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This paper studies full-scale gas neutron tube modeling, including the following processes: gas discharge combustion in a Penning ion source, particle motion in an ion optical system, and modeling the in-target processes, such as sputtering, diffusion, thermal desorption of hydrogen isotopes, and nuclear reactions. Plasma modeling in quadrupole electric and axial magnetic fields was based on the electrostatic particle-in-cell method with molecular kinetic processes. The TechX Vsim software package was used. The neutron tube element sputtering by ions was simulated using SRIM/TRIM software based on Monte–Carlo methods. The OpenFOAM, as an open integrated platform for numerical simulation in continuum mechanics, was used to calculate the hydrogen thermal desorption activated by ion irradiation. The time-dependent neutron yield modeling was performed using the Geant4 software based on Monte–Carlo methods with CHIPS-TPT VNIIA-developed library. In addition, an experimental study of a gas neutron tube with a Penning ion source was conducted here as well. Details are given on the experiment and measurement technique used in this study. The operating characteristics for the gas neutron tube, including amplitude-time characteristics of current flashes (discharge and extraction currents), were determined. The neutron flux dependencies on the discharge current at various accelerating voltages were also obtained. Finally, a comparison between the experimental and calculated results is presented. [ABSTRACT FROM AUTHOR]

The composition and ion energy distributions of the main ion species of an ion beam were recorded and analyzed. The RF-type broad beam ion source was operated with a mixture of CHF 3 and O 2. A plasma bridge neutralizer operating with Ar was employed for ion beam neutralization. The data were collected with an energy-selective mass spectrometer (ESMS). The mass spectrum showed numerous ion species, beginning with ionized molecules, dissociation products of the process gases and products from reactions with background gas and the plasma discharge vessel, and the extraction system. For a quantification of the ion beam composition, the mass dependent transmission functions for two ESMS were determined. The ion energy distributions show that, in comparison to operation with inert gases, there are additional slower ions present. These ions can be related to dissociation processes outside of the ion beam source. As a result of their typically lower etching yield, these slower ions affect the etching behavior. [ABSTRACT FROM AUTHOR]

The composition and ion energy distributions of the main ion species of an ion beam were recorded and analyzed. The RF-type broad beam ion source was operated with a mixture of CHF 3 and O 2. A plasma bridge neutralizer operating with Ar was employed for ion beam neutralization. The data were collected with an energy-selective mass spectrometer (ESMS). The mass spectrum showed numerous ion species, beginning with ionized molecules, dissociation products of the process gases and products from reactions with background gas and the plasma discharge vessel, and the extraction system. For a quantification of the ion beam composition, the mass dependent transmission functions for two ESMS were determined. The ion energy distributions show that, in comparison to operation with inert gases, there are additional slower ions present. These ions can be related to dissociation processes outside of the ion beam source. As a result of their typically lower etching yield, these slower ions affect the etching behavior. [ABSTRACT FROM AUTHOR]

Pulsed discharge nozzles (PDNs) have been successfully used for decades to produce rotationally cold (Trot ∼ 20 K) radicals and ions of astrophysical interest and to characterize these species spectroscopically. In this work, an evolution of the PDN, the piezostack pulsed discharge nozzle (P2DN), is used for the first time to investigate the characteristics of the still poorly understood supersonic plasma expansion. The P2DN allows for a better control of the reservoir pressure of which an accurate measurement is required to characterize the plasma expansion. This new source, thus, gives the opportunity to further optimize the plasma conditions and extend its use to new target species. The spatial distribution of an argon plasma and the effect of the supersonic flow for different pressures are studied by combining a two-dimensional extended fluid model (extFM) and a direct simulation Monte Carlo (DSMC) method. The combined simulation is validated with experimental results obtained through emission spectroscopy associated with a group-code collisional-radiative model to retrieve the plasma parameters. The validated numerical approach (DSMC-extFM) allows for an accurate characterization of the plasma structure in our typical experimental conditions (a reservoir pressure ranging from 90 to 905 mbar). Thus, this simulation will be used in future studies to improve the plasma conditions to favor the synthesis of (transient) hydrocarbon species as found in space, by seeding the argon gas with a suitable precursor, such as acetylene. [ABSTRACT FROM AUTHOR]

The microwave electrothermal thruster (MET) utilizes wave-exited microdischarges to heat gas flows, enhancing the specific impulse of the thruster. Our computational study investigates a 17.5 GHz helium-propellant MET, employing a two-dimensional, axisymmetric fluid model of plasma coupled with electromagnetic wave and gas flows. The discharges operate in the glow regime, remaining weakly ionized, and in thermal non-equilibrium. The plasma densities reach approximately 10 20 m − 3 , and the gas temperature is around 2000 K. Even a slight off-resonant frequency operation results in a significantly lower plasma density and gas temperature. Gas heating, primarily driven by electromagnetic Joule heating, plays a critical role in influencing the overall discharge behavior. The measured peak thrust and specific impulse are 8.24 mN and 292 s, respectively, at a mass flow rate of 3.2 mg/s with 30 W of power. Compared to a cold gas thruster, there is a significant increase in the specific impulse by a factor of approximately 1.7. The enhanced performance trades off with propulsive efficiency, which decreases by a factor of 1.5 from the peak 65% at 10 W. This is due to higher energy losses to cavity walls from heat conduction with increased power. These findings underscore the critical balance between the input power and mass flow rate to improve the MET performance, highlighting the importance of power management to maximize thrust and efficiency. Furthermore, the predicted thrust and specific impulse agree well with experimental values for nominally similar MET thruster studies in the literature. [ABSTRACT FROM AUTHOR]

Current–voltage characteristics, space- and time-resolved optical emission spectroscopy, and 1D fluid modeling are used to examine the effect of dual-frequency sheath oscillations on the ion and electron transport in dielectric barrier discharges sustained by a combination of low frequency (LF, 50 kHz, 650 V) and radiofrequency (RF, 5.3 MHz, 195 V) voltages, exhibiting the α-to-γ mode transition. On one hand, when polarities of the LF and RF voltages are opposite, an electric field near the LF cathode (due to LF cathode sheath) drives the secondary electrons to the plasma bulk and an opposite electric field between the sheath edge and the LF anode attracts the electrons toward the LF cathode (to maintain quasi-neutrality in the plasma bulk). At the sheath edge, electrons become trapped and ions drift toward the cathode and the anode simultaneously according to their position in the gap. On the other hand, when the RF voltage has the same polarity as the LF voltage, the total applied voltage increases and this yields to enhanced production of electrons and ions in the sheath. To maintain quasi-neutrality in the bulk, the electric field along the gap exhibits the same polarity as the one in the sheath, allowing electrons created in the sheath to be evacuated toward the LF anode. The behavior of the LF cathode is, therefore, controlled by the LF sheath, and, thus, by the LF voltage amplitude, while the behavior in the bulk and at the anode alternates on the time scale of the RF voltage. [ABSTRACT FROM AUTHOR]

