Laser ignition experiments were performed in a heptane spray using an Nd:YAG laser to investigate its ignitability. In the first set, Laser-Induced Breakdown Spectroscopy (LIBS) and imaging were employed to quantify H-alpha/O ratios, kernel size, and kernel number at various locations within the spray. While these parameters generally followed the spray profile, they did not reliably predict ignition. In the second set, OH chemiluminescence, LIBS, and high-speed imaging was utilized to understand the effect of laser energy and ignition location on ignitability. Two distinct modes of ignition failure - short and long - were identified based on kernel extinction time. For the spray conditions studied, ignition was achieved at laser energy of 250 mJ, while long-mode failure occurred at 80 mJ, and short-mode failure at 30 mJ. Optical intensities of OH and CH showed that higher laser energies generated more radical species and sustained the flame long enough to establish stable ignition. Additionally, kernel trajectories extracted from high-speed images showed that ignition is more probable for cases where the spark is generated in or moves into recirculation zone. These findings enhance our understanding of spray ignitability and can inform the development and validation of models for laser or plasma-assisted combustion in sprays.

A common dielectric gas used for high voltage switch operation is sulfur hexafluoride (SF6). While having desirable performance, SF6 is cost prohibitive and presents significant supply chain challenges. Thus, it is desirable for SF6 to be removed from current systems. This contribution presents experimental data for the operation of a high voltage switch system using stainless steel electrodes spaced 3.5 mm apart. The system is pressurized in ranges from 80-180 kPa with various mixtures of zero air and SF6. The system load is tuned with an aqueous liquid resistor filled with a sodium chloride and deionized water solution. Resistive measurements are used to generate various Paschen curves (breakdown voltages) for these gas mixtures. The linearity of these created curves is examined. Additional long data series measurements are performed to examine the system performance over many shots with each gas mixture. The data time series are examined by Weibull analysis. The experimental results can be used to quantitatively support possible gas changes within the switches.

This contribution presents hydrogen detection by coherent anti-Stokes Raman spectroscopy using a single nanosecond pulsed Nd:YAG laser and a Raman shifter cell. The frequency-doubled Nd:YAG laser beam at 532 nm is used to pump a Raman shifter cell filled with hydrogen to generate the Stokes beam at 683 nm. The beams are combined and focused in the detection cell filled with hydrogen diluted in nitrogen to generate the anti-Stokes beam that is collected and analyzed to infer the hydrogen number density. The parameters of the Raman shifter cell, mainly the laser seeding, cell pressure, and input energy, are optimized in order to maximize the detected signal. The optimum conditions were determined to be a seeded pump beam with a pressure slightly below atmospheric pressure. The optimized Raman cell was used to generate a Stokes beam for coherent anti- Stokes Raman spectroscopy in second cell filled with varying concentrations of hydrogen. Hydrogen is then detected in atmospheric pressure mixtures with nitrogen where the hydrogen concentration is diluted down to similar to 100 part-per-million (ppm) molar concentration.

This study investigates the non-equilibrium temperature dynamics in air within a femtosecond laser plasma filament using coherent anti-Stokes Raman spectroscopy (CARS). Hybrid femtosecond/picosecond CARS is demonstrated for measuring the ro-vibrational CARS spectra for a range of delays relative to the femtosecond filament. Ro-vibrational CARS fitting is used to extract the vibrational temperature temporal evolution from the experimental data by differentiating between the hot plasma region and the colder surrounding gas. This also allows for measurements of gas filament number density based on the intensity of the ro-vibrational spectrum. The rotational temperature dynamics are inferred from the measured density and calibrated using spontaneous Raman measurements.

