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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Effect of Approach Speed and Electrode Geometry on Electrostatic Discharges Off Floating Dielectrics

With the damaging effects of electrostatic discharges (ESDs), it is important to study them within various parameter regimes relevant to real-world scenarios. One such scenario studied here is a floating dielectric with no nearby ground plane with a grounded object approaching at a high rate of speed. One may encounter this scenario when moving objects by hand. Discharge current and radiated fields are captured for discharges drawn from polytetrafluoroethylene (PTFE) and poly methyl methacrylate (PMMA) with approach speeds ranging from 20 to 100 mm/s. Previous studies have shown that at lower speeds in metal-to-metal or metal-to-ground backed dielectric geometries, the peak current and discharge distance decrease with increasing speed. However, in the speed range studied here, an increase in distance and current are observed for increasing speed. Namely, the highest speeds coincide with the highest peak currents and discharge distances of approximately 800 mA and up to 24 mm. Further, with no grounded backing, as opposed to setups in other ESD studies, the electric field between the dielectric and an approaching electrode has a more uniform distribution with highest fields at the electrode, which is elucidated to be the driving factor in the differences revealed in this study. Two spheres of differing diameters and a conical electrode are used to draw discharges off the dielectric surfaces. Captured radiated fields via B-dot sensor, plasma imagery via intesified charge-coupled device (ICCD), and mapping of surface potential reveal that a majority of the energy lost during a discharge goes to gas losses and radiated fields.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High resolution identification and quantification of diffuse deep groundwater discharge in mountain rivers using continuous boat-mounted helium measurements

Discharge of deeply sourced groundwater to streams is difficult to locate and quantify, particularly where both discrete and diffuse discharge points exist, but diffuse discharge is one of the primary controls on solute budgets in mountainous watersheds. The noble gas helium is a unique identifier of deep groundwater discharge because groundwater with long residence times is commonly enriched in helium. In this study, a portable mass spectrometer was used to measure longitudinal variation in dissolved helium concentrations in two mountainous rivers at high spatial resolution not feasible with traditional sampling techniques. Helium profiles were then simulated using a mass-balance model to quantify longitudinal variation in groundwater discharge to the receiving rivers. Results indicate helium concentrations were enriched by multiple orders of magnitude above atmospheric equilibrium in both rivers and that this persisted for up to 18 km below observed pulse inputs in the Colorado River. Helium mass-balance models match observed longitudinal patterns with the exception of sharp initial increases in helium observed in the rivers. Increased longitudinal groundwater discharge rates correspond to mapped geologic structures in both watersheds that likely transport deep geothermal water. Models show variable sensitivity to spatial assignment of input variables representing the groundwater source, illustrating the importance of collecting data from discrete groundwater discharges where possible. The methodology shows promise for field experiments designed to assess air–water exchange rates and to quantify total groundwater discharge from a combination of discrete and diffuse sources.

Deep groundwater↗
DOE OSTI2012

X-ray emission from a nanosecond-pulse discharge in an inhomogeneous electric field at atmospheric pressure

This paper describes experimental studies of the dependence of the X-ray intensity on the anode material in nanosecond high-voltage discharges. The discharges were generated by two nanosecond-pulse generators in atmospheric air with a highly inhomogeneous electric field by a tube-plate gap. The output pulse of the first generator (repetitive pulse generator) has a rise time of about 15 ns and a full width at half maximum of 30-40 ns. The output of the second generator (single pulse generator) has a rise time of about 0.3 ns and a full width at half maximum of 1 ns. The electrical characteristics and the X-ray emission of nanosecond-pulse discharge in atmospheric air are studied by the measurement of voltage-current waveforms, discharge images, X-ray count and dose. Our experimental results showed that the anode material rarely affects electrical characteristics, but it can significantly affect the X-ray density. Comparing the density of X-rays, it was shown that the highest x-rays density occurred in the diffuse discharge in repetitive pulse mode, then the spark discharge with a small air gap, and then the corona discharge with a large air gap, in which the X-ray density was the lowest. Therefore, it could be confirmed that the bremsstrahlung at the anode contributes to the X-ray emission from nanosecond-pulse discharges.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Growth of tungsten nanoparticles in direct-current argon glow discharges

