Conversion of methane in a coaxial microwave torch

A microwave coaxial plasmatron (microwave torch) is used as a plasmachemical converter of methane into hydrogen and hydrocarbons. The measured energy cost of methane decomposition is close to its minimum theoretical value. Such a low energy cost is unsurpassed for reactors operating at atmospheric pressure. A model of the plasmachemical converter is constructed. The results of calculations in the frame-work of this model agree well with experimental data.     

Microwave torch as a plasmachemical generator of nitric oxides

The possibility of using a microwave coaxial plasmatron (a microwave torch) as an efficient plasmachemical generator of nitric oxides in an air jet has been studied experimentally. A plasmachemical model of the generator is developed. Results of calculations by this model do not contradict experimental results. A conclusion about the mechanisms governing NO_x production in a plasma torch is drawn by comparing the experimental and calculated results.     

Synthesis of Nitrogen Oxides in a Subthreshold Microwave Discharge in Air and in Air Mixtures with Methane

Plasma Physics Reports - A subthreshold discharge excited by a microwave beam in air at pressures close to atmospheric is studied as a plasmachemical method of nitrogen oxide (NOx) production. It...     

Plasma chemistry of the sealed-off slab CO laser active medium pumped by radio-frequency discharge with liquid-nitrogen-cooled electrodes

The long-term time behavior of the output power of a sealed-off cryogenic slab CO laser pumped by a repetitively pulsed RF discharge and operating on the overtone (λ = 2.6–3.5 μm) vibrational?rotational transitions of the CO molecule was studied experimentally. It is shown that adding of an anomalously large amount of oxygen (up to 50% with respect to the CO concentration) to the initial gas mixture CO : He = 1 : 10 leads to a manyfold (by several tens of times) increase in the duration of the laser operating cycle (until lasing failure due to the degradation of the active medium). In this case, the laser life-time without replacement of the active medium reaches 10⁵–10⁶ pulses. Using various diagnostics (including luminescence spectroscopy and IR and UV absorption spectroscopy), regularities in the time-behavior of the concentrations of the main component of the active medium (CO molecules) and the products of plasmachemical reactions (O₃, CO₂) generated in the discharge gap during the laser operating cycle are revealed. Time correlation between the characteristics of the active medium and the laser output power are analyzed. A phenomenological approach to describing the entirety of plasmachemical, purely chemical, gas-dynamic, and diffusion processes determining the behavior of the laser output characteristics throughout the laser operating cycle is offered.     

Kinetics of Atomic Recombination on Silicon Samples in Chlorine Plasma

The recombination kinetics of chlorine atoms on the wall of a plasmachemical reactor and on silicon samples in the positive column of a glow discharge in Cl₂ has been studied experimentally. The rate constants and probabilities of the heterogeneous recombination of chlorine atoms on the plasma limiting surfaces, as well as of the chemical interaction of chlorine atoms with silicon, are calculated. The temperature and time dependences of the probabilities of the chemical interaction of chlorine atoms with silicon are analyzed, and optimal conditions for conducting pulse relaxation experiments are determined.     

Radiation of nitrogen molecules in a dielectric barrier discharge with small additives of chlorine and bromine

Spectral and energy characteristics of nitrogen molecule radiation in dielectric barrier discharges in Ar-N₂, Ar-N₂-Cl₂, and Ar-N₂-Br₂ mixtures were investigated experimentally. Small additives of molecular chlorine or bromine to an Ar-N₂ mixture are found to increase the radiation intensity of the second positive system of nitrogen. The conditions at which the radiation spectrum predominantly consists of vibronic bands of this system are determined. Using a numerical model of plasmachemical processes, it is shown that, at electron temperatures typical of gas discharges (2–4 eV), a minor additive of molecular chlorine to an Ar-N₂ mixture leads to an increase in the concentrations of electrons, positive ions, and metastable argon atoms. In turn, collisional energy transfer from metastable argon atoms to nitrogen molecules results in the excitation of the N₂(C ³Π _u ) state.     

Dynamics of the water molecule density in a discharge chamber filled with a low-pressure humid gas

The dynamics of the H₂O molecule density in a metal gas-discharge chamber filled with low-pressure water vapor or its mixtures with noble gases was investigated by manometric and spectral methods. Regimes both with and without discharge excitation were studied. In the absence of a discharge, the molecule density dynamics is governed by the heterogeneous interaction of molecules with the chamber walls. In the presence of a discharge, in addition to the heterogeneous interaction, fast plasmachemical molecule dissociation also contributes to the initial stage of H₂O molecule loss. The role of heating of the chamber walls is discussed.     