Plasma-reforming of ammonia represents a possible strategy to leverage non-equilibrium plasma kinetics to achieve chemical mixtures that are favorable for combustion applications. Numeric modeling has shown that the amidogen radical (NH2) is an important radical species in the overall ammonia decomposition kinetics important in both reforming and combustion applications, yet quantitative measurements of NH2 radical remain elusive. The present contribution reports on the development of a compact optically-accessible ammonia plasma reactor driven by nanosecond high-voltage pulses. We also report on the development and initial measurement results of NH2 by the highly sensitive cavity ring-down spectroscopy (CRDS) absorption diagnostic. The cylindrical shaped plasma reactor is operated with high voltage pulses (5.9 kV) of duration 15 ns delivered at frequencies of 10 kHz. Fast high-voltage probes are used to measure current/voltage traces. The CRDS system is based on a pulsed dye-laser system operating at 566.9-570.9 nm and probing the.. (..,....,..)-(..,..,..) vibronic band of the. transition. We provide survey spectra showing several NH2 rotational lines from the R-branch. Specific transitions have been identified and wavelengthintegrated absorptions have been determined. Results will benefit numerical modeling efforts focused on NH3-plasma systems with applications related to both fuel reforming and plasma assisted combustion.

The present study addresses advanced monitoring techniques for particles and airborne molecular contaminants (AMCs) in cleanroom environments, which are crucial for ensuring the integrity of semiconductor manufacturing processes. We focus on quantifying particle levels and a representative AMC, hydrogen chloride (HCl), having known detrimental effects on equipment longevity, product yield, and human health. We have developed a compact laser sensor based on open-path cavity ring-down spectroscopy (CRDS) using a 1742 nm near-infrared diode laser source. The sensor enables the high-sensitivity detection of HCl through absorption by the 2-0 vibrational band with an Allan deviation of 0.15 parts per billion (ppb) over 15 min. For quantifying particle number concentrations, we examine various detection methods based on statistical analyses of Mie scattering-induced ring-down time fluctuations. We find that the ring-down distributions’ 3rd and 4th standard moments allow particle detection at densities as low as ~105 m−3 (diameter > 1 μm). These findings provide a basis for the future development of compact cleanroom monitoring instrumentation for wafer-level monitoring for both AMC and particles, including mobile platforms.

A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of ∼ 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at ∼ 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected.

We present the development of a portable Thomson scattering diagnostic system allowing simultaneous spatially and temporally resolved plasma property measurements for low density plasmas. The setup uses a compact pulsed Nd:YAG laser (532 nm) as the light source with suppression by two volume Bragg grating notch filters and dispersion with a single-stage spectrometer before measurement with an intensified camera. A key issue is the detailed light collection and how it impacts the sensitivity and elastic light suppression, for which we have investigated two optical configurations, one based on a 7 x 1 linear fiber bundle and the other based on a slit spatial-filter. We find that the configuration with the slit spatial-filter provides a higher sensitivity by a factor of similar to 2 along with more uniform spatial response. We have developed a custom pulsed-plasma setup with a modulation at 20 kHz, representative of the Hall thruster breathing mode oscillation, to show the possibility of temporally resolved measurements for electric propulsion applications. We have successfully recorded the variations in electron number density and temperature with sub-mm spatial resolution and capturing ten temporal points over the 50 mu s modulation period. The detection limit of electron density (with the spatial-filter configuration) is similar to 1.6 x 1017 m-3, which is similar to 1/10 of the plasma density in the acceleration channel of Hall thrusters.

A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of similar to 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at similar to 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected.

In this study, we examined the impact of droplet size and laser energy on droplet fragmentation and the resulting species composition due to laser irradiation of an acoustically levitated heptane droplet. Using shadowgraphy and spatially resolved laser-induced breakdown spectroscopy (LIBS), we observed two different fragmentation regimes for the conditions studied. The experiments demonstrated that low laser energy densities (<~70 mJ/mm3), designated as regime 1, resulted in a single plasma breakdown event accompanied by broadband emission and C2 Swan bands, suggesting weak plasma formation. Conversely, high energy densities (>~70 mJ/mm3), designated as regime 2, resulted in multiple plasma breakdowns that resulted in the emission of Hα, O, and N, implying a full laser breakdown in the gaseous reactive mixture. Additionally, in regime 2, we calculated the electron density using Stark broadening of the Hα line and temperature using Boltzmann analysis of O lines at 715 nm and 777 nm. We found that the electron densities and temperatures within the air spark and heptane droplets are quite similar. The findings from this research could impact the design of spray ignition systems and may also aid in validating the modeling efforts of aerosols, droplet breakdown, and ignition.

