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We present a feasibility study on different tomographic algorithms to overcome the issues of finite sets of projection data, limited viewing angles, and noisy data, which cause the tomographic reconstruction to be an ill-posed inversion problem. We investigated three approaches: single angle Abel inversion, two angle approach, and multiple angle 2D plasma tomography. These methods were tested on symmetric and asymmetric sample functions and on experimental results from a supersonic flowing argon microwave plasma sustained in a cylindrical quartz cavity. The analysis focused on the afterglow region of the microwave flow where a plasmoid-like formation was observed. We investigated the effects of the uniform random noise added to the simulated data by applying smoothing techniques. The quality of reconstructed images was assessed by using peak signal-to-noise ratio and universal quality image measures. The results showed that the Abel inversion approach could be employed only when the system is radially symmetric, while the systems with slight asymmetry could be reconstructed with the two angle approach. In the complete absence of symmetry, full 2D tomographic reconstruction should be applied. The data analysis showed that the best results were obtained by employing either the filtered back projection or the simultaneous algebraic reconstruction technique. The total variation minimization method proved to be the best denoising technique. Each approach was used to obtain the spatial distributions of argon excited states taken at three positions along the plasmoid-like structure. The results indicated that the plasma was asymmetric with argon populating the cavity surface.

A comparative analysis of two popular spectroscopy techniques is conducted in a coaxial cylindrical capacitively coupled discharge designed for the plasma processing of superconducting radio frequency (SRF) cavities. The density of the metastable and resonant levels in Ar is measured in both Ar and Ar/Cl 2 discharges to properly characterize the unique discharge system and aid in the development of a cavity etching routine. The first method, deemed the “branching fraction method,” utilizes the sensitivity of photon reabsorption of radiative decay to measure the lower state (metastable and resonant) densities by taking ratios of spectral lines with a common upper level. This method has been gaining popularity as it does not require any a priori knowledge about the electron energy distribution. The second method is a tunable diode laser absorption spectroscopy technique that measures the thermal Doppler broadening of spectral lines, from which the neutral gas temperature and lower state density of the transition can be evaluated. The two methods were conducted in tandem, while external parameters that were empirically determined to be important to the etching mechanism of SRF cavities are varied. Relationships between the excited state densities and the external parameters are presented for both spectroscopy methods and conclusions about the effects of these parameters on the discharge are stated when appropriate.

Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces.Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is ...

A kinetic model for Martian atmospheric entry plasma (MAEP) is presented. The model was calculated based on data from Viking, Pathfinder, and the Mars Exploration Rovers Opportunity and Spirit. Calculations of the density, temperature, and electron density across the shock front were made. We employed steady-state and nonsteady-state kinetic models to describe the rate of dissociation of CO2 in the Martian atmosphere as well as the production of O2. Water vapor was included in the models since it represents 0.03% of the surface atmospheric composition. With the addition of small amounts of water vapor, we found a decrease in the dissociation of CO2 in the Martian air as well as an increase in the production of O2. This paper is to report an analysis of MAEP and its interactions with the shock front which forms in the front of the Martian Landers.

Plasma-based Nb surface treatment provides an excellent opportunity to eliminate surface imperfections and increase the cavity quality factor in important applications such as particle accelerators and cavity quantum electrodynamics, as well as Josephson junctions. In this study, plasma etching of bulk Nb is performed on the surface of disk-shaped samples with the goal of eliminating nonsuperconductive pollutants in the penetration depth region and the mechanically damaged surface layer. The authors have demonstrated that in the microwave glow discharge, an etching rate of 1.5 μm/min can be achieved using Cl2 as a reactive gas. The influence of plasma parameters such as input power, pressure, and concentration of the reactive gas on the etching rate is determined. Simultaneously, plasma emission spectroscopy was used to estimate the densities of Cl, Cl+, and Cl2 under various plasma conditions.

