Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session SF3: Optical Diagnostics II |
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Chair: David Smith, General Electric Room: 305 AB |
Friday, October 16, 2015 8:00AM - 8:30AM |
SF3.00001: Challenges in collisional-radiative modelling for low-temperature plasmas: EEDF, species density profiles, and collisional cross section data Invited Speaker: Ximing Zhu Collisional-radiative (CR) models in low-temperature plasmas are a widely investigated topic. These models can predict the metastable density, radical density, and the VUV photon flux from resonance states. Also, they can relate the emission line-ratios from excited species to the plasma parameters (e.g. electron density and temperature) when using optical emission spectroscopy. Although the CR models for low-temperature plasmas have been developed for several decades, they still face several challenges: (a) a Maxwellian EEDF is assumed in many models for simplicity but a large error can be introduced under more typical conditions with non-Maxwellian EEDFs; (b) homogenous density profiles for excited species are often used, though bounded plasmas are generally inhomogeneous; (c) the collisional cross section data for these models may have too large uncertainties. In this work, some recent progress in the research of CR models is reviewed, which attempts to overcome these challenges. A CR model with a variable EEDF profile (possibly Maxwellian or non-Maxwellian) has been developed. With this model the OES line-ratio method can obtain EEDFs in good agreement with those by Langmuir probe in low-pressure inductive/capacitive discharges. Further, a self-consistent CR model that additionally includes the radiation transfer equations is built. In this way, the assumption of a homogeneous density profile of excited species is avoided. The model can predict the actual density profile. At last, we discuss the recent works on measurements of collisional cross section using plasmas and lasers. In particular, we propose a novel approach, i.e. the pulsed laser induced photoelectron beam method, which in principle overcomes several limitations in the previous measurements. Based on these efforts, a new generation of CR models with a more accurate description of EEDF, species densities, and collisional process rates is supposed to come out in the future. [Preview Abstract] |
Friday, October 16, 2015 8:30AM - 8:45AM |
SF3.00002: Heavy particle temperatures in low pressure N$_{2}$ / H$_{2}$ discharges determined via OES Stefan Briefi, David Rauner, Ursel Fantz A widespread method for obtaining the gas temperature in low pressure discharges is the determination of the rotational population of a molecule via measuring its emission spectra. This can be done either directly if the particular emission lines of the emission band can be resolved or indirectly via simulating an emission band and adjusting the simulation to the measurement if the lines cannot be resolved. The first method is usually applied to the Fulcher transition of hydrogen (d $^{3}\Pi_{u}\to $ a $^{3}\Sigma_{g}^{+}$, located between 590 and 650 nm); a prominent example of the second method is the second positive system of nitrogen (C $^{3}\Pi_{u}\to $ B $^{3}\Pi_{g}$, located between 280 and 450 nm). In order to compare the results obtained by these methods, spectroscopic high resolution measurements have been carried out in H$_{2}$ / N$_{2}$ discharges in a pressure range of a few Pa. The experimental setup is an ICP with a cylindrical discharge vessel (length 40 cm, diameter 10 cm, made out of quartz glass). Besides the determination of rotational and vibrational temperatures of the nitrogen and hydrogen molecule the emission line profile of the Balmer lines is analyzed to obtain the temperature of atomic hydrogen. [Preview Abstract] |
Friday, October 16, 2015 8:45AM - 9:00AM |
SF3.00003: Experiments and numerical simulations to estimate the accuracy of probe assisted laser photo-detachment for negative ion density and temperature measurements Nishant Sirse, Noureddine Oudini, Bert Ellingboe Pulsed laser photo-detachment is the most commonly used technique to measure negative ion density and temperature in electronegative plasmas. The technique is based on measuring the excess electron current produced by the photo-detachment of negative ions. It is considered that the negative ion density is proportional to a rise in electron current following laser pulse, whereas, the temperature of negative ions is estimated based on the recovery of electron current to its value prior to the photo-detachment. During the photo-detachment process it is assumed that the background plasma remains unchanged. However, an electrostatic potential barrier is formed between the laser column (electropositive column) and the surrounding electronegative plasma in order to prevent the outward flow of electrons from the electropositive plasma column. The strength of the potential barrier depends on the various parameters such as electronegative ($\alpha =$n$_{-}$/n$_{\mathrm{e}})$, laser wavelength etc. Neglecting potential barrier leads to an erroneous estimation of negative ion density and temperature. In the present work we have investigated the above effects by using computer simulation which is further verified by experiments in an inductively coupled oxygen plasma. [Preview Abstract] |
Friday, October 16, 2015 9:00AM - 9:15AM |
SF3.00004: Electric Field Measurements in AC Double Dielectric Barrier Discharge Overlapped With Ns Pulse Discharge Benjamin Goldberg, Ivan Shkurenkov, Igor Adamovich, Walter Lempert Time-resolved electric field measurements by picosecond CARS / 4-wave mixing are carried out in a double dielectric barrier discharge in H2 between two plane electrodes covered by quartz plates and separated by a 3 mm gap, at a pressure of 300 Torr. The discharge is sustained by an AC voltage waveform (amplitude 4 kV, frequency 500 Hz), overlapped with nanosecond pulses (peak voltage 9 kV, pulse FWHM 100 ns), generated when the AC voltage is zero and operated at twice the frequency. Time and spatial resolution of electric field measurements are 10 $\mu$s and 1 cm, respectively. Absolute calibration of the diagnostics is done using a sub-breakdown AC sine wave. Measurements taken in the AC discharge without ns pulses show that electric field remains nearly constant during the entire AC discharge period. Adding ns pulses to the AC waveform results in large-volume breakdown generated in the entire electrode gap every half-period, with a well reproduced time delay after each pulse. Each of these ``regular'' AC breakdowns results in significant electric field reduction in the entire discharge volume. Basically, diffuse plasma produced by ns pulses neutralizes surface charge accumulated during the AC discharge and generates nearly uniform volume ionization, which results in subsequent large-volume breakdown when the AC voltage is applied. The results show that combining the AC waveform with ns pulses transforms the AC discharge from a superposition of random, small scale micro-discharges to regular, large volume discharges. [Preview Abstract] |
Friday, October 16, 2015 9:15AM - 9:30AM |
SF3.00005: Two-dimensional Measurement of N$_{2}$ Rotational Temperature Distribution in Atmospheric Positive DC Glow Discharge using Spectroscopic Imaging Takao Matsumoto, Ryo Sasamoto, Hideaki Orii, Yasuji Izawa, Kiyoto Nishijima An experimental method of determining a two-dimensional image of the N$_{2}$ rotational temperature in stationary atmospheric non-thermal plasma by spectroscopic imaging was presented. In the experiment, a steady-state glow corona discharge was generated by applying a positive DC voltage to a rod-plane electrode in synthetic air. Spectral images of a DC glow discharge were taken using a gated ICCD camera with ultra narrow band-pass filters, corresponding to the head and tail of a N$_{2}$ second positive system band (0-2). The qualitative N$_{2}$ rotational temperature was obtained from the emission intensity ratio between the head and tail of the N$_{2}$ second positive system band (0-2). The directions of observation were toward the lateral side and hemisphere sides of the rod electrode. The change in the distribution of rotational temperature in a positive DC glow discharge due to the amplitude of applied voltage was investigated. As a result, it was confirmed the rotational temperature and its distribution in a positive DC glow corona respectively increased and spread diffusely with increasing applied voltage. In particular, a distinctly high temperature region was formed in positive DC glow corona near the tip of the rod electrode just below the sparkover voltage. [Preview Abstract] |
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