Hollow cathodes (HC) are important components of several electric propulsion devices acting as electron sources and neutralizers. Widely used barium oxide (BaO) HCs are prone to evaporation and sputtering due to plasma-heating and collisions of ions within the channel of the HC. Measuring the number density of the barium atoms that are emitted in the plasma plume of the HC will help inform models of HC physics and lifetime. In this work, cavity ringdown spectroscopy (CRDS) measurements of barium from the thermionic emitter of a heaterless BaO HC are presented. The CRDS scheme employs a pulsed laser at ~553.7 nm (vacuum wavelength) to access a ground state transition of barium and allows a Ba density detection limit of ~4x10^5 (cm-3). We have performed spatial scans of barium density within the plasma plume at a position 0.5 cm downstream from the orifice of the cathode at a current of 5 A and flow-rate of 10 sccm. An Abel inversion was used to find radial Ba density yielding a peak value of ~1.8x10^6 cm-3. We have also performed initial measurements of barium density versus axial distance, anode current, and krypton flow.

The following contribution describes the Colorado State University supersonic wind tunnel design, manufacture, assembly, and validation experiments. The facility is dedicated to studying supersonic combustion physics for future air-breathing hypersonic aircraft engines. An indraft-type tunnel was built with a simple, modular, and low capital investment design which allows for future expansions. Its main advantages are large windows (3 pairs of 3.7 '' x 7.4 '') for advanced optical diagnostics, modular experimental setup, and cycle times under 15 minutes. The test section size is adjustable, with a maximum rectangular cross section of 5.25 '' x 5.25 '' and a length of similar to 25 ''. By varying the height of the test section, we can obtain experimental times from 1 second (using the full test section size) up to 5 seconds (using a 1.57" x 5.25 '' test section) at Mach=2.4. This study focused primarily on freestream Mach characterization using three experimental methods: supersonic wedge, spherical blunt body, and total pressure measurements. The visualization of the shocks in the first two diagnostics was enabled by a high-speed folded z-type schlieren optical imaging technique. In its current configuration, all three flow diagnostics methods confirmed that the tunnel operates at Mach=2.4 at the nozzle exit, with the Mach number slowly decreasing to Mach similar to 1.5 at the exit of the test section. Ultimately, the tunnel will be used in different configurations, including to simulate combustor internal flow (i.e., "direct-connect"), as well as for simulating external supersonic flows. This facility will allow the study of laser ignition inside of a supersonic air cross-stream, plasma flame stabilization inside of a cavity, and shock-boundary layer interactions. Experiments in this tunnel will provide valuable insights into the fundamental principles of supersonic combustion.

In the present study, we designed a controlled experimental setup to investigate the microscopic interaction between a laser plasma and a single fuel droplet (diameter similar to 1mm). To achieve this, we utilized a single-axis acoustic levitator to suspend a fuel droplet and irradiate it with the fundamental wavelength ( 1064 nm) of an Nd:YAG laser at two different energies (200 mJ and 35 mJ). We employed shadowgraphy to gain insights into the impact of the incident laser energy on the fragmentation of the droplet. Additionally, we conducted spatially resolved laser-induced breakdown spectroscopy (LIBS) to quantify and analyze the variations in electron density and temperature in different regions of the droplet breakdown. At 200 mJ, we observed a forward and backward plume (consisting of droplet fragments) from the droplet and two blast waves, while at 35 mJ we observed only a forward plume and a single blast wave. We noted atomic lines of H-alpha, N I, O I, and molecular bands of C-2 from the droplet at 200 mJ. While the atomic lines were absent at 35 mJ, we observed C-2 swan bands at 470 nm, 516 nm, and 563 nm. We conducted measurements of droplet electron density and observed that, in the region where H-alpha lines were detected, the electron density was close to that of the surrounding air when the laser energy was set at 200 mJ. However, the electron temperature exhibited a lower value for the droplets as opposed to the surrounding air. This discrepancy can be attributed to the heat dissipated to the droplet during the processes of heating and vaporization. At 35 mJ, there were no prominent emission lines for H-alpha, N I, and O I, rendering it challenging to compute the plasma parameters. The findings from this research may have direct implications for spray ignition, particularly in addressing the challenge of rapid, high-altitude relight. Furthermore, our results can contribute to the development of precise quantitative calibrations for determining the local equivalence ratio.