A microwave torch is designed to produce fan-shaped plasma, which extends about 140 mm laterally. This torch produces an abundance of reactive atomic oxygen in the plasma effluent as evidenced by its emission spectroscopy. The results of the spectral intensity measurements show that the produced atomic oxygen outside the microwave cavity distributes quite uniformly over a width of about 80 mm and reaches out more than 10 mm. An experiment applying this plasma to kill Bacillus cereus contained in an envelope has been performed. The kill rate is presented.

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured., 8 pages, 10 figures

Pulsed electric discharges were used to demonstrate the validity of plasma surface treatment of superconducting radio-frequency cavities. The experiments were performed on disc-shaped Nb samples and compared with identical samples treated with buffer chemical polishing techniques. The results of several standard surface analytical techniques indicate that plasma-treated samples have comparable or superior properties regarding the surface roughness and composition.

The development of a plasma torch, which carries desirable features to be an ignition aide within a supersonic combustor, is studied. The desirable features include: 1) a microwave adaptor arrangement to couple additional power (from microwave as well as arc discharge) to the plasma torch; 2) an additional port for fuel injection; and 3) compactness (including the power supply), portability, and light weight. The plasma jet generated by this torch is described by the cycle energy of the discharge and by the imaging of its plume under various conditions, supply air pressures, and microwave (on or off) to the torch. This torch system, with its high-voltage discharge, is operated in periodic (60-Hz) mode. The capacitors in the circuit of the power supply are charged at a line frequency of 60 Hz, resulting in a cyclical discharge. The cycle energy reaches up to 12 J. The microwave electric field has a profound effect on the arc discharge. It intensifies the emission and increases the volume of the arc loop region. It also enhances the electron temperature significantly, as shown by the emission spectroscopy of the torch. The spatial distribution of the microwave electric field outside the nozzle of the torch module is not affected by the supersonic crossflow, which is a favorable feature for increasing the penetration depth of the torch plume into the supersonic crossflow in the combustor

The air plasma effluent of an arc-seed microwave torch is used to kill bacterial spores contained inside an envelope. The torch is operated at a 60-Hz periodic mode (with about 40% duty cycle) and runs stably at a low airflow rate (e.g., 0.393 l/s). The images of plasma torch plumes show that the arc loop of the discharge prolongs from the electrodes by nearly 3 cm and microwave energizes the charge particles along the arc loop considerably. The emission spectroscopy of the torch indicates that the plasma effluent contains an abundance of reactive atomic oxygen. Bacillus cereus is chosen for the biological agent in the decontamination experiment. The experiment and the decontamination efficacy of this torch are presented. The averaged temperature inside the envelope is measured to be less than 40degC, thus ruling out the thermal decontamination mechanism

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.

The design and operation of a portable microwave plasma torch is presented. An arc plasma torch running at 60 Hz and generated by a torch module is installed on the bottom wall in the narrow section of a tapered S-band rectangular cavity, and is used to seed the microwave discharge at the location where the microwave electric field is at a maximum. This tapered cavity is designed to support the TE/sub 103/ mode. With seeding, only low Q cavity and moderate microwave power (time average power of 700 W) are needed. The microwave-enhanced discharge increases the size, cycle energy, and duty cycle of the torch plasma considerably. This torch can be run without introducing gas flow to stabilize the arc and microwave discharges. Adding gas flow can increase not only the size of the torch plasma, but also its cycle energy which reaches a plateau of about 12 J/per cycle for a gas flow rate exceeding 0.393 l/s. The electron density and excitation temperature, and the composition of torch species are determined by emission spectroscopy. It is shown that, at the bottom of the torch close to the cavity wall, electrons distribute quite uniformly across the core of the torch with density and excitation temperature determined to be about 7/spl times/10/sup 13/ cm/sup -3/ and 8000 K, respectively. It is also found that this torch produces an abundance of reactive atomic oxygen.