Hollow cathodes are an integral part of ion, and Hall thrusters are used for electric propulsion in deep space missions and in commercial communications satellites. Hollow cathodes are known to operate in a quiescent "spot mode" and in a noisy "plume mode" in which plasma instabilities generate erosive energetic ions. The onset of the plume mode in hollow cathodes has been defined historically as when the keeper voltage oscillation values exceed 5 Vpp (peak-to-peak). Using a LaB6 hollow cathode in a vacuum chamber setup that simulates operation in ion and Hall thrusters, a set of emissive and Langmuir probes have been used to investigate the plasma properties associated with plume mode onset as a function of discharge current and gas flow rate. We find that the plume mode onset occurs at even less than 2 Vpp of the keeper voltage for the 5–75 A hollow cathode investigated here and starts at higher gas flow rates than expected from the traditional 5 Vpp metric used by those in the field. Mode competition and coupling between three different instabilities observed in the near-cathode plume affect the overall plasma oscillation levels that are correlated to energetic ion production. We find that the plasma oscillation levels measured by in situ plasma diagnostics are more indicative of the presence of oscillations and the generation of energetic ions than indirect keeper voltage measurements. [ABSTRACT FROM AUTHOR]

The unstable plume mode of hollow cathodes should be avoided in practical applications because it severely degrades the overall cathode lifetime. In this study, we investigate the spot-plume transition and plasma stability characteristics of an unused segmented lanthanum hexaboride emitter. The expansion of the unstable plume mode region is observed during a discharge experiment. Subsequently, the segmented emitter is retrieved, the inner surface of the emitter is observed, and the work function on the surface is measured at room temperature. The emitter surface exhibits color variations with oxygen and carbon detection. The downstream edge shows the original purple color and almost no degradation in the work function. The high temperature in this region promotes the desorption of carbon and oxygen. In the spot mode, this region mainly contributes to thermionic electron emission; therefore, the discharge voltage in the spot mode does not change during the discharge experiment. Carbon or carbide is detected in the middle of the axial direction on the emitter surface, where the surface temperature is not sufficiently high to desorb carbon during discharge. Based on the surface analysis results, the dominant substance in the region where carbon is detected was lanthanum carbide. An increase in the work function is indicated in the region, which appears to increase the plasma instability. According to previous studies, an increase in the work function results in a rise in the potential in the emitter, and an increase in the electron temperature in the outside plume region induces the plasma instability. Further investigation is needed to understand the mechanism connecting the rise in the work function and the rise in the electron temperature in the plume region. [ABSTRACT FROM AUTHOR]

Conventional magnetron sputter deposition of B 4 C uses Ar as the working gas. Here, we explore the magnetron sputter deposition of B 4 C with a Ne plasma, which is expected to exhibit larger sputtering yields than Ar. We study properties of films deposited with different substrate tilt angles with the magnetron source operated in either direct-current (DC) or radio-frequency (RF) mode in an Ar or Ne plasma. Results show that the B 4 C film properties are determined by a combination of sputtering ballistics and effects of the working gas on the plasma discharge and gas phase scattering of depositing species flux. At constant discharge power, deposition rates for Ar and Ne plasmas are similar, which is attributed to balancing effects of a higher ballistic sputtering yield of Ne and lower ion flux to the target. Both depositing B and C neutral species and bombarding ions have higher energies for the case of Ne plasmas. Films deposited with the RF-driven Ne plasma exhibit a uniform non-columnar structure, lowest oxygen impurity content, and highest mass density and mechanical properties at a cost of Ne incorporation and larger compressive residual stress. [ABSTRACT FROM AUTHOR]

Laser Thomson scattering is a minimally intrusive diagnostic technique for determining electron temperature, density, and bulk velocity in plasma systems. Advances in technology have made possible the application of Thomson scattering to electric propulsion-relevant plasma systems, with reported electron number-density detection limits as low as 1 × 10 16 m − 3 , and electron temperatures from one-to-tens eV. However, the implementation of laser Thomson scattering in large vacuum testing facilities, wherein electric propulsion devices are tested, remains a challenge. This work presents the implementation of a laser Thomson scattering system in a large vacuum test facility at the Georgia Tech High Power Electric Propulsion Laboratory. The diagnostic was optimized for maximum light-collection efficiency and ease of re-alignment while the facility is at vacuum. The high light-collection efficiency allowed reduced accumulation times to achieve the target detection limit of 1 × 10 17 m − 3 . The diagnostic is used to measure axial electron property profiles in the near-field plume of a lanthanum hexaboride hollow cathode operating at 25 A on krypton at a background pressure of 1.3 × 10 − 6 Torr—Kr. The diagnostic is quantitatively compared to similar systems in the literature. The resulting axial points, collected from 2 to 8 mm downstream of the cathode keeper orifice, are qualitatively and quantitatively compared with simulations and experimental measurements made with electrostatic probes and laser-induced fluorescence. The main quantitative difference between measured values and results is the one to two order of magnitude difference in the peak electron density, being attributed to the relative size and location of the external anode with respect to the cathode keeper. [ABSTRACT FROM AUTHOR]

Underwater pulsed spark discharge has been widely used in industrial fields as a source of shock waves or acoustic waves, and numerical modeling of the discharge and pressure wave characteristics is necessary to improve the application performance. In this paper, a numerical model is proposed that couples the circuit equation, the mass and energy conservation equations, and a momentum conservation equation based on the Rankine–Hugoniot conditions. A tabulated wide range equation of state and conductivity data of water are used, and various physical processes during the plasma channel expansion are considered, including the energy flow and mass exchange between the channel and the surrounding water due to thermal radiation, evaporation, and condensation. The model self-consistently solves the circuit current and voltage, the plasma channel parameters including composition, temperature, conductivity, pressure, etc., and the pressure profile at a certain distance from the discharge channel. The calculated results show good consistency with the experimental measurements, and three sets of experimental results from other literature are tested to further verify the applicability and effectiveness of the model. [ABSTRACT FROM AUTHOR]

The topology of the applied magnetic field is an important design aspect of Hall thrusters. For modern Hall thrusters, the magnetic field topology most often features curved lines with a concave (negative) curvature upstream of the field's peak and a convex (positive) curvature downstream. Additionally, the advent of the magnetic shielding technique has resulted in Hall thruster designs with non-conventional field topologies that exhibit high degrees of concavity upstream of the field's peak. In this article, a detailed study is carried out on the effects that the magnetic field curvature has on the plasma discharge in a 2D configuration representative of a Hall thruster's radial–azimuthal cross section. The analyses are performed for discharges of three propellants of high applied interest: xenon, krypton, and argon. For each propellant, high-fidelity electrostatic reduced-order particle-in-cell (PIC) simulations are performed with various degrees of positive and negative curvatures of the magnetic field. Corresponding 1D radial PIC simulations are also performed for xenon to compare the observations against the 2D results. Most notably, it is observed that the instability spectra in the positive-curvature cases are mostly dominated by electron cyclotron drift instability, whereas the modified two stream instability is dominant in the negative-curvature cases. The distributions of electron and ion temperatures, in particular, as well as the contribution of various mechanisms to electrons' cross-field transport show notable variations between the positive and negative curvature values. Finally, the field curvature is observed to majorly influence the ion beam divergence along the radial and azimuthal coordinates. [ABSTRACT FROM AUTHOR]

