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We have developed a technique for absolute measurements of electron density in pulse-repetitive microwave discharges in air. The technique is based on the time-resolved absolute intensity of a nitrogen spectral band belonging to the Second Positive System, the kinetic model and the detailed particle balance of the N2C3Πu (ν = 0) state. This new approach bridges the gap between two existing electron density measurement methods (Langmuir probe and Stark broadening). The electron density is obtained from the time-dependent rate equation for the population of N2C3Πu (ν = 0) using recorded waveforms of the absolute C3Πu → B3Πg (0-0) band intensity, the forward and reflected microwave power density. Measured electron density waveforms using numerical and approximated analytical methods are presented for the case of pulse repetitive planar surface microwave discharge at the aperture of a horn antenna covered with alumina ceramic plate. The discharge was generated in air at 11.8 Torr with a X-band microwave generator using 3.5 μs microwave pulses at peak power of 210 kW. In this case, we were able to time resolve the electron density within a single 3.5 μs pulse. We obtained (9.0 ± 0.6) × 1013 cm-3 for the peak and (5.0 ± 0.6) × 1013 cm-3 for the pulse-average electron density. The technique presents a convenient, non-intrusive diagnostic method for local, time-defined measurements of electron density in short duration discharges near atmospheric pressures. [ABSTRACT FROM AUTHOR]

In this article the refraction effects in the weak shock wave (SW) dispersion on an interface with a temperature variation between two mediums are described. In the case of a finite-gradient boundary, the effect of the SW dispersion is remarkably stronger than in the case of a step change in parameters. In the former case the vertical component of velocity for the transmitted SW (the refraction effect) must be taken into account. Results of comparative calculations based on the two-dimensional model corrected for the refraction effect show significant differences in the shapes of the dispersed SW fronts. [ABSTRACT FROM AUTHOR]

In this paper we are presenting the full characterization of a supersonic flowing CO2/N2/Ar discharge at static pressures of 1–20 Torr and Mach number 2.15. In all aspects besides the flow speed and gas temperature, these conditions correspond to Martian entry plasma. Plasma parameters were determined by optical emission spectroscopy techniques. The gas and vibrational temperature were found from analysis of the rotational and vibrational spectra of the CO B 1Σ+-A 1Π Ångstrom system, respectively. The electron density was determined from the absolute intensity of the N2 C 3Πu-B 3Πg second positive system. A kinetic model for the discharge was developed to calculate the electron density and compared with experimental data. [ABSTRACT FROM AUTHOR]

Detailed characterization of a microwave cavity discharge in the supersonic flow of Ar/H2/Air mixtures at static pressures of 1–10 Torr and Mach number 2 is mostly based on emission spectroscopy techniques. In the conditions close to real combustion environments, effects of hydrogen and air admixture to plasma parameters and population of excited species in the discharge are demonstrated. The effects resulting in ionization loss are discussed from the aspects of dominant mechanisms and consequences for the plasma assisted hydrogen oxidation. [ABSTRACT FROM AUTHOR]

We study a strong spherical expanding shock wave interacting with the finite-gradient interface between neutral cold gas and weakly ionized plasma. We want to see how the interaction with the interface can alter the shock structure compared to the case of its free propagation through the media with the exponentially varying density. From our comparative calculations based on the 2D model, we found substantial difference in the shock structure including strong deformation of the shock front followed with its gradual flattening and the redirection in its propagation. There are a number of factors that can be used to control this phenomenon in order to strengthen or lessen the effect. The calculations can be made on any scale, limited with the requirement for the shock wave to be strong. The study points at the possibility in certain applications to avoid the shock wave with its redirection rather than attenuation. The results can be applicable to optimization of the energy deposition into the supersonic flux, the drag reduction in hypersonic flight, in the detonation theory, and combustion through the control of the ignition conditions, and for environmental improvements through sonic boom reduction. Cartesian coordinates were used in order to visualize the phenomenon. [ABSTRACT FROM AUTHOR]

A nonthermal low temperature air plasma jet is characterized and applied to study the plasma effects on oral pathogens and biofilms. Experiments were performed on samples of six defined microorganisms' cultures, including those of gram-positive bacteria and fungi, and on a cultivating biofilm sample of Streptococcus mutans UA159. The results show that the plasma jet creates a zone of microbial growth inhibition in each treated sample; the zone increases with the plasma treatment time and expands beyond the entire region directly exposed to the plasma jet. With 30s plasma treatment twice daily during 5 days of biofilm cultivation, its formation was inhibited. The viability of S. mutans cells in the treated biofilms dropped to below the measurable level and the killed bacterial cells concentrated to local regions as manifested by the fluorescence microscopy via the environmental scanning electron microscope. The emission spectroscopy of the jet indicates that its plasma effluent carries an abundance of reactive atomic oxygen, providing catalyst for the observed plasma effect. [ABSTRACT FROM AUTHOR]