Results of a hydrogen Balmer line-shape study on microwave-induced plasma discharges operated with pure hydrogen and with argon–hydrogen or helium–hydrogen mixtures are reported. Plasma is generated in a rectangular or coaxial microwave cavity in two separate experiments. In both cases, the emission profiles of the Balmer lines did not show excessive broadening as reported by Mills et al. [J. Appl. Phys. 92, 7008 (2002)].

The reduction of the asymmetry in the plasma sheath voltages of a cylindrical coaxial capacitively coupled plasma is crucial for efficient surface modification of the inner surfaces of concave three-dimensional structures, including superconducting radio frequency cavities. One critical asymmetry effect is the negative dc self-bias, formed across the inner electrode plasma sheath due to its lower surface area compared to the outer electrode. The effect on the self-bias potential with the surface enhancement by geometric modification on the inner electrode structure is studied. The shapes of the inner electrodes are chosen as cylindrical tube, large and small pitch bellows, and disc-loaded corrugated structure (DLCS). The dc self-bias measurements for all these shapes were taken at different process parameters in Ar/Cl2 discharge. The reversal of the negative dc self-bias potential to become positive for a DLCS inner electrode was observed and the best etch rate is achieved due to the reduction in plasma asymmetry., Comment: 5 pages, 8 figures

An apparatus and a method for etching of the inner surfaces of superconducting radio frequency (SRF) accelerator cavities are described. The apparatus is based on the reactive ion etching performed in an Ar/Cl2 cylindrical capacitive discharge with reversed asymmetry. To test the effect of the plasma etching on the cavity rf performance, a 1497 MHz single cell SRF cavity was used. The single cell cavity was mechanically polished and buffer chemically etched and then rf tested at cryogenic temperatures to provide a baseline characterization. The cavity’s inner wall was then exposed to the capacitive discharge in a mixture of Argon and Chlorine. The inner wall acted as the grounded electrode, while kept at elevated temperature. The processing was accomplished by axially moving the dc-biased, corrugated inner electrode and the gas flow inlet in a step-wise manner to establish a sequence of longitudinally segmented discharges. The cavity was then tested in a standard vertical test stand at cryogenic temperatures. The rf tests and surface condition results, including the electron field emission elimination, are presented.

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 in order to account for the measured propagation time. This is an outcome of detailed analysis of planar shock waves propagating along a cylindrical glow discharge [B. N. Ganguly, P. Bletzinger, and A. Garscadden, Phys. Lett. A 230, 218 (1997)]. The analysis was restricted to shock front propagation time to the two well-defined positions at the centerline where the motion can be considered one dimensional. Although there is not yet a comprehensive interpretation of the anomalous propagation velocity, it seems to increase wherever number density of excited states increases. Intensified local ionization due to excessive presence of excited states combined with the excitation of ion acoustic waves could be the mechanism for anomalous shock propagation velocity.

The story about a properly oriented outdoor globe in the hands and minds of Eratosthenes, Jefferson, Milankovic and science educators is presented. Having the same orientation in space as the Earth, the Day Night Year Globe (DING) shows in real time the pattern of illumination of the Earth's surface and its diurnal and seasonal variations. It is an ideal object for the visualization of knowledge and increase in knowledge about: the form of the Earth, Earth's rotation, Earth's revolution around the Sun, the length of seasons, solstices, equinoxes, the longitude problem, the distribution of the Sun's radiation over the Earth, the impact of this radiation on Earth's climate, and how to use it efficiently. By attaching a movable vane to the poles, or adding pins around the equator to read time, DING becomes a spherical/globe-shaped sundial. So, the DING is simultaneously useful for teaching physics, geophysics, astronomy, use of solar energy and promoting an inquiry-based learning environment for students and the public.

Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate nonsuperconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk niobium (Nb) presents an alternative surface preparation method to the commonly used buffered chemical polishing and electropolishing methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on flat samples. We have achieved an etching rate of 1.7 μm/min⁡ using only 3% chlorine in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the sample surface to the environment. We intend to apply the optimized experimental conditions to the preparation of single cell cavities, pursuing the improvement of their rf performance.