The growth of nanoparticles from the sputtering of a tungsten cathode in DC argon glow discharges is reported. The study was performed at fixed argon pressure and constant discharge current. The growth by successive agglomerations is evidenced. First, tungsten nanocrystallites agglomerate into primary particles, the most probable size of which being {approx}30 nm. Primary particles of this size are observed for all plasma durations and always remain the most numerous in the discharge. Primary particles quickly agglomerate to form particles with size up to {approx}150 nm. For short plasma duration, log-normal functions describe accurately the dust particle size distributions. On the contrary, for long discharge durations, a second hump appears in the distributions toward large particle sizes. In the meantime, the discharge voltage, electron density, and emission line intensities strongly evolve. Their evolutions can be divided in four separate phases and exhibit unusual distinctive features compared to earlier observations in discharges in which particles were growing. The evolution of the different parameters is explained by a competition between the surface state of the tungsten cathode and the influence of the growing nanoparticles. The differences with sputtering glow discharges and chemically active plasmas suggest that the nanoparticle growth and its influence on discharge parameters is system and material dependent.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Specific features of a single-pulse sliding discharge in neon near the threshold for spark breakdown

Experimental data on the spatial structure of a single-pulse sliding discharge in neon at voltages below, equal to, and above the threshold for spark breakdown are discussed. The experiments were carried at gas pressures of 30 and 100 kPa and different polarities of the discharge voltage. Photographs of the plasma structure in two discharge chambers with different dimensions of the discharge zone and different thicknesses of an alumina dielectric plate on the surface of which the discharge develops are inspected. Common features of the prebreakdown discharge and its specific features depending on the voltage polarity and gas pressure are analyzed. It is shown that, at voltages below the threshold for spark breakdown, a low-current glow discharge with cathode and anode spots develops in the electrode gap. Above the breakdown threshold, regardless of the voltage polarity, spark channels directed from the cathode to the anode develop against the background of a low-current discharge.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Parametric Study of Two Stable Forms of Discharge Burning in a Hall-Effect Thruster

In this paper, we experimentally studied the behavior of the integral operation parameters of a Hall-effect thruster (thrust and specific thrust impulse) in two stable discharge glow modes significantly different from each other in anodic efficiency. The studies were conducted using a laboratory model of a Hall-effect thruster with an extended layer with an average discharge channel diameter of 77 mm in the discharge voltage range of 500–900 V and in the gas-flow rate range from 2 to 5 mg/s. The most striking distinguishing features of the observed glow regimes are the plasma jet shape and the discharge current value at almost identical input parameters (gas flow, discharge voltage, and magnetic field). In the entire studied range of input parameters when changing from the optimal mode (in terms of efficiency) to suboptimal, the main integral characteristics of the engine are shown to undergo a stepwise change: the discharge current increases by 10‒30% with a simultaneous relative drop in thrust by 5–15% and in efficiency by 20–40%. A detailed study of the anodic efficiency structure showed that, at abrupt rebuilding, the efficiency of using the electron current (the ratio of the ion current to the discharge current) changes, i.e., the electron conductivity in the engine discharge channel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2006

Analytical Ion Thruster Discharge Performance Model

A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows the original work of Brophy in the development of global 0-D discharge models that utilize conservation of particles into and out of the thruster and conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam and plasma radiation. The present model is significantly expanded over Brophy's original work by including self-consistent calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance and electrical inputs such as discharge voltage and currents, etc. to produce accurate performance curves of discharge loss versus mass utilization efficiency. The model has been benchmarked against the NEXIS Laboratory Model (LM) and Development Model (DM) thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be presented and results showing ion thruster performance and stability given.

