Propagation Of Ionization Waves Research Articles

Spatio-temporal electric field distributions in an atmospheric plasma jet impinging on a microchannel array surface

The electric field distribution in the ionization waves (IWs) propagating over a microchannel array dielectric surface, with the channels either empty or filled with distilled water, is measured by ps electric field induced second harmonic generation. The surface IW is initiated by the atmospheric pressure N2–Ar plasma jet impinging on the surface vertically and powered by ns pulse discharge bursts. The results show that the electric field inside the microchannels, specifically its horizontal component, is enhanced by up to a factor of 2. The field enhancement region is localized within the channels. The vertical electric field inside the channels lags in time compared to the field measured at the ridges, indicating the transient reversal of the IW propagation direction across the channels (toward the jet). This is consistent with the phase-locked plasma emission images and confirmed by the kinetic modeling predictions, which show that the IW ‘jumps’ over the empty channels and propagates into the channels only after the jump between the adjacent ridges. When the channels are filled with water, the wave speed increases by up to 50%, due to the higher effective dielectric constant of the surface. No evidence of a significant electric field enhancement near the dielectric surface (ceramic or water) has been detected, within the spatial resolution of the present diagnostic, ∼100 μm.

Manipulating the propagation of ionization wave by pulsed electrical potential in atmospheric plasma jet

A noninvasive method was developed to measure the electrical potential in an atmospheric pressure plasma jet. The spatiotemporal evolution of electrical potential was measured by a wire electrode in experiment and simulated by a two-dimensional self-consistent numerical model, which demonstrates that the electrical potential along with the traveling ionization wave outside the discharge tube in ambient air plays a significant role on the propagation of ionization wave. It furtherly shows that the enhancement and restriction of ionization wave propagation are dependent on the amplitude of electrical potential on the auxiliary electrode, which proposes the way to manipulate the propagation of ionization wave in atmospheric pulsed discharge plasma jet.

Ionization wave propagation and cathode sheath formation due to surface dielectric-barrier discharge sustained in pulsed mode

This work deals with the experimental study of a surface dielectric-barrier discharge, as a part of the ongoing interest in the control of plasma induced electro-fluid dynamic effects (e.g. plasma actuators). The discharge is generated using a plasma reactor consisting of a fused silica plate which is sandwiched between two printed circuit boards where the electrodes are developed. The reactor is driven by narrow high voltage square pulses of asymmetric rising (25 ns) and falling (2.5 μs) parts, while the discharge evolution is considered in a temporarily and spatially resolved manner over these pulses. That is, conventional electrical and optical emission analyzes are combined with high resolution optical emission spectroscopy and ns-resolved imaging, unveiling main characteristics of the discharge with a special focus on its propagation along the dielectric-barrier surface. The voltage rising part leads to cathode-directed ionization waves, which propagate with a speed up to 105 m s−1. The voltage falling part leads to cathode sheath formation on the driven electrode. Τhe polarization of the dielectric barrier appears critical for the discharge dynamics.

Kinetics of the fast ionization waves with runaway electrons

In nanosecond gas breakdown, runaway electrons are responsible for the interelectrode space pre-ionization, which supports the propagation of fast ionization waves or streamers. However, a detailed understanding of mechanisms of how this happens is still elusive because of the wide range of energies of runaway electrons, from the hundreds of eV to the hundreds of keV. While the low-energy part of the runaway electron population can only ionize the background gas, the high-energy part can produce x-rays that can also ionize the gas. The objective of this paper is to understand the influence of different electron groups on the fast ionization wave dynamics for the conditions typical for runaway electrons generation. We find that there are two main groups, one having energies below 100 eV and another having energies above 1 keV. The first group defines the establishment of the fast ionization waves and their plasma parameters, while the second group defines the speed of the ionization front propagation.

Ionization wave propagation of a surface dielectric barrier discharge with a flexible-structure plasma sheet

Plasma sources based on flexible substrates are receiving attention due to their unique adaptability to irregular surfaces and large range of plasma coverage, which endows them with irreplaceable advantages in the fields of material processing and biomedical treatment. Numerous studies have been carried out focusing on the application effects of these flexible plasma sources, while their surface discharge characteristics and mechanisms still lack revelation. In this work, a flexible plasma sheet with a surface dielectric barrier discharge structure is realized via a printed circuit board, and its multiple surface ionization wave (SIW) propagation on curved gas–solid interfaces is studied via experiments and 2D fluid simulation models. Qualitative agreement is achieved between the experiments and simulations. It is found that a positive and a negative discharge are generated at the rising and falling edges of the excitation pulse, respectively. In the positive discharge, SIWs originate at the grounded mesh edge and then propagate to the center in a petal-like pattern, which is shaped by the space electric field. Controlled by electron collision reactions, the development of the excited states of N2 and O2 molecules is similar to that of electrons. In the negative discharge, electrons dissipate and no SIW is generated. The evolution of heavy particles shows differences in this period, which is attributed to the disparate rate coefficients of their consumption reactions. Further study shows that when the plasma sheet changes from convex to concave, the electron density and electron temperature above its surface increase, but the petal patterns of the SIW propagation have no variation. The electron density, electron temperature, and electron impact ionization source will rise as a result of the increasing pulse amplitude or the decreasing duration of the pulse rising edge.

