Measurement of ions in H2 - N2 capacitively coupled discharge
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Abstract:
Neutral molecules and especially positive ions and their energy
distributions were measured in low-pressure (ca 4 Pa) high
frequency (13,56 MHz) capacitively coupled discharge ignited in
the mixtures of hydrogen and nitrogen. Especially, the
hydrogen-rich mixtures were investigated. Firstly, the energy
dependence of the spectrometer was calculated and the ion
energy distributions were corrected. The dominant ions were H3+
and HN2+. Ions without hydrogen (N2+ and N+) were significant
only at low hydrogen concentration (less than 10%). Also a high
amount of ions NH4+ was detected. The nitrogen addition caused
a quite steep decrease of the dominant hydrogen ion H3+.
Further, an unusual behaviour of NH4+ and H+ was observed.
Concerning creation of neutral stable molecules, only a small
amount of ammonia was detected.Cite
The low-temperature plasma (LTP) jet is a potentially effective method to accelerate "ion-mediated nucleation" in the lower troposphere by providing lots of ions. In this article, besides the basic characteristics of helium-based LTP such as the current-voltage value and optical emission spectroscopy, the positive and negative ion densities away from the LTP jet were measured by the ion counter. The positive ion density was higher than negative ion density. The increasing applied voltage increased the ion density substantially. The mass spectrometry measurements indicate that for the positive ions, although LTP jet generated lots of ions such as He + , He 2 + , O 2 + , and N 2 + , the final dominant ions were hydronium ions and their water clusters H 3 O + (H 2 O) n ; for the negative ions, the dominant ions were OH - , CO 3 - , and NO 3 - , and their water clusters. The production mechanism of these ions was also summarized. Finally, the larger flow rate was found to be able to enhance the generation of ions.
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In this study, we have carried out the experimental observation and modeling of negative ion, atomic and molecular ions in hydrogen sheet plasma. On the experiment, hydrogen sheet plasma was produced by a linear plasma device TPD-Sheet IV. The electron density and temperature were measured by Langmuir probe. The density profiles of hydrogen ions were measured by omegatron mass spectrometer. In the experimental result, it has been found that negative ions are produced at periphery region of sheet plasma. When the hydrogen gas pressure was 0.2-0.3Pa, negative ion density became maximum (NH− ∼1017 m−3). To model the ion density in this experiment, a zero-dimensional model is developed for solving the system of rate balance equations for ion and gas species. In evaluating the rate coefficients, the reactions involving H+, H2+, H3+, and H- are vibrationally resolved in the model. In the calculate result, it has been found that negative ions are produced at periphery region of sheet plasma. When the hydrogen gas pressure was 0.2-0.3Pa, negative ion density became maximum (NH− ∼1016 m−3). The experiments and the model calculate results indicate that production of negative ions in sheet plasma depends on the gas pressure and location from the plasma column.
Langmuir Probe
Electron temperature
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Summary form only given. Recently, due to the potential application of carbon nanotubes in field emission flat panel displays and nanoelectronics, the inductively coupled hydrocarbon plasmas such as acetylene plasma were used to grow carbon nanotubes directly on substrates. We developed a two-dimensional fluid simulation model to study the inductively coupled acetylene/hydrogen plasmas. The simulated plasma chamber has a cylindrical shape with a height of 18 cm. The upper part has a diameter of 8.5 cm, and three inductive coils surround the cylindrical quartz wall. The lower part has a diameter of 17 cm. Our model included 16 neutral species and 10 ionic species. Simulation was done for a gas pressure of 100 mtorr and a gas flow rate of 100 sccm for pure acetylene plasma, and 25 sccm C 2 H 2 and 75 sccm H 2 for C 2 H 2 /H 2 plasma. The absorption power is set at 500 W Simulation results show that the plasma density is equal to 4.39times10 17 l/m 3 for the C 2 H 2 /H 2 case. This plasma density is much smaller than the 5.94times10 18 l/m 3 for the pure acetylene plasma. Due to the addition of hydrogen gas, much energy is used to dissociate hydrogen molecules instead of ionization. In addition, electron temperature is higher for the case of C 2 H 2 /H 2 plasma since the ionization rate constants of hydrogen atom and hydrogen molecule are lower and both hydrogen atom and hydrogen molecule are more abundant in the C 2 H 2 /H 2 case than in the case of pure acetylene. For the C 2 H 2 /H 2 plasma, H, CH, C 2 H, C 4 H, C 6 H and C 8 H are concentrated near the coil region. These species are, however, concentrated in the chamber center for the case of pure acetylene plasma. For C 2 H 2 /H 2 plasma, it was found that the electron temperature is higher near the coil region and thus a higher production rate in that region. In the C 2 H 2 /H 2 plasma, the concentrations of H, H 2 , CH, CH 2 , C 2 H, C 4 H, C 6 H and C 8 H are higher compared with the case of pure acetylene plasma. On the other hand, the concentrations of other neutral species are lower in the C 2 H 2 /H 2 case. It was found that the addition of hydrogen gas would change the concentration distribution profile and electron temperature distribution as well as the concentration of neutral species. Simulation results can provide a better understanding of the complicated chemical kinetics in acetylene/hydrogen ICP discharges and help to elucidate the growth mechanism of carbon nanotubes.
