Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session SF4: Microwave Glows |
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Chair: Antoine Rousseau, Ecole Polytechnique Room: 162 |
Friday, October 8, 2010 8:30AM - 9:00AM |
SF4.00001: Simulations of plasma formation and sustainment in an RLSA (radial-line slot antenna) microwave plasma source Invited Speaker: Microwave surface-wave plasma sources are promising alternatives to capacitively- or inductively-coupled sources. The source and wafer are decoupled so that the wafer may be independently biased without affecting the source. Also, they are known to produce relatively dense, quiescent, low-temperature plasmas near the wafer surface, minimizing wafer damage. Our device consists of an RLSA that transmits 2.45 GHz microwaves into a large quartz resonator disk that then couples to the plasma. Because of natural modes in the disk, regions (nodes) of enhanced electric fields exist at the plasma/dielectric interface. For sufficient input power the density at these nodes can rise to the cutoff density ($\omega =\omega _{pe})$. It is well known that resonances then occur in which the electromagnetic fields grow to large amplitudes and convert to electrostatic plasma waves, which in turn transfer the field energy into electron energy. We investigate these phenomena through simplified 2-D PIC (particle-in-cell) simulations using the Tech-X VORPAL code. The simulations include ionization using a Monte-Carlo model with an energy-dependent cross section. We specifically study the complex interaction of the plasma waves with electrons. We observe that the electrons are strongly accelerated into the underdense regions ($\omega \quad < \quad \omega _{pe})$ and raise the density there to the cutoff density as well through enhanced ionization. For sufficient power, the density is driven to an overdense state ($\omega \quad > \quad \omega _{pe})$ everywhere. We present plots of the $E$-field, density, electron phase-space, and EEDF's (electron energy distribution functions) throughout the resonance period. [Preview Abstract] |
Friday, October 8, 2010 9:00AM - 9:15AM |
SF4.00002: Distributed Discharge Limiter Studies for X-band High Power Microwaves John Scharer, David Holmquist, Matt Kirley, Brian Kupczyk, John Booske The design, fabrication, measurements and analysis of an X-band high power microwave (HPM) limiter are presented. The microwave discharge test chamber is an L-band rectangular waveguide with Lexan microwave windows. The chamber is illuminated by the output of an X-band waveguide pressed against the chamber window. The objective is to study conditions and configurations that enable rapid ($<$ 50 ns) discharge formation above a pre-set power density threshold. A 25 kW X-band magnetron (9.38 GHz) with a 0.8 microsec pulse width is used to produce the breakdown. Incident, reflected and transmitted microwave powers and optical emission intensity are measured to observe the discharge breakdown and extinction rates. Pressures used are in the 1 to 760 Torr range. Transmission, reflection and attenuation for Ar and Ne are examined. Modeling of experimental results and analyses are presented. [Preview Abstract] |
Friday, October 8, 2010 9:15AM - 9:30AM |
SF4.00003: Generation of Suprathermal Electrons in Argon Plasmas Sustained by Surface-Waves Luc Stafford, Olivier Boudreault, Stefano Mattei, Frederique Piche, Joelle Margot, Vincent Donnelly In plasmas produced by propagating electromagnetic surface-waves (SW), the spatially averaged plasma frequency $\omega _{p}$ is larger than the wave frequency $\omega $ and this ensures the condition for SW propagation. However, due to spatial plasma density inhomogeneity, local plasma resonances at which $\omega _{p}=\omega $ can occur over the radial density profile close to the discharge walls. This yields to large and sharp peaks of the SW electrical field and could perhaps play an important role on electron heating. We used trace-rare-gas-optical-emission-spectroscopy (TRG-OES) to measure the electron energy distribution function (EEDF) in a 50 mTorr Ar plasma sustained in a 8 mm quartz discharge tube. For $\omega $/2$\pi $=100 MHz, the EEDF was maxwellian with an electron temperature T$_{e}$=4eV. At higher excitation frequencies ($>$1 GHz), the EEDFs could be segmented into 3 temperatures, with Te-low$>$Te-high$<$Te-tail. Similar results were obtained across the discharge radius. However, deep in the expansion region near the end of the plasma column where $\omega _{p}<\omega $ (i.e. where the wave does no longer propagate), the EEDF became maxwellian. This suggests that local plasma resonances near the discharge wall boundaries play an important role on the observed fast electron generation in the main plasma region. [Preview Abstract] |
Friday, October 8, 2010 9:30AM - 9:45AM |
SF4.00004: Production of O$_{2}$(a$^{1}\Delta _{g})$ molecules in flowing Ar-O$_{2}$ surface-wave microwave discharges Kinga Kutasi, Vasco Guerra, Paulo Sa The possibility of development of an electric discharge oxygen-iodine laser stimulates nowadays many studies on the production of O$_{2}$(a) in discharges. The laser would operate on the electronic transition of the iodine atom at 1315~nm, the population of the upper laser state occurring in the reaction of O$_{2}$(a) with I. In order to achieve population inversion between the iodine upper and lower states and thus positive gain, the yield of O$_{2}$(a), [O$_{2}$(a)]/[O$_{2}$(X)], should be at least equal to the threshold yield, which at 400 K is 24{\%}. In this work we develop a discharge model to investigate the O$_{2}$(a) yield in an Ar-O$_{2}$ flowing surface-wave microwave discharge created in a 1~cm diameter tube and follow its evolution in the afterglow downstream the discharge. Our calculations show that the O$_{2}$(a) threshold yield necessary for positive gain can be achieved at 10~mbar, the yield increasing with pressure and reaching 50{\%} at 50~mbar. Furthermore, the yield increases with Ar addition into O$_{2}$, as a result of the enhancement of O$_{2}$ dissociation with the Ar content. We also show that the density of O atoms in the afterglow, which can quench the upper laser state, can be tuned to the desired levels. [Preview Abstract] |
Friday, October 8, 2010 9:45AM - 10:00AM |
SF4.00005: Surface Wave Discharges as Sources of ``Super Hot'' Hydrogen Atoms Elena Tatarova, Edgar Felizardo, Francisco Dias, Julio Henriques, Carlos Ferreira, Boris Gordiets Surface waves discharges are sustained by the electric field of a propagating surface wave, which creates its own propagation structure as it travels ``Super hot'' (with kinetic energy in the range 4 - 10 eV) and ``hot'' (kinetic energy $\sim $ 0.3 eV) hydrogen atoms were detected throughout the volume of surface wave (350 MHz, 500 MHz) generated H$_{2}$ plasma columns, at pressures p = 0.01 -- 0.2 mbar, from the analysis of the H$_{\beta}$, H$_{\gamma }$, H$_{\delta }$and H$_{\varepsilon }$ emission line profiles. These line profiles were found to evolve from single Gaussian to bi-Gaussian shapes towards the plasma column end. The energy of ``super hot'' atoms in the discharge operating at 350 MHz is higher than that at 500 MHz. Moreover, the measured profiles change significantly along the radius. The broader base expands towards the wall, indicating that H atoms are accelerated in that region. These results provide confidence to the hypothesis that ``super hot'' H atoms originate from charge acceleration in the radial dc space-charge field followed by recombination with electrons at the wall and subsequent emission of fast excited atoms back into the plasma. Population inversion between the levels 5$\to $4 and 6$\to $4 was detected from the measured relative intensities of transitions within the Balmer series. [Preview Abstract] |
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