Discharge characteristics of atmospheric pressure pulsed microwave He/N2 plasma jet

AbstractIt is difficult to generate jet‐shaped He microwave plasma jets in ambient air. In this study, a dual‐channel pulsed microwave coaxial resonant discharge device is constructed. An atmospheric pressure pulsed microwave He plasma jet is formed when He passes through the central copper tube. And a spatially layered He/N2 plasma jet is produced when He and N2 are aerated through the central copper tube and the resonant cavity, respectively. The spatiotemporal ionization evolution process of the He/N2 plasma jet is determined in experiments. The ionization characteristics could be attributed to the combining interactions between the resonant excitation of locally enhanced electric fields, the ionization wave propagation, and the various particle states in the He/N2 plasma jet.

Analysis of preionization effect of excimer laser

Excimer laser plays an important role in the national economic production because of its short wavelength, high repetition frequency and large energy. In the output characteristics of excimer lasers, high beam quality and high energy stability are the characters of high-end excimer light source, and whether the excimer laser system can sufficiently develop the preionization process is the key to output laser pulses with high beam quality and stable energy. This paper is mainly to study the preionization process of ArF excimer laser, and establish one-dimensional simulation calculation model and two-dimensional simulation calculation model. Based on the fluid calculation method of Comsol Multiphysics software, the numerical simulation analysis and calculation are carried out, with the study focusing on the ionization wave propagation process in the preionization process and the relationship between preionization and main discharge. The results show that the spatial structure of corona rod and main electrode should be considered comprehensively when an excimer laser system is designed. Appropriate preionization structure and voltage can make the main discharge triggered during the maintenance of free electrons, which can effectively reduce the discharge voltage, avoid streamer and arc discharge, and improve the output beam quality and system energy stability.

Diagnostics of a nanosecond atmospheric plasma jet. Electron and ro-vibrational excitation dynamics

Atmospheric repetitive He discharge with 10 ns current peak width and V s−1 voltage front rise working in jet geometry is studied. The first part of the study is devoted to electrical and optical discharge characterization including voltage-current behavior, emission dynamics, as well as ro-vibrational dynamics of N2, N and OH molecules. It is found that He atoms get excited at the very early stage, as a result of ionization wave formation. This process follows by excitation of O, N2 and N. It is also shown that a rather small (0.1%–1%) air admixtures facilitate gas breakdown, as revealed by shortening of the discharge current risetime. The rotational excitation of N always overtakes He excitation by about 5 ns, with rotational temperature peaking at about 650 K and decaying afterwards, whereas rotational temperature of N2 always remains constant equal to about 300 K. This value is associated with gas kinetic temperature since the electron-rotational (e-R) excitation process is too slow, so it cannot be activated during the plasma phase and no additional change in rotational excitation of N2 happens. Nitrogen molecules remain vibrationally excited in the discharge, post-discharge and jet areas after the plasma pulse which is likely a result of ionization wave propagation. Upon water vapor injection the apparent OH rotational distributions reveal double Boltzmann slope representing thermalized and non-thermalized OH groups. Rotational temperature of the thermalized group correlates with the one of N showing, however, much longer relaxation time, whereas for the non-thermalized group it remains above 1 eV at all conditions. The obtained results confirm that the studied ns- discharge is a good candidate for temperature-sensitive applications, such as the bio-sample treatment. Further analysis related to the ionization waves, electron density and electric field behavior is undertaken in the second part.

