Showing papers by "Richard B. Miles published in 2006"

Modeling of Interaction Between Weakly Ionized Near- Surface Plasmas and Gas Flow

Abstract: Detailed physical model for asymmetric dielectric barrier discharge (DBD) in air is developed. Modeling of DBD with applied sinusoidal voltage is carried out. The leading role of charging the dielectric surface by electrons in the cathode phase is shown to be critical, acting as a harpoon that pulls positive ions forward and accelerates the gas in the anode phase. The positive ion motion back towards the exposed electrode is shown to be a major source of inefficiency in the sinusoidal or near-sinusoidal voltage cases. Based on understanding of the DBD physics, an optimal voltage waveform is proposed, consisting in high repetition rate short (a few nanoseconds in duration) negative pulses combined with positive dc bias applied to the exposed electrode. The velocity of near-surface gas jet produced by the DBD actuator thus optimized is shown to be considerably (potentially – by 1-2 orders of magnitude) greater than that for a sinusoidal signal with similar parameters.

Microwave diagnostics of laser-induced avalanche ionization in air

Abstract: This work presents a simplified model of microwave scattering during the avalanche ionization stage of laser breakdown and corresponding experimental results of microwave scattering from laser breakdown in room air. The model assumes and measurements confirm that the breakdown regime can be viewed as a point dipole scatterer of the microwave radiation and thus directly related to the time evolving number of electrons. The delay between the laser pulse and the rise of the microwave scattering signal is a direct measure of the avalanche ionization process.

Magnetohydrodynamic Power Generation Using Externally Ionized, Cold, Supersonic Air as Working Fluid

Abstract: Magnetohydrodynamic (MHD) power extraction from cold air has been demonstrated using short-duration, high-repetition rate, high-voltage pulses (2 ns, 100 kHz, 5 kV/cm) to ionize a Mach 3 (600 m/s), 0.04 kg/m 3 flow. Because the power used to ionize the flow using such a method was less than 1% of the total flow enthalpy, the flow was not heated significantly. A few tens of milliwatts were extracted from the 3-cm cube region of ionization, which scales to hundreds of kilowatts of power in higher velocity, larger-scale devices that would be appropriate for flight applications. Peak electron number densities between 5 × 10 11 and 10 12 cm -3 are reported from complementary measurements using microwave absorption in a variable magnetic field. The Hall parameter was estimated from the electrical properties of the MHD channel. Modeling predictions were found to be in agreement with experimentally extracted Faraday current measured between the high-voltage pulses. Modeling also confirmed that the electrons have low energy between the pulses and that the resultant cathode and anode voltage falls are quite low, on the order of 1 V.

Steering Moments Creation in Supersonic Flow by Off-Axis Plasma Heat Addition

Abstract: The change in aerodynamic forces as a result of local plasma heat addition to supersonic flow (Mach 3.0), upstream of a cone, is studied numerically by solving the three-dimensional compressible Euler equations. In principle, such an effect on the forces and moments can be used for vehicle steering as well as drag reduction. Local energy addition to the flow is achieved by the use of microwave radiation as a heating-source and an electron beam to control the air conductivity and consequently the location of the energy deposition. This approach requires heating only in a localized preionized region, and so the strength of the microwave field has to be much lower than the critical value of the electric field at breakdown. Results show the potential effects of heat addition on the aerodynamic forces. The corresponding power and the optimized location required to achieve these effects are discussed.

