Session DT3: Microdischarges I

Physical properties of colliding microplasma bullets

Cold plasma jets produced by pulsed discharges have recently attracted attention because of their physical properties enabling the development of new applications, such as plasma medicine. ICCD pictures of these jets revealed that they were composed of plasma bullets propagating at high velocity. When two plasma bullets interact in two counter-propagating helium flows surrounded by ambient air, a secondary bullet is produced as shown in [1]. But, when the two bullets are produced within only one gas flow, a transient pink glow is observed, in between the two plasma bullets, a few nanoseconds after their complete collapse. The size and the location of this glow exactly correspond to the area free of previous bullet propagation. Based on time-resolved spectroscopic investigation, correlations between this transient glow and the well-know pink afterglow of nitrogen will be discussed. \\[4pt] [1] C. Douat, M. Fleury, M. Laroussi, and V. Puech, IEEE Trans. Plasma Sci. To be published 2011, DOI 10.1109/TPS.2011.2109740   

STUDENT AWARD FINALIST: Characteristics of Arrays of Independently Controlled RF Micro-Dielectric Barrier Discharges

Micro dielectric barrier discharges (mDBD's) are being developed for high pressure, non-thermal plasma sources. The micro-plasma devices (10-100 $\mu $m) of interest are RF-excited arrays where individually controlled apertures are used for charge extraction to treat or pattern surfaces. When using mDBDs to produce plumes of charged species, there are potential interactions between the mDBD devices. In this presentation, we discuss properties of atmospheric pressure mDBD's arrays using results from a 2D simulation. The devices consist of sandwich structures of dc and rf biased electrodes to help shape the plume. The model solves Poisson's equation, transport equations for charged and neutral species, the electron energy equation and Green's function propagator for radiation transport. A Monte Carlo simulation tracks sheath accelerated electrons. We find that the adjacency of the mDBDs and the dielectric properties of the materials being treated are important to operation. Charge extraction and the shape of the plume can be optimized by choice of gas composition and pressure. Scaling laws will be presented for mDBD arrays as a function of frequency and phasing of the arrays.   

Low-Pressure Microwave Excited Microplasmas as Sources of VUV Photons and Metastable Excited Atoms: Experimental Measurements

Microplasma discharges are typically operated at high pressure due to pd scaling. However, there are a number of potential applications for which lower pressure operation offers advantages. These applications, including spatially precise surface processing, treatment of soft materials, and chemical analysis, require energetic plasma products such as excited state species, VUV photons, or high-energy ions while taking advantage of the small size and high specific power a microdischarge offers. To this end, microwave-excited microplasma sources in rare gases operating at pressures of $<$ 10 Torr are being developed. The microplasmas are sustained in ceramic cavities having cross sectional dimensions of $\leq$ 1 mm, excited by a split-ring resonator antenna operated at 2.45 GHz at power levels of a few W. Experimental measurements, focused on the production of energetic plasma products, will be discussed. These will include emission spectroscopy, photodiode measurements of total VUV emission, diode laser absorption measurements of excited-state densities, and measurements of electron density through RF reflectometry. The effects of varying gas flow rate, composition, and RF power will be explored.   

Observation of Plasma Propagation in an Array of Microchannels

Discharge dynamics in an array of microscale channels fabricated in Al/Al$_{2}$O$_{3}$ has been investigated at atmospheric pressure in He and Ar. Microplasmas in channels having a width of 200-300 $\mu $m, a length up to 50 mm, and a volume of 1-15 mm$^{3}$ have been generated on a static or flowing gas basis. Fabricated by micromachining and wet chemical processes, these channels are situated in a dielectric barrier structure fabricated in 125-250 $\mu $m thick Al foil and having a buried electrode geometry. Spatiotemporally-resolved optical emission profiles, recorded with a gated CCD camera and a telescope, reveal a plasma propagation speed (and direction of excited emission along the microchannel) which varies with gas inlet pressure. For 1 atm of Ar, the velocity of the excitation wave is $\sim $15 km/s.   

