Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session HT3: Optical Diagnostics I |
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Chair: Marc Schaepkens, Momentive Performance Materials Room: Saratoga Hilton Ballroom 3 |
Tuesday, October 20, 2009 1:30PM - 2:00PM |
HT3.00001: Laser Thomson scattering diagnostics of low-temperature plasmas Invited Speaker: Laser Thomson scattering (LTS) is the light scattering by free electrons and one can derive electron density and energy distribution from the intensity and profile of a light scattering spectrum. To apply LTS technique to diagnostics of low-temperature plasmas, one needs to detect narrow ($\sim $a few nm) and extremely weak light scattering spectra against orders-of-magnitude stronger background stray scattering. This difficulty has been overcome by the development of a triple grating spectrograph [1], which produces a light scattering spectrum on its output focal plane with the stray component highly suppressed (10$^{-6})$ with the aid of an internal spatial filter. Imaging detection of the spectrum with a gated ICCD camera with photon-counting-level sensitivity enables one to carry out measurements within a reasonable time. The technique has been applied to a number of cases where a conventional Langmuir probe method is difficult to use, including measurements of EEDF near the plasma-dielectric interface in a surface wave plasma (where strong microwave field interferes with the probe), high spatial resolution measurements for atmospheric pressure microdischarge, etc. Other applications of the LTS measurement system could be negative ion density measurements (with the aid of laser photodetachment effect) and Raman scattering measurements, giving local gas temperature and local gas species concentration. To make reliable LTS measurements, one should be careful about electron production due to multiphoton ionization caused by strong laser field in the focal region. Direct measurements of multiphoton ionization yields for various gas species indicate that metastable rare-gas atoms are ionized with a high probability and even ground-state atoms and molecules are ionized with a probability exceeding the ionization degree of the plasma under study, depending on the gas species, plasma conditions, and laser focusing conditions, which should be controlled to minimize the laser perturbation.\\[4pt] [1] Kono and Nakatani, Rev. Sci. Instrum. 71 (2000) 2716. [Preview Abstract] |
Tuesday, October 20, 2009 2:00PM - 2:15PM |
HT3.00002: Two-dimensional mapping of electron densities and temperatures using laser-collisonal fluorescence Edward Barnat, Kraig Frederickson We discuss the application of the laser-collisonal induced fluorescence technique to produce two-dimensional maps of both electron densities and electron temperatures in a helium plasma. A collisonal-radiative model is used to describe the evolution of electronic states after laser excitation. We discuss generalizations to the time dependant results that are used to simplify data acquisition and analysis. Calibration of the predictions made by the model is achieved using an cw rf discharge that is periodically perturbed via a high voltage pulse. We then demonstrate the capability of the technique by producing images of electron density and temperature of the sheath region formed around a biased electrode. [Preview Abstract] |
Tuesday, October 20, 2009 2:15PM - 2:30PM |
HT3.00003: Absorption spectroscopy diagnostics of a dual-frequency capacitive dielectric etch tool using Ultraviolet Light-Emitting Diodes Jean-Paul Booth, Jerome Bredin Dual-frequency capacitively-coupled etch reactors using Ar/fluorocarbon/O2 mixtures are widely employed for etching of dielectric films for integrated circuit manufacture. CF2 radicals play an important role in the gas-phase and surface chemistry controlling etching and polymer deposition, and their density can be measured by UV absorption via the A-X band (230-270 nm). Previously Xe arc lamps have been used as the light source, but they are rather unstable, limiting the sensitivity of the technique, as well as being cumbersome and relatively expensive. We have successfully replaced the Xe arc with UV light-emitting diodes. The CF2 density was determined as a function of gas composition and power in a modified 2 + 27MHz commercial etch reactor operating in Ar/C4F8/O2. The CF2 density decreases rapidly as the O2/C4F8 ratio is increased, and increases with RF power at both frequencies, but is most affected by 27 MHz power. There is speculation that CF2 may play an important role in the creation and destruction of F- negative ions. However, we did not find any simple correlation between CF2 density and electro-negativity. [Preview Abstract] |
