainly determined by the heavy particles. In order to obtain information on the electron density, the discharge was operated in a pulsed repetitive mode with pulse duration varying between 100 ps and 50 ms.