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 UF3: Nonequilibrium Kinetics of Low-Temperature Plasmas |
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Chair: Julian Schulze, West Virginia University Room: 305 AB |
Friday, October 16, 2015 1:30PM - 1:45PM |
UF3.00001: Discussion on Electron Temperature of Low-Pressure Discharge Oxygen Plasma with Non-Maxwellian EEDF Based on Statistical Physics Hiroshi Akatsuka, Yoshinori Tanaka We reconsider electron temperature of non-equilibrium oxygen plasmas based on thermodynamics and statistical physics by the relationship between entropy and mean energy of electron gas. First, we solve the Boltzmann equation to obtain electron energy distribution function (EEDF) $F(\epsilon)$ of the oxygen plasma for a given reduced electric field $E/N$. We also simultaneously solve kinetic equations to determine some essential excited species in the oxygen plasma, since the EEDF should be self-consistently solved with the densities of collision partners. Next, we calculate the electron mean electron energy $U=\langle\epsilon\rangle=\int_0^\infty \epsilon F(\epsilon) \mathrm{d}\epsilon$ and entropy $S=-k\int_0^\infty F(\epsilon)\ln [F(\epsilon)] \mathrm{d}\epsilon$ for each value of the reduced electric field $E/N$. Then, we can obtain the electron temperature calculated as $T_{\mathrm{e}}^{\mathrm{th}} =[\partial S/\partial U]^{-1}$. After that, we discuss the difference between $T_{\mathrm{e}}^{\mathrm{th}}$ and the kinetic temperature $T_{\mathrm{e}}^{\mathrm{k}} \equiv (2/3) \langle\epsilon\rangle$, as well as the temperature given as a slope of the calculated EEDF for each value of $E/N$ from the viewpoint of statistical physics as well as elementary processes. [Preview Abstract] |
Friday, October 16, 2015 1:45PM - 2:00PM |
UF3.00002: Influences of electron-electron and metastable collisions in electrical breakdown of air John Lowke, Eugene Tam, Anthony Murphy To predict the time development of electrical breakdown in air accurately, detailed knowledge is required of the dominant ionization processes that occur between initial ionization, which requires an electric field of $\sim$ 25 kV/cm at 1 bar, and the final arc stage, which can be maintained by electric fields of only $\sim$ 20 V/cm. This paper discusses two collision processes which increase ionization by changing the energy distribution function of the electrons. At 5 kV/cm at 1 bar, there are effectively zero electrons at high enough energies to produce ionization, so the ionization coefficient is zero, largely due to the large energy losses of electrons exciting the many vibrational states of nitrogen. This situation is markedly changed by collision processes that make the energy distribution more Maxwellian, introducing more electrons at ionization energies. One such process is electron-electron collisions, which can dominate for high degrees of ionization of the air. It is shown that this process can increase streamer speeds and lengths about a factor of two. A second process is the presence of large populations of metastable states, for example of nitrogen vibrational states, which can increase electron energies by electrons de-exciting these states. But the electric field and discharge times need to be large enough to allow for the development of sufficient population densities of these states. The discharge development times appear to be not large enough to explain lightning initiation at low fields. [Preview Abstract] |
Friday, October 16, 2015 2:00PM - 2:15PM |
UF3.00003: Modelling the influence of neutral gas heating mechanisms on particle densities in inductively coupled chlorine discharges Andrew Gibson, Timo Gans, Mickael Foucher, Daniil Marinov, Pascal Chabert, Vasco Guerra, Mark Kushner, Jean-Paul Booth Inductively coupled plasmas produced in reactive electronegative gases, such as chlorine (Cl2), are commonly used for the etching of nanometre scale structures in the semiconductor industry. However, despite their widespread usage, the dominant energy transport mechanisms in these systems are often not well known. In particular, neutral gas heating is an important factor in determining many plasma parameters, such as the densities of electrons or atomic chlorine radicals (Cl). In this context the dissipation of electron energy by collisions with neutral species, for example through Franck-Condon heating or indirectly by vibrational excitation followed by v-t transfer, is of key importance. In this study the influence of such heating mechanisms on important species densities has been investigated using the Hybrid Plasma Equipment Model (HPEM). By comparison with experimental data, it is found that the electron density can be underestimated and its radial profile poorly reproduced if the proper heat transfer mechanisms are not included in the model. This in turn has important effects on other plasma parameters, such as the charged and neutral particle densities. The inclusion of both Franck-Condon heating and v-t transfer significantly improves agreement with experimental data. [Preview Abstract] |