The effects of the electrical parameters, including storage capacitance, additional inductance, charging voltage, and electrode gap, on the shock wave induced by spark discharge in gas were experimentally investigated. The results showed that the shock waves induced by spark discharge conform to the attenuation law for weak spherical shock waves outside the spark core. The shock wave amplitude is approximately proportional to the electrode gap and storage energy and decreases with increasing inductance. The effect of the charging voltage on the shock wave amplitude can be almost ignored if the storage energy is the same. The average power in the first quarter cycle of spark discharge was found to be closely related to the shock wave amplitude. An empirical equation was given between the shock wave amplitude and the average discharge power, which provides convenient access to set appropriate electrical parameters to generate shock waves of specified amplitude induced by spark discharge. [ABSTRACT FROM AUTHOR]

The Air-breathing Microwave Plasma CAThode (AMPCAT) has been developed for air-breathing electric propulsion in very-low Earth orbit. In this study, the standalone AMPCAT plasma characteristics are analyzed by means of several diagnostic tools and operation on xenon is compared to a conventional hollow cathode. A transition of AMPCAT extracted current from a lower (< 0.1 A) to higher-current (> --> 0.5 A) mode, triggered by increasing the negative cathode bias voltage, is accompanied by a significant rise in internal electron density and external electron temperature. The AMPCAT is coupled with a cylindrical Hall thruster in the 100–300 W power-level running on 0.5–0.7 mg/s of xenon, and the thrust is directly measured for cathode operation with both xenon and air. Stable thruster operation is demonstrated for the AMPCAT running on both propellants. For xenon, the performance is compared to a hollow cathode, which reveals matching discharge current profiles but a significantly higher thrust for the AMPCAT at low discharge voltages, approximately two times higher at 200 V. Langmuir probe measurements highlight a 30–40 V lower plasma potential in the cathode vicinity for the AMPCAT with xenon compared to both the hollow cathode and AMPCAT with air. This indicates a significantly improved coupling of cathode electrons to the thruster discharge, yielding an increased degree of ionization. Faraday probe and Wien filter results show that a larger current utilization efficiency drives the observed performance difference at low discharge voltages, rather than a significant change in ion acceleration or plume divergence. [ABSTRACT FROM AUTHOR]

A one-dimensional implicit particle-in-cell/Monte Carlo collision simulation was employed to study the effects of a uniform weak magnetic field and negative direct-current (DC) voltage on a radio frequency capacitively coupled plasma (CCP). The simulation results indicate that the application of a magnetic field to RF/DC hybrid power-driven CCP discharge can increase the plasma density and cause it to exhibit an asymmetric distribution. When the magnetic field strength increases, pronounced striations can be observed within the DC sheath in the spatiotemporal plots of an electron heating rate and an ionization rate. This is attributed to the generation of a large number of secondary electrons by the DC electrode. These secondary electrons are accelerated by the sheath voltage and undergo E × B drift motion. When the energy of these electrons reaches the ionization threshold of an argon gas, ionization occurs. At this point, the electrons are still situated within the DC sheath, and hence, they repeatedly undergo this process until they exit the DC sheath. Additionally, the electron energy distribution function reveals that an increase in a magnetic field can cause a transition from stochastic heating to ohmic heating. The simulation results of magnetized CCP discharge under the influence of negative DC voltage show that increasing negative DC voltage can effectively improve plasma density. The application of negative DC voltage and magnetic field strength has similar effects on the heating stripe phenomenon. As the negative DC voltage increases, the striation phenomenon becomes more pronounced. [ABSTRACT FROM AUTHOR]

We investigate the temporal correlation between very-high-frequency (VHF, at frequencies of the order of 10–100 MHz), ultrahigh-frequency (UHF, at frequencies within 1–6 GHz), and x-ray (with photon energies more than 10 keV) emissions, which accompany the development of a high-voltage discharge initiated in a long gap at voltages up to 1 MV. The x-ray and UHF emissions are found to emerge starting approximately from the prepulse current onset observed before the discharge gap breakdown and both gradually decay coming to this instant. The UHF emission spectrum is represented with frequencies up to 6 GHz, with the highest spectral power being achieved within 1–2 GHz. The radio emission power drops sharply at frequencies below 1 GHz and increases closer to 150 MHz with the highest spectral power of VHF radiation being reached within 60–90 MHz. The VHF emission can appear before the discharge current and UHF emission onsets in the form of 100-ns-long prepulses, and its intensity significantly increases as the UHF emission starts. The analysis of the temporal correlation between x rays, VHF, and UHF radiations, discharge current, and voltage waveforms indicates that the generation mechanisms of the discharge electromagnetic radiations are difficult to be interpreted in terms of the developing or colliding streamer concepts. [ABSTRACT FROM AUTHOR]

Although two-dimensional (2D) transition metal oxide monolayers have shown potential for applications in metal-ion batteries, the heterostructures of this family are yet to be studied in details for energy storage applications. In this work, we have made the heterostructure by taking half-metallic ferromagnetic 2D transition metal oxide CoO 2 and semiconducting MnO 2 monolayers and demonstrated its potential application as a cathode material in lithium and sodium-ion batteries by performing first-principles calculations using density functional theory approach. We have systematically studied the electronic structure and stability of the MnO 2 / CoO 2 heterostructure. We have carefully examined the adsorption and diffusion behavior of metal ions (lithium and sodium). Our structure has offered a maximum adsorption energy of − 3.84 eV, which is greater than the adsorption energy of individual monolayers. We found that the lowest diffusion barrier is 0.4 eV for lithium ion and 0.32 eV for sodium ion. Also, our system has shown a maximum open circuit voltage of 2.18 V for lithium ion battery and 0.32 V for Na-ion battery. The specific capacity is found to be 584 mAh g − 1 for lithium ion and 529 mAh g − 1 for sodium ion battery. These findings can serve as a proof that the MnO 2 / CoO 2 heterostructure should be considered as a potential cathode for lithium- and sodium-ion batteries. [ABSTRACT FROM AUTHOR]