discharge chambers↗
DOE OSTI2015

Groundwater Discharge along a Channelized Coastal Plain Stream

In the Coastal Plain of the southeastern USA, streams have commonly been artificially channelized for flood control and agricultural drainage. However, groundwater discharge along such streams has received relatively little attention. Using a combination of stream- and spring-flow measurements, spring temperature measurements, temperature profiling along the stream-bed, and geologic mapping, we delineated zones of diffuse and focused discharge along Little Bayou Creek, a channelized, first-order perennial stream in western Kentucky. Seasonal variability in groundwater discharge mimics hydraulic-head fluctuations in a nearby monitoring well and spring-discharge fluctuations elsewhere in the region, and is likely to reflect seasonal variability in recharge. Diffuse discharge occurs where the stream is incised into the semi-confined regional gravel aquifer, which is comprised of the Mounds Gravel. Focused discharge occurs upstream where the channel appears to have intersected preferential pathways within the confining unit. Seasonal fluctuations in discharge from individual springs are repressed where piping results in bank collapse. Thereby, focused discharge can contribute to the morphological evolution of the stream channel.

58 GEOSCIENCES↗
DOE OSTI2021

Generation of excited species in a streamer discharge

At or near atmospheric pressure, most transient discharges, particularly in molecular gases or gas mixture containing molecular gases result in a space-charge dominated transport called a streamer discharge. The excited species generation in such discharges forms the basis for plasma chemistry in most technological applications. In this paper we simulate the propagation of streamers in atmospheric pressure N 2 to understand the energy partitioning in the formation of various excited species and compare the results to a uniform Townsend discharge. The model is fully two-dimensional with azimuthal symmetry. Results show a significantly larger fraction of the energy goes into vibrational excitation of the N 2 ground state in a streamer-type discharge in comparison to a Townsend discharge. For lower applied voltages, the anode-directed (negative) steamer is slightly more efficient in channeling energy for excited species production in comparison to a cathode-directed (positive) streamer. Near 70% overvoltage both types of streamers show very similar energy partitioning but quite different from a Townsend discharge.

42 ENGINEERING↗
DOE OSTI2019

Time-resolved populations of N 2 (A 3 Σ u + ,v) in nanosecond pulse discharge plasmas

Absolute time-resolved populations of N 2 in the excited A 3 Σ u + electronic state that are generated in a repetitive ns pulse discharge in nitrogen have been measured by Cavity Ring Down Spectroscopy (CRDS) and Tunable Diode Laser Spectroscopy (TDLAS). CRDS measurements of N 2 (A 3 Σ u + ,v = 0–2) populations are made in the discharge afterglow at pressures of 22 and 39 Torr. The data reduction procedure takes into account the linewidth of the pulsed laser source, which is comparable with the absorption linewidth and results in a non-single exponential ring down decay. Peak N 2 (A 3 Σ u + ,v = 0–2) populations after a 10-pulse ns discharge burst are in the range of 10 12 –10 13 cm -3 . In the afterglow, these populations exhibit a relatively slow decay with the characteristic time of approximately 500 μs, most likely due to the quenching by N atoms. TDLAS data have been taken at a higher pressure of 132 Torr. Absolute time-resolved N 2 (A 3 Σ u + ,v = 0,1) number densities are measured during ns pulse discharge bursts up to 5 pulses long and in the afterglow, peaking at 6·10 12 cm -3 and 3·1013 cm -3 , respectively. The results indicate that N 2 (A 3 Σ u + ) is generated after every discharge pulse on a 20–50 μs time scale, much longer compared to the discharge pulse duration of ~100 ns, and subsequently decays between the pulses. The decay rate increases during the discharge burst. In the afterglow, N 2 (A 3 Σ u + ,v = 0,1) population decay significantly more rapidly compared to the low-pressure CRDS conditions, with the characteristic time of approximately 160 μs after a 5-pulse discharge burst.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

The dependence of extracted current on discharge gas pressure in neutral beam ion sources on HL-2A tokamak