A global model of the self-pulsing regime of micro-hollow cathode discharges

A global model of the self-pulsing regime of Micro-Hollow Cathode Discharges (MHCD's) working in argon gas is proposed. The power balance is done using an equivalent circuit model of the discharge that allows the current and voltage dynamics to be calculated. The fraction of the total power dissipated in the discharge that contributes to electron heating is deduced from a sheath model. The particle balance is first done in a very simplified reaction scheme involving only electrons, argon atomic ions and argon molecular ions. In a second step, the excited states are included in the particle balance equations. The models are compared to experiments and several conclusions are drawn. The model without excited states underestimates the electron density and does not capture well the trends in pressure. The model with the excited states is in better agreement which shows that multi-step ionization plays a significant role. The time-evolution of the electron density follows closely that of the discharge current but the excited states density presents two peaks: (i) the first at the early stage of the current peak due to direct excitation with high electron temperature, (ii) the second at the end of the current (and electron density) peak due to large production of excited states by electron-ion recombination at very low electron temperature.   

Low-Pressure Microwave Excited Microplasmas as Sources of VUV Photons and Metastable Excited Atoms: Modeling

Low pressure plasmas sustained in rare gases and rare gas mixtures can be efficient sources of VUV light from resonant optical transitions. Many applications would benefit from having small, inexpensive sources of plasma produced VUV light. To address this need, microwave wave excited microplasma sources in rare gases operating at pressures of $<$ 10 Torr are being developed. The microplasmas are sustained in ceramic cavities having cross sectional dimensions of $\le $ 1 mm, excited by a split-ring resonator antenna operated at 2.45 GHz. Power deposition is a few W. Hybrid computer modeling of microplasmas sustained in Ar has been performed to develop scaling laws for increasing the efficiency of VUV light production. The model includes a Monte Carlo simulation for the electron energy distribution and for radiation transport. Results from those studies will be discussed for plasma densities, electron energy distributions, VUV light production and excited state densities as a function of power, pressure and aspect ratio of the microplasma cavities. Modeling results will be compared to laser absorption spectroscopy of Ar excited state densities.   

Kinetics of metastable He atom at middle pressure in micro hollow cathode discharge

Hollow cathode discharges has been studied as light sources. Conventional hollow cathode discharges has been operated in low pressure, while micro hollow cathode discharge in near atmospheric pressure. In this study, the behavior of metastable He atoms (2$^{3}$S$_{1}$-2$^{3}$P$_{0}$ ; 1082.909 nm) in micro hollow cathode discharge in the middle range of the pressure from 5 to 30 kPa were fundamentally studied. The translational temperature and density of metastable He atoms were measured by diode laser absorption spectroscopy. The spectral line profile was analyzed and the pressure-broadening effect was investigated. The absorption profile of metastable He atoms was obtained by scanning the wavelength of the diode laser. The absorption profile of metastable He atoms shits from a Gaussian to Voigt profile at around pressure of 10 kPa. The metastable He atoms temperatures were estimated to be 900 to 1150 K, and the densities were measured to be 1.3 x 10$^{13}$ cm$^{-3}$ to 7.6 x 10$^{12}$ cm$^{-3}$ in the range of 5 to 30 kPa. The translational temperatures and densities of metastable He atoms were decreased with increasing pressure below 10 kPa, on the other hand, increased with increasing pressure in the range from 10 to 30 kPa.   

Reduction of pulsed voltage for generating atmospheric microdischarges by using a small-current DC pre-discharge

The reduction of pulsed voltage for generating atmospheric microdischarges with miniature helium gas flow was established by using a small-current DC pre-discharge. An electrode system was consisted of three electrodes: a nozzle one, a plate one with a hole of 0.6-mm in diameter, and a plain plate one. The DC glow pre-discharge, which was generated between the nozzle and plate with the hole electrodes, created electrons and radicals, which were provided on the helium gas flow to the gap between the plain plate and plate with the hole electrodes. These particles contributed to reduce the pulsed dielectric breakdown voltage, which was affected by the helium gas flow rate, the DC discharge current, and electrode polarities. The mode of pulsed microdischarges depended on the polarity of pulsed voltage significantly. Pulsed arc and glow microdischarges were obtained according to the positive and negative pulsed voltages, respectively. In addition to the glow pre-discharge, we discussed a corona pre-discharge, which was obtained by enlarging the hole-diameter to 1.2 mm. The consumed power of the corona pre-discharge was lower than that of the glow one.   

