Development of a high-intensity, nanosecond, pulsed vacuum ultraviolet light source Final technical report, Oct. 1966 - Jan. 1968
High intensity, nanosecond, pulsed vacuum ultraviolet light source
Weissler, G. L.↗
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On the origins of the continuum radiation of an underwater nanosecond pulsed discharge: an absolute-intensity optical emission spectroscopy study
The plasma properties of an underwater nanosecond pulsed discharge remain not fully understood despite being extensively studied for several decades. In this work, we focus on the continuum radiation generated in such discharges. The discharge is characterized by power measurements as well as by absolute emission spectroscopy. When observed, Stark broadenings of H α , H β and O (777 nm) are employed for electron number densities measurements. The discharge was generated by a 10 ns main voltage pulse followed by multiple secondary pulses, which last up to 4 μs after the primary pulse. It is shown that a peak power of 3.5 MW and energy of 35 mJ is coupled during the main voltage pulse. A quantitative estimation of the different possible continuum radiation sources is performed through analytical calculations. This includes emission (blackbody, free–bound and free–free bremsstrahlung radiations) and absorption (electron–ion and electron–neutral free–free inverse bremsstrahlung) mechanisms. Our results suggest that electron–neutral free–free bremsstrahlung is the principal mechanism responsible for the strong continuum radiation observed experimentally during the primary pulse. We also show that self-absorption through electron–neutral (and electron–ion) inverse bremsstrahlung plays an important role in the main discharge pulse. Further, our results indicate the non-negligible additional contribution of the H 2 continuum during the first reflected pulse which is likely ignited in bubbles generated by the first discharge pulse.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Plasma thermal-chemical instability of low-temperature dimethyl ether oxidation in a nanosecond-pulsed dielectric barrier discharge
Plasma stability in reactive mixtures is critical for various applications from plasma-assisted combustion to gas conversion. To generate stable and uniform plasmas and control the transition towards filamentation, the underlying physics and chemistry need a further look. Here, this work investigates the plasma thermal-chemical instability triggered by dimethyl-ether (DME) low-temperature oxidation in a repetitive nanosecond pulsed dielectric barrier discharge. First, a plasma-combustion kinetic mechanism of DME/air is developed and validated using temperature and ignition delay time measurements in quasi-uniform plasmas. Then the multi-stage dynamics of thermal-chemical instability is experimentally explored: the DME/air discharge was initially uniform, then contracted to filaments, and finally became uniform again before ignition. By performing chemistry modeling and analyzing the local thermal balance, it is found that such nonlinear development of the thermal-chemical instability is controlled by the competition between plasma-enhanced low-temperature heat release and the increasing thermal diffusion at higher temperature. Further thermal-chemical mode analysis identifies the chemical origin of this instability as DME low-temperature chemistry. This work connects experiment measurements with theoretical analysis of plasma thermal-chemical instability and sheds light on future chemical control of the plasma uniformity.
repetitive nanosecond pulses↗
Measurements of atoms and metastable species in N 2 and H 2 –N 2 nanosecond pulse plasmas
We report time-resolved, absolute number densities of metastable N 2 (A 3 Σ u + , v = 0, 1) molecules, ground state N 2 and H atoms, and rotational–translational temperature have been measured by tunable diode laser absorption spectroscopy and two-photon absorption laser-induced fluorescence in diffuse N 2 and N 2 –H 2 plasmas during and after a nanosecond pulse discharge burst. Comparison of the measurement results with the kinetic modeling predictions, specifically the significant reduction of the N 2 (A 3 Σ u + ) populations and the rate of N atom generation during the burst, suggests that these two trends are related. The slow N atom decay in the afterglow, on a time scale longer than the discharge burst, demonstrates that the latter trend is not affected by N atom recombination, diffusion to the walls, or convection with the flow. This leads to the conclusion that the energy pooling in collisions of N 2 (A 3 Σ u + ) molecules is a major channel of N 2 dissociation in electric discharges where a significant fraction of the input energy goes to electronic excitation of N 2 . Additional measurements in a 1% H 2 –N 2 mixture demonstrate a further significant reduction of N 2 (A 3 Σ u + , v = 0, 1) populations, due to the rapid quenching by H atoms accumulating in the plasma. Comparison with the modeling predictions suggests that the N 2 (A 3 Σ u + ) molecules may be initially formed in the highly vibrationally excited states. The reduction of the N 2 (A 3 Σ u + ) number density also diminishes the contribution of the energy pooling process into N2 dissociation, thus reducing the N atom number density. The rate of N atom generation during the burst also decreases, due to its strong coupling to N 2 (A 3 Σ u + , v) populations. On the other hand, the rate of H atom generation, produced predominantly by the dissociative quenching of the excited electronic states of N 2 by H 2 , remains about the same during the burst, resulting in a nearly linear rise in the H atom number density. Comparison of the kinetic model predictions with the experimental results suggests that the yield of H atoms during the quenching of the excited electronic state of N 2 by molecular H 2 is significantly less than 100%. The present results quantify the yield of N and H atoms in high-pressure H 2 –N 2 plasmas, which have significant potential for ammonia generation using plasma-assisted catalysis
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Quantitative phase microscopy monitors subcellular dynamics in single cells exposed to nanosecond pulsed electric fields