Ionization Waves, Propagating in Opposite Directions, as in Red Sprites

The paper is devoted to the study of a pulsed streamer discharge, similar to that in the Earth’s upper atmosphere. An experimental setup providing the formation of two ionization waves propagating in opposite directions from a region filled with a plasma formed by the capacitive discharge in low-pressure atmospheric air was created. In a physical experiment, the process of propagation of red ionization waves (streamers) was simulated. It was established that the average propagation velocities of their fronts correspond to those of red sprites. This was shown as a result of spectral studies that at air pressures of 0.4-3 Torr, the radiation color radiation observed visually and captured on an integral photograph from the region of passage of ionization waves is determined by the spectral transitions of the first positive system (FPS) of nitrogen molecules, similar to what occurs for red sprites. In this case, the spectral energy density of radiation in the most intense band of the second positive system (SPS) of the nitrogen molecule with a wavelength of 337.13 nm is an order of magnitude or higher than that in the most intense band of the FPS with the wavelength of 775.32 nm. Using the emission spectra and methods of optical emission spectroscopy (OES), the main parameters of the discharge plasma are estimated. Thus, the created setup makes it possible to simulate the process of the formation of red sprites propagating in opposite directions under laboratory conditions.

Pulse-width-dependent dynamic charge deposition on quartz dielectrics interacted with atmospheric pressure He plasma jet

Surface processing with atmospheric pressure plasma jet (APPJ) is attracting attention from both academia and industry for its capability of flexible plasma release onto targets. It is significant to understand the characteristics and mechanism of interfacial phenomenon in plasma surface interaction. This paper reports the fundamental surface charging phenomenon on quartz dielectrics by a He APPJ excited by μs-width pulses with duty ratios from 0.5% to 75%. The targeted quartz plate is attached to an electro-optic crystal grounded in the back for charge measurement. The evolution of surface charges is detected with a reflective-type optical measurement platform based on Pockels effect and the fast charge inversion on 2D-FTA. Results show that ionization waves and quartz dielectric interaction generates a round positive charge deposition, whose lifetime greatly depends on the pulse widths. Under a fixed pulse width, higher voltage magnitudes encourage more charges with a higher deposition rate. An interesting polarity reversal of surface charging occurs when the pulse width increases to over half-cycle, with a reduced positive charge on pulse-on and more negative charges on pulse-off. Efforts are devoted to exploring the mechanisms of charge maintenance on pulse-on and the polarity reversal under longer pulses of several hundred microseconds. These results reveal that the unchanged charge density in dark channel is due to the much lower rate of charge decay compared with the fast ionization wave propagation. The abrupt decrease at the falling edge results from the charge neutralization in the negative conductive channel formed by the secondary discharge. By measuring the initial evolution during the foremost 1000 periods, we prove that the accumulative difference in dissipated charges between pulse-on and pulse-off periods is the main cause of the occurrence of polarity reversal under long pulses. This study provides a more compatible tool for in-depth insight into the charging process of plasma interacting with multiple dielectrics, and exhibits a potential strategy of regulating surface charging by adjusting pulse parameters, which are of interest to the plasma community.

On self organization: model for ionization wave propagation with targets of varying electrical properties

This work presents a model for an atmospheric helium plasma interacting with normal and cancer cells. This interaction is simulated through the expansion and impingement of a gaseous jet onto targets with varying electrical permittivity. Simulation results show that for a plasma jet impinging onto two targets with different permittivity placed axis-symmetrically relative to the stagnation point of impingement, the jet is biased toward the target with lower permittivity when the target acts as a floating potential. This trend is reversed when the back surface of the target is grounded. In the case of a floating target, higher target permittivity yields a higher positive surface potential as the material experiences higher polarization in response to the net flux of electrons from the plasma onto the surface. Because of this higher surface potential, targets with higher permittivity generate a smaller electric field in the discharge column relative to materials with lower permittivity. When the back surface of the target is ground, the trend is reversed, with polarization occurring primarily on the back surface due to the response to the reservoir of positive charges introduced by ground. In the ground case, the material experiences more negative charging the front surface which induces a lower electric potential. As a result, the material with higher permittivity and a grounded back surface attracts plasma organization at the interface because of the higher local electric field. These numerical findings support experimental results presented by other researchers, which demonstrate selectivity of plasma jets toward some cancer cells more than others. The mechanism introduced here may help inform targeted treatment of specific cells, including those reported to be more resistant to plasma jets.