Effect of the scheme of plasmachemical processes on the calculated characteristics of a barrier discharge in xenon

The dynamics of a repetitive barrier discharge in xenon at a pressure of 400 Torr is simulated using a one-dimensional drift-diffusion model. The thicknesses of identical barriers with a dielectric constant of 4 are 2 mm, and the gap length is 4 mm. The discharge is fed with an 8-kV ac voltage at a frequency of 25 or 50 kHz. The development of the ionization wave and the breakdown and afterglow phases of a barrier discharge are analyzed using two different kinetic schemes of elementary processes in a xenon plasma. It is shown that the calculated waveforms of the discharge voltage and current, the instant of breakdown, and the number of breakdowns per voltage half-period depend substantially on the properties of the kinetic scheme of plasmachemical processes.     

Study of charged particle motion in fields of different configurations for developing the concept of plasma separation of spent nuclear fuel

The concept of plasma separation of spent nuclear fuel in a plane perpendicular to the magnetic field in an electric potential of special configuration is developed. A specific feature of the proposed approach consists in using an accelerating potential for reducing energy and angular spread of plasma ions at the entrance to the separator chamber and a potential well for the spatial separation of ions with different masses. The trajectories of ions of the substance imitating spent nuclear fuel are calculated. The calculations are performed for azimuthal and axial magnetic fields and model electric field configurations corresponding to different geometries of the separator chamber. It is shown that, using magnetic fields with a characteristic strength of 1 kG and electric potentials of up to 1 kV inside a region with a linear size less than 100 cm, it is possible to separate ions of spent nuclear fuel with energies from 0.2 to 3 eV. The calculations were performed for a collisionless mode in the single-particle approximation. Possible variants of the experimental facility for plasma separation of spent nuclear fuel are proposed.     

Fragmentation of Ar 3 + : The Role of Rotational and Vibrational Predissociation Dynamics

The quantum study of the fragmentation dynamics of argon trimer ions has been carried out using two different ab-initio potential energy surfaces. Different computational methods have been employed and the effects of the total angular momentum of the system have been evaluated. It is found that the observed metastability can be explained with the lengthening of the lifetimes produced by the centrifugal barrier.     

A Monte Carlo code for the simulation of heavy-ion tracks in water

TILDA, a new Monte Carlo track structure code for ions in gaseous water that is valid for both high-LET (approximately 10(4) keV/microm) and low-LET ions, is presented. It is specially designed for a comparison of the patterns of energy deposited by a large range of ions. Low-LET ions are described in a perturbative frame, whereas heavy ions with a very high stopping power are treated using the Lindhard local density approximation and the Russek and Meli statistical method. Ionization cross sections singly differential with energy compare well with the experiment. As an illustration of the non-perturbative interaction of high-LET ions, a comparison between the ion tracks of light and heavy ions with the same specific energy is presented (1.4 MeV/nucleon helium and uranium ions). The mean energy for ejected electrons was found to be approximately four times larger for uranium than for helium, leading to a much larger track radius in the first case. For electrons, except for the excitation cross sections that are deduced from experimental fits, cross sections are derived analytically. For any orientation of the target molecule, the code calculates multiple differential cross sections as a function of the ejection and scattering angles and of the energy transfer. The corresponding singly differential and total ionization cross sections are in good agreement with experimental data. The angular distribution of secondary electrons is shown to depend strongly on the orientation of the water molecule.     

Experimental verification of the method for detection of water microleakages in plasma vacuum chambers by using the hydroxyl spectrum

Experimental determination of the sensitivity of the method for detection of water microleakages in the cooling systems of the plasma vacuum chambers of complex electrophysical devices (such as tokamaks, fuel elements of nuclear reactors, and plasmachemical reactors) is considered. It was shown that the spectroscopic method for detection of water microleakages by using the hydroxyl radiation spectrum makes it possible to detect leakages at a level of 10⁻⁵ Pa m³ s⁻¹. The spatial resolution of the method allows one to localize defects with an accuracy of several centimeters.     

Responses of a direct ion storage dosimeter (DIS-1) to heavy charged particles.

