Glow Discharge Experiments Research Articles

Insights into the limitations to vibrational excitation of CO2: validation of a kinetic model with pulsed glow discharge experiments

Vibrational excitation represents an efficient channel to drive the dissociation of CO2 in a non-thermal plasma. Its viability is investigated in low-pressure pulsed discharges, with the intention of selectively exciting the asymmetric stretching mode, leading to stepwise excitation up to the dissociation limit of the molecule. Gas heating is crucial for the attainability of this process, since the efficiency of vibration–translation (V–T) relaxation strongly depends on temperature, creating a feedback mechanism that can ultimately thermalize the discharge. Indeed, recent experiments demonstrated that the timeframe of V–T non-equilibrium is limited to a few milliseconds at ca. 6 mbar, and shrinks to the μs-scale at 100 mbar. With the aim of backtracking the origin of gas heating in pure CO2 plasma, we perform a kinetic study to describe the energy transfers under typical non-thermal plasma conditions. The validation of our kinetic scheme with pulsed glow discharge experiments enables to depict the gas heating dynamics. In particular, we pinpoint the role of vibration–vibration–translation relaxation in redistributing the energy from asymmetric to symmetric levels of CO2, and the importance of collisional quenching of CO2 electronic states in triggering the heating feedback mechanism in the sub-millisecond scale. This latter finding represents a novelty for the modelling of low-pressure pulsed discharges and we suggest that more attention should be paid to it in future studies. Additionally, O atoms convert vibrational energy into heat, speeding up the feedback loop. The efficiency of these heating pathways, even at relatively low gas temperature and pressure, underpins the lifetime of V–T non-equilibrium and suggests a redefinition of the optimal conditions to exploit the ‘ladder-climbing’ mechanism in CO2 discharges.

A Model of Titan-like Chemistry to Connect Experiments and Cassini Observations

Abstract A numerical model is presented for interpreting the chemical pathways that lead to the experimental mass spectra acquired in the Titan Haze Simulation (THS) laboratory experiments and for comparing the electron density and temperature of the THS plasma to observations made at Titan by the Cassini spacecraft. The THS plasma is a pulsed glow-discharge experiment designed to simulate the reaction of N2/CH4-dominated gas in Titan's upper atmosphere. The transient, one-dimensional model of THS chemistry tracks the evolution of more than 120 species in the direction of the plasma flow. As the minor species C2H2 and C2H4 are added to the N2/CH4-based mixture, the model correctly predicts the emergence of reaction products with up to five carbon atoms in relative abundances that agree well with measured mass spectra. Chemical growth in Titan's upper atmosphere transpires through ion–neutral and neutral–neutral chemistry, and the main reactions involving a series of known atmospheric species are retrieved from the calculation. The model indicates that the electron density and chemistry are steady during more than 99% of the 300 μs long discharge pulse. The model also suggests that the THS ionization fraction and electron temperature are comparable to those measured in Titan's upper atmosphere. These findings reaffirm that the THS plasma is a controlled analog environment for studying the first and intermediate steps of chemistry in Titan's upper atmosphere.

An 18th century glow discharge experiment to model an aurora

We discuss an experiment first performed in 1789 designed to reproduce the glow of an aurora. The experiment was partially based on ideas of Benjamin Franklin.

Oxygen glow discharge cleaning in ASDEX Upgrade

Oxygen-containing glow discharges have been proposed for the removal of redeposited hydrocarbon films in fusion experiments. After the 2004/2005 experimental campaign a 49h He/O2 glow discharge experiment was performed in ASDEX Upgrade. A total of 25g of carbon was removed as CO and CO2. In order to probe erosion in various locations in the vessel 48 well defined laboratory-deposited a-C:H film samples were distributed poloidally and toroidally. In principle, the films were successfully removed on all types of surfaces directly facing the discharge. Additionally, two sets of tungsten-coated tiles were removed from the vessel, one prior to and one after the cleaning discharge. Analysis of films on the tiles revealed that they consisted mainly of boron with only little carbon and deuterium. Neither erosion of these films nor any further reduction of their carbon and deuterium content due to the glow discharge could be detected.