Non-premixed counterflow methane flames in DC/AC/NS electric fields

The response of counterflow diffusion flames to sub-breakdown DC and AC electric fields, as well as their superposition with ns pulse discharge waveforms, is studied in the plane-to-plane electrode geometry. Sub-breakdown DC and low-frequency AC electric fields cause the flame displacement toward the grounded electrode, in the direction of the applied field, indicating that the body force on the positive ions exceeds that on the electrons and negative ions. As the AC frequency increases, the flame response becomes less pronounced, due to the reduction of the electrohydrodynamic (EHD) body force impulse over the AC half-period. The electric field in the electrode gap is determined by ps Electric Field Induced Second Harmonic (E-FISH) generation, with absolute calibration using sub-breakdown ns pulses overlapped with the measured electric field waveform. The results show that the electric field distribution across the flame in the current saturation regime follows the Laplacian field. This indicates that the space charge density in the gap is too low to distort the applied DC or AC field, consistent with the kinetic modeling predictions. Combining a nanosecond pulse discharge with a sub-breakdown DC field generates a diffuse plasma across the entire gap. Time-resolved and spatially resolved measurements of the electric field in the discharge indicate the ionization wave propagation between the electrodes. The present results do not exhibit a detectable flame displacement enhancement by ns discharge pulses combined with a sub-breakdown field, observed previously. Kinetic modeling calculations show that the absence of this effect in the plane-to-plane geometry is due to the rapid plasma self-shielding. This indicates that alternative electrode geometries limiting the self-shielding would be more effective for the plasma / electric field enhanced flameholding and flame stabilization applications.

Ionization waves (striations) in a low-current plasma column revisited with kinetic and fluid models

A one-dimensional particle-in-cell Monte Carlo collisions method has been used to model the development and propagation of ionization waves in neon and argon positive columns. Low-current conditions are considered, that is, conditions where stepwise ionization or Coulomb collisions are negligible (linear ionization rate). This self-consistent model describes the development of self-excited moving striations, reproduces many of the well-known experimental characteristics (wavelength, spatial resonances, potential drop over one striation, and electron “bunching” effect) of the ionization waves called p, r, and s waves in the literature, and sheds light on their physical properties and on the mechanisms responsible for their existence. These are the first fully kinetic self-consistent simulations over a large range of conditions reproducing the development of p, r, and s ionization waves. Although the spatial resonances and the detailed properties of the striations in the nonlinear regime are of kinetic nature, the conditions of existence of the instability can be obtained and understood from a linear stability analysis of a three-moment set of quasi-neutral fluid equations where the electron transport coefficients are expressed as a function of electron temperature and are obtained from solutions of a 0D Boltzmann equation. An essential aspect of the instability leading to the development of these striations is the non-Maxwellian nature of the electron energy distribution function in the uniform electric field prior to the instability onset, resulting in an electron diffusion coefficient in space much larger than the energy diffusion coefficient.

Formation and evolution of striation plasma in high-pressure argon glow discharge

The self-organized striation phenomenon in the positive column region of glow discharge plasma is a basic problem in gas discharge physics, which involves rich physics such as electron dynamics, transport process, discharge instability and nonlinear phenomenon. It is an important topic in basic physics and practical application. In this work an argon glow discharge striation plasma at high pressure is reported. The electrical, optical and ionization wave propagation characteristics of the striation plasma, and the evolution of the striation plasma with pressure and impurity gas are investigated experimentally. The generation and quenching mechanism of argon striation plasma are analyzed. The results show that the striation length is about 1.5 mm, and decreases with pressure increasing, and the velocity and frequency of the ionization wave are estimated at 1.87 m/s and 1.25 kHz, respectively. The measurement of optical emission spectrum shows that the generation of striation plasma is probably related to the argon metastable atoms. The stepwise ionization process caused by metastable atoms triggers off an ionization instability. The instability propagates in the form of ionization wave, which leads the plasma parameters to be modulated longitudinally, thus, forming an alternating bright and dark striation plasma. The adding of nitrogen can effectively quench metastable argon atoms and change the electron energy distribution function, which destroys the instability conditions of the plasma, therefore, the striation plasma disappears. This work provides a new insight into the understanding of the formation and annihilation mechanism of glow discharge striation plasma at high pressure.

Features of a negative ionization wave development preceding the full breakdown in a long capillary tube wrapped with a continuous or fine-sectioned grounded electrode

The experimental results on the study of negative ionization wave propagating along a long capillary tube are presented. The ionization wave was initiated by high-voltage pulse of negative polarity. The propagation of this surface ionization wave precedes and influences the establishment of complete electric breakdown within the tube. The spreading of this wave is accompanied by the surface charge deposition. The usage of the fine-sectioned outer electrode allows one to find out the general features of a negative ionization wave. The main of them is the tight correlation between local currents determining the formation of local surface charge and visual discharge images taken by the fast camera characterizing the pace of the ionization wave propagation.

Ionization wave propagation in a long capillary tube in the presence of a low-current glow discharge in low-pressure helium

The experimental results on the study of the positive ionization wave propagating along a long capillary tube are presented. The ionization wave was initiated by eigther high-voltage pulse of positive polarity applied over a long capillary tube with helium or high-voltage pulse applied over the same tube but in presence of low-current glow discharge. The spreading of this wave is accompanied by the surface charge deposition. The usage of the fine-sectioned outer electrode allows one to find out the features of the positive ionization wave in both cases (with or without low-current discharge).