We investigate the performance of a cavity microwave discharge (MWD) operating in tandem with a dielectric barrier discharge (DBD). Tandem discharge operates in helium/oxygen mixtures, where metastable molecular oxygen can be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, DBD provides the metastablesrich mixture to MWD, thereby providing seeding and modifying the discharge kinetics. Both discharges operate in synchronized pulse repetitive mode, which is tailored to maximize power efficiency of the oxygen metastable production. The system operates at pressures up to 350 Torr at average power between 3 to 20 Watt.

Summary form only given. Particle accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate non-superconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the non-equilibrium plasma treatment of bulk polycrystalline Nb presents a viable surface preparation method due to possibility to use plasma-generated radicals and due to the inherent anisotropy of the etching action. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples and achieved etching rate of 1.7 μm/min using up to 3% Cl2 in the reactive Ar/Cl2 mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the fresh sample surface to the environment.The geometry of SRF cavities made of bulk polycrystalline Nb requires the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the sheath attached to the cavity surface. The driven electrode geometry is optimized to achieve homogeneous sheath conditions at the surface. Specially designed single cell cavity is used to study these asymmetric discharges which contain 20 sample holder holes symmetrically placed over the cell. These sample holder holes can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.

In this paper we discuss the properties of the hot, ionized gas behind the bow shock during the Martian descent and/or aerobraking mostly from the perspective the system being a plasma reactor that processes large volume of the dilute atmospheric weakly ionized gas in the freestream. It becomes compressed, heated, and ionized in the shock layer. There are a few concepts of small subsystems that could be used to harvest a part of the kinetic energy converted during the entry or aerobraking phase, or to redistribute the heat flux, thereby acting as a thermal protection system. We describe the results of a kinetic model created based on free stream gas temperature, density, and velocity data from the Viking, Pathfinder, and MER Opportunity Landers. Altitudinal distribution of atmospheric composition was taken from Viking data. The density and temperature across the shock front were calculated for each probe. A temperature spread was observed due to the imprecision in the atmospheric models and data for the free stream temperature. Conditions in Mars ionosphere provide seeding electrons, which contribute to the formation of the ionizing wave in Martian atmospheric entry plasma (MAEP). Electron energy distribution function was calculated for the Martian atmosphere by solving the Boltzmann transport equation. For atmospheric gas mixtures, electron temperature and dissociation rate coefficients were then calculated. The electron density was determined from the Saha equation for a simple model with the main species for the ionized gas. A gas composition model was then used to estimate the dissociation of C0 2 in the Martian atmosphere for steady state and non-steady state conditions and a comparison was made. All this triggers many ideas for subsystems utilizing MAEP as plasma reactor.

The tomographic reconstruction of local plasma parameters for nonequilibrium plasma sources is an indispensable tool for understanding the fundamental processes and phenomena that follow the dynamics of plasmas with complex geometry. We present practical approaches that are developed on the case-to-case basis and applied to several plasma sources in our laboratories.

Local measurements of the electron density in high pressure discharges have commonly been performed through use of an electrical probe, such as a Langmuir probe, or by the Stark broadening of the hydrogen lines. However in supersonic flowing discharges electrical probes can cause shocks to form, which is unwanted. In addition, these types of discharges do not often contain the hydrogen needed to determine the electron density through Stark broadening where the measurements are hampered by the lack of intensity and breadth of the hydrogen Balmer lines. An alternative approach is to use the intensity of the rotational bands of the N 2 second positive system. We performed detailed measurements of the population densities of the N 2 C3n u -B3n g system and the hydrogen Balmer lines in a supersonic flow of weakly ionized Ar/H 2 /Air. Gases were premixed in the stagnation chamber at room temperature by adding up to 10% hydrogen and up to 45% air to pure argon. A cylindrical cavity was used to sustain a discharge in the pressure range of 100-700 Pa. Absolute emission spectroscopy was used to determine the gas temperature in the flow from the N 2 system. Comparison was made between the results obtained from the N 2 band intensity technique and Stark broadening of the hydrogen Balmer lines. The difference between line broadening technique and the nitrogen band absolute intensity technique is found to be by a factor of two to three.