The responses of a direct ion storage dosimeter (DIS-1) to energetic heavy charged particles were examined using (4)He, (12)C, (40)Ar and (56)Fe ion beams at the HIMAC at the National Institute of Radiological Sciences. The efficiency of the DIS-1 on the basis of absorbed dose was almost unity for the helium and carbon ions and was slightly decreased for the argon and iron ions. The linearity in the dose response and the angular independence for these heavy ions were fairly good. Although further studies are necessary, these results suggest that the DIS-1 would be a suitable passive dosimeter for measurements of absorbed dose in a field dominated by heavy charged particles such as the space environment.     

Computer tomography imaging of fast plasmachemical processes

Results are presented from experimental studies of the interaction of a high-enthalpy methane plasma bunch with gaseous methane in a plasmachemical reactor. The interaction of the plasma flow with the rest gas was visualized by using streak imaging and computer tomography. Tomography was applied for the first time to reconstruct the spatial structure and dynamics of the reagent zones in the microsecond range by the maximum entropy method. The reagent zones were identified from the emission of atomic hydrogen (the H_α line) and molecular carbon (the Swan bands). The spatiotemporal behavior of the reagent zones was determined, and their relation to the shock-wave structure of the plasma flow was examined.     

The functional form of angular forces around transition metal ions in biomolecules.

A method for generating angular forces around sigma-bonded transition metal ions is generalized to treat pi-bonded configurations. The theoretical approach is based on an analysis of ligand-field and small-cluster Hamiltonians based on the moments of the electron state distribution. The functional forms that are obtained involve a modification of the usual expression of the binding energy as a sum of ligand-ligand interactions, which, however, requires very little increase in CPU time. The angular interactions have simple forms involving sin and cos functions, whose relative weights depend on whether the ligands are sigma- or pi-bonded. They describe the ligand-field stabilization energy to an accuracy of about 10%, and the interaction energy of covalently bonded systems to an accuracy of better than 4%. The resulting functional forms for the force field are used to model the structure of small clusters, including fragments of the copper blue protein structure. Large deviations from the typical square copper coordination are found when pi-bonded ligands are present.     

Accelerating structural life science by paramagnetic lanthanide probe methods

We describe the recent progress of structural analysis methods exploiting paramagnetic lanthanide ions. In NMR spectroscopy, the paramagnetic effects induced by the trivalent lanthanide ions provide long-range (~40 Å) distance and angular information that can be exploited in protein structure determination, ligand screening, structure-based resonance assignment, and in-cell observation. The paramagnetic lanthanide ions can also be utilized in EPR spectroscopy, providing nanometer-scale distance measurement. These applications of the paramagnetic lanthanide probe are becoming more widespread by the use of a variety of lanthanide binding tags. Here, we introduce the basics of paramagnetic effects, several examples of lanthanide tags, and recent applications of paramagnetic lanthanide ions in NMR and EPR spectroscopy. Collectively, we show how the paramagnetic lanthanide probe accelerates research in protein science and drug design, and consequently life science.     

Inhibition of carboxypeptidase A by excess zinc: analysis of the structural determinants by X-ray crystallography

Pancreatic metallocarboxypeptidases are inhibited by a millimolar excess of zinc together with other exo- and endometalloproteases. We have analyzed the structure of bovine carboxypeptidase A inhibited by an excess of zinc ions using X-ray crystallography at 1.7 Å overall resolution. Under these conditions, a second zinc is observed to bind to the enzyme active site, establishing a distorted tetrahedrally coordinated complex which involves Glu-270 (the general base for catalysis), a water molecule, a chloride ion, and a hydroxide ion. This hydroxide ion forms a 114° angular bridge between the inhibitory and the catalytic zinc ions, which are at a distance of 3.3 Å from one another. The inhibitory zinc holds the hydroxide at nearly the same location as a previously observed active site water molecule (W571) and probably perturbs the substrate positioning and stereochemical rearrangements required for substrate cleavage during catalysis.     