Polyimide‐Based Flexible Plasma Sheet and Surface Ionization Waves Propagation (Adv. Electron. Mater. 11/2021)

Non-Thermal Plasma Guanjun Zhang and colleagues realize a polyimide-based flexible sheet to generate non-thermal plasma in atmosphere and study the propagation of ionization waves along its surface in article number 2100369. Multiple waves propagate in a petal pattern, which is impervious to different curvature of the sheet. Bending the flexible plasma sheet will not significantly affect its utility in the fields of material processing and biomedical application.

Numerical verification of the two-spike-current behavior in the initial stage of plasma formation in a pulsed surface dielectric barrier discharge

Nanosecond pulsed surface dielectric barrier discharge (SDBD) is a hot topic in many fields, but the mechanisms of discharge evolution and micro-channel propagation are still not clearly understood. In this paper, a plasma fluid model of nanosecond pulsed SDBD is established, and the deductions of the two current spikes are verified and improved by simulation. The two current spikes correspond to the two stages of the discharge: the ionization wave propagation and the repeated re-ignition in the gap between the ionization wave and the dielectric surface. In the first stage, the ionization wave develops rapidly at first and propagates very slowly in the end, which produces the first current spike with a very short rise time and a tailing falling edge. The curve profile of the ionization wave velocity is very similar to that of the first current spike. A certain distance is maintained between the bottom of the ionization wave and the dielectric surface in this stage, which forms the gap for the next stage. In the second stage, the charged particle cloud induced by the quasi-uniform electric field in the middle of the gap and the new micro-channel from the edge of the high-voltage electrode propagate at first, and then merge together to establish a new discharge channel. The reverse electric field in the gap induced by the accumulated charges on the dielectric surface restricts the expansion velocity of the second discharge channel, which results in a longer rise time and falling time of the second current spike.

Turbulence generated small-scale structures as PMWE formation mechanism: Results from a rocket campaign

Simultaneous measurements of rocket-borne and ground-based instruments in a common volume were performed from Andøya, Norway (69° N, 16° E) in frame of the recent PMWE-project, devoted to clarify the formation mechanism behind polar mesosphere winter echoes (PMWE). This article focuses on measurements of April, 13th 2018. Despite low solar activity, we observe several radar echoes, giving the launch criterion. Combining precise in-situ ionization gauge and wave propagation measurements with ground-based radar measurements on 53.5 and 3.17 MHz, as well as lidar, we were able to measure key parameters of PMWE formation. Carefully analyzing the atmospheric background (i.e., temperature, viscosity, Brunt-Väisälä frequency, and scale heights of electron and neutral density), deriving turbulence parameters by means of radar and rocket, as well as estimating particle sizes of meteor smoke particles (MSP), we got a deep insight into the physical processes behind the PMWE phenomenon. Measurements clearly show that the coherent structures in refractive index variations (forming PMWE) are accompanied by neutral air turbulence, which is reflected in small-scale structures (down to some meters) of neutral and electron density. We analyze and discuss the temporal development of the radar echos by means of spectral width and wind measurements. We show that the behavior of the structures under investigation together with the atmospheric background is consistent with the interpretation, that PMWE were created by turbulence. Furthermore, it becomes clear that charged Meteor Smoke Particles (MSP) and background electron density can only enhance SNR, while turbulence is a prerequisite for their formation.

On the Development of Ionization Waves Preceding Breakdown in a Long Capillary Tube Filled with Helium at Low Pressure

Experiments that revealed the main trends in the development of complete breakdown in a long capillary tube for different electrode configurations are reported. It is demonstrated that, in the presence of an additional external electrode around the tube, a fast stage completing the breakdown is preceded by a slow stage caused by development of a weak-current surface barrier discharge inside the tube. This discharge is realized due to propagation of forward (from the high-voltage electrode) and backward (from the grounded electrode) surface ionization waves. The presence of preliminarily, locally deposited charge of the sign opposite to polarity of the voltage pulse leads to acceleration of propagation of ionization waves in the tube. In the absence of the external electrode, the breakdown occurs much faster but is characterized by a large scatter in the delay time (up to complete lack of breakdown) relative to the applied voltage pulse. Unstable operation of the gas-discharge device determined by development of breakdown represents a much more serious problem in many applications compared to the situation in which it turns on slowly but stably. Therefore, the results demonstrating the possibility of controlling the breakdown are of great scientific and practical value.