Bulletin of the American Physical Society
67th Annual Gaseous Electronics Conference
Volume 59, Number 16
Sunday–Friday, November 2–7, 2014; Raleigh, North Carolina
Session DT1: Plasma Diagnostics I |
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Chair: Jean-Paul Booth, Ecole Polytechnique Room: State EF |
Tuesday, November 4, 2014 10:00AM - 10:15AM |
DT1.00001: Surface wave discharge in helium: evolution of metastable density and temperatures with operating parameters Ahmad Hamdan, Joelle Margot, Fran\c{c}ois Vidal Metastable and resonant-state atoms play an important role in the kinetics of gas discharges ($e.g.$ stepwise ionization and excitation processes). In this contribution, we study a surface-wave discharge in helium. Properties of the plasma such as metastable density, gas temperature and excitation temperature were studied as a function of the operating parameters (pressure, power and axial position z). Rotational temperatures of OH, NH and N$_{2}^{+}$ (impurities) are estimated by fitting the experimental rotational spectra by synthetic spectra. It was observed that the rotational temperature of N$_{2}^{+}$ is far to be in thermal equilibrium with the gas. The temperature of the latter T$_{\mathrm{g}}$ is better described by the rotational temperature of the OH radical. Its evolution was studied as a function of z, power and pressure. T$_{\mathrm{g}}$ was found to change from 400 to 1000 K, depending of discharge conditions. The excitation temperature was estimated to be about 0.55 eV using the Bolzmann plot method. The corresponding electron temperature and density were assumed to be 3 - 4 eV and 1 - 4 10$^{12}$ /cm$^{3}$, respectively, based on the results of collisional-radiative models presented in literature. The metastable density n* in the 2$^{3}$S level was determined using absorption spectroscopy. It was observed that n* depends neither of the power nor of the axial position. However, an important dependence of the pressure was observed. n* decreases from 10$^{11}$ to 10$^{10}$ /cm$^{3}$ when the pressure increases from 5 to 50 Torr. [Preview Abstract] |
Tuesday, November 4, 2014 10:15AM - 10:30AM |
DT1.00002: Spectroscopic Examination of Vibrational and Rotational Properties of NO A$^{2}\Sigma^{+}$ Metastable State from NO $\gamma $-Band Spectra in N$_{2}$-O$_{2}$ Mixture Microwave Discharge Hao Tan, Atsushi Nezu, Haruaki Matsuura, Hiroshi Akatsuka The spectra are observed in our microwave discharge plasma experiments. N$_{2}$-O$_{2}$ mixture plasma is generated by using a rectangular waveguide. We measured the spectra at 0, 60, 100 and 140 mm with the discharge pressure several Torrs. From these results, we can find that both NO and N$_{2}$ molecules experience a cooling down process both on vibrational and rotational temperatures as the plasma flows to the downstream direction. And NO molecule has always a higher rotational temperature than N$_{2}$. Meanwhile, we can see that in this nonequilibrium plasma, both NO and N$_{2}$ molecules tend to get higher energy for vibrational motion than for rotational motion. We also change the gas partial pressure rate, when O$_{2}$ molar ratio of the mixture increases, the NO experiences an increasing vibrational temperature. This is because that the NO A$^{2}\Sigma ^{+}$ metastable state is excited from two main paths: N$_{2}$(A $^{3}\Sigma _{\mathrm{u}}^{+})+$NO(X $^{2}\Pi ) \to $N$_{2}$(X $^{1}\Sigma _{\mathrm{g}}^{+})+$NO(A $^{2}\Sigma ^{+})$, (1) NO(X $^{2}\Pi )+$e$^{-} \to $NO(A $^{2}\Sigma^{+})+$e$^{-}$ (2) When O$_{2}$ or N$_{2}$ is the majority of the discharge species, reaction (2) or (1) dominates the excitation process of NO A$^{2}\Sigma^{+}$, respectively. Therefore, under our plasma conditions, vibration-vibration energy transition of the reaction (1) results in a strong vibrational relaxation of NO A$^{2}\Sigma ^{+}$ state molecules when N$_{2}$ is the majority in the discharge gas. In conclusion, the admixture of N$_{2}$ gas can lead to the reduction of average vibrational temperature significantly. [Preview Abstract] |
Tuesday, November 4, 2014 10:30AM - 10:45AM |