Oxygen glow discharge experiment to remove deposited layers and to release trapped hydrogen isotopes in HT-7 superconducting tokamak

An experiment to remove re-deposited layers and to release hydrogen using a glow discharge in oxygen (O-GDC) has been performed in the HT-7 superconducting tokamak. In the absence of magnetic fields, the O-GDC wall conditioning had produced rapid, controlled co-deposit removal. Average removal rates, 5.2 × 10 22 H-atoms/h, 5.65 × 10 21 D-atoms/h and 5.53 × 10 22 C-atoms/h, respectively, were obtained during 145 min O-GDC experiment in the pressure range 0.5–1.5 Pa. The corresponding removal rate of co-deposited films was ∼1.19 μm/day (26.5 g/day for carbon) based on an area of 12 m 2. Compared to thermo-oxidation and O-ICR experiment, high pressure O-GDC wall conditioning promoted the oxidation and improved the C and D atoms removal. In the O-GDC experiment, the removal rates of H-atoms and D-atoms as H 2O, HDO and D 2O were higher than that of H 2 and D 2 by factors of about 20 and 50, respectively. During the 145 min O-GDC experiment, about 14.5% O-atoms were converted into carbon oxides and hydroxides, and about 5.37 × 10 22 O-atoms were adsorbed on the walls corresponding to a coverage of 4.5 × 10 21 O/m 2 on an wall area of 12 m 2. In a 100 min helium glow discharge (He-GDC) following the O-GDC experiment, 1.53 × 10 22 O-atoms, about 28.5% oxygen retained on the walls, were removed. The removal rate of H-atoms in He-GDC cleaning after O-GDC experiment was lower than that in He-GDC cleaning before O-GDC experiment, which indicates that the O-GDC wall conditioning had effectively reduced hydrogen retention on the walls.

Analyses of metallic first mirror samples after long term plasma exposure in Tore Supra

Metallic mirrors are foreseen in ITER diagnostic systems as optical elements directly viewing the plasma radiation. In the frame of an EFDA contract, metallic mirror samples have been exposed for long pulse plasma discharges in Tore Supra (TS) in order to investigate surface modifications caused by erosion and re-deposition processes. Three different materials have been selected: mono-crystalline molybdenum (mc-Mo), polycrystalline stainless steel (SS) and copper (Cu). The mc-Mo samples showed after TS exposure almost no surface roughness modifications and the lowest net-erosion. A slight reflectivity reduction, most pronounced in the near UV, is attributed to light absorption in a thin carbon deposit. Cu mirrors showed by far the highest surface roughness, erosion and diffusive reflectivity. Comparative laboratory glow discharge experiments with virgin reference samples and numerical simulations of erosion/deposition confirm the dominant contribution of conditioning procedures to erosion of mirrors exposed (without shutter protection) in Tore Supra.

Measurement of Ion Energy Distribution in a Cylindrical Inertial Electrostatic Confinement Fusion (C-IECF) Device

In recent researches, an assisted glow discharge experiment using an external ion source has been tried to reduce operation gas pressure. As results, operating gas pressure has been successfully reduced from 1.5 Pa to 0.3 Pa, and the neutron production rate has increased.These results are considered to be due to an increase of ion energy. However, it is necessary to measure the ion energy distribution of the Cylindrical Inertial Electrostatic Confinement Fusion (C-IECF) device in order to confirm this. To do this, we have measured the distribution of the neutral particle beam energy (relative to ion energy distribution). These experimental results demonstrate that a decrease of operation gas pressure (from 1.7 Pa to 0.3 Pa) contributes to the increase in ion energy.

Preliminary Results of Low Pressure Discharge Experiments of a Cylindrical Inertial Electrostatic Confinement Fusion Device Aimed for a Small Neutron Source

The effects of operating gas pressure on a Cylindrical Inertial Electrostatic Confinement Fusion (IECF) device are investigated. First we did glow discharge experiments, and we obtained 1.8 × 106 neutrons production per second with 45-kV, 44-mA discharge at 1.2 Pa. Next we modified the device and tried to reduce the operating gas pressure with an ion source, aimed to increase neutron production. Although the discharge currents are small, we can make steady discharges at less than 0.1 Pa. The neutron production rates per current are larger than those of glow discharge at higher pressure. We consider it should suggest the validity of reducing operating pressure in IECF devices.