Tuesday, October 20, 2009 2:30PM - 3:00PM |
HT3.00004: Integration of PIC simulations and magnetic-sublevel atomic kinetics for plasma polarization spectroscopy Invited Speaker: Interactions of high-intensity ultrashort-duration laser pulses with matter often result in the creation of highly non-equilibrium plasmas. Electrons and other particles can be accelerated to high velocities and can be very directional. Studies of such anisotropic plasmas are important both from the fundamental research point of view and also because of their relevance to such problems as fast ignition. In this work we study some aspects pertaining to the spectroscopic modeling of anisotropic plasmas. We investigate the possible diagnostic value of polarized line emissions driven by energetic plasma particles. We build on our previous experience with magnetic sublevel atomic kinetics models for polarized line emissions [1,2]. In particular, we discuss the details of constructing more accurate sublevel models that incorporate results of PIC simulations [3] of laser-matter interactions and the formation of directed energetic particles. New analytic expressions for the calculations of sublevel collisional atomic rates are given and their performance evaluated. These techniques make the calculation of collisional rates more accurate and realistic than before. Finally, we discuss the degrees of polarization of selected spectral lines and their sensitivity to the details of the particle distributions. \\[4pt] [1] P. Hakel et al., Phys. Rev. A 76, 012716 (2007).\\[0pt] [2] P. Hakel et al., Phys. Rev. E 69, 056405 (2004).\\[0pt] [3] Y. Sentoku and A. J. Kemp, J. Comput. Phys. 227, 6846 (2008). [Preview Abstract] |
Tuesday, October 20, 2009 3:00PM - 3:15PM |
HT3.00005: Diagnostic based Modelling for Determination of Absolute Atomic Oxygen Densities in Cold Atmospheric Pressure Plasmas Stephan Reuter, Kari Niemi, Lucy M. Graham, Jochen Waskoenig, Timo Gans The present study introduces a novel diagnostic technique for the determination of absolute atomic oxygen densities in rf atmospheric pressure plasmas, which combines easy to apply optical emission spectroscopy (OES) with a relatively simple 1D numerical simulation. Atomic oxygen ground state densities are determined from the intensity ratio of the lambda = 750.4 nm argon and the lambda = 844 nm atomic oxygen line. The effective excitation rate coefficients k$^{\ast }_{e}$ of the upper Ar(2p$^{1})$ and O(3p$^{3}$P) states, adequately describing the time and space integrated optical emission measurements, are calculated on basis of the time and space averaged EEDF from the numerical simulation. The method is applied on a low temperature rf-driven atmospheric pressure plasma jet operated in helium with small admixtures of oxygen and argon. The results were confirmed by reliable independent two-photon laser-induced fluorescence measurements. [Preview Abstract] |
Tuesday, October 20, 2009 3:15PM - 3:30PM |
HT3.00006: Nonlinear Zeeman Spectroscopy of Nitric Oxide in Strong Magnetic Field Andrei Kotkov, Andrei Ionin, Yurii Klimachev, Andrei Kozlov In our experiments the Zeeman splitting of several rotational-vibrational spectral lines of nitric oxide in pulsed magnetic field up to 14~T was measured with frequency-tunable CO-laser. We developed computational model for calculation of Zeeman splitting in different orders of perturbation theory for nitric oxide molecules. We took into account nonlinear Zeeman splitting in third order of perturbation theory only when the comparison demonstrated a satisfactory agreement between the experimental and calculated data on time histories of measured and calculated absorption coefficients in pulsed magnetic field. Our study demonstrated that nonlinear Zeeman spectroscopy of nitric oxide could measure a strong magnetic field up to 15T in plasma. [Preview Abstract] |
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