Emergence of disinhibition-induced synchrony in the CA3 region of the guinea pig hippocampus in vitro

Abstract: Suppressing inhibition mediated by GABAA receptors induces rhythmic bursts of synchronous firing in the CA3 region of the hippocampus. Extracellular and intracellular records were made from guinea pig hippocampal slices to examine the emergence of this synchrony. We found that application of GABAA receptor antagonists initiated a sequence of changes in the activity of the CA3 neuronal population. First, the frequency of firing detected in multiunit records increased. Then, firing began to oscillate with increases followed by decreases in firing that occurred at intervals of 0.5–2 s. The coherence of the rhythmic activity at a single site increased with time, and discharges at distant sites in the CA3 region became correlated. Fluctuations in firing were associated with extracellular field potentials. Finally, epileptiform events associated with large field potentials began to recur at intervals of 5–10 s. The onset of fully synchronous events was sudden and correlated with a large increase in the amplitude of the field potential. Thus the CA3 population can express states of partial population synchrony preceding the onset of epileptiform discharges. A similar activity was induced and maintained by applying low doses of GABAA receptor antagonists. Intracellular records suggest that inhibitory signalling mediated by GABAB receptors contributes to the emergence of this activity. States of partial synchrony in the CA3 region exposed to GABAA receptor antagonists therefore depend on alternating periods of firing, presumably dependent on excitatory synaptic mechanisms, and silence, mediated in part by the activation of GABAB receptors.

Measurements of hydrocarbon flame speed enhancement in high-Q microwave cavity

Abstract: In this work we demonstrate that a small amount of microwave power below its breakdown threshold can be locally absorbed into a flame combustion zone. The absorbed microwave power can significantly change the flame speed of both laminar and turbulent flames. PIV technique was employed to measure the laminar flame speed. It was found that microwave assisted flame speed enhancement was greatly dependent on Q of the microwave cavity. Due to the unsteady nature of interaction, microwave assisted flame speed measurements were difficult to make, however, preliminary observations of the flame luminosity indicated that there was energy addition occurring without microwave breakdown and the flame speed was increased.

The subiculum comes of age

Abstract: The subiculum has long been considered as a simple bidirectional relay region interposed between the hippocampus and the temporal cortex. Recent evidence, however, suggests that this region has specific roles in the cognitive functions and pathological deficits of the hippocampal formation. A group of 20 researchers participated in an ESF-sponsored meeting in Oxford in September, 2005 focusing on the neurobiology of the subiculum. Each brought a distinct expertise and approach to the anatomy, physiology, psychology, and pathologies of the subiculum. Here, we review the recent findings that were presented at the meeting. © 2006 Wiley-Liss, Inc.

Radar return enhanced by a grating of species-selective multiphoton ionization as a probe for trace impurities in the atmosphere

Abstract: Two-color time-ordered dyads of short laser pulses induce a spatially periodic modulation of the refractive index of the atmosphere through resonance-enhanced multiphoton ionization (REMPI), enhancing the back reflection of radio waves The carrier frequencies and the timing of laser pulses in a dyad provide a unique code for selectively accessing the manifold of energy levels of impurity molecules, inducing REMPI gratings of the refractive index only in the presence of impurity molecules, thus suggesting an attractive strategy for the radar-based stand-off detection of trace impurities in the atmosphere

Method and apparatus for remotely monitoring properties of gases and plasmas

Abstract: A method and apparatus for remotely monitoring properties of gases and plasmas is disclosed. A laser beam is focused at a desired region within a gas or plasma to be analyzed, generating an ionized sample region in the gas or plasma. A beam of microwave radiation is directed toward the ionized sample region, and a portion of the microwave radiation is scattered by the ionized sample region and Doppler-shifted in frequency. The scattered, frequency-shifted microwave radiation is received by a microwave receiver, and is processed by a microwave detection system to determine properties of the gas or plasma, including velocities, temperatures, concentrations of molecular species, and other properties of the gas or plasma.