Effects of water vapor presence in Martian atmospheric entry plasma (MAEP) were calculated based on entry data from the Viking, Pathfinder, and the Mars Exploration Rover Opportunity Landers. Calculations of the temperature and electron density across the shock front were made. We employed steady state and non-steady state kinetic models to describe the rate of dissociation of CO2 in the Martian atmosphere as well as the production of O2. Water vapor was included in the models since it represents 0.03% of the surface atmospheric composition. With the addition of small amounts of water vapor, we found a decrease in the dissociation of CO2 in the Martian air as well as an increase in the production of O2.

Recently we introduced a new mechanism of the shock wave SW) dispersion on a boundary between two mediums with different temperatures [1]. The need of bringing up this mechanism was to explain the odd shapes of the deflection signals from the shock wave as it entered the hot discharge regions with strong temperature gradients. Now we consider same effect but under specific conditions on the interface with weak gradients at the interface, but stronger in the bulk. These conditions appear more realistic for most applications, and what is also important, the SW dispersion in this case becomes stronger. We derive all relations using the approach similar to [1]. Then the model applied to calculate the shock wave dispersion on a plasma sphere numerically and compared with the existing experiments. The strength of the effect is demonstrated by comparing results of calculation for different conditions on the interface. Comparative calculations show significant difference in the shock front shapes and they are in complete agreement with the experiments. The advantage of the proposed model is that it offers rather simple method of calculations which can cut the computational cost substantially by eliminating the need to employ laborious and sophisticated numerical procedures. More results are underway which can contribute to understanding of basic phenomena in weakly ionized gases, combustion, and fluid dynamics on the interfaces.

In the references [1, 2] a new mechanism of the shock wave dispersion has been introduced. The model of the shock wave dispersion proposed there can explain not only a whole complex of features of the shock wave - discharge interaction observed in experiments, but also offers a way to control the shape of the shock wave front. In this paper we discuss the possibility to change the shock wave front shape by use the superposition of the two thermal mechanisms of the shock wave dispersion. By appropriate choice of the boundary shape or the temperature distribution, the shock front can be modified to any shape required by an application. Application to the case of hollow cathode region of D.C. discharge demonstrates such a possibility of the shock front modification. The proposed solution is not limited to the mechanisms of the shock wave dispersion considered in this paper, but rather offers a broad possibility of the shock front modification with other known mechanisms.

Since the first observations that a very small ionized fraction (order of 1 ppm) could strongly affect the gas flow, numerous experiments with partially or fully wall‐free discharges have demonstrated the dispersion of shock waves, the enhancement of lateral forces in the flow, the prospects of levitation, and other aerodynamic effects with vast potential of application. A review of physical effects and observations are given along with current status of their interpretation. Special attention will be given to the physical problems of energy efficiency in generating wall‐free discharges and the phenomenology of filamentary discharges. Comments and case examples are given on the current status of availability of necessary data for modelling and simulation of the aerodynamic phenomena in weakly ionized gas.

The ionization rates of the electron-impact ionization and dissociative ionization of ethylene for two typical cases: thermal plasmas with an electron temperature in the range between 1000 and $24\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; and nonthermal plasmas with the reduced electric field in the range between 10 and $200\phantom{\rule{0.3em}{0ex}}\mathrm{Td}$ are presented. Electron-impact dissociative ionization rates were calculated for 11 fragment ions of ethylene in the case of low temperature thermal plasma, and in a case of nonthermal ionized mixture of argon and ethylene. Dissociative ionization cross sections were calculated using a semiempirical binary-encounter bethe (BEB) model [Y. K. Kim and M. E. Rudd, Phys. Rev. A 50, 3954 (1994)], with each of the four most dominant fragments, ${\mathrm{C}}_{2}\mathrm{H}_{4}{}^{+}$, ${\mathrm{C}}_{2}\mathrm{H}_{3}{}^{+}$, ${\mathrm{C}}_{2}\mathrm{H}_{2}{}^{+}$, and ${\mathrm{H}}^{+}$, being associated with a single molecular orbital. Calculated cross sections are used in this analysis due to the fact that the existing experimental data are the least accurate in the threshold region and the calculated results can improve the accuracy in that region. Also, the procedure may be extended to molecules for which experimental data are not available.