The discharge gas pressure is a key factor to influence the extracted current of ion source. In this paper, the dependence of extracted current on discharge gas pressure was investigated in detail at different arc discharge currents. The discharge gas pressure with a very broad range (0.1 Pa-2.7 Pa) was scanned for the first time. It is turned out that, with the increasing of discharge gas pressure, the extracted current increases and the arc voltage decreases at different arc currents; however, when the discharge gas pressure exceeds a certain value, the extracted current decreases. For the same discharge gas pressure, the higher the arc current, the higher the arc voltage and the extracted current are. The arc efficiency was also calculated, and its dependence on gas pressure was almost the same with the dependence of extracted current on gas pressure, but at the same discharge gas pressure, the lower the arc current, the higher the arc efficiency is and the lower the extracted current is.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Mode transition of a Hall thruster discharge plasma

A Hall thruster is a cross-field plasma device used for spacecraft propulsion. An important unresolved issue in the development of Hall thrusters concerns the effect of discharge oscillations in the range of 10–30 kHz on their performance. The use of a high speed Langmuir probe system and ultra-fast imaging of the discharge plasma of a Hall thruster suggests that the discharge oscillation mode, often called the breathing mode, is strongly correlated to an axial global ionization mode. Stabilization of the global oscillation mode is achieved as the magnetic field is increased and azimuthally rotating spokes are observed. A hybrid-direct kinetic simulation that takes into account the transport of electronically excited atoms is used to model the discharge plasma of a Hall thruster. The predicted mode transition agrees with experiments in terms of the mean discharge current, the amplitude of discharge current oscillation, and the breathing mode frequency. It is observed that the stabilization of the global oscillation mode is associated with reduced electron transport that suppresses the ionization process inside the channel. As the Joule heating balances the other loss terms including the effects of wall loss and inelastic collisions, the ionization oscillation is damped, and the discharge oscillation stabilizes. A wide range of the stable operation is supported by the formation of a space charge saturated sheath that stabilizes the electron axial drift and balances the Joule heating as the magnetic field increases. Finally, it is indicated from the numerical results that there is a strong correlation between the emitted light intensity and the discharge current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Three distinct modes in a surface micro-discharge in atmospheric pressure He + N{sub 2} mixtures

A surface micro-discharge in atmospheric pressure He + N{sub 2} mixtures is studied in this paper with an emphasis on the discharge modes. With the N{sub 2} admixture increasing from 0.1% to 20%, the discharge evolves from a spatially diffuse mode to a filamentary mode during positive half-cycles of the applied voltage. However during the negative half-cycles, an additional patterned mode emerges between the diffuse and the filamentary modes, which has not been reported before to exist in surface micro-discharges. In the diffuse and patterned modes, the plasmas cover almost the entirety of the mesh area during one cycle after plasma ignition in all mesh elements, and the discharge power increases linearly with the applied voltage. In contrast, plasma coverage of the mesh area is only partial in the filamentary mode and the plasma is more unstable with the discharge power increasing exponentially with the applied voltage. As the surface micro-discharge evolves through the three modes, the density of excited species changes significantly, for instance, the density of N{sub 2}{sup +}(B) drops by ∼20-fold from [N{sub 2}] = 0.2% to 20%. The N{sub 2}{sup +}(B) is predicted to be generated mainly through successive processes of Penning ionization by helium metastables and electron-impact excitation of N{sub 2}{sup +}(X), the latter is most responsible for the density decrease of N{sub 2}{sup +}(B) because much more N{sub 2}{sup +}(X) is converted to N{sub 4}{sup +}(X) as the increase of N{sub 2} fraction. Also, the electron density and electron temperature decrease with the discharge mode transition.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

How does a probe inserted into the discharge influence the plasma structure?

