Abstract Plasma stratification has been studied for more than a century. Despite the many experimental studies reported on this topic, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed kinetics of electrons and vibrationally excited molecules is employed to simulate moderate-pressure (i.e. a few Torrs) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model also considers ambipolar diffusion to account for the radial loss of ions and electrons to the wall. The proposed model predicts self-excited standing striations in nitrogen for a range of discharge currents. The impact of electron transport parameters and reaction rates obtained from a solution of local two-term and a multi-term Boltzmann equation on the predictions are assessed. In-depth kinetic analysis indicates that the striations result from the undulations in electron temperature caused due to the interaction between ionization and vibrational reactions. Furthermore, the vibrationally excited molecules associated with the lower energy levels are found to influence nitrogen plasma stratification and the striation pattern strongly. A balance between ionization processes and electron energy transport allows the formation of the observed standing striations. Simulations were conducted for a range of discharge current densities from ∼0.018 to 0.080 mA cm −2 , for an operating pressure of 0.7 Torr. Parametric studies show that the striation length decreases with increasing discharge current. The predictions from the model are compared against experimental measurements and are found to agree favorably.
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Scaling of pulsed nanosecond capillary plasmas at different specific energy deposition
Abstract Nano-second, capillary discharges (nCDs) are unique plasma sources in their ability to sustain high specific energy deposition ω dep approaching 10 eV/molecule in molecular gases. This high energy loading on short timescales produces both high plasma densities and high densities of molecular exited states. These high densities of electrons and excited states interact with each other during the early afterglow through electron collision quenching and associative ionization. In this paper we discuss results from a two-dimensional computational investigation of a nCD sustained in air at a pressure of 28.5 mbar and with a voltage amplitude 20 kV. Discharges were investigated for two circuit configurations—a floating low voltage electrode and with the low voltage electrode connected to ground through a ballast resistor. The first configuration produced a single ionization wave from the high to low voltage electrode. The second produced converging ionization waves beginning at both electrodes. With a decrease of the tube radius, the velocity of the ionization fronts decreased while the shape of the ionization wave changed from the electron density being distributed smoothly in the radial direction, to being hollow shaped where there is a higher electron density near the tube wall. For sufficiently small tubes, the near-wall maxima merge to have the higher density on the axis of the capillary tube. In the early afterglow, the temporal and radial behavior of the N 2 (C 3 Π u ) density is a sensitive function of ω dep due to electron collision quenching. These trends indicate that starting from ω dep ⩾ 0.3 eV/molecule, it is necessary to take into account interactions of electrons with electronically excited species during the discharge and early afterglow.
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- Award ID(s):
- 1902878
- NSF-PAR ID:
- 10437798
- Date Published:
- Journal Name:
- Plasma Sources Science and Technology
- Volume:
- 29
- Issue:
- 12
- ISSN:
- 0963-0252
- Page Range / eLocation ID:
- 125006
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We have performed hybrid kinetic-fluid simulations of a positive column in alternating current (AC) argon discharges over a range of driving frequencies
f and gas pressurep for the conditions when the spatial nonlocality of the electron energy distribution function (EEDF) is substantial. Our simulations confirmed that the most efficient conditions of plasma maintenance are observed in the dynamic regime when time modulations of mean electron energy (temperature) are substantial. The minimal values of the root mean square electric field and the electron temperature have been observed atf/p values of about 3 kHz Torr−1in a tube of radiusR = 1 cm. The ionization rate and plasma density reached maximal values under these conditions. The numerical solution of a kinetic equation allowed accounting for the kinetic effects associated with spatial and temporal nonlocality of the EEDF. Using thekinetic energy of electrons as an independent variable, we solved an anisotropic tensor diffusion equation in phase space. We clarified the role of different flux components during electron diffusion in phase space over surfaces of constanttotal energy. We have shown that the kinetic theory uncovers a more exciting and rich physics than the classical ambipolar diffusion (Schottky) model. Non-monotonic radial distributions of excitation rates, metastable densities, and plasma density have been observed in our simulations atpR > 6 Torr cm. The predicted off-axis plasma density peak in the dynamic regime has never been observed in experiments so far. We hope our results stimulate further experimental studies of the AC positive column. The kinetic analysis could help uncover new physics even for such a well-known plasma object as a positive column in noble gases. -
