Plasma Filaments Research Articles

Nanosecond plasma actuation by a bending actuator mounted on a sharp edge in quiescent air

An experimental investigation of the actuation characteristics of a repetitive nanosecond pulsed dielectric barrier discharge from bending plasma actuators mounted on a sharp edge is conducted. Four bending actuators with different bending angles are tested and compared with a planar actuator in quiescent air using schlieren imaging and electrical measurements. The results show that when fed by the same pulse, the plasma morphology, current waveform, and energy consumption of the bending and planar actuators are very similar. However, the thermal perturbations and induced flows differ significantly. In this experiment, at a low load voltage (V = 10–14 kV), when the plasma discharge is in the diffuse mode, the bending actuator can induce a stronger vortex and near-wall jet than the planar actuator, and as the bending angle decreases, the strength of the induced jet increases rapidly. At a higher load voltage (V = 18–20 kV), when the discharge is in the constricted mode, the plasma filaments produce hot plumes with both the bending and planar actuators, but the hot plumes from the bending actuator are injected into the air with a larger incidence. During the streamer-to-filament transition (V = 16 kV), induced flows from small-bending-angle (30° and 60°) actuators are characterized by a thickened near-wall jet, while those from the planar actuator are characterized by hot plumes.

Effect of plasma actuator-based control on flow-field and acoustics of supersonic rectangular jets

We perform a computational study on the effects of localized arc filament plasma actuator based control on the flow field and acoustics of a supersonic 2:1 aspect ratio rectangular jet. Post validation of the baseline jet, effects of control in the context of noise reduction are studied at experimentally guided forcing parameters, including frequencies $St=0.3, 1.0$ and $St=2.0$ with duty cycles of $20\,\%$ and $50\,\%$ . In general, high-frequency forcing reduces noise in the downstream direction, with the actuator signature appearing mostly in the sideline direction. Here $St=1$ , ${\rm DC}=50\,\%$ yields an optimum balance between peak noise reduction (of ${\sim }1.5$ dB) and actuator tones, with control being most effective on the major axis plane that bisects the shorter edges of the nozzle. Shear layer response to the most effective forcing includes generation of successive arrays of mutually interacting staggered lambda vortices, which eventually energize streamwise vortical elements. Causal mechanisms of noise mitigation are further elucidated as follows. First, the control reduces the energy within the supersonic phase speed regime of peak radiating frequencies by redistributing a part of it into a high-frequency band. Second, it enhances azimuthal percolation of energy into the first and second helical modes at frequencies where noise reduction is seen, thus weakening the radiatively efficient axisymmetric mode. Finally, sound-producing intermittent events in the jet are significantly reduced, thereby minimizing the high-intensity acoustic emissions. This small-perturbation-based control strategy results in only minor variations in the mean flow properties. However, reduced production and enhanced convection attenuate turbulent kinetic energy within the spreading shear layer in the controlled jet.

Backward THz Emission from Two-Color Laser Field-Induced Air Plasma Filament.

Two-color laser field-induced plasma filaments are efficient broadband terahertz (THz) sources with intense THz waves emitted mainly in the forward direction, and they have been investigated intensively. However, investigations on the backward emission from such THz sources are rather rare. In this paper, we theoretically and experimentally investigate the backward THz wave radiation from a two-color laser field-induced plasma filament. In theory, a linear dipole array model predicts that the proportion of the backward emitted THz wave decreases with the length of the plasma filament. In our experiment, we obtain the typical waveform and spectrum of the backward THz radiation from a plasma with a length of about 5 mm. The dependence of the peak THz electric field on the pump laser pulse energy indicates that the THz generation processes of the forward and backward THz waves are essentially the same. As the laser pulse energy changes, there is a peak timing shift in the THz waveform, implying a plasma position change caused by the nonlinear-focusing effect. Our demonstration may find applications in THz imaging and remote sensing. This work also contributes to a better understanding of the THz emission process from two-color laser-induced plasma filaments.

Single-shot spatiotemporal plasma density diagnosis using an arbitrary time-wavelength-encoded biprism interferometer

Single-shot ultrafast imaging reveals the transient dynamics of ultrafast phenomena. Herein, we introduce a new approach for capturing ultrafast micron-scale dynamics using an arbitrary time-wavelength-encoded biprism interferometer (TWEBI). In TWEBI, a time-wavelength-encoded pulse train is generated using a set of cascaded frequency-doubling crystals with slightly different phase-matching angles and independent delay lines, which have the advantages of independent temporal resolution, time window, and frame interval parameters. Phase measurement of TWEBI is achieved using a plug-and-play biprism interferometer. In the experiment to capture the transient dynamic of femtosecond laser-induced plasma filament, we demonstrated that the TWEBI setup can capture a total of 12 frames in a single shot, and its effective frame rate is 5 trillion frames per second (Tfps), corresponding to a temporal resolution of 200 fs and a spatial resolution of 4 μm. Moreover, the time window of the TWEBI setup can be adjusted from sub-picosecond to nanosecond timescales. Thus, TWEBI has the advantages of high-precision temporal-spatial resolution, high frame rate, adjustable time window, and no reference arm, thus providing a practical and feasible diagnostic scheme for complex transient dynamics.