For plasma window (PW) applications, we developed the cascade arc discharge device with an indirectly heated hollow cathode. The 8-mm channel diameter hollow cathode made of a lanthanum hexaboride (LaB 6) was heated by the C/C composite heater surrounding the cathode to increase the thermionic electron emission. The PW developed successfully separated 2.4 kPa and 16 Pa, and the pressure separation capability was sustained for over 1 h. H- β Stark broadening measurement and the Thomson scattering measurement showed that the electron density and temperature inside the channel reached 10 19 – 10 20 m − 3 and 2.0 eV, respectively. The power balance analysis on the electron thermal energy revealed that the neutral density and temperature inside the channel were as high as 10 23 m − 3 and 4000 K, respectively. The relation between the pressure separation capability and the neutral temperature showed that the flow inside the channel of the PW had the molecular flow feature. The SEM-EDX analysis on the LaB 6 cathode showed that boron diffused to the molybdenum (Mo) shaft during plasma operation, which supported the LaB 6 cathode. Mo shaft became brittle after more than 50 h of operation, exhibiting the necessity of buffer material between the LaB 6 cathode and Mo shaft for long-duration operation. [ABSTRACT FROM AUTHOR]

Rotating plasma structures, or "spokes," in magnetized discharges characterized by perpendicular electric and magnetic fields have been seen in a growing number of studies and are believed to be the result of gradient-driven drift instabilities. Under certain conditions, we have shown [Ito and Cappelli, Appl. Phys. Lett. 94, 211501 (2009)] the spoke's rotation to be opposite to the E × B direction, i.e., retrograde in its expected direction. Recently [Marcovati et al., J. Appl. Phys. 127, 223301 (2020)], we have linked such counter-intuitive rotation to a local inversion of the electric field. Here, we give further experimental evidence for this inversion and attempt to provide an explanation for a relatively distinct transition seen between retrograde and positive (prograde) drift. In the experiments, a partially magnetized plasma forms inside a magnetron device of ≈ 10 mm radius operated with argon. Discharge current–voltage measurements are acquired for a range of argon fill pressure and inter-electrode spacing. We find two branches of operation—a low current branch of negative resistance, coinciding with the retrograde spoke rotation, and a higher current branch of positive resistance, coincident with prograde spoke rotation. We postulate that at low discharge currents, high magnetic field confinement leads to a large density gradient, causing more electron transport to the anode than that demanded by the external circuit. At higher currents, anomalous axial electron transport (across the magnetic field lines) becomes dominant, eliminating the conditions for field inversion. The current thresholds for the field inversion are found to be sensitive to the argon pressure and inter-electrode spacing. [ABSTRACT FROM AUTHOR]

In this work, as a promising laboratory-based extreme ultraviolet (EUV) radiation source, microwave (MW)-induced helium discharge is studied and analyzed based on spectroscopic measurements at low pressures. The helium emission spectrum in the EUV wavelength range is presented to show all the relatively strong EUV lines. A Maxwellian shape is assumed for the electron energy distribution, and a corona model is applied to evaluate the plasma parameters under low-pressure conditions. The intensities of a pair of emission lines at 30.38 and 58.43 nm, which are the strongest in the spectrum and of great astrophysical interest, are studied under gradient discharge powers and pressures. After correcting for plasma reabsorption, the intensity ratio of the two lines is used to obtain the electron temperature (Te), which is found to vary within the range of 3.7–5.6 eV. Electron density (ne) is deduced from a global discharge model, which is of the order of magnitude of 1010 cm−3. From experimental determination and mechanism analyses, the optimal discharge pressure is found to be within 1.45–2.18 mbar for the 34.38 nm line and the vicinity of 3.45 mbar for the 58.43 nm line. This work explores the dependency of physical behavior of discharge on different working conditions based on a EUV spectroscopic study and theoretical analyses and determines the optimal working condition to produce the strongest EUV emission lines of the low-pressure MW-induced helium discharge. [ABSTRACT FROM AUTHOR]

A pulsed microwave surfaguide-type discharge used for nitrogen fixation in N 2 –O 2 gas mixtures is characterized by optical emission spectroscopy. Results show that both rotational and vibrational temperatures are elevated in the active zone near the waveguide, decaying along the discharge tube in both upstream and downstream. The characteristic length of optical emission from NO(A - X) transition gets contracted when pressure increases, specifically at P ≥ 2 Torr. The degree of vibrational non-equilibrium (defined as the ratio between vibrational and rotational temperatures) is decreased by a factor of two when pressure changes from 0.6 to 10 Torr. Non-equilibrium likely disappears as the discharge pressure rises, resulting in a gas temperature elevation. A correlation between gas residence time, pulse duration, and characteristic times for different energy transfer channels is discussed. The rotational–vibrational dynamics differs for NO and N 2 during the pulse. Both species lose vibrational excitation at the beginning of the pulse, whereas N 2 gets re-excited again during the second half of the pulse, which may occur as a result of an efficient pumping-up effect through the vibrational–vibrational energy transfer. At the same time, vibrational relaxation of NO takes place primarily due to a strong vibrational–translational exchange via NO–O 2 and NO–O collisions. [ABSTRACT FROM AUTHOR]

The micro-discharge with a hollow cathode and a second anode with different geometries is experimentally and numerically studied. Effects of the second anode on the micro-hollow cathode sustained discharge (MCSD) in helium are investigated. Three kinds of electrodes with different geometries are used as the second anode. The results show that when needles are inserted into the plate to be used as the second anode, the formation of MCSD can be promoted, and MCSD can be obtained at a low cathode current. Moreover, the distribution of needles has an important influence on the formation of MCSD. The cathode breakdown threshold currents for the three cases of the second anode from low to high are as follows: 13 needle-plate, five needle-plate, and plate with no needles. At the same cathode current, a stronger MCSD and a higher second anode current can be obtained when needles are inserted into the second anode plate compared with that when only a plate is used as the second anode. In the present experiment, the volume of MCSD reaches approximately 10 cm3 with an electron density of 4–5 × 1017 m−3 and an electron temperature of 2–3 eV. The experimental and simulated results show that the MCSD originates from cooperative formation between the micro-hollow cathode discharge in the cavity and the discharge around the second anode. Compared with that when only a plate is used as the second anode, when needles are inserted into the second anode, the electric field, electron temperature, and electron production rates near the second anode increase, and a stronger discharge is generated near the second anode. When the needle-plate is used as the second anode, the electron density in both the axial and radial directions in the MCSD region is higher than that when the plate without needles is used as the second anode. Before the formation of MCSD, the electrons generated by the strong discharge near the second anode extend toward the first anode, thus promoting the formation of MCSD. [ABSTRACT FROM AUTHOR]

Numerical simulation is an essential way to investigate the discharge behaviors of atmospheric low-temperature plasmas (LTPs). In this study, a deep neural network (DNN) with multiple hidden layers is constructed to surrogate the fluid model to investigate the discharge characteristics of atmospheric helium dielectric barrier discharges (DBDs) with very high computational efficiency, working as an example to show the ability and validity of DNN to explore LTPs. The DNN is trained by the well-formed training datasets obtained from a verified fluid model, and a designed loss function coupled in the DNN program is continuously optimized to achieve a better prediction performance. The predicted data show that the essential discharge characteristics of atmospheric DBDs such as the discharge current waveforms, spatial profiles of charged particles, and electric field can be yielded by the well-trained DNN program with great accuracy only in several seconds, and the predicted evolutionary discharge trends are consistent with the previous simulations and experimental observations. Additionally, the constructed DNN shows good generalization performance for multiple input attributes, which indicates a great potential promise for vastly extending the range of discharge parameters. This study provides a useful paradigm for future explorations of machine learning-based methods in the field of atmospheric LTP simulation without high-cost calculation. [ABSTRACT FROM AUTHOR]