Interaction of shock waves with a weakly ionized gas generated by discharges has been studied. An additional thermal mechanism of the shock wave dispersion on the boundary between a neutral gas and discharge has been proposed [A. Markhotok, S. Popovic, and L. Vuskovic, Proceedings of the 15th International Conference on Atomic Processes in Plasmas, March 19–22, 2007 (NIST, Gaitersburg, MD, 2007)]. This mechanism can explain a whole set of thermal features of the shock wave-plasma interaction, including acceleration of the shock wave, broadening or splitting of the deflection signals and its consecutive restoration. Application has been made in the case of a shock wave interacting with a laser induced plasma. The experimental observations support well the results of calculation based on this model. [ABSTRACT FROM AUTHOR]

Highlights the results of a detailed analysis of planar shock waves propagating along a cylindrical glow discharge. Evidence that shock propagation velocity along the centerline of a cylindrical glow discharge has to be much higher in the cathode region than in the positive column to account for the measured propagation time; Mechanism for anomalous shock propagation velocity.

A tomographic numerical method based on the two-dimensional Radon formula for a cylindrical cavity has been employed for obtaining spatial distributions of the argon excited levels. The spectroscopy measurements were taken at different positions and directions to observe populations of excited species in the plasmoid region and the corresponding excitation temperatures. Excited argon states are concentrated near the tube walls, thus, confirming the assumption that the post discharge plasma is dominantly sustained by travelling surface wave. An automated optical measurement system has been developed for reconstruction of local plasma parameters of the plasmoid structure formed in an argon supersonic flowing microwave discharge. The system carries out angle and distance measurements using a rotating, flat mirror, as well as two high precision stepper motors operated by a microcontroller-based system and several sensors for precise feedback control. [ABSTRACT FROM AUTHOR]

The levitation of objects above a microwave horn is demonstrated. High-power microwave pulses generate a low-temperature, diffuse plasma on the surface of the horn window. The thermal effect of the surface plasma brings about a localized increase in the pressure and results in a vertical flow of air, thus levitating the object. [ABSTRACT FROM AUTHOR]

An innovative type of subcritical streamer discharge, generated by bursts of focused microwave (MW) radiation in transverse electromagnetic mode with wavelength of 10 cm, pulse duration of 40 µs, and intensity of 5.6 × 10[sub 4] W/cm², was initiated at the surface of a polymer foil at 420 Torr in air. The MW field of a quasioptical beam was three times lower than the critical breakdown field. Exposure of a polymer foil to the discharge caused a transition from hydrophobicity to hydrophilicity. [ABSTRACT FROM AUTHOR]

We determined the law of shock wave refraction for a class of extended interfaces with continuously variable gradients. When the interface is extended or when the gas parameters vary fast enough, the interface cannot be considered as sharp or smooth and the existing calculation methods cannot be applied. The expressions we derived are general enough to cover all three types of the interface and are valid for any law of continuously varying parameters. We apply the equations to the case of exponentially increasing temperature on the boundary and compare the results for all three types of interfaces. We have demonstrated that the type of interface can increase or inhibit the shock wave refraction. Our findings can be helpful in understanding the results obtained in energy deposition experiments as well as for controlling the shock-plasma interaction in other settings. [ABSTRACT FROM AUTHOR]

The kinetics of the reaction of N(3) (+) with O(2) has been studied from 120 to 1400 K using both a selected ion flow tube and high-temperature flowing afterglow. The rate constant decreases from 120 K to approximately 1200 K and then increases slightly up to the maximum temperature studied, 1400 K. The rate constant compares well to most of the previous measurements in the overlapping temperature range. Comparing the results to drift tube data shows that there is not a large difference between increasing the translational energy available for reaction and increasing the internal energy of the reactants over much of the range, i.e., all types of energies drive the reactivity equally. The reaction produces both NO(+) and NO(2) (+), the latter of which is shown to be the higher energy NOO(+) linear isomer. The ratio of NOO(+) to NO(+) decreases from a value of over 2 at 120 K to less than 0.01 at 1400 K because of dissociation of NOO(+) at the higher temperatures. This ratio decreases exponentially with increasing temperature. High-level theoretical calculations have also been performed to compliment the data. Calculations using multi-reference configuration interaction theory at the MRCISD(Q)/cc-pVTZ level of theory show that singlet NOO(+) is linear and is 4.5 eV higher in energy than ONO(+). A barrier of 0.9 eV prevents dissociation into NO(+) and O((1)D); however, a crossing to a triplet surface connects to NO(+) and O((3)P) products. A singlet and a triplet potential energy surface leading to products have been determined using coupled cluster theory at the CCSD(T)/aug-cc-pVQZ level on structures optimized at the Becke3-Lee, Yang, and Parr (B3LYP)/aug-cc-pVTZ level of theory. The experimental results and reaction mechanism are evaluated using these surfaces.