Polarization control of terahertz waves generated by a femtosecond three-color pulse scheme

Polarization characteristics of terahertz waves generated from a short air plasma excited by femtosecond three-color pulses with a frequency ratio of 1:2:3 have been theoretically investigated, and the results show that flexible and effective control of terahertz polarization can be achieved by means of changing the polarization combination and relative phase of three-color pulses, which is related to the electric field spatiotemporal distribution of the synthetic pulse formed via three-color pulse superposition. The complicated spatiotemporal distribution can be made clear by analyzing the projection component of the electric field in the three-dimensional Cartesian coordinate system. For terahertz waves generated from a short air plasma filament, the proposed method of terahertz polarization control on the basis of a three-color pulse scheme can be realized by ordinary multi-cycle laser pulses and overcome the disadvantage of few-cycle laser pulses utilized to obtain nearly circularly polarized intense terahertz waves or elliptically polarized intense terahertz waves with large ellipticity in the two-color pulse scheme.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation.

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

Directivity Control of Terahertz Radiation from Single-Color Filament Plasma with Polypropylene Pipe

Two-dimensional angular distribution of terahertz emission from single-color filamentation of laser pulses is experimentally studied. The use of a polypropylene pipe makes it possible to transform the angular pattern of terahertz radiation from the original hollow-cone one to a more preferable unimodal structure within the terahertz frequency range. It has been shown that propagation in a dielectric pipe can significantly (up to several orders of magnitude) increase the terahertz radiation amplitude at a required distance. We have also experimentally demonstrated that bending the pipe allows us to control the direction of the terahertz radiation propagation.

Stochastic modeling of blob-like plasma filaments in the scrape-off layer: Theoretical foundation

A stochastic model for a superposition of uncorrelated pulses with a random distribution of amplitudes, sizes, and velocities is presented. The pulses are assumed to move radially with fixed shape and amplitudes decaying exponentially in time due to linear damping. The pulse velocities are taken to be time-independent but randomly distributed. The implications of a distribution of pulse amplitudes, sizes, and velocities are investigated. Closed-form expressions for the cumulants and probability density functions for the process are derived in the case of exponential pulses and a discrete uniform distribution of pulse velocities. The results describe many features of the boundary region of magnetically confined plasmas, such as high average particle densities, broad and flat radial profiles, and intermittent large-amplitude fluctuations. The stochastic model elucidates how these phenomena are related to the statistical properties of blob-like structures. In particular, the presence of fast pulses generally leads to flattened far scrape-off layer profiles and enhanced intermittency, which amplifies plasma–wall interactions.

Effect of electromagnetic wave reflection from conducting surfaces on blob dynamics in the tokamak scrape-off layer

Electromagnetic dynamics of blobs in hot scrape-off-layer plasmas of the tokamak are affected by excitation of the Alfvén waves and their subsequent propagation to the machine first wall along open magnetic field lines. In this study, the interaction of electromagnetic perturbations with the conducting tokamak wall and the resulting impact of these perturbations on the motion of filaments at the tokamak edge are analyzed. The model describing blob dynamics is presented. To describe the reflection of the Alfvén waves from the tokamak wall, the new form of sheath boundary conditions for the parallel current and electrostatic potential at the plasma–sheath interface is proposed. It is demonstrated that depending on the wall resistivity, the waves can be either absorbed or reflected by the wall, influencing the excitation of electromagnetic fluctuations inside the filament plasma. The theoretical conclusions of the study are supported with the BOUT++ numerical modeling of blob dynamics at the edge of the DIII-D and NSTX tokamaks. It is shown that taking the reflective boundary conditions into account leads to the excitation of the standing Alfvén waves in the filament, periodically canceling the electrostatic currents inside the blob.