A time-dependent global model is developed for deep oscillation magnetron sputtering (DOMS) with a Cr target to characterize the plasma using the working Ar gas at a pressure of 0.8 Pa. An input of the global model is the target voltage and current waveforms at charging voltages from 260 to 400 V. The plasma densities vary synchronously with the oscillations of target power. During the on-time of a micropulse, the gas atoms are ionized first to ignite the plasma and subsequently the metal ions follow to sustain the discharge. During the off-time, the gas ions dominate the afterglow. The DOMS possesses a characteristic of alternating gas/metal discharge in the time domain. The peak values of plasma densities increase linearly with charging voltages, which is mainly attributed to the higher Cr+ ion density. The discharge transits from gas dominated to metal dominated at high charging voltage. The working gas rarefaction in DOMS discharge is enhanced at higher charging voltages. At charging voltages higher than 360 V, the metal self-sputtering comes into the runaway regime temporarily as indicated by the self-sputtering parameters exceeding unity, generating the dense and metal-rich plasma. The metal self-sputtering is promoted by gas rarefaction. The electrons tend to ionize the metal atoms after the gas atoms are depleted. The decreased grain size and elevated nano-hardness of the Cr thin films are explained by structure transition from zone I to zone T due to the efficient metal ion bombardment to the growing thin films as the charging voltage increases. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms of electrically induced light emission in nanoscale junctions is crucial in molecular-scale optoelectronics. In this study, we investigated the light emission from nanogap electrodes with an insulating and nonluminescent material (polystyrene). Electrically induced light emission was observed over a wide wavelength range of 400–600 nm. The threshold voltages for the light emission ranged from 3 to 15 V, depending on the device, indicating that the electric field induced at the nanogap, rather than the applied potential, is the driving force for the light emission. The intensity of the light emission varies exponentially with the current with an exponent higher than 1.6, and the light emission is driven by a thermally activated current. These characteristics are inconsistent with the mechanisms reported for solid-state nanogap devices. We conclude that light emission in the nanogap electrodes is attributed to nanoscale vacuum discharge with metal plasma. The light emission from this mechanism can conflict with the other mechanisms ever reported since it occurs at such low voltages. The results also suggest intrinsic instability in molecular-scale electronic devices based on nanogap electrodes because the vacuum discharge is accompanied by the plasticization of the electrode. [ABSTRACT FROM AUTHOR]

Broadened hydrogen Balmer lines are observed in a DC glow discharge sustained in Ar–SiH4–N2 gas mixture. This broadening is due to the Doppler frequency shift. H atoms' translational energy distribution is calculated in different positions between the two electrodes of the discharge along the interelectrode axis and in different gas mixtures. Three main components are observed at 0.6(±0.5) eV, 12(±3) eV, 62(±6) eV, 1(±1) eV, 28(±6) eV, and 105(±12) eV for Hα and Hβ, respectively. They correspond to hydrogen dissociation by electron impact producing H(n = 3, 4) atoms and dissociative recombination of the ion in the cathode fall or backscattered fast H atoms on the cathode surface. The density ratio nH2/nAr ranges from 44% to 13% or from 10% to 3% when a = nH/nH2 = 0.01 and 0.1, respectively, in function of the gas mixture. So, large hydrogen density could be due to hydrogen desorption from the wall and cathodes and/or to the gas recirculation within the reactor. A new low energy component is observed in the Hα line profile, when silane and nitrogen concentrations decrease and increase, respectively. It could also be due to the H atom desorbing from the reactor wall or NHx species dissociated by an electron impact within the reactor. [ABSTRACT FROM AUTHOR]

We have investigated the deposition characteristics of silicon oxide (SiOx) layers in atmospheric pressure (AP) argon (Ar)-based plasma at a substrate temperature of 120 °C. A 150 MHz, very high-frequency (VHF) power is effectively used for exciting stable and uniform hexamethyldisiloxane (HMDSO)-oxygen (O2) fed plasma under AP. The microstructure of the SiOx layers is discussed in comparison with that by using helium (He)-based plasma. In the case of depositions with He/HMDSO/O2 plasma, SiO2-like films with uniform thickness, which have sufficient compactness to be used as the gate dielectrics of bottom-gate thin film transistors, can be obtained by moving substrate at a constant speed during the plasma operation. However, the decrease in the total gas flow rate (increase in the gas residence time in the plasma) causes the increase in the participation of nanoparticles to the film growth, which might result in the deterioration of film quality. Shortening the electrode length is effective for avoiding the incorporation of nanoparticles into the growing SiOx films. On the other hand, when Ar/HMDSO/O2 plasma is used, no deterioration of film compactness is observed irrespective of the gas flow rate. The results obtained in this study demonstrate the effectiveness of the VHF excitation of AP plasma on the generation of stable and uniform glow discharge without using He, which will lead to the development of a highly efficient and reduced cost formation process of good-quality SiOx films. [ABSTRACT FROM AUTHOR]

Interactions at the interface between atmospheric pressure plasmas and liquids are being investigated to address applications ranging from nanoparticle synthesis to decontamination and fertilizer production. Many of these applications involve activation of droplets wherein the droplet is fully immersed in the plasma and synergistically interacts with the plasma. To better understand these interactions, two-dimensional modeling of radio frequency (RF) glow discharges at atmospheric pressure operated in He with an embedded lossy dielectric droplet (tens of microns in size) was performed. The properties of the sheath that forms around the droplet were investigated over the RF cycle. The electric field in the bulk plasma polarizes the dielectric droplet while the electron drift in the external electric field is shadowed by the droplet. The interaction between the bulk and sheath electric fields produces a maximum in E/N (electric field/gas number density) at the equator on one side of the droplet where the bulk and sheath fields are aligned in the same direction and a minimum along the opposite equator. Due to resistive heating, the electron temperature Te is maximum 45° above and below the equator of the droplet where power deposition per electron is the highest. Although the droplet is, on the average, negatively charged, the charge density on the droplet is positive on the poles and negative on the equator, as the electron motion is primarily due to diffusion at the poles but due to drift at the equator. [ABSTRACT FROM AUTHOR]