A substantial body of literature exists to study the dynamics of single cells exposed to short duration (<1 μs), high peak power (~1 MV/m) transient electric fields. Much of this research is limited to traditional fluorescence-based microscopy techniques, which introduce exogenous agents to the culture and are only sensitive to a single molecular target. Quantitative phase imaging (QPI) is a coherent imaging modality which uses optical path length as a label-free contrast mechanism, and has proven highly effective for the study of single-cell dynamics. In this work, we introduce QPI as a useful imaging tool for the study of cells undergoing cytoskeletal remodeling after nanosecond pulsed electric field (nsPEF) exposure. In particular, we use cell swelling, dry mass and disorder strength measurements derived from QPI phase images to monitor the cellular response to nsPEFs. We hope this demonstration of QPI's utility will lead to a further adoption of the technique for the study
59 BASIC BIOLOGICAL SCIENCES↗
Modeling Nanosecond-Pulsed Spark Discharge and Flame Kernel Evolution
Dilute combustion, either using exhaust gas recirculation or with excess air, is considered a promising strategy to improve the thermal efficiency of internal combustion engines. However, the dilute air-fuel mixture, especially under intensified turbulence and high-pressure conditions, poses significant challenges for ignitability and combustion stability, which may limit the attainable efficiency benefits. In-depth knowledge of the flame kernel evolution to stabilize ignition and combustion in a challenging environment is crucial for effective engine development and optimization. To date, a comprehensive understanding of ignition processes that result in the development of fully predictive ignition models usable by the automotive industry does not yet exist. Spark-ignition consists of a wide range of physics that includes electrical discharge, plasma evolution, joule-heating of gas, and flame kernel initiation and growth into a self-sustainable flame. In this study, an advanced approach is proposed to model spark-ignition energy deposition and flame kernel growth. To decouple the flame kernel growth from the electrical discharge, a nanosecond-pulsed high-voltage discharge is used to trigger spark-ignition in an optically accessible small ignition test vessel with a quiescent mixture of air and methane. Initial conditions for the flame kernel, including its thermodynamic state and species composition, are derived from a plasma-chemical equilibrium calculation. The geometric shape and dimension of the kernel are characterized using a multi-dimensional thermal plasma solver. Here, the proposed modeling approach is evaluated using a high-fidelity computational fluid dynamics procedure to compare the simulated flame kernel evolution against flame boundaries from companion Schlieren images.
42 ENGINEERING↗
Repetitively pulsed nanosecond discharge plasma decay in propane–oxygen gas mixture in the presence of a heating electric field
Plasma decay in the afterglow of a repetitively pulsed nanosecond discharge in a stoichiometric propane–oxygen mixture was experimentally investigated when a weak heating DC electric field was applied and in its absence. The discharge was ignited at room gas temperature and a pressure of 1–2 Torr and was characterized by low specific energy inputs (<0.004 eV per molecule in one pulse). Using microwave interferometry, the temporal evolution of the electron density during plasma decay was studied, and the effective recombination coefficients were obtained from data processing. It was shown that the rate of plasma decay behaved in a non-monotonic manner with increasing degree of propane oxidation; at first the decay rate grew, then passed through a maximum, fell and saturated in the limit of a large (~2000) number of pulses. In this limit, the effect of the heating DC electric field on the plasma decay decreased with approaching chemical equilibrium. Numerical simulation of the observed effects was performed for low and high oxidation degrees of propane taking into account changes in the composition of positive ions in the plasma. Good agreement was obtained between measurements and calculations of the electron density during plasma decay in these cases. Here it was shown that the formation of cluster ions in the discharge afterglow plays a fundamental role. The plasma decay was controlled by electron recombination with hydrocarbon cluster ion at low oxidation degree of propane and with water cluster (hydrated) ions at high oxidation degree. A hypothesis was proposed to explain the observed nonmonotonic behavior of the plasma decay rate with an increase in the propane oxidation in the discharge, based on the formation of hydrated hydrocarbon ions C x H y + (H 2 O) k at moderate oxidation degrees.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Breakdown and Discharge Characteristics of High Repetition Frequency Nanosecond Pulsed Air Dielectric Barrier Discharge
This study explores the breakdown characteristics and discharge modes in a parallel plate air dielectric barrier discharge system using repetitive nanosecond pulses, with pulse repetition frequencies (PRFs) from 0.1 to 100 kHz. It examines how pulse parameters—PRF, pulse number and gas pressure—affect the memory effect, leveraging current and voltage measurements, fast imaging and optical emission spectroscopy. The findings show that higher PRFs lead to a reduction in breakdown voltage well below the streamer breakdown threshold. We argue that this effect may be attributed to the cumulative buildup of metastable species and negative ions in the discharge gap which could sustain free electrons in‐between the voltage pulses. Despite observing increased energy deposition with higher PRFs, the impact on filament formation was minimal, highlighting a strong dependence of discharge morphology on the accumulation of plasma‐produced species. This research studies provides valuable insights for controlling discharge regimes in applications such as plasma‐assisted combustion, surface treatment and air treatment by clarifying the interactions between discharge mechanisms at different PRFs and pressures.