Development of a plasma torch, which carries desirable features for being an ignition aide within a supersonic-combustor, is studied. The desirable features include 1) a microwave adaptor arrangement to couple additional power (from microwave as well as arc discharge) to the plasma torch; 2) an additional port for fuel injection; and 3) compact (including the power supply), portable, and light weight. The plasma jet generated by this torch is described by the cycle energy of the discharge and through imaging of its plume under various conditions, the supply air pressure and microwave (on or off) to the torch. This torch system, with its high voltage discharge, is operated in periodic (60-Hz) mode. The capacitors in the circuit of the power supply are charged at the line frequency of 60 Hz, resulting in a cyclical discharge. The cycle energy is up to 12 J. The microwave electric field has a profound effect on the arc discharge. It intensifies the emission and increases the volume of the arc loop region. It also enhances the electron temperature significantly as shown by the emission spectroscopy of the torch. The spatial distribution of the microwave electric field outside the nozzle of the torch module is not affected by the supersonic cross flow, a favorable feature for increasing the penetration depth of torch plume into the supersonic crossflow in the combustor.

Aerodynamic effects in ionized gases, often neglected phenomena, have been subject of a renewed interest in recent years. After a brief historical account, we discuss a selected number of effects and unresolved problems that appear to be relevant in both aeronautic and propulsion applications in subsonic, supersonic, and hypersonic flow. Interaction between acoustic shock waves and weakly ionized gas is manifested either as plasma‐induced shock wave dispersion and acceleration or as shock‐wave induced double electric layer in the plasma, followed by the localized increase of the average electron energy and density, as well as enhancement of optical emission. We describe the phenomenology of these effects and discuss several experiments that still do not have an adequate interpretation. Critical for application of aerodynamic effects is the energy deposition into the flow. We classify and discuss some proposed wall‐free generation schemes with respect to the efficiency of energy deposition and overall gene...

The study of weakly ionized gas at atmospheric and near-atmospheric pressure has been attracting an increased interest due to the enormous potential for applications. Atomic and molecular processes drive its complex phenomenology. Excited atoms, especially ones with a long lifetime, have a very important role acting as both an energy reservoir and as an agent of energy transfer. We are using the exemplary case of ionizing shock waves in neon to illustrate how a complete knowledge of the collisional dynamics involving excited atoms is crucial for fundamental understanding of the properties of high-pressure, weakly ionized gas.

A periodic series of microwave-induced plasmoids is generated using the outgoing wave from a microwave horn and the reflected wave from a nearby on-axis concave reflector. The plasmoids are spaced at half-wavelength separations according to a standing-wave pattern. The plasmoids are enhanced by an effective focusing in the near field of the horn (Fresnel region) as a result of a diffractive narrowing. Optical imaging, electron density, and rotational temperature measurements characterize the near field plasma region. Volumetric microwave discharges may have application to combustion ignition in scramjet engines.

Various types of high‐pressure non‐thermal discharges are increasingly drawing attention in view of many interesting applications. These, partially ionized media in non‐equilibrium state, tend to generate complex effects that are difficult to interpret without a detailed knowledge of elementary processes involved. Electronically excited molecules and atoms may play an important role as intermediate states in a wide range of atomic and molecular processes, many of which are important in high‐pressure discharges. They can serve also as reservoirs of energy or as sources of high energy electrons either through the energy pooling or through superelastic collisions. By presenting the analysis of current situation on the processes involving excited atoms and molecules of interest for high‐pressure gas discharges, we will attempt to draw attention on the insufficiency of available data. In the same time we will show how to circumvent this situation and still be able to develop accurate models and interpretations...