Shielding the bias applied to the probe by the sheath formed around it and determination of parameters of unperturbed plasmas are in the basis of the probe diagnostics. The results from a two-dimensional model of a discharge with a probe inserted in it show that the probe influences the spatial distribution of the plasma parameters in the entire discharge. The increase (although slight) in the electron temperature, due to the increased losses of charged particles on the additional wall in the discharge (mainly the probe holder), leads to redistribution of the plasma density and plasma potential, as shown by the results obtained at the floating potential of the probe. The deviations due to the bias applied to the probe tip are stronger in the ion saturation region of the probe characteristics. The pattern of the spatial redistribution of the plasma parameters advances together with the movement of the probe deeper in the discharge. Although probe sheaths and probe characteristics resulting from the model are shown, the study does not aim at discussions on the theories for determination of the plasma density from the ion saturation current. Regardless of the modifications in the plasma behavior in the entire discharge, the deviations of the plasma parameters at the position of the probe tip and, respectively, the uncertainty which should be added as an error when the accuracy of the probe diagnostics is estimated do not exceed 10%. Consequently, the electron density and temperature obtained, respectively, at the position of the plasma potential on the probe characteristics and from its transition region are in reasonable agreement with the results from the model of the discharge without a probe. Being in the scope of research on a source of negative hydrogen ions with the design of a matrix of small radius inductive discharges, the model is specified for a low-pressure hydrogen discharge sustained in a small-radius tube.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Transition from Townsend to radio-frequency homogeneous dielectric barrier discharge in a roll-to-roll configuration

The aim of this paper is to better understand the transition from Townsend to radio-frequency homogeneous dielectric barrier discharge (DBD) at atmospheric pressure. The study is done in an Ar/NH{sub 3} Penning mixture for an electrode configuration adapted to roll-to-roll plasma surface treatment. The study was led in a frequency range running from 50 kHz up to 8.3 MHz leading to different DBD modes with a 1 mm gas gap: Glow (GDBD), Townsend (TDBD), and Radio-frequency (RF-DBD). In the frequency range between TDBD and RF-DBD, from 250 kHz to 2.3 MHz, additional discharges are observed outside the inter-electrode gas gap. Because each high voltage electrode are inside a dielectric barrel, these additional discharges occur on the side of the barrel where the gap is larger. They disappear when the RF-DBD mode is attained in the 1 mm inter-electrode gas gap, i.e., for frequencies equal or higher than 3 MHz. Fast imaging and optical emission spectroscopy show that the additional discharges are radio-frequency DBDs while the inter-electrode discharge is a TDBD. The RF-DBD discharge mode is attained when electrons drift becomes low enough compared to the voltage oscillation frequency to limit electron loss at the anode. To check that the additional discharges are due to a larger gas gap and a lower voltage amplitude, the TDBD/RF-DBD transition is investigated as a function of the gas gap and the applied voltage frequency and amplitude. Results show that the increase in the frequency at constant gas gap or in the gas gap at constant frequency allows to obtain RF-DBD instead of TDBD. At low frequency and large gap, the increase in the applied voltage allows RF-DBD/TDBD transition. As a consequence, an electrode configuration allowing different gap values is a solution to successively have different discharge modes with the same applied voltage.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Transition characteristics of low-pressure discharges in a hollow cathode

Based on a two-dimensional (2-D) fluid model, the transition processes of discharges in a hollow cathode at low pressure are observed by changing three parameters, i.e., applied voltage U 0 , gas pressure p, and external circuit ballast resistance R b . The voltage-current characteristic curves, electron density distributions, and electric potential distributions of different discharge operating points in a hollow cathode are obtained. The transition processes are characterized by the voltage-current characteristic curves, the electron density distributions, and the electrical potential distributions. The transition modes observed from the voltage-current characteristics include the low-current abnormal mode, normal mode, and high-current abnormal mode. Increasing the applied voltage U0 can have a similar effect on the discharge transition processes to decreasing the ballast resistance. By increasing U0 from 200V to 500V and decreasing Rb from 5000 kX to 100 kX independently, it is observed that the discharge transfers from the outside to the inside of the hollow cavity, thus forming the virtual anode potential. By increasing the gas pressure p from 50 Pa to 5 kPa, the discharge also moves into the hollow cavity from the outside; however, a further increase in the gas pressure leads to the discharge escaping from the hollow cavity. Simulation results and characterizations for different parameters are presented for the transition properties of low-pressure discharges in a hollow cathode. It is verified that the hollow cathode discharge only exists under certain ranges of the above parameters.