In the present study, we designed a controlled experimental setup to investigate the effects of droplet size, laser energy, and laser-droplet positioning on the laser-induced fragmentation and resulting species composition of acoustically levitated heptane droplets. Utilizing shadowgraphy and spatially resolved laser induced breakdown spectroscopy (LIBS) diagnostics, we observed that varying laser energies and droplet sizes influenced the number and type of laser breakdown events during the laser-droplet interaction. At 80 mJ with a 1.0 mm droplet diameter, we noted multiple breakdowns and spectra with atomic lines of H-alpha, N I, O I. In contrast, for the same pulse energy with 1.5 mm droplets, we observed a single breakdown and spectra characterized by C-2 Swan bands at similar to 516 nm and similar to 563 nm without atomic lines. Similar studies at other conditions show that droplet breakup dynamics are significantly influenced by the ratio of laser energy to droplet volume. Temperature measurements based on spectral analysis show that at same laser energy, the fragmentation of larger droplets leads to a lower droplet temperature by 2000 K. These results provide insights for precisely controlling laser-droplet fragmentation and may have direct implications for spray ignition, particularly in addressing the challenge of rapid, high-altitude relight.

We present a novel sensing approach for ambient ozone detection based on deep-ultraviolet (DUV) cavity-enhanced absorption spectroscopy (CEAS) using a laser driven light source (LDLS). The LDLS has broadband spectral output which, with filtering, provides illumination between ~230–280 nm. The lamp light is coupled to an optical cavity formed from a pair of high-reflectivity (R~0.99) mirrors to yield an effective path length of ~58 m. The CEAS signal is detected with a UV spectrometer at the cavity output and spectra are fitted to yield the ozone concentration. We find a good sensor accuracy of <~2% error and sensor precision of ~0.3 ppb (for measurement times of ~5 s). The small-volume (<~0.1 L) optical cavity is amenable to a fast response with a sensor (10–90%) response time of ~0.5 s. Demonstrative sampling of outdoor air is also shown with favorable agreement against a reference analyzer. The DUV-CEAS sensor compares favorably against other ozone detection instruments and may be particularly useful for ground-level sampling including that from mobile platforms. The sensor development work presented here can also inform of the possibilities of DUV-CEAS with LDLSs for the detection of other ambient species including volatile organic compounds.

Krypton tagging velocimetry (KTV) is an emerging flow diagnostic that investigates the bulk movement of high-speed, low-pressure gas flows. The present contribution expands the typical KTV method to the lower density (vacuum) conditions of interest in electric propulsion research. Our KTV scheme utilizes a two-photon (Write) excitation at 214.7 nm from a pulsed dye laser, followed by (Read) re-excitation of the ensuing metastable at 769.5 nm with an optical parametric oscillator. For the case of cold krypton gas expanding from a hollow cathode into vacuum, we find a bulk velocity of 452 ± 37 m/s and temperature of 20 ± 16 K. We also study the flow in the plume with the hollow cathode plasma operating for which we find bulk velocity of 1200 ± 130 m/s and temperature of 880 ± 370 K. Measurements are performed at background pressures down to ∼10−5 Torr.