Enzyme systems for biodegradation of polychlorinated dibenzo-p-dioxins

The angular dioxygenase, cytochrome P450, lignin peroxidase, and dehalogenase are known as dioxin-metabolizing enzymes. All of these enzymes have metal ions in their active centers, and the enzyme systems except for peroxidase have each distinct electron transport chain. Although the enzymatic properties of the angular dioxygenase, lignin peroxidase, and cytochrome P450 have been studied well, the information about dehalogenase is much less than other enzyme systems due to its instability under the aerobic conditions. However, this enzyme system appears to be quite promising from the viewpoint of practical use for bioremediation, because dehalogenases are capable of degradation of polychlorinated dibenzo-p-dioxins (PCDDs) with more than four chlorine substituents, whereas the other three enzyme systems prefer low-chlorinated PCDDs. On the other hand, protein engineering of angular dioxygenase, lignin peroxidase, and cytochrome P450 based on their tertiary structures has great potential to generate highly efficient dioxin-metabolizing enzymes. Actually, we successfully generated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-metabolizing enzyme by site-directed mutagenesis of cytochrome P450. We hope that recombinant microorganisms harboring genetically engineered dioxin-metabolizing enzymes will be used for bioremediation of soil contaminated with PCDDs and polychlorinated dibenzofurans in the near future.     

Investigation of the angular distribution and energy spectrum of fast ions produced during irradiation of targets by short laser pulses in the SOKOL-P facility

Results are presented from experimental investigations of the angular distributions and energy spectra of fast ions produced in deuterium polyethylene targets under irradiation by picosecond laser pulses with intensities of up to 2 × 10¹⁸ W/cm² in the SOKOL-P facility. The parameters of ion fluxes were measured by time-of-flight spectrometers based on semiconductor detectors.     

Bacterial degradation of aromatic compounds via angular dioxygenation

Dioxygenation is one of the important initial reactions of the bacterial degradation of various aromatic compounds. Aromatic compounds, such as biphenyl, toluene, and naphthalene, are dioxygenated at lateral positions of the aromatic ring resulting in the formation of cis-dihydrodiol. This "normal" type of dioxygenation is termed lateral dioxygenation. On the other hand, the analysis of the bacterial degradation of fluorene (FN) analogues, such as 9-fluorenone, dibenzofuran (DF), carbazole (CAR), and dibenzothiophene (DBT)-sulfone, and DF-related diaryl ether compounds, dibenzo-p-dioxin (DD) and diphenyl ether (DE), revealed the presence of the novel mode of dioxygenation reaction for aromatic nucleus, generally termed angular dioxygenation. In this atypical dioxygenation, the carbon bonded to the carbonyl group in 9-fluorenone or to heteroatoms in the other compounds, and the adjacent carbon in the aromatic ring are both oxidized. Angular dioxygenation of DF, CAR, DBT-sulfone, DD, and DE produces the chemically unstable hemiacetal-like intermediates, which are spontaneously converted to 2,2',3-trihydroxybiphenyl, 2'-aminobiphenyl-2,3-diol, 2',3'-dihydroxybiphenyl-2-sulfinate, 2,2',3-trihydroxydiphenyl ether, and phenol and catechol, respectively. Thus, angular dioxygenation for these compounds results in the cleavage of the three-ring structure or DE structure. The angular dioxygenation product of 9-fluorenone, 1-hydro-1,1a-dihydroxy-9-fluorenone is a chemically stable cis-diol, and is enzymatically transformed to 2'-carboxy-2,3-dihydroxybiphenyl. 2'-Substituted 2,3-dihydroxybiphenyls formed by angular dioxygenation of FN analogues are degraded to monocyclic aromatic compounds by meta cleavage and hydrolysis. Thus, after the novel angular dioxygenation, subsequent degradation pathways are homologous to the corresponding part of that of biphenyl. Compared to the bacterial strains capable of catalyzing lateral dioxygenation, few bacteria having angular dioxygenase have been reported. Only a few degradation pathways, CAR-degradation pathway of Pseudomonas resinovorans strain CA10, DF/DD-degradation pathway of Sphingomonas wittichii strain RW1, DF/DD/FN-degradation pathway of Terrabacter sp. strain DBF63, and carboxylated DE-degradation pathway of P. pseudoalcaligenes strain POB310, have been investigated at the gene level. As a result of the phylogenetic analysis and the comparison of substrate specificity of angular dioxygenase, it is suggested that this atypical mode of dioxygenation is one of the oxygenation reactions originating from the relaxed substrate specificity of the Rieske nonheme iron oxygenase superfamily. Genetic characterization of the degradation pathways of these compounds suggests the possibility that the respective genetic elements constituting the entire catabolic pathway have been recruited from various other bacteria and/or other genetic loci, and that these pathways have not evolutionary matured.