42 ENGINEERING↗
Effects of frequency and pulse width on electron density, hydrogen peroxide generation, and perfluorooctanoic acid mineralization in a nanosecond pulsed discharge gas-liquid plasma reactor
Plasma electron density and temperature were characterized in a continuous flowing gas-liquid film reactor with argon carrier gas by time-resolved optical emission spectroscopy. The plasma parameters were studied as a function of time for varying pulse widths and frequencies. Pulse frequency was varied between 1 and 10 kHz at 16 kV (input voltage) and 40 ns (pulse width) using an Eagle Harbor Technologies, Inc. (EHT) power supply and 5–100 kHz using an Airity Technologies, LLC (AT) power supply. The pulse width was varied between 40 and 200 ns at 16 kV, 2 kHz with the EHT power supply. Optimal frequencies of 5 and 20 kHz were observed for peak electron density with EHT and AT power supplies, respectively. The peak electron density increased with increasing pulse width between 40 and 200 ns using the EHT power supply. Hydrogen peroxide exiting the reactor in the liquid phase increased with discharge power irrespective of the power supply or pulse parameters. Mineralization of 12.5, 50, and 200 ppm perfluorooctanoic acid (PFOA) dissolved in DI water to fluoride (F - ) correlated to the peak electron density. Glycerol, a liquid-phase hydroxyl radical scavenger, depleted hydrogen peroxide but did not affect PFOA mineralization. CO, a gas-phase hydroxyl radical scavenger, led to a reduction in the formation of F - production, suggesting hydroxyl radicals in the gas-liquid film play a necessary, but not singular, role in mineralization of PFOA.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Laser damage of crazed electron-beam high-reflectors following infrared and ultraviolet irradiation in the nanosecond pulse regime
Laser damage of optical components can be a limiting factor in scaling the energetics of high-peak and average power laser systems. Specifically for optical coatings, damage under nanosecond pulsed irradiation is initiated by pre-existing defects in the coating layers, including those that cause discontinuities in the structure, like craze lines. Crazing or cracking in a multilayer dielectric optical coating is induced when the overall coating stress is sufficiently tensile, and is an occasionally observed issue when employing more porous deposition techniques like electron-beam evaporation. Here, in this study, electron-beam high-reflectors were fabricated utilizing process parameters that are known to induce crazing based on prior processing history to systematically evaluate the impact of crazing on reflector damage performance for 1064 and 355 nm lasers. The crazing that was observed was apparently nucleated at nodular defects. When the cross-section of these nodules was investigated, it was observed that there were cracks into the fused silica substrate of approximately 5 µm in depth. The craze lines were irradiated with 1064 and 355 nm light at fluences slightly above the onset of damage initiation fluence of the coating. The 1064 nm irradiated sub-apertures exhibit laser damage but with no spatial correlation with the craze line, whereas the 355 nm irradiated area exhibited many damage sites along the craze line. Finite-difference time-domain electric-field simulations were conducted, and ∼2× field amplification in hafnia was observed for the 355 nm wavelength case. The laser damage can be attributed to a slight electric-field intensification coincidental with an area where UV damage-prone precursors are known to occur. The 355 nm laser damage in uncoated fused silica substrates has been previously correlated to initiate through localized UV absorption at the broken silica bonds in the tips of fractures.