We present a new approach to measure population densities of Ar I metastable and resonant excited states in low temperature Ar plasmas at pressures higher than 1 Torr. This approach combines the time resolved laser induced fluorescence technique with the kinetic model of Ar. The kinetic model of Ar is based on calculating the population rates of metastable and resonant levels by including contributions from the processes that affect population densities of Ar I excited states. In particular, we included collisional quenching processes between atoms in the ground state and excited states, since we are investigating plasma at higher pressures. We also determined time resolved population densities of Ar I 2 p excited states by employing optical emission spectroscopy technique. Time resolved Ar I excited state populations are presented for the case of the post-discharge of the supersonic flowing microwave discharge at pressures of 1.7 and 2.3 Torr. The experimental set-up consists of a pulsed tunable dye lase...

Summary form only given. Electrodeless microwave-excited rare-gas halide lamps are presently among the most powerful ultraviolet light sources, capable of emitting cw radiation power of several kilowatts in the spectral range between 200 and 350 nm. Development and optimization of high-pressure microwave-excited excimer lamps depends on the knowledge of the excimer plasma parameters, particularly electron temperature and density. Non-coherent excimer light sources are relatively new devices and the information on their plasma parameters is still incomplete. Standard diagnostic techniques of high density discharges are inapplicable in this case, because of the molecular excited state structure and absence of self-absorption. We will present description of two techniques for determination of plasma temperature and electron density. Temperature measurement is based on the equilibrium population of certain vibrational levels of excimer excited states. Electron density was determined from the measurements of Stark profiles of H/sub /spl beta// radiation from a small amount of hydrogen mixed with rare-gas halides. Lamps were specially designed and constructed to meet the basic diagnostic requirements, to permit axial radiation detection preserving microwave coupling characteristics of the commercial microwave cavity. Description of lamps, diagnostic apparatus and the method will be presented. Variation of plasma parameters depending on the initial partial pressures of gas components will be shown.

We studied shock wave propagation and dispersion through a transverse electric field cavity microwave discharge. Electrodeless discharges are convenient active plasma media for shock dispersion studies in weakly ionized gas since there are no solid objects in the path of the shock wave that may distort the shock structure. Discharge was generated in a cylindrical cavity powered by one or two magnetrons operating at the commercial S-band frequency. Shock diagnostics include planar laser sheet deflection system, designed to determine radial and axial distribution of shock front velocity, as well as the transversal dispersion effects. Discharge diagnostics included spectroscopic determination of electron density and temperature, electric field, and vibrational and rotational temperatures of the nitrogen molecules. Our measurements indicate a correlation between the distribution of plasma density and the intensity of dispersion and propagation effects in the discharge.

We are reporting experimental results on a microwave discharge operating at resonant frequency in a multi-cell radio frequency (RF) accelerator cavity. Although the discharge operated at room temperature, the setup was constructed so that it could be used for plasma generation and processing in fully assembled active superconducting radio-frequency cryo-module. This discharge offers a mechanism for removal of a variety of contaminants, organic or oxide layers, and residual particulates from the interior surface of RF cavities through the interaction of plasma-generated radicals with the cavity walls. We describe resonant RF breakdown conditions and address the issues related to resonant detuning due to sustained multi-cell cavity plasma. We have determined breakdown conditions in the cavity, which was acting as a plasma vessel with distorted cylindrical geometry. We discuss the spectroscopic data taken during plasma removal of contaminants and use them to evaluate plasma parameters, characterize the process, and estimate the volatile contaminant product removal.