Diagnostics by RADAR REMPI: Microwave Scattering from Laser-induced Small-volume Plasmas

Abstract: This work presents predictions and measurements of microwave scattering properties of laser sparks in air. This work is part of an ongoing study of Resonant Enhanced Multiphoton Ionization (REMPI) for the generation of a species selective ionization region and the use of microwaves (Radar) to detect the presence of that ionization. From a single laser induced breakdown, the model assumes and measurements confirm that the breakdown regime can be viewed as a point dipole scatterer of the microwave radiation and thus the detected microwave signal is directly related to the time evolving number of electrons. The delay between the laser pulse and the rise of the microwave scattering signal is a direct measure of the avalanche ionization process. If the volume is exposed to two sequential pulses, the experiments show that the signal amplitude increases significantly if the pulses are separated by less that a few tens of nanoseconds. Tens of microsecond delay can be used to extend the lifetime of the plasma. An examination of two simultaneous laser induced breakdowns separated in space shows that the microwave scattering from multiple breakdowns is coherent.

Rayleigh scattering measurements for obtaining spatiallyresolved absolute gas densities in large scale facilities

Abstract: A systematic approach for the use of Rayleigh scattering to determine spatially resolved (along a line) gas densities is presented. Lessons learned from earlier (2003) measurements on the MARIAH II Radiatively-Driven Hypersonic Wind Tunnel (RDHWT) are incorporated into an approach which seeks to address all of the previous difficulties encountered, providing the requisite spatial resolution, suppression of scattered light, calibration and correction for laser shot-to-shot fluctuations in order to obtain data of sufficient quality to validate the predictive MARIAH II models for a 1 MW electron beam coupled into a supersonic flow. A statistical analysis is used to ascertain the necessary measurement accuracy, based on the final required accuracy for the density for its particular application. Laboratory tests are carried out on a free jet expansion using various test gases - air, helium, methane, ethylene and propane – to simulate the density variations expected in the actual experiments. The results are processed and analyzed quantitatively to verify the approach. Although developed with the above-mentioned wind tunnel in mind, the method is generally and universally applicable to any large scale facility.

Microwave Diagnostics of Laser-induced Small-volume Plasma

Abstract: *† ‡ This work presents the preliminary results of an ongoing study of microwave scattering from a laser-induced, small-volume plasma in room air. Microwave scattering from the precursor and after-spark evolution reveal the dynamic characteristics of the laser spark formation, growth and decay. In particular, the avalanche ionization phase is detected by microwave scattering in the precursor part, during and shortly after the laser pulse. A simplified model of the microwave scattering from this early stage of laser breakdown is presented and shown to qualitatively fit the experimental results. Subsequent features of the microwave signal provide further insight into the after-spark evolution and are still under study. This work also discusses the possibilities and limitations of measuring the local electron number density by laser breakdown enhanced microwave scattering.

Microwave diagnostics of a repetitive, short-pulse-sustained, weakly ionized, air plasma under the influence of a magnetic field

Abstract: A microwave-transmission-based diagnostic method is presented here, applicable to plasmas having electron collision frequencies up to about twice their electron plasma frequency, and under the influence of an applied magnetic field. This technique is capable of measuring both electron number density and collision frequency. By varying the intensity of the applied magnetic field, the frequency of the upper hybrid resonance for transmission of extraordinary waves, as predicted by the Asher-Appleton-Hartree dispersion relation, is scanned through the microwave diagnostic frequency. Qualitatively, the zero transmission location of the resonant band depends on the electron number density, and the existence of the band depends on the collision frequency. Because there is essentially zero transmission through the resonant band, the measurement is accomplished by determining the ranges of microwave frequencies and magnetic field intensities that yield no transmission. In contrast to more standard techniques, the results presented here do not rely on accurate measurement of the transmission fraction or phase shift. The technique relies instead on measuring the applied field strengths and diagnostic frequencies that yield an opaque plasma. The opacity of the plasma can be robust with respect to the refraction, diffraction, multiple reflections, and impedance matching that can plague accurate measurements of microwave transmission fraction and phase shift, particularly in the case of small plasmas with near field geometries

Modeling of Thermionic Devices with Nonequilibrium Inert Gas Plasmas

Abstract: A novel concept of thermionic energy conversion with inert gas filled cells is proposed. The low-pressure weakly ionized plasma in the cell eliminates the space charge limit on the current. Nonequilibrium ionization can allow the use of low-pressure inert gases instead of cesium vapor. Preliminary modeling of plasma dynamics and kinetics shows that under certain conditions the electric field-induced heating of plasma electrons can result in selfsustained ionization sufficient for the argon-filled device functioning without external ionization. The self-sustained nonequilibrium ionization regime is also characterized by oscillations of electron temperature and density. At relatively low voltages, the heating of plasma electrons is not sufficient to sustain the ionization, and repetitive short pulses can be used to operate the inert gas filled device.