Analytical models of rotating magnetized gas filaments

ABSTRACT Several rotating molecular gas filaments with striking helical morphology have been observed in the Central Molecular Zone, including the Double Helix Nebula (DHN), the Galactic Centre Tornado (GCT), and the Pigtail Nebula (PTN). We present a model for isothermal, rotating, magnetized, pressure-truncated, non-self-gravitating filaments, as a model for these objects. We find a novel analytical stationary solution for these molecular tornadoes by assuming a propagating torsional Alfvén wave resulting in balance between toroidal magnetic stress and centrifugal force, as well as constant flux to mass ratio for the poloidal magnetic field. Our model allows for differential rotation and the density profile for the stationary solution goes asymptotically as r−1 at large radius for a special case where the flux to mass ratios for the poloidal and toroidal fields are constant inside the filament. We subject our stationary solutions to a virial analysis and show that they satisfy the virial equation for cylindrical symmetry $2\mathcal {K}+\mathcal {M}=0$. Finally, we estimate the ratio of gravitational potential energy per unit length, $\mathcal {W}$, to the bulk kinetic energy per unit length, $\mathcal {K}_0$, and calculate $|\mathcal {W}|/\mathcal {K}_0$ for the DHN, GCT, and PTN. Our estimates suggest that self-gravity is negligible in the DHN and the PTN and GCT are at most weakly self-gravitating. We also briefly address an application of our model to filamentary plasma lenses in the interstellar medium.

Terahertz shaping technology based on coherent beam combining

The generation of terahertz (THz) waves by focusing a femtosecond pulsed laser beam at a distance is able to overcome the strong absorption properties of air and has rapidly attracted the attention of industry. However, the poor directionality of the THz wave radiation generated by this method is not conducive to THz wave applications. By controlling the morphology of the ultrafast laser-excited plasma filament and its electron density distribution through coherent beam combining technology, we achieve direct THz beam shaping and are able to obtain THz wave radiation of Gaussian or arbitrary transverse distribution. The novel experimental approach proposed in this paper opens up the research field of direct THz wave shaping using plasma. Moreover, it innovates multi-parameter convergence algorithms and, by doing so, has the potential to find beam patterns with higher energy conversion efficiency and break the energy limit of THz waves emitted by lasers at high power.

On different modes of a filamentary atmospheric pressure plasma jet: modeling, laser schlieren deflectometric and high-speed camera investigations

We investigated a filamentary non-thermal atmospheric pressure plasma jet used for coating applications. The plasma jet exhibits two different modes, the stationary mode and the locked mode, in which the filaments rotate around the plasma jet axis. We present a model for the modes and verify it by measurements with high-speed camera and laser schlieren deflectometry. Laser schlieren deflectometry investigations reveal a drift of the rotation frequency over time towards lower values. Additionally, relations between the filament rotation frequency and gas flow as well as input power, respectively, are investigated by the laser schlieren deflectometry. Correlation between rotation frequency and gas flow show a linear dependence and with increasing power the rotation frequency increases as well. All the results are in agreement with the proposed model.Graphical abstract

3D modelling of cavity-free lasing in nitrogen plasma filaments.

In this article we show results on cavity-free lasing in nitrogen filaments using our 3D, time-dependent Maxwell-Bloch code, Dagon. This code was previously used to model plasma-based soft X-ray lasers and it has been adapted to model lasing in nitrogen plasma filaments. In order to assess the predictive capabilities of the code, we have conducted several benchmarks against experimental and 1D modelling results. Afterwards, we study the amplification of an externally seeded UV beam in nitrogen plasma filaments. Our results show that the phase of the amplified beam carries information about the temporal dynamics of amplification and collisional processes inside the plasma, along with information about the spatial structure of the amplified beam and the active region of the filament. We thus conclude that measuring the phase of an UV probe beam, in combination with 3D Maxwell-Bloch modelling, might be an excellent method for diagnosing electron density value and gradients, mean ionization, density of N2+ ions and the magnitude of collisional processes inside these filaments.

Mechanisms of reducing energy costs for nitrogen fixation using air-based atmospheric DBD plasmas over water in contact with the electrode

In this study, a plasma-assisted nitrogen fixation (NF) process, using dielectric barrier discharge (DBD) air and N2 plasmas ignited over a water surface is presented. We describe the influential factors on the plasma properties through plasma diagnostics such as electrode spacings, applied voltages, and feedstocks to reduce the energy cost for NF. In order to maximize the energy yield and reduce the energy cost for NF as NO3-, the gas phase NOx emitted through the gas outlet is trapped into water. Plasma diagnostics is performed to explain the underlying mechanisms for NOx formation with a reduced energy cost. The analyzed results suggest that decreasing the electrode gap over water surface can effectively reduce the energy cost for NF, and that this can be attributed to the increased number of distributed microdischarges instead of more intense plasma filaments that dominate at higher gaps. This system offers a promising result in NO3− synthesis, with a selectivity of 98 %, and an energy cost of 20.7 MJ/mol.