Harthcock, Colin [Lawrence Livermore National Labo↗
Townsend to glow discharge transition for a nanosecond pulse plasma in helium: space charge formation and resulting electric field dynamics
Stark polarization spectroscopy is used to investigate the temporal evolution of the electric field distribution in the cathode region of a nanosecond pulsed discharge in helium at 120 Torr. The measurements are performed on the He I transition at 492.19 nm, during the early stages of the discharge formation. The experimental results are compared with the predictions of a 1D fluid model. Time-resolved ICCD images show that the discharge develops as a diffuse, cathode-directed ionization wave with a Townsend-like feature before transitioning into a glow-like structure. Near anode instabilities characterized by filament formation were observed near the high voltage electrode. Within 30 ns, a reduction of the sheath thickness to about 250 μm is observed, coinciding with a gradual increase of the discharge current and proportional increase in electric field at the cathode. Additionally, the cathode electric field corresponding to this sheath with a thickness of 250 μm is about 40 kV cm -1 . A subsequent steep increase of the discharge current leads to a further reduction of the sheath width. The electric field evolution as obtained by the fluid model is in excellent agreement with the measurements and shows that an enhanced ionization near the cathode is causing the space charge formation responsible for the increase in electric field.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Hollow plasma structure during the breakdown phase of nanosecond pulsed pin–pin discharges generated with overvoltage
Atmospheric pressure nanosecond repetitive pulsed discharges are of interest for applications in combustion, material processing, plasma medicine and gas treatment. Here, the observation of a hollow structure at the ignition stage of such discharge in a pin–pin geometry is reported. Plasma images show that the phenomenon occurs in a variety of gases (helium, humid helium, argon and air) suggesting that it is in first approximation non-species specific. The phenomenon is investigated in detail in pure helium discharges for peak applied voltages between 3 and 6 kV and 200 ns voltage pulse duration by performing time-resolved optical emission spectroscopy measurements of electron density, gas temperature and electric field. All the measurements were performed at the mid-gap during the early stages of the discharge, near the breakdown. The transition from a homogeneous filament to a hollow structure occurs between 4 and 5 kV in helium. Electron density measurements by Stark broadening of the He I line at 492.2 nm show no significant difference in electron densities for the applied voltage between 3 and 6 kV. In all cases, a maximum electron density of about 3.5 × 10 21 m –3 was found. Electric field measurements performed by Stark polarization spectroscopy of the He I transition at 492.2 nm and of its forbidden counterpart show electric field strengths higher than the DC breakdown field of helium at atmospheric pressure consistent with the generation of the discharges with overvoltage. Moreover, a correlation between electric field values and the appearance of the hollow structure is observed and, remarkably, the phenomenon is not sensitive to the pre-ionization level (memory effect). Possible mechanisms for the phenomenon are discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Impact of CH 4 addition on the electron properties and electric field dynamics in a Ar nanosecond-pulsed dielectric barrier discharge
Non-equilibrium plasmas derive their low temperature reactivity from producing and driving energetic electrons and active species under large electric fields. Therefore, the impact of reactants on the plasma properties including electron number density, electric field, and electron temperature is critical for applications such as plasma CH 4 reforming. Due to experimental complexity, electron properties and the electric field are rarely measured together in the same discharge. Here, we combine time-resolved Thomson scattering and electric field induced second harmonic generation (EFISH) to probe electron temperature, electron density, and electric field strength in a 60 Torr CH 4 /Ar nanosecond-pulsed dielectric barrier discharge (ns-DBD) while varying the CH 4 mole fraction from 0 to 8%. These measurements are compared to a 1-D numerical model to benchmark its predictions and identify areas of uncertainty. Nonlinear coupling between CH 4 addition, electron temperature, electron density, and the electric field was directly observed. Contrary to previous measurements in He, the electron temperature increased with CH 4 mole fraction. This rise in electron temperature is identified as electron heating by residual electric fields that increased with larger CH 4 mole fraction. Moreover, the electron number density has been found to decrease rapidly with the increase of methane mole fraction. Comparison of these measurements with the model yielded better agreement at higher CH 4 mole fractions and with the usage of ab initio calculated Ar electron-impact cross-sections from the B-spline R-matrix (BSR) database. Furthermore, the calculated plasma properties are shown to be sensitive to the residual surface charge implanted on the quartz dielectric surfaces. Without considering surface charge in the simulations, the calculated electric field profiles agreed well with the measurements, but the electron properties were underpredicted by more than a factor of three. Therefore, measurements of either the electric field or electron properties measurements alone are insufficient to fully validate modelling predictions.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence
The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. In this work, we report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0, 0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0, 0), A-X(1, 1), and B-X(0, 1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
High-pressure CO 2 dissociation with nanosecond pulsed discharges
Herein we investigate the conversion of CO 2 into CO and O 2 with nanosecond repetitively pulsed (NRP) discharges in a high-pressure batch reactor. Stable discharges are obtained at up to 12 bar. By-products are measured with gas chromatography. The energy efficiency is determined for a range of processing times, pulse energy, and fill pressures. It is only weakly sensitive to the plasma operating parameters, i.e the extent of CO 2 conversion is almost linearly-dependent on the specific energy invested. A conversion rate as high as 14% is achieved with an energy efficiency of 23%. For long processing times, saturation in the yield and a drop in efficiency are observed, due to the increasing role of three-body recombination reactions, as described by zero-dimensional detailed kinetic modeling. The modeling reveals the presence of three-stage kinetics between NRP pulses, controlled by electron-impact CO 2 dissociation, vibrational relaxation, and neutral elementary kinetics. Transport effects are shown to be important for CO 2 conversion at high pressures. For fill pressures beyond 10 bar, CO 2 may locally transit into supercritical states. The supercritical plasma kinetics may bypass atomic oxygen pathways and directly convert CO 2 into O 2 . This work provides a detailed analysis of plasma-based high-pressure CO 2 conversion, which is of great relevance to future large-scale sustainable carbon capture, utilization, and storage.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Experimental study of atmospheric pressure single-pulse nanosecond discharge in pin-to-pin configuration
We present an experimental study of nanosecond high-voltage discharges in a pin-to-pin electrode configuration at atmospheric conditions operating in single-pulse mode (no memory effects). Discharge parameters were measured using microwave Rayleigh scattering, laser Rayleigh scattering, optical emission spectroscopy enhanced with a nanosecond probing pulse, and fast photography. Spark and corona discharge regimes were studied for electrode gap sizes 2-10 mm and discharge pulse duration of 90 ns. The spark regime was observed for gaps < 6 mm using discharge pulse energies of 0.6-1 mJ per mm of the gap length. Higher electron number densities, total electron number per gap length, discharge currents, and gas temperatures were observed for smaller electrode gaps and larger pulse energies, reaching maximal values of about 7.5 x 10 15 cm -3 , 3.5 x 10 11 electrons per mm, 22 A, and 4,000 K (at 10 microseconds after the discharge), respectively, for a 2 mm gap and 1 mJ/mm discharge pulse energy. Initial breakdown was followed by a secondary breakdown occurring about 30-70 ns later and was associated with ignition of a cathode spot and transition to cathodic arc. Here, a majority of the discharge pulse energy was deposited into the gas before the secondary breakdown (85-89%). The electron number density after the ns-discharge pulse decayed with a characteristic time scale of 150 ns governed by dissociative recombination and electron attachment to oxygen mechanisms. For the corona regime, substantially lower pulse energies (~0.1 mJ/mm), peak conduction current (1-2 A), and electron numbers (3-5 x 10 10 electrons per mm), and gas temperatures (360 K) were observed.
42 ENGINEERING↗
Increasing the deuterated potassium dihydrogen phosphate crystal laser resistance by an additional conditioning with nanosecond pulses
Laser conditioning with 355-nm sub-nanosecond laser light is a well-known procedure to increase the bulk laser-induced damage resistance of deuterated potassium dihydrogen phosphate (DKDP) crystals. In this study, we investigate a new process to further increase the bulk damage resistance of DKDP crystals by performing additional conditioning with a 6.7-ns 355-nm laser after first conditioning with a 500-ps 355-nm laser. Here, damage tests (using both small and large beams) show that the second nanosecond conditioning raster increased the fluence required to produce the same density (large beam test) and probability (small beam test) of bulk damage by ∼30%.
Crystals↗
Measurements of OH radical concentrations in uniform and non-uniform nanosecond-pulsed dielectric barrier discharge plasma
In this work we studied OH radical generation in nanosecond dielectric barrier discharge in humid He/air gas. The measurements were carried out by means of laser-induced fluorescence (LIF) method at the PPPL Low Temperature Plasma Research Facility, directed by Dr. Shurik Yatom. Application of LIF towards OH density measurements is a widely used approach in plasmas generated in humid gas, particularly when temporal evolution of OH density is of interest. Here we examined the OH densities generated in the discharge ignited by application of nanosecond high-voltage pulses with amplitudes varying between 10 to 20 kV and interelectrode gap length varying between 0.5-3 mm. The peak voltage and the gap length determine the mode in which discharge is generated: homogenous or filamentary. The measurements of OH in the gas phase were accompanied by the measurements of hydrogen peroxide (H 2 O 2 ) in liquid water, downstream of the plasma and the relationship between these two were examined.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