Modeling of Thermionic Devices With Nonequilibrium Inert Gas Plasmas

Abstract: A novel concept of thermionic energy conversion with inert gas filled cells is proposed. The low-pressure weakly ionized plasma in the cell eliminates the space charge limit on the current. Nonequilibrium ionization can allow the use of low-pressure inert gases instead of cesium vapor. Preliminary modeling of plasma dynamics and kinetics shows that under certain conditions the electric field-induced heating of plasma electrons can result in selfsustained ionization sufficient for the argon-filled device functioning without external ionization. The self-sustained nonequilibrium ionization regime is also characterized by oscillations of electron temperature and density. At relatively low voltages, the heating of plasma electrons is not sufficient to sustain the ionization, and repetitive short pulses can be used to operate the inert gas filled device.

One GHz linewidth, 33 line per mm, wide angle imaging filter at the potassium resonant line

Abstract: This paper presents a narrow linewidth, high resolution, and high quantum efficiency imaging transmission filter based on optical trapping of resonance radiation in potassium vapor. The filter can be used to image radiation over a bandwidth narrow enough to fall within a Fraunhofer dark zone in the solar spectrum, and it can be applied to the imaging of flames, plumes or discharges containing potassium. It may also be applicable to the imaging of Raman scattering from a tunable laser. The spectral and imaging properties of the filter are demonstrated with a 1 cm aperture optically thick potassium cell illuminated by a narrow linewidth tunable laser. The spectral width at the potassium D2 line wavelength, 766.5 nm, is shown to be 1 to 2 GHz (.002 nm). At the line center, the quantum efficiency is better than 60% and the imaging resolution is better than 30 line pairs per mm. By employing a 200 micron “thin” potassium vapor cell, it is also shown that the filter maintains the high quantum efficiency (~50%) and good imaging capability (~20 lines per mm) across the 2 GHz spectral bandwidth of the cell. The “thin” cell has an out-of-band rejection of better than 1000. Its operation is demonstrated with a tunable laser as well as with broad band light from a potassium lamp and from a potassium chloride seeded flame.

Seymour M. Bogdonoff and the Princeton Gasdynamics Laboratory

Abstract: The Princeton Gasdynamics Laboratory was founded in about 1950 by Lester Lees and Seymour Bogdonoff. Bogdonoff became its Director in 1953, and remained in that post until 1989. Under his direction the Laboratory became a national powerhouse in aeronautical research. At its peak, the laboratory employed about six or seven faculty, maybe 50 graduate students, and 10 or 12 research associates and technicians, all working with a large range of wind tunnels designed by Bogdonoff, often aided by his close associate Irwin Vas. Here, we review some of the history and accomplishments of the Gasdynamics Laboratory.

COMMENTARY The Subiculum Comes of Age

Abstract: The subiculum has long been considered as a simple bidirectional relay region interposed between the hippocampus and the temporal cortex. Recent evidence, however, suggests that this region has specific roles in the cognitive functions and pathological deficits of the hippocampal formation. A group of 20 researchers participated in an ESF-sponsored meeting in Oxford in September, 2005 focusing on the neurobiology of the subiculum. Each brought a distinct expertise and approach to the anatomy, physiology, psychology, and pathologies of the subiculum. Here, we review the recent findings that were pre- sented at the meeting. V V C 2006 Wiley-Liss, Inc.