Experimental observation of dust-acoustic wave turbulence

Spontaneously excited dust acoustic oscillations are studied in a dusty plasma cloud that is in the strongly coupled fluid regime. As the gas pressure in this DC glow discharge experiment is reduced in a controlled manner the oscillations are seen to change from a coherent to a turbulent state through higher harmonic excitations. A simple explanation of the nonlinear features of the saturated spectrum is provided using a viscoelastic theory of dusty plasmas.

Direct-current glow discharges in atmospheric pressure air plasmas

Investigations have been conducted to experimentally validate the mechanisms of ionization in two-temperature atmospheric pressure air plasmas in which the electron temperature is elevated with respect to the gas temperature. To test a predicted S-shaped dependence of steady-state electron number density on the electron temperature and its macroscopic interpretation in terms of current density versus electric field, direct-current (dc) glow discharge experiments have been conducted in flowing low temperature, atmospheric pressure air plasmas. These experiments show that it is feasible to create stable diffuse glow discharges with electron number densities in excess of 1012 cm−3 in atmospheric pressure air plasmas. Electrical characteristics were measured and the thermodynamic parameters of the discharge were obtained by spectroscopic measurements. The measured gas temperature is not noticeably affected by whether or not the dc discharge is applied. The discharge area was determined from spatially resolved optical measurements of plasma emission during discharge excitation. The measured discharge characteristics are found to be in good agreement with the predicted electrical characteristics.

The preliminary study on the chemical synthesis between graphite and water by glow-discharge experiments

Glow-discharge experiments were performed against water using a graphite rod as the anode and a silver thread as the cathode under an Ar atmosphere. UV-spectrum, FT-IR, 1H-NMR spectrum and GC-MS analyses of the solutions irradiated with plasma were studied. It was found that a small amount of carbon was transferred from anode into water and some organic compounds were produced by reaction between carbon and water. Among those, formic acid, acetic acid and propanoic acid were identified as the main products.

Experimental evidence for a torus breakdown in a glow discharge plasma

A global bifurcation scenario for a two-frequency torus breakdown depicted by Baptista and Caldas [Physica D 132, 325 (1999)] is observed on a glow-discharge experiment. The torus is broken through a crisis with an unstable periodic orbit. The torus section before the bifurcation is a sided polygon that has a number of edges equal to the period of the unstable orbit. Since the discharge is an extended system the two-frequency torus breakdown is shown to be a possible way to space-time chaos.

Etching of UO 2 in NF 3 RF plasma glow discharge

A series of single effect, RF plasma, glow discharge experiments were conducted using NF 3 gas to decontaminate depleted uranium dioxide from stainless-steel substrates. In the experiments, the plasma absorbed power was varied from 25 to 210 W and the pressure from ∼10 to 40 Pa. The results demonstrated that UO 2 can be completely removed from stainless-steel substrates after several minutes processing at under 100 W with initial etch rates ranging from 0.2 to 7.4 μm/min. A primary etch mechanism is proposed in which F atoms created in the plasma diffuse to the UO 2 surface and react to form successive intermediates of uranium fluorides and/or oxyfluorides with reactions continuing to form volatile UF 6 which desorbs into the gas phase to be pumped away. Ions created in the plasma are too low in concentration to be the primary etch mechanism, yet they can deliver enough energy to enhance the reaction process. UO 2 etching is a self-limiting process due to the formation of non-volatile uranium oxyfluorides and fluorides which form over the UO 2 surface, slowing or completely blocking the reaction to UF 6.