The importance of gas discharges for numerous applications with increasingly small device size motivates a more fundamental understanding of breakdown mechanisms. Gas breakdown theories for these gap sizes unify field emission with the Townsend avalanche, which depends on Townsend's first ionization coefficient α ; however, the ratio of the electric field E to gas pressure p for microscale gas breakdown exceeds the range of validity for the typical empirical equation. While some studies have used particle-in-cell simulations to assess α in this range, they only examined a narrow range of experimental conditions. This work extends this approach to characterize ionization in microscale gaps for N2, Ar, Ne, and He for a broader range of pressure, gap distance d, and applied voltage V. We calculated α at steady state for 0.75 ≤ d ≤ 10 μ m and p = 190, 380, and 760 Torr. As expected, α / p is not a function of reduced electric field E / p for microscale gaps, where the electron mean free path is comparable to d and E / p is high at breakdown. For d < 2 μ m , α / p scales with V and is independent of p. For d > --> 10 μ m , α / p approaches the standard empirical relationship for E / p ≲ 1000 V Tor r − 1 c m − 1 and deviates at higher levels because the ionization cross section decreases. We develop a more rigorous semiempirical model for α , albeit not as universal or simple, for a wider range of d and p for different gas species that may be incorporated into field emission-driven breakdown theories to improve their predictive capability. [ABSTRACT FROM AUTHOR]

Microdischarges (MDs) have attracted increasing attention recently due to their widespread applications. The electron beam injection as an external source can affect the formation and characteristics of microdischarges. Aimed at exploring the kinetic properties of the atmospheric-pressure microdischarges purely driven by electron beam injection without external voltage, the one-dimensional implicit particle-in-cell/Monte Carlo collision model is developed. The monoenergetic electron beam is injected from the left electrode with a current of 0.001–0.05 A and an emission energy of 20–80 eV. It is found that similar to voltage and current-driven MDs, electron beam driven MDs can sustain steady glow discharge with high density (10 21 – 10 22 m − 3 ) but has much lower plasma potential (∼ 0.15 − 0.30 V) and electron temperature (<1 eV) due to the absence of an external field. The electron energy distribution function is composed of a low-energy group with two-temperature distribution and a high-energy group with a discrete distribution. In addition, the injected electron beam current and energy can influence the plasma properties significantly, such as plasma density, electron temperature, plasma potential, etc. The characteristics of ion bombardment can also be modulated by the beam energy and current, resulting in achievement of low energy and high flux. By enlarging the gap between the electrodes, the parameter difference on both sides can be realized. [ABSTRACT FROM AUTHOR]

The effects of gas pressure and gas component on the power transfer efficiency η and transition power threshold P th during the E–H mode transition in inductively coupled plasmas are studied. The evolutions of η during the E–H mode transition in pure Ar and Ar/O2 discharges are similar, i.e., in E-mode discharge, η slightly increases with raising the applied power (η is below 30%), whereas, an abruptly upward jump of nearly two to four times for η occurs when the discharge transits to H-mode, and then η monotonously increases with increasing the applied power. In addition, as the pressure rises, η increases rapidly first and then slowly in pure Ar discharge. However, η in Ar/O2 mixture gases discharge shows a non-monotonic behavior. It reveals a tendency of first increase at low pressures and then decreases at high pressures, which is because the resistive component of the plasma varies with the electron density n e and the electron effective collision frequency ν eff . Furthermore, a non-monotonic behavior of transition power threshold P th with the pressure is found, i.e., with increasing the pressure, P th first decreases and then increases. It may be caused by the threshold electron density, which is constant at low pressures (ν eff / ω < 1) but increases at high pressures (ν eff / ω > 1). [ABSTRACT FROM AUTHOR]

Diode-pumped metastable rare gas lasers are showing their potential for high-energy laser systems with intensive investigations carried out to understand their lasing characteristics. Here, we demonstrated pulsed lasing of a diode-pumped argon metastable laser based on a pulsed DC glow discharge in argon and helium mixtures for different discharge voltages and pressures. A transient rate equation model was established and solved to explain the cause of the pulsed lasing in our experiment. The insufficient production rate of argon metastable atoms (1s5) and the slow transfer rate out of the 1s4 state in the plasma during the lasing process were found to be the key reasons. This work may shed some light on the future design of plasma sources that have higher metastable atom production rates to achieve highly efficient continuous wave lasing. [ABSTRACT FROM AUTHOR]

A statistical analysis is conducted to identify which physically relevant non-dimensional parameters influence the total (neutral, ion, and electron) static pressure inside thermionic orificed hollow cathodes. It is critical to uncover and order the importance of the physical mechanisms that affect the pressure inside hollow cathodes because it influences the plasma attachment length, the electron temperature, and the sheath potential. These plasma parameters, in turn, affect the emitter lifetime. A principal component analysis of total pressure data obtained from the literature reveals that four non-dimensional variables can account for most of the variation in the total-to-magnetic pressure ratio over five orders of magnitude. The relevant variables are identified with a backward stepwise selection process and an exhaustive grid search and include, by order of importance: the gasdynamic-to-magnetic pressure ratio, the ratio of the mass flow rate to the discharge current, the orifice-to-insert diameter ratio, and the orifice Reynolds number. It is also shown, using various models and regression analyses, that empirical, Poiseuille, or isentropic flow models should not be used for predictive cathode design work. The data-driven study suggests that, while viscous effects may be important, the variation in those effects between cathodes is negligible compared to the effects of the modification of the gas constant due to the plasma, the transitional flow, the flux of heavy species on the orifice plate, and the Lorentz force. [ABSTRACT FROM AUTHOR]

Plasma discharges are traditionally associated with a gaseous phase; however, discharges can be generated in all phases. For liquids with a charge relaxation time shorter or in the same order of the plasma characteristic time, such as tap water, the liquid behaves as a conductor, and the EHD flow induced in the gas phase dominates the flow in the liquid phase. The Perspective by Vanraes and Bogaerts[9] focuses on that same plasma-liquid interface but emphasizes the gas phase processes rather than liquid-phase processes. Yang I et al i .[25] found correlations between self-organized anode layers at the plasma-liquid interface and the plasma-induced flow field. [Extracted from the article]

This article reports on the characterization of a laboratory model 100 A-class hollow cathode with a sintered lanthanum hexaboride ( LaB 6) emitter for high-power Hall thrusters. The cathode has been fired up to 70 A with xenon as working gas. The cathode architecture, test setup, ignition procedure, and power consumption are described first. The second part of this contribution comments on the current–voltage characteristics and the discharge modes obtained for discharge currents in the 30–70 A range and flow rates in the 15–30 SCCM range. The cathode operates in a spot mode at high discharge currents and in a plume mode with large oscillations at low currents and low gas flow rate. Spectral analysis shows that most frequencies reside in the 10–200 kHz range with flat and sharp distributions in plume and spot modes, respectively. Finally, we present electron temperatures and densities measured in the cathode plasma plume by means of incoherent Thomson scattering. The two quantities decrease along the axis. The density is large (up to ∼ 10 19 m − 3 ) and increases with both the ion current and the gas flow rate. The electron temperature increases with the current and decreases with the gas flow rate. The temperature remains relatively low (<1.5 eV) in spite of large currents and applied powers. [ABSTRACT FROM AUTHOR]

Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86 μ g s − 1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided. [ABSTRACT FROM AUTHOR]