DT1.00003: Diagnostics of Pulsed Hydrogen Plasmas Jerome Dubois, Gilles Cunge, Olivier Joubert, Maxime Darnon, Laurent Vallier, Nicolas Posseme Hydrogen plasmas present a great potential interest for new materials such as graphene or C-nanotubes. To modify or clean such ultrathin layers without damaging the material, low ion energy bombardment is required (conditions such as those obtained in pulsed ICP reactor). By contrast, for other applications the ion energy must be high, to get a significant etch rate for example. To assist the development of innovative processes in H2 plasmas, we have thus analyzed systematically CW and pulsed H2 plasmas both with and without RF bias power. In particular, we carry out time-resolved ion flux, and time-averaged ion energy measurements in different pulsing configurations. A large variety of ion energies and shapes of IVDF are reported depending on pulsing parameters. The IVDF are typically very broad (due to the low ion transit time of low mass ion through the sheath) and either bi or tri-modal (H$+$, H2$+$ and H3$+$ contributions). The time variations of the ion flux in pulsed plasmas also presents peculiar features that will be discussed. Finally, we show that a specific issue is associated to H2 plasmas: they reduce the chamber walls material therefore releasing impurities (O atoms\textellipsis ) in the plasma with important consequences on processes. [Preview Abstract] |
Tuesday, November 4, 2014 10:45AM - 11:00AM |
DT1.00004: Characterization of a Diverging Cusped Field Thruster Operating on Krypton Natalia MacDonald-Tenenbaum, Landon Tango, William Hargus, Jr., Michael Nakles The Diverging Cusped Field Thruster (DCFT) is a low-power plasma with a cusped magnetic field profile. The magnetic fields have strong gradients that cause energetic electrons to mirror back and forth within the discharge chamber, enhancing propellant ionization. Radial portions of the magnetic field are seen only at magnet interfaces, thereby mitigating the ion impingement and heat flux to the channel walls that reduces thruster lifetime. The DCFT has been studied extensively while operating on xenon. This work represents the initial efforts at characterizing the DCFT operating on krypton. Krypton has gained interest in recent years as an alternate propellant for plasma propulsion, mainly because its lower cost has the potential to provide great savings for satellite missions. The results presented include a mapping of changes in the DCFT's discharge current with varying applied anode voltages and propellant mass flow rates, and frequency analysis of the discharge current oscillations. Additionally, time-averaged and time-synchronized laser induced fluorescence velocimetry are used to examine the ionization and acceleration regions of the discharge channel in an effort to better understand the dynamics of the thruster operation on krypton. [Preview Abstract] |
Tuesday, November 4, 2014 11:00AM - 11:30AM |
DT1.00005: Optical emission spectroscopy at different timescales: nanoseconds, microseconds, milliseconds Invited Speaker: John B. Boffard Analysis of plasma optical emissions can provide a simple, non-invasive way of measuring key plasma parameters such as the electron temperature and electron density. Due to the short radiative lifetimes of excited states, the plasma emissions can be used to track the near-instantaneous state of time-varying plasmas. Using a small set of argon emission lines along with a low-resolution spectrometer we have monitored the effective electron temperature, electron density, and number densities of long-lived excited Ar($3p^54s$) atoms in near real-time (update rate 10~Hz, $T$=100~ms) for an inductively-coupled plasma (ICP) under a wide variety of plasma conditions.\footnote{J. Vac. Sci. Technol. A \textbf{31} (2013) 021303.} When this same set of Ar emission lines are measured with a faster time-response by using a monochromator/PMT, the plasma conditions on a microsecond timescale can be monitored in pulsed plasmas. Time-resolved measurements of neon emission lines at an even higher time resolution ($\sim 5$~ns) have been used as a probe for the presence of high energy electrons which occur during only select portions of the 13.56~MHz rf cycle in Ne/Ar ICP discharges.\footnote{J. Phys. D: Appl. Phys. \textbf{45} (2012) 382001.} [Preview Abstract] |
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