Plasma Decontamination Of Depleted Uranium Oxide From Stainless Steel Surfacesa

ABSTRACT: A series of single effect, RF plasma, glow discharge experiments were conducted using NF3 gas to decontaminate depleted uranium dioxide from stainless‐steel substrates. In the experiments, the plasma absorbed power was varied from 25 to 210 W, the pressure from ∼10 to 40 Pa, and the NF3 flow rate from 3 to 18.5 SCCM. Results demonstrated that UO2 can be completely removed from stainless‐steel substrates after several minutes processing at under 100 W with initial etch rates ranging from 0.2 to 7.4 μm/min. A primary etch mechanism is proposed in which F atoms created in the plasma diffuse to the surface UO2 and react to form successive intermediates of uranium fluorides and/or oxyfluorides with reactions continuing to form the volatile UF6 which desorbs into the gas phase to be pumped away. Ions created in the plasma are too low in concentration to be the primary etch mechanism, yet they can deliver enough energy to enhance the reaction process. UO2 etching is a self‐limiting process due to the formation of non‐volatile uranium oxyfluorides and fluorides which form over the UO2, slowing or completely blocking the reaction to UF6.

Nuclear Reaction by Three-Body Recombination between Deuterons and the Nuclei of Lattice-Trapped D2 Molecules

A hypothesis is proposed where the main low-energy nuclear reactions in glow discharge experiments involve three-body recombination between a deuteron and the nuclei of a D2 molecule trapped in a dense lattice of a chemical compound of transition metal and impurity. Two D’s fuse to 4He, and the energy is “converted” by expulsion of the third deuteron. Three boson (efimov) interactions can have a longer range than two boson interactions. The scheme accounts for the low reproducibility and short duration of the effect because of rapid destruction of the active structure by sputtering, radiation damage, bubble formation, or chemical changes, and it conforms to the reported prevalence of 4He >> tritium >> neutrons.

Electron assisted glow discharges for conditioning fusion tokamak devices

Glow discharge conditioning of tokamaks with graphite plasma-facing surfaces has been used to reduce impurities and obtain density control of the plasma discharge. However, a major operational disadvantage of glow conditioning is the high pressure required to initiate the glow discharge, e.g., ≊70 mTorr for helium in DIII-D, which requires isolating auxiliary components that can not tolerate the high pressure. An electron-gun-assisted glow discharge can lower breakdown pressure, possibly eliminating the necessity of isolating these auxiliary systems during glow discharge conditioning and allowing glow discharge operation at lower pressures. An electron-assisted glow discharge experiment has been carried out in a small vacuum vessel to evaluate whether such a system can be employed in the DIII-D tokamak. With an electron gun to produce an initial source of electrons, the pressure at which a helium glow discharge can be initiated has been decreased by two orders of magnitude. The glow was produced in a 0.40 m3 Inconel test chamber (3.5 m2 surface area) and was pumped with a 330 ℓ/s turbomolecular pump. The electron gun consists of a tungsten filament and grid assembly. An electron current of up to 18 mA with energies up to 1.6 kV has been used. The pressure is measured with a capacitance manometer, the gas composition with a residual gas analyzer and the electron temperature, density, and plasma floating potential with a Langmuir probe. With the addition of an electron current of 10 mA, the initiation pressure was reduced from 165 to 1.6 mTorr. The lowest sustaining pressure dropped from 3.0 mTorr in the absence of electron assist to 1.5 mTorr with 10 mA of electron current.

Studies on the Materials Obtained in the Reactor from the Glow Discharge Reaction of Benzene and Glow Discharge Reactions of Benzene-Water and Benzene-Methanol

In the reactions of benzene in glow discharge, the polymeric materials were dbtained in glow column chromatography, napthalene and biphenyl were isolated and the presence of fluorene, phenanthrene and or anthracene, and biacenaphthylene was also estimated by the studies of the mass spectra of another spots. A yellowish polymer, obtained by means of the reprecipitation in methanol of the latter portion, was estimated to be a polymer having a skeleton of some condensed-ring compun ds, On the basis of its IR spectru. and its eleme ntary analysis.The glow discharge experiments of both benzene water and benzene-Tmethanel were also undertaken. ln the former case, the presence of phenol and o, m p-cteole was confirmed along with that of products, obtained by glow discharge reaction of benzene, in terms of GLC analysis of liquid product and, in addition, benzyl alcchol was isolated in the latter case.