Acetylene
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Negative atomic hydrogen ion (H$^{-}$) densities were measured in a pulsed low-pressure E-mode inductively-coupled radio-frequency (rf) driven plasma in hydrogen by means of laser photodetachment and a Langmuir probe. This investigation focuses on the influence of different metallic surface materials on the volume production of H$^{-}$ ions. The H$^{-}$ density was measured above a thin disc of either tungsten, stainless steel, copper, aluminium, or molybdenum placed onto the lower grounded electrode of the plasma device as a function of gas pressure and applied rf power. For copper, aluminium, and molybdenum the H$^{-}$ density was found to be quite insensitive to pressure and rf power, with values ranging between 3.6x10$^{14}$ to 5.8x10$^{14}$ m$^{-3}$. For stainless steel and tungsten, the H$^{-}$ dependency was found to be complex, apart from the case of a similar linear increase from 2.9x10$^{14}$ to 1.1x10$^{15}$ m$^{-3}$ with rf power at a pressure of 25 Pa. Two-photon absorption laser induced fluorescence was used to measure the atomic hydrogen densities and phase resolved optical emission spectroscopy was used to investigate whether the plasma dynamics were surface dependent. An explanation for the observed differences between the two sets of investigated materials is given in terms of surface reaction mechanisms for the creation of vibrationally excited hydrogen molecules.
Langmuir Probe
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The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3–1 mbar) and low coupled power (3–13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 × 109 cm−3, corresponding to ionization degrees from 2 × 10−8 to 4 × 10−7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 × 1013 cm−3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical–neutral collisions (ex: N2H+, NH4+, H3+), while an increase of the coupled power favours ions formed by direct ionization (ex: N2+, NH3+, H2+).
Electron temperature
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Surface production of negative hydrogen (H-) ions due to hydrogen atom impact has been confirmed in the actual ion source operating condition. Three types of atomic sources injected beams of atomic hydrogen (H0) into a cesiated/uncesiated hydrogen plasma confined in a small multicusp ion source. Hydrogen atoms produced by a thermal cracking and a microwave capacitively coupled plasma (CCP) sources decreased the amount of H− ions under the uncesiated conditions. Injection of hydrogen atoms from the thermal cracking source enhanced the H− ion current corresponding to the surface production at the plasma grid under the cesiated condition. Meanwhile, the H− ion enhancement by low temperature H0 injection from microwave discharge plasma was less pronounced. A series of experiments concluded that the electrons in high energy tails of the velocity distribution functions of produced hydrogen atoms from the CCP source and that from the thermal cracking source are capable to realize H− surface production.
Plasma cleaning
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Saturation (graph theory)
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In this contribution RF excited atmospheric pressure glow discharges are investigated in He–water mixtures in a parallel metal plate reactor by mass spectrometry. Positive and negative ion fluxes to the electrode are investigated as a function of varying water concentration and discharge power. The dominant positive ions are H3O+ (and its clusters), OH+, O+, , , HeH+, and . Negative ions are detectable from a concentration of 900 ppm water in He onwards. Coinciding with the emergence of the negative ions, there is a drop in positive ion flux to the mass spectrometer and a significant increase in applied voltage indicating increasing electron loss by attachment and ion loss by mutual (three and two body) positive–negative ion recombination. The dominant negative ions are OH− and its clusters. The negative ion flux increases with increasing water concentration. Positive and negative ion cluster formation increases with decreasing discharge power and increasing concentration of water vapour at constant power. It is shown that the size of the sampling orifice of the inlet of the mass spectrometer is important for sampling atmospheric pressure active plasmas due to the presence of the narrow sheath.
Glow discharge
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HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not.The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Diagnostics of low-pressure hydrogen discharge createdin a 13.56 MHz RF plasma reactor
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Langmuir Probe
Electron temperature
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The chemical composition of a low-pressure hydrogen dc plasma produced in a hollow cathode discharge has been measured and modeled. The concentrations of H atoms and of H(+), H(2)(+) and H(3)(+) ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures (p approximately 0.008-0.2 m bar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction ( approximately 0.1+/-0.05) of H atoms, indicative of a relatively small wall recombination, was observed. Low ionization degrees (<10(-4)) were obtained in all cases. In general, the ionic composition of the plasma was found to be dominated by H(3)(+), except at the lowest pressures, where H(2)(+) was the major ion. The key physicochemical processes determining the plasma composition were identified from the comparison of experimental and model results, and are discussed in the paper.
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