42 ENGINEERING↗
DOE OSTI2024

Quantifying the thermal effect and methyl radical production in nanosecond repetitively pulsed glow discharges applied to a methane-air flame

In this work, we investigated non-equilibrium plasma produced by nanosecond repetitively pulsed glow discharges applied across a lean premixed methane-air flame. The flame is stationary, axisymmetric, and laminar. The discharges are applied on the symmetry axis crossing the reactant gases, flame front, and product gases, allowing phase-locked averaged measurements and comparisons with axisymmetric numerical simulations. The thermal effect and methyl radical production are quantified in the discharge in the reactant gas region. One-dimensional, two-beam, hybrid, femtosecond-picosecond, coherent anti-Stokes Raman scattering is used to acquire spatial and temporal profiles of temperature and oxygen-to-nitrogen concentration ratio. Photo-fragmentation laser-induced fluorescence is used to acquire quantitative two-dimensional profiles of methyl radicals in the discharge providing the first quantitative imaging of methyl produced ahead of a flame by plasma-induced methane dissociation. The spatial profiles of temperature and oxygen-to-nitrogen concentration ratio are in steady state, indicating that individual discharges have an insignificant heating effect. Upper and lower bounds of the produced mole fraction of methyl radicals in the plasma are obtained due to uncertainties in the collisional quenching rates of excited state methylidyne radicals in the plasma. The discharges produce a maximum of 600–1100 ppm of methyl radicals upstream of the flame front within 25 ns. This amount is similar to the predicted methyl mole fraction for the flame without plasma and thus represents a significant chemical perturbation to the reactants upstream of the flame front. The produced methyl follows an exponential decay in the first microsecond after the discharge with a decay constant of 8 µs close to the flame, and 0.8 µs further from the flame. The decay then deviates from the exponential curve and the methyl persists for tens of microseconds. The results suggest that for the tested configuration, the thermal effect of individual discharges through fast gas heating is negligible, while active chemical species are produced in large quantities in the reactant gases, upstream of the flame front.

hybrid fs-ps CARS↗
DOE OSTI2018

Study of the Formation Time of a Self-Sustained Subnanosecond Discharge at High and Ultrahigh Gas Pressures

The formation times of self-sustained subnanosecond discharges in nitrogen at pressures of 1‒40 atm and in hydrogen at pressures of 1–60 atm are analyzed in terms of the avalanche model. In experiments, a subnanosecond voltage pulse with an amplitude of 102 ± 2 kV was applied to a 0.5-mm-long discharge gap with a uniformly distributed electric field (the curvature radii of both the cathode and anode ends were 1 cm). The rise time of the voltage pulse from 0.1 to 0.9 of its amplitude value was about 250 ps. Breakdown occurred at the leading edge of the pulse. The discharge formation time was measured at different gas pressures with a step of 5–10 atm. Analysis of the experimental results shows that, in nitrogen at pressures of 10–40 atm and in hydrogen at pressures of 20–50 atm, breakdown occurs earlier than the electron avalanche reaches its critical length and that the critical avalanche length lies in the range of (2–8) × 10{sup –2} mm, which is one order of magnitude shorter than the discharge gap length. This means that the avalanche–streamer model is inapplicable in this case. The fast formation of a conducting channel under these conditions can be explained by ionization of gas by runaway electrons. In this case, the conducting column develops as a result of simultaneous development of a large number of electron avalanches in the gas volume. An increase in the hydrogen pressure from 50 to 60 atm leads to an abrupt increase in the discharge formation time by about 50%. As a result, the growth time of the electron avalanche to its critical length becomes shorter than the discharge formation time. In this case, the electrons cease to pass into the runaway regime and the discharge is initiated from the cathode due to field emission from microinhomogeneities on its surface. Under these conditions, the discharge formation time is well described by the avalanche–streamer model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