Friday, October 16, 2015 2:15PM - 2:30PM |
UF3.00004: Controlling the Electron Energy Distribution Function Using a Biased Electrode Scott Baalrud, Benjamin Yee, Matthew M. Hopkins, Edward V. Barnat Positively biased electrodes inserted into plasmas influence the EEDF by providing a sink for low energy electrons that would otherwise be trapped by ion sheaths at the chamber walls. In hot filament generated discharges, the EEDF is nominally characterized by a cool trapped population at energies below the sheath energy and a comparatively warm tail population associated with the filament primaries. Inserting a positively biased electrode has little influence if it is so small that it collects a negligible fraction of the total electron current exiting the plasma. However, as the electrode area approaches $\sqrt{2.3m_e/m_i}A_w$, where $A_w$ is the chamber wall area, it collects most of the electrons leaving the plasma. This drastically reduces the density of the otherwise trapped population, and causes the electron temperature to increase as the distribution approaches a temperature associated with the energetic filament primaries. A global model is developed based on current and power balance, which shows the interconnected nature of the electron temperature, density and the plasma potential. This model is compared with Langmuir probe measurements in a dc filament generated plasma [1], and with 2D PIC simulations.\\[4pt] [1] Barnat, Laity and Baalrud, Phys. Plasmas 21, 103512 (2014). [Preview Abstract] |
Friday, October 16, 2015 2:30PM - 2:45PM |
UF3.00005: Global model of oxygen plasmas: A benchmark study and the role of the vibrational quanta of O2 Efe Kemaneci, Jan van Dijk, Thomas Mussenbrock, Ralf Brinkmann Oxygen plasmas are investigated based on a global modelling approach with a focus on the inductive radio-frequency discharges in both continuous and pulse-modulated modes. A throughout benchmark study is performed mainly with respect to the experimental data available in literature. The experimental data is preferred to cover a wide range of energy coupling modes: (asymmetric) capacitive, inductive as well as microwave modes; and an agreement is obtained in both continuous and pulse-modulated power inputs. In a benchmark case of a microwave-induced reactor plasma, a spatially-resolved plasma fluid model is also developed that is self-consistently coupled to the microwave propagation and the data is compared with the results of the corresponding global model simulation. The role of the vibrational quantum levels of molecular oxygen is analysed, where a set of chemical kinetics is proposed. The chemical kinetics includes 41 vibrational quanta as well as the e-V, V-V and V-T interactions. A ladder-like dissociation mechanism is also incorporated, where the highest vibrational quanta are set to be a pseudo-level and it is assumed to dissociate immediately. [Preview Abstract] |
Friday, October 16, 2015 2:45PM - 3:00PM |
UF3.00006: Revisiting Pierce Instability: Bandwidth Structure of Growth Rate of Two-Stream Instability of an Electron Beam Propagating in a Bounded Plasma Igor D. Kaganovich, Dmytro Sydorenko The two-stream instability of an electron beam propagating in finite-size plasma placed between two electrodes is studied analytically and numerically. It is shown that the growth rate in such a system is much smaller than that of infinite plasma or finite size plasma with periodic boundary conditions. We show that even if width of the plasma matches the resonance condition for standing waves; standing waves do not develop and transform into spatially growing wave, whose growth rate is small compared to that of the standing wave in a system with periodic boundary conditions; this growth rate is approximately described by $\gamma \approx \raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {13}}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${13}$}\omega _{pe} \left( {n_{b} /n_{p} } \right)\left( {L\omega_{pe} /v_{b} } \right)\ln \left( {L\omega_{pe} /v_{b} } \right)$, where $\omega_{pe} $ is the electron plasma frequency, $n_{b} $and $n_{p} $ are beam and plasma densities, respectively, and $v_{b} $ is the beam velocity, $L$ is the plasma width. The frequency and growth rate as a function of plasma width form a bandwidth structure. [Preview Abstract] |
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