Determination of 915-MHz Atmospheric Pressure Air Microwave Plasma Torch (MPT) Parameters

The gas temperature, electron temperature, and electron density are critical parameters that affect microwave plasma chemistry processes in industrial applications. Using optical emission spectroscopy (OES), the gas temperature and electron density of a 915-MHz atmospheric pressure air microwave plasma torch (MPT) were investigated at various absorbed power levels and radial positions. In addition, the variation in the electron temperatures of 915-MHz atmospheric pressure argon microwave plasma filaments was analyzed as a function of the absorbed power. The experimental results showed that: 1) the absorbed power had little effect on the radial gas temperature or electron density; 2) the skin effect was present in the 915-MHz atmospheric pressure air MPT, where it causing caused hollowing in the center of the radial gas temperature distribution; 3) the electron density reached approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times10$ </tex-math></inline-formula> 13 cm−3 and was almost insensitive to variations in the absorbed power in the center of the 915-MHz atmospheric pressure air MPT; and 4) the upper limit of the 915-MHz atmospheric pressure microwave plasma filament electron temperature increased from approximately 1 to approximately 3 eV as the absorbed power increased from 2.0 to 6.3 kW.

Dynamics of Discharge Development in a Homogeneous DC Field Initiated by a Plasma Filament Created by a Femtosecond Laser Pulse

Dynamics of Discharge Development in a Homogeneous DC Field Initiated by a Plasma Filament Created by a Femtosecond Laser Pulse

Cumulative Effects in 100 kHz Repetition‐Rate Laser‐Induced Plasma Filaments in Air

Cumulative effects are crucial for applications of laser filaments, such as for the remote transfer of energy and the control of electric discharges. Up to now, studies of cumulative effects in the air of high‐repetition‐rate pulse trains have been performed at lower rates than 10 kHz. Herein, the nonlinear effects associated with short plasma filaments produced by pulses of moderate energy (0.4 mJ per pulse) and repetition rates up to 100 kHz are experimentally characterized. With increasing repetition rate, a decrease in absorption, fluorescence emission, and breakdown voltage and concurrently an increase in peak intensity and third‐harmonic‐generation efficiency are observed. Hydrodynamic simulations of the heated gas show that the observed decreases are directly related to a quasi‐stationary state of reduced gas density in the filament. However, further investigations are required to fully understand the physics underpinning the observed sharp reduction of the breakdown voltage at 100 kHz repetition rates. The results may prove relevant for energy and information delivery applications by laser‐induced air waveguide or electric discharge and lightning control.

Electron Polarization in Ultrarelativistic Plasma Current Filamentation Instabilities

Plasma current filamentation of an ultrarelativistic electron beam impinging on an overdense plasma is investigated, with emphasis on radiation-induced electron polarization. Particle-in-cell simulations provide the classification and in-depth analysis of three different regimes of the current filaments, namely, the normal filament, abnormal filament, and quenching regimes. We show that electron radiative polarization emerges during the instability along the azimuthal direction in the momentum space, which significantly varies across the regimes. We put forward an intuitive Hamiltonian model to trace the origin of the electron polarization dynamics. In particular, we discern the role of nonlinear transverse motion of plasma filaments, which induces asymmetry in radiative spin flips, yielding an accumulation of electron polarization. Our results break the conventional perception that quasisymmetric fields are inefficient for generating radiative spin-polarized beams, suggesting the potential of electron polarization as a source of new information on laboratory and astrophysical plasma instabilities.

Peculiarities of the formation of a filamentary structure of a microwave discharge in an argon flow

This paper presents the results of an experimental study of the spatial structure of a microwave discharge maintained in an argon flow by gyrotron radiation in a continuous mode with a frequency of 24 GHz at atmospheric pressure. In the structure of the plasma plume, stationary filamentary channels are observed, elongated along the direction of the argon flow, regardless of the orientation of the external electric field of the wave, surrounded by a diffusion halo. Measurements of the electron density, vibrational and rotational temperatures of gas molecules in plasma filaments have been carried out. The role of gas-dynamic mechanisms responsible for the formation of the inhomogeneous static structure of the plasma torch and the maintenance of a substantially nonequilibrium distribution of temperature characteristics in the discharge is discussed. Keywords: high-pressure microwave discharge, plasma torch, argon, filamentous plasma channels, filaments.

Особенности формирования нитевидной структуры микроволнового разряда в потоке аргона

This paper presents the results of an experimental study of the spatial structure of a microwave discharge maintained in an argon flow by gyrotron radiation in a continuous mode with a frequency of 24 GHz at atmospheric pressure. In the structure of the plasma plume, stationary filamentary channels are observed, elongated along the direction of the argon flow, regardless of the orientation of the external electric field of the wave, surrounded by a diffusion halo. Measurements of the electron density, vibrational and rotational temperatures of gas molecules in plasma filaments have been carried out. The role of gas-dynamic mechanisms responsible for the formation of the inhomogeneous static structure of the plasma torch and the maintenance of a substantially nonequilibrium distribution of temperature characteristics in the discharge is discussed.