A one-dimensional f ow model for weakly ionized g&‘is described. It is based on a conventional model, modified to account for the separation of charge and detailed description of non-steady state ionization-recombination process. The model was applied to simulate excited-state population profile across the shock structure in ionizing neon. Excited-state population of precursor and relaxation region of the shock structure obtained in the experiment was several orders of magnitude higher tbau predicted by conventional model. Present analysis has shown that the dominant mechanism in the modification of precursor and relaxation regions is the difbrsion of radiation from the equilibrium region, across the shock structure.

Martian atmospheric gas processed by a radio frequency (RF) discharge result in a mixture of predominantly oxygen and carbon monoxide, with a minor concentration of carbon dioxide and other residual components. Recombination of oxygen and carbon monoxide into carbon dioxide is slow enough to allow the extraction of oxygen, the exhaust stream can approach 90% carbon monoxide. Its value as a propellant is recognized and an oxidizer/fuel production system analyzed. In addition, dust removal capability of RF discharge is described.

We have investigated the performance of a microwave-cavity discharge (MWD) operating in tandem with a fast rise-time pulsed dielectric-barrier discharge (DBD). The tandem discharge operated in a helium/oxygen mixture, where metastable molecular oxygen could be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, a DBD operating at high E/N provided a metastables-rich mixture, thereby modifying the discharge kinetics of the MWD, which operated in the E/N range centered around 10?Td. Both discharges operated in synchronized pulse-repetitive mode, which was tailored to maximize the oxygen metastable production efficiency. The system operated at pressures up to 350?Torr with an average power between 3 and 20?W.

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 treatedbiofilms 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.

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.

The effectiveness and mechanism of a low temperature air plasma torch in clotting blood are explored. Both blood droplets and smeared blood samples were used in the tests. The treated droplet samples reveal how blood clotting depends on the distance at which the torch operated, and for how long the droplets have been exposed to the torch. Microscopy and cell count of smeared blood samples shed light on dependencies of erythrocyte and platelet counts on torch distance and exposure time. With an increase of torch distance, the platelet count of treated blood samples increases but is less than that of the control. The flux of reactive atomic oxygen (RAO) and the degree of blood clotting decreased. With an increase of exposure time, platelet count of treated samples decreased, while the degree of clot increased. The correlation among these dependencies and published data support a blood clotting mechanism that RAO as well as other likely reactive oxygen species generated by the plasma torch activate erythrocyte-platelets interactions and induces blood coagulation.

Cavity quantum electrodynamics (QED) is a powerful experimental tool for investigating the fundamental aspects of coherence and decoherence in quantum mechanics. The core of each cavity QED experimental set-up is an open cavity made of two superconducting niobium (Nb) mirrors that are facing each other. For cavity QED application, the surface imperfection of Nb mirrors contributes to lowering the achievable cavity quality factor and therefore decreasing the field energy damping time of the cavity. Therefore, particular attention must be devoted to the cavity surface preparation. Here, we present the advantages of bulk Nb compared with thin film mirrors and other processes involved in surface preparation. Special attention is devoted to plasma-based Nb surface treatment that provides an excellent opportunity to eliminate surface imperfections and increase the cavity quality factor. We present ongoing work on Nb surface preparation using the microwave glow discharge system with Cl2 as a reactive gas.

Singlet oxygen, O2(a 1Δg), is generated using a low pressure, low power continuous microwave discharge operating at 2.45 GHz with a flow of helium seeded with 1–10% molecular oxygen. The absolute concentration of O2(a 1Δg) is measured using off-axis integrated cavity output spectroscopy to probe the Q-branch transition of the (1, 0) band of the Noxon system. In order to remove other energetic species from the flow, the post-discharge flow is passed through a coarse fritted quartz filter. The use of the quartz frit takes advantage of the substantially lower surface sticking probability of O2(a 1Δg) in comparison with other excited species on the flow. Up to 6% of the total oxygen passing through the filter remains in the a 1Δg state, and absolute densities of 2.5 × 1014 cm−3 are obtained using this method. This preparation method and transport is important in developing sources of singlet oxygen for kinetic and spectroscopic studies.

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.