The work is concerned with the modeling of low-current quasi-stationary discharges, including the Townsend and corona discharges. The aim is to develop an integrated approach suitable for the computation of the whole range of existence of a quasi-stationary discharge from its inception to a non-stationary transition to another discharge form, such as a transition from the Townsend discharge to a normal glow discharge or the corona-to-streamer transition. This task includes three steps: (i) modeling of the ignition of a self-sustaining discharge, (ii) modeling of the quasi-stationary evolution of the discharge with increasing current, and (iii) the determination of the current range where the quasi-stationary discharge becomes unstable and the non-stationary transition to another discharge form begins. Each of these three steps is considered in some detail with a number of examples, referring mostly to discharges in high-pressure air. [ABSTRACT FROM AUTHOR]

Cold plasma as a new anticancer agent is described by Freund I et al. i [25] The anticancer effect of oxidized saline generated using a spark discharge is investigated using CT26 colorectal cancer cells I in vitro i and I in vivo i and compared against oxidized saline generated by the well-investigated medical plasma jet I kINPen i . In summary, atmospheric plasmas play an important role in many current and emerging fields, including plasma medicine, agriculture, plasma processing, catalysis, and aerospace engineering. Plasma generated in contact with liquid is charactered by Ng I et al i .[35] It is shown that the plasma can be tailored to tune the composition of plasma-functionalized liquids. The significant effect of radiation trapped photon emission on ionization wave dynamics demonstrates the need to consider these effects in plasma reactors where self-absorbed radiation is typically ignored. [Extracted from the article]

Lanthanum hexaboride ( LaB 6) hollow cathodes have demonstrated a capability for long life operation, which is critical to many space exploration missions. Thermal characterization of LaB 6 hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches a steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass flow rates of 5–25 SCCM in cathodes with three different orifice diameters. Thus, the currently accepted value of the work function for polycrystalline LaB 6 , 2.67 eV, does not describe well the emission characteristics of LaB 6 hollow cathodes operating with internal gas discharges at a steady state. We use empirically measured temperatures combined with a model of the hollow cathode emitter and xenon discharge to estimate the value of the work function, yielding a value ranging from 2.1 to 2.44 eV. This lower work function value implies that LaB 6 hollow cathodes are expected to have even longer lifetimes than previously anticipated, further establishing them as a more suited alternative to other conventional cathode technologies for the task of long duration travel. Direct measurements of the work function as a function of depth on a hollow cathode emitter using x-ray photoelectron spectroscopy and ion beam milling indicate that the work function decreases with depth. We postulate several mechanisms that could explain the observed work function enhancement. Altogether, our results have important implications to the design, study approach, and operation of LaB 6 cathodes and potentially other cathodes with hollow configuration. Finally, our work opens the question of why the work function is reduced upon interaction with Xe plasma. [ABSTRACT FROM AUTHOR]

Atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition of silicon (Si) films was numerically simulated. The AP plasma used for Si depositions was excited by a 150-MHz very high-frequency (VHF) electric power, which was capable of generating continuous glow discharges covering the entire electrode surface. The experimental film thickness profiles could be well fitted by the simulations by adjusting the electron density in the plasma. The results showed that, although neutral–neutral reactions proceed very rapidly due to the frequent collisions between the gas species, the dissociation of the source SiH4 molecules by electron impact is the key factor that governs the chemistry occurring in the AP-VHF plasma and promotes the film growth on the substrate. The input power dependences of electrical property of the Si films could be explained by the contribution of SiH3 radical to the deposition. It was also shown that, even though the plasma was continuous glow, the electron density changed in the direction of gas flow, suggesting that the very rapid nucleation of clusters and their growth into nanoparticles were occurring in the AP-VHF plasma. [ABSTRACT FROM AUTHOR]

Hollow cathode plasma discharges are a fundamental part of a large variety of applications in industry, academia, and space. From surface processing and coatings deposition to plasma–surface interaction research and electric propulsion, advances in hollow cathode modeling and performance are critically important to the progress and evolution of these and other areas of technology. This paper describes perspectives on the progress that has been made in recent years in the capabilities and modeling of hollow cathodes used in plasma discharges. While many of the developments have been driven by the demanding requirements of electric propulsion applications, the information provided applies to all thermionic hollow cathodes and their applications. In the paper, we describe the status of 2D global simulations of hollow cathode plasmas, hollow cathode plume instabilities, and the development of higher current cathodes and low-current heaterless cathode technologies. Advances in our understanding and technology in these areas and some of the challenges that still need to be addressed and solved are discussed. [ABSTRACT FROM AUTHOR]

Recently, solution-electrode glow discharges (SEGDs) have shown great utility as ionization sources for mass spectrometry (MS). The solution composition of SEGD electrodes is pivotal to their performance as it influences analyte-ion formation. The performance of electrospray ionization is heavily dictated by spray-solution composition, which can alter ionization efficiency and pathways. While SEGDs produce Taylor cones similar to electrospray ionization, the influence of solution-electrode composition on molecular-ion formation has not been studied. Here, we examine how additives to an atmospheric-pressure solution-cathode glow discharge (SCGD) influence molecular ionization and fragmentation. The impact of several additives to the acidic solution of an SCGD ionization source was evaluated based on mass-spectrometric performance. Addition of methanol increased molecular- and fragment-ion signals for peptide angiotensin II. This effect is likely due to improved desolvation and a greater interaction of analyte molecules with glow-discharge species. Several high-boiling-point reagents were tested to examine changes in the ion signal, the average charge state, and the degree of fragmentation. Overall, these additives inhibited fragmentation but significantly lowered intact molecular-ion signals. Interestingly, loss of fragment ions trended with the boiling point of the reagent used. We hypothesize that analyte molecules become trapped in droplets produced at the solution-cathode surface. These droplets do not fully desolvate before escaping the discharge region, sparing analyte molecules from fragmentation. For low volatility additives, droplets do not desolvate, even as they enter the heated MS, which yields a loss in molecular ions. The changing composition of the SCGD solution alters analytical performance, but also provides insight into analyte ionization and fragmentation processes. [ABSTRACT FROM AUTHOR]

The direct interaction between a non-equilibrium gas discharge and a liquid volume leads to the generation of a plasma activated liquid. This interaction induces a flow in both the gas above the liquid and within the liquid volume. The physical mechanisms behind the induced flows are complex. In this work, a two-dimensional experimentally validated numerical model was developed to determine the dominant mechanism driving the liquid flow at the plasma–liquid interface. The model followed the evolution of the plasma and the flow fields in both phases, describing a pin-water discharge configuration operating in air, which was used to treat a de-ionized water sample and a tap water sample. Two potential physical mechanism were investigated, the electrohydrodynamic (EHD) flow induced in the gas phase and the electric surface stresses across the interface. It was found that the dominant mechanism driving the liquid flow is correlated with the charge relaxation time of the liquid. For liquids with a charge relaxation time longer than the characteristic time of the plasma, such as de-ionized water, the liquid behaves as a dielectric, and the electric surface stresses dominate the flow in the liquid phase. For liquids with a charge relaxation time shorter or in the same order of the plasma's characteristic time, such as tap water, the liquid behaves as a conductor, and the EHD flow induced in the gas phase dominates the flow in the liquid phase. [ABSTRACT FROM AUTHOR]