Influence of microwave electric field on H$_{\beta }$ broadening in atmospheric pressure microwave plasma

Atmospheric pressure plasmas have been given much attention because of its cost performance and various possibilities for industrial applications. Although Stark broadening of emission line is commonly used for the electron density measurement in atmospheric pressure plasmas, pulsed operation may influence the line width due to the applied electric field, especially at the early stage of the plasma ignition. In this study, temporal and spatial variations of H$_{\beta }$ spectra from an atmospheric pressure microwave plasma was measured using optical multi-channel analyzer with a gated CCD detector. From the time-resolved measurement, difference of the line widths between parallel and perpendicular polarizations of the H$_{\beta }$ emission was observed at the early stage of plasma ignition, suggesting the influence of applied electric field on the H$_{\beta }$ spectra. Spatial variation of H$_{\beta }$ spectra across the discharge gap was measured and increase of both the microwave electric field and the emission intensity was observed in the vicinity of electrodes.   

High efficacy plasma display utilizing opposite electrode discharge cell structure with long electrodes gap

Recently, applications of plasma display to the large public display and transparent display gain much attention. With this background, we report characteristics of opposite electrodes discharge cell with long electrode gap in comparison with conventional co-planar surface discharge. The cell size of test panel is 2950 $\mu$m $\times$ 840 $\mu$m, which corresponds to that of the display having diagonal size of 130'' with XGA resolution. Electrode gap of co-planar and opposite electrode structure are 240 $\mu$m and 500 $\mu$m respectively. These gap dimensions provide similar driving voltage windows. Experimental results show that opposite discharge provides approximately four fold higher luminous efficacy compared with that of the surface discharge. Resulting efficacy is found to be higher than 20 lm/W in green phosphor with 10 KHz continuous pulse operation. Spatio-temporally resolved, three-dimensional ICCD image using prism side wall reveal that high efficacy is correlated with positive column formation. Simulation study based on the drift diffusion approximation show that improvement on the electron heating efficiency and excitation efficiency related with low electron energy is responsible for high efficacy nature of the proposed cell structure.   

Diagnostics of Dielectric Barrier Microdischarges Using Laser Thomson Scattering

We have been developing a laser Thomson scattering technique to apply for dielectric barrier pulsed discharges. The light source of LTS is the second harmonics of a Nd:YAG laser with a energy of 8 mJ. Also a triple grating spectrometer which is equipped with a spatial filter for eliminating Rayleigh and stray lights is used to measure LTS spectra. The dielectric barrier discharge is generated in neon gas at around atmospheric pressure of 400 Torr by applying the bipolar pulses at a frequency of 50 Hz with a peak value of 3 kV. The electrode set in this experiment is consisted of a needle electrode and a hemispherical electrode with an inter-electrode gap of 0.5 mm. Teflon as a dielectric layer is coated on the hemispherical electrode with a thickness of 200 $\mu $m. In this experiment, the peak current of discharge was about 3 A and the total electric charge that flows through the discharge channel was estimated to be 20-30 nC. Finally, we applied LTS successfully as a diagnostics method in DBD. Temporal evolution of electron density and temperature could also be measured. It has been investigated that the peak values of electron density and electron temperature at the center of the pulsed filament discharge to be (1.0 \underline {+}0.1)$\times $10$^{22}$ m$^{-3}$ and 2.62 (\underline {+}0.2) eV.   

Vacuum Ultraviolet Emission from Magnetized Low-pressure Micro Plasma

High ionizing level micro plasma has been developed for the low-pressure and short gap length conditions by using resonantly confinement effect of electrons at the second harmonic ECR condition, $\omega _{ce}$/$\omega $=0.5. So, it is expected that the high-efficiency VUV source with less self-absorption of VUV emissions. In the present study, the VUV emissions from the low-pressure micro plasma source have been investigated by using VUV spectroscopic measurement. The maximum 147nm VUV peak intensity was obtained at the second harmonic ECR condition as same as the maximum electron density and minimum electron temperature. Furthermore, the 147nm emission intensity was increased with decreasing the operational gas pressure. The self absorption of 147nm VUV emission by Xe atom could be decreased with decreasing the operational gas pressure.