null (Ed.)Dielectric barrier discharges are receiving increasing attention as sampling/ionization sources for ambient mass spectrometry. Nevertheless, the underlying mechanisms are not completely understood, particularly when the plasma plume is exposed to a sample surface. Herein, an atmospheric pressure helium micro-dielectric barrier discharge (μDBD), flowing into open-air and onto a sample surface (floating Cu or LDPE), is studied via optical emission spectroscopy (OES) with radial information extracted through Abel's inversion. Radially resolved optical emission profiles along the axis are shown to shift with respect to the line-of-sight counterparts, for some species. The OES images, as well as vibrational and rotational temperature maps, indicate the energy transfers mainly via Penning ionization of N 2 with He metastables to produce N 2 + , as the plasma plume exits the capillary into open air, while charge transfer with He 2 + is dominant further downstream, as well as toward the periphery of the plume. In addition, the sample surface is shown to play an important role in the energy transfer mechanisms. For the LDPE sample, the spatial distribution sequence of excited species is similar to the free-flowing counterpart but disappears into the surface, which indicates that excited N 2 further downstream/outer periphery is produced from electron recombination with N 2 + . On the other hand, the presence of the floating Cu sample results in an intensity peak at the plasma/surface interface for most species. We propose that the temporal evolution of the half-cycle dynamics have a great effect, where the resulting higher electron temperature and density towards the surface of metallic samples favors electron impact excitation. Furthermore, profilometry of the resulting craters in the floating Cu samples revealed a close correlation between their diameter and the width of the N 2 + emission.more » « less
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Abstract Self-Organized Patterns (SOPs) at plasma-liquid interface in atmospheric pressure plasma discharges refer to the formation of intricate and puzzling structures due to the interplay of electrodynamic and hydrodynamic processes. Studies conducted to date have shown that this phenomenon results in the formation of distinctive patterns such as circular ring, star, gear, dots, spikes, etc., and primarily depends on working gas, electrolyte type, gap distance, current, conductivity, etc. However, an adequate understanding of how these patterns change from one type to another is still not available. This study aims to elucidate the influence of initial liquid conductance ( σ i ) on the temporal evolution of SOPs in liquid-anode discharges. The discharge was generated in a pin-to-liquid anode configuration at a constant helium (He) flow rate of 500 sccm and DC applied voltage of 6 kV at a gap distance of 12 mm. Through the gradual increment of σ i from 1.8 μ S to 4820 μ S, we observe that the trend in the evolution of SOPS takes place as solid discs, spikes, dots, rings, double rings, and stars. The continuous formation of reactive species onto the liquid anode in all conductive solutions results in a decrease in pH, an increase in bulk liquid temperature, and an increase in total dissolved solutes, and these have been confirmed through experimental measurements. Observations using optical emission spectroscopy show that the electrons at the plasma-liquid interface participate in the reduction of cations followed by their excitation & ionization due to which electron density as well as emissions from excited species (mainly hydroxyl radicals & excited nitrogen) decrease with time. Our investigation provides experimental evidence on the presence of cations at the plasma-liquid interface required for SOP formation.more » « less
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The gasdynamic electron cyclotron resonance (ECR) ion source is a type of the device in which the ionization efficiency is achieved primarily due to a high plasma density. Because of a high particle collision rate, the confinement is determined by a gasdynamic plasma outflow from a magnetic trap. Due to high efficiency of resonant heating, electrons gain energy significantly higher than that in inductively or capacitively coupled plasmas. As a consequence of such a parameter combination, the gasdynamic ECR plasma can be a unique source of low to medium charged ions, providing a high current and an ultimate quality of an ion beam. One of the most demanded directions of its application today is a development of high-current proton injectors for modern accelerators and neutron sources of different intensities. Special plasma parameters allow for the use of diagnostic techniques, traditional for multiply charged ECR plasmas as well as for other types of discharges with a high plasma density. Among the additional techniques, one can mention the methods of numerical simulation and reconstruction of the plasma density and temperature from the parameters of the extracted ion beams. Another point is that the high plasma density makes it possible to measure it from the Stark broadening of hydrogen lines by spectroscopy of plasma emission in the visible range, which is a fairly convenient non-invasive diagnostic method. The present paper discusses the main physical aspects of the gasdynamic ECR plasma, suitable diagnostic techniques, and possibilities and future prospects for its various applications.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1361-6595/abc413