The effects of an external magnetic field on the solution-cathode glow-discharge (SCGD) and solution-anode glow-discharge (SAGD) are investigated. The SCGD is atmospheric-pressure glow discharge sustained between a metal pin and a liquid cathode electrode in the ambient atmosphere, and it is often used for trace elemental analysis by atomic emission spectroscopy. Here, the SCGD is modified to allow an external permanent magnetic field to be applied, either in a static orientation or as a rotating field, as a means of stabilizing the SCGD plasma and modulating atomic emission from the discharge. The effect of the external magnetic field on the physical structure, electrical characteristics, and spectroscopic response of the SCGD and SAGD are investigated. A rotating external magnetic field was found to change both SAGD and SCGD structure and spatial emission pattern. Analytical figures of merit are examined, and a lock-in amplifier is used to discriminate analytical atomic emission from background emission, improving limits of detection. [ABSTRACT FROM AUTHOR]

In plasma–liquid interactions, the phenomenon of induced liquid flow that originates at the plasma–liquid contact point is important in that it influences mass, charge, and heat transport from the source to the surrounding bulk fluid. Such stimulated flows have been observed in 1 atm glows with a liquid anode. Because the plasma contact point in such discharges is patterned, a natural question is what is the relationship between the observed self-organized patterns and the induced flow field? It is, therefore, of great interest to investigate the coupling mechanism between the self-organization patterns in an atmospheric pressure dc helium glow discharge with a liquid anode and the induced flow circulation. Particle imaging velocimetry is used to probe the flow fields in the plane normal and parallel to the plasma–liquid interface. A strong ascending flow with maximum speed up to 1.5 cm/s and circulation vortices nearby are observed in the plane normal to the interface centered at the plasma attachment. The experiment results suggested that the ascending flow is caused by water evaporation and the vortices are formed by viscous stress. With a self-organization pattern formed, the flow structures become non-static and the circulation vortices are observed to periodically form and decay. In the plane parallel to the interface, a strong swirl flow was found to exist only when the plasma attachment is self-organized. The analysis revealed that the driving mechanism could be the electrohydrodynamics force. Averaged flow velocity over time in the field of view was found to scale linearly with increasing input power and increasing liquid conductivity. [ABSTRACT FROM AUTHOR]

When electrons from a plasma enter a water surface, they collide with the water molecules and lose their energy, eventually becoming hydrated electrons, which are an important species for radical formation in liquid water. Although gas discharge plasmas in contact with water are an easy source of hydrated electrons, their production processes are not clearly understood. In this study, Monte Carlo simulations of electrons in liquid water were performed to investigate the production of a hydrated electron and radical species by low-energy electron irradiation of the water surface from an atmospheric-pressure plasma. The incident electrons were assumed to have a Maxwellian energy distribution with mean electron energy ranging from 1 to 10 eV. The production of hydrated electrons by electron irradiation of 3 eV reached a peak near the gas–liquid interface, and the electrons spread to a depth of approximately 17 nm. OH and H are also generated near the water surface. [ABSTRACT FROM AUTHOR]

This work deals with the numerical and experimental investigation of atmospheric pressure dielectric barrier discharges (DBDs). In the experiment, the current and voltage of DBD sustained in an argon flow in coaxial discharge cell are measured. Numerical models are based on the drift-diffusion theory of gas discharges. Different modelling approaches, where the electron kinetics is determined on the basis of the Maxwellian electron energy distribution function (EEDF), vs models with more realistic EEDF obtained from the solution of the electron Boltzmann equation are implemented. The effect of energy loss due to heating of the gas is considered. [ABSTRACT FROM AUTHOR]

The optically pumped rare gas laser (OPRGL) is an innovative type of gas laser, which has the potential to be a candidate for high-energy lasers (HELs), in which metastable rare gas atoms generated by gas discharge are used as active particles. To realize these kinds of lasers as HELs in field application, high efficiency and low cost are necessary. For this purpose, we demonstrated a closed-chamber operation with repetitively pulsed gas discharge for an OPRGL of argon metastables. The pulsed discharge was powered by a home-made LC generator, which provided a high efficiency for the deposition of electric energy. The metastable density produced was higher than 1012 cm−3, quite enough for efficient laser operation. The dependencies of laser radiation on the parameters of discharge voltage, gas pressure, and argon concentration were studied, which showed an optimized argon concentration of 1%. Long-time operations were also performed, which demonstrated the practicability of the operation of OPRGL in a closed chamber, with the avoiding of the running away of rare gases in the open cycle of flowing operation. [ABSTRACT FROM AUTHOR]

Determination of the key chemical reaction pathways in cold atmospheric plasmas (CAPs) is of great importance not only for understanding the spatiotemporal evolutions of the key plasma parameters during discharges but also for improving the plasma materials processing qualities. In this paper, a novel chemical reaction reduction method (CRRM) is proposed by using the global fluid model coupled with the genetic algorithm and the dynamic programming technique. With the aid of this newly developed CRRM, the key chemical reaction pathways can be automatically screened with a high computational efficiency under a pre-set critical calculation accuracy for the atmospheric pure helium and helium–nitrogen glow discharge plasmas. By comparing the calculated key plasma parameters, e.g., the species number densities, electron temperatures, voltage–current characteristics, based on the simplified models and their corresponding full models with those of the experimentally measured data, the reliability of the CRRM itself and the established key chemical reaction database for the atmospheric pure helium and helium–nitrogen CAPs are validated. This research also provides a general method for screening the key chemical reaction pathways for various low-temperature plasma sources. [ABSTRACT FROM AUTHOR]

A self-consistent unified model of direct current (DC) gas discharges in molecular gases describing the processes both in the discharge gap and in the electrodes is developed. Numerical simulations of the formation of parameters of various modes of DC discharge in nitrogen at atmospheric pressure in a wide range of current densities have been carried out in the one-dimensional approximation. As a result of numerical experiments, the dependence of voltage drop across the discharge on current density is obtained, which describes various discharge modes: from Townsend through normal and abnormal glow to arc discharge. The distributions of the main mechanisms of heating the electrodes, the values of their temperatures at the boundary with the discharge, depending on the current density, and all plasma parameters for various modes of a dc discharge are presented. It was shown that in the plasma region the dominant role in gas heating belongs to vibrational-translational relaxation, as well as to the reactions of dissociation of N2 molecules by electron impact and quenching of electronically excited N2 molecules. In the cathode layer, the dominant role in gas heating belongs to Joule heating by ion current. The scenario of formation of an arc discharge with free cathode mode when electrons are the predominant charged particles in the cathode layer was shown. [ABSTRACT FROM AUTHOR]