Characterization of the single nanosecond pulsed surface dielectric barrier plasma actuator: Geometric and electric effects

The influence of the electrode length and the voltage pulses on the discharge characteristics of the surface dielectric barrier discharge actuators were investigated numerically by using the plasma kinetic model. The governing equations including the coupled continuity plasma discharge equation, drift-diffusion equation, electron energy equation, Poisson's equation, and Navier–Stokes equation were solved in quiescent air at atmospheric pressure. The results show that the shorter pulse rising time results in higher discharge characteristics, more intense discharge, and bigger discharge region. Differently, the compared discharge characteristics for the electrodes with different lengths prove that the length of the powered and ground electrodes has little effect on the surface dielectric barrier discharge driven by nanosecond pulsed voltage.     

Surface dielectric barrier discharge: Effect of encapsulation of the grounded electrode on the electromechanical characteristics of the plasma actuator

Active flow control is a rapidly developing topic because the associated industrial applications are of immense importance, particularly for aeronautics. Among all the flow control actuators, plasma-based devices are very promising. In most cases, the plasma actuator is based on a surface dielectric barrier discharge (DBD) established between two electrodes flush mounted on both sides of a dielectric layer. In this paper, the effects of the encapsulation of the grounded electrode on the electric wind produced by a surface DBD are investigated by Pitot velocity measurements. Moreover, the current versus time is statistically analysed in order to count the number of streamers, their duration and the electrical charge they transfer. Unfortunately it was not possible to detect and separate the glow-type synchronous current because its amplitude is of the same order as noise. On one hand, the encapsulation of the grounded electrode results in an increase of the electromechanical efficiency of the surface DBD plasma actuator because the electrical power consumption is divided by two. On the other hand, statistical analysis of the discharge current versus time has shown that the encapsulation results in a decrease of the streamer number and an increase of the charge transferred by each current streamer because the duration of streamers is longer.     

Compared ionic wind measurements on multi-tip corona and DBD plasma actuators

Two sets of actuators with triangular tips on their active electrodes (13 corona and 15 DBD) are studied in the laboratory. Far field ionic wind velocity, mass flow and efficiency are measured for all the actuators. The best electrode shape as function of tip sharpness and tips number/unit length is determined for each of the above measurements. The gas velocity increases downstream of the tips in all the actuators, but the DBD flow has a three-dimensional structure more complicated than for coronas. The tips improve the efficiency of all the actuators, and the stability of coronas is remarkably improved.     

Enhancement of thin air jets produced by ring-shaped dielectric barrier discharges using an auxiliary electrode

A ring-shaped dielectric barrier discharge (DBD) was explored as a small form factor ionic wind device. Using a concentric ring electrode geometry, the DBD produced a converging ionic wind that leads to a vertical flow away from the DBD electrodes. The vertical flow was channeled through an outlet nozzle to produce a thin air jet, and a grounded auxiliary electrode was placed at the nozzle to enhance the exit velocity. The inner diameter of the ring-shaped DBD electrode and the auxiliary electrode ranged 3.18–9.54 mm and 1.0–4.0 mm, respectively. Results showed that the auxiliary electrode generated an ionic wind velocity up to 3.7 m/s and increased the conversion efficiency from discharge to flow power by a factor of 30 by strengthening the electric field where the ions are accelerated.     

同轴介质阻挡放电发生器介质层等效电容和负载特性研究

大气压介质阻挡放电(DBD)可以在常压下产生非平衡等离子体,已经成为热点研究领域.通过脉冲或交变电源激发放电,研究电源输出特性、电源与放电发生器负载间的匹配和外界条件对放电的影响对于理解放电现象和提高放电效率具有重要意义.本文采用Lissajous图形法,分别研究了驱动电压、气流速率等因素影响同轴DBD发生器介质层等效电容及负载幅频特性的规律.结果表明,气流速率和驱动电压等外界条件影响DBD发生器的负载特性:介质层等效电容随气流速率增大而减小,随驱动电压增大而增大;幅频特性曲线均表现出RLC回路谐振现象,谐振频率随气流速率增大而增大,随驱动电压增大而减小.通过对比发现,介质层等效电容随频率的变化曲线与幅频特性曲线具有一致的特征,介质层等效电容是影响电路谐振频率动态变化的主要因素.提出了一种有关介质层等效电容的形成机制.     

Electrohydrodynamic force produced by a corona discharge between a wire active electrode and several cylinder electrodes – Application to electric propulsion

Low-speed electric propulsion systems for long-duration near-space travels by using solar energy could be based on the electrohydrodynamic force produced inside a corona discharge. This paper is a contribution to a better understanding of these types of thrusters, in order to enhance the produced thrust and their electromechanical effectiveness. Three different simple designs are experimentally studied and compared. The first one is composed of a wire active electrode and a single cylinder grounded one. For the second three-electrode design, the single grounded cylinder is replaced by two cylinders. Finally, the last design is composed of an active wire supplied by a positive voltage, two grounded electrodes and two others cylinders at a negative voltage. On one hand, results show that the use of two grounded electrode instead of a single one results in an increase of the discharge current. Moreover, whatever the electrode gap d, the current-to-thrust conversion is more effective with the three-electrode design. It changes from 31 to 58 N/A (+87%), from 74 to 85 N/A (+15%), and from 104 to 120 N/A (+15%), for electrode gaps d = 10, 20 and 30 mm, respectively. The thruster effectiveness θ is improved by 2 mN/W. On the other hand, the use of two collecting electrodes supplied by a negative high voltage does not result in an effectiveness enhancement because the power consumption is significantly increased.     

An atmospheric-pressure nitrogen-plasma jet produced from microdischarges in a porous dielectric

This report presents an atmospheric-pressure nitrogen-plasma jet generated from microdischarges in a porous dielectric. A plasma jet with a length of 42 mm was produced by feeding nitrogen gas through a porous alumina installed between an outer electrode and a hollow inner electrode and by applying 60 Hz sinusoidal voltage wave to the electrodes. Microdischarges in the porous alumina are ejected as a plasma jet from the outer electrode through a 1 mm hole by increasing the applied voltage, showing that the temperature of the jet decreases to a value close to room temperature. Even at a frequency as low as 60 Hz, the plasma that evolves from a large amount of microdischarge inside a porous dielectric can have characteristics that are similar to those generated at several hundreds of kilohertz. From the electrical measurements, it is expected that not only the steady generation but also the frequency of the pulses resulting from the microdischarges in the porous dielectric play an important role in obtaining a stable plasma jet. We also identified the various excited plasma species produced from the plasma jet by an optical emission spectroscopy.     

Electrical characterization of dielectric barrier discharge driven by repetitive nanosecond pulses in atmospheric air

Dielectric barrier discharge (DBD) is an important method to produce non-thermal plasma, which has been widely used in many fields. In the paper, a repetitive nanosecond-pulse generator is used for the excitation of DBD. Output positive pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 30–40 ns, and pulse repetition frequency varies from single shot to 2 kHz. The purpose of this paper is to experiment the electrical characteristics of DBD driven by repetitive nanosecond pulses. The variables affecting discharge conditions, including air gap spacing, dielectric thickness, barrier fashion, and applied pulse repetition frequency, are investigated. The relationship between electric field, discharge current, instantaneous discharge power across air gap, and estimated electron density with the length of air gap, dielectric thickness, barrier fashion, and pulse repetition frequency is obtained respectively, and the experimental results are also discussed. In addition, two typical images exhibiting homogeneous and filamentary discharges are given with different experimental conditions.     

Cathode-sheath driven low-speed aerodynamic flow actuation using direct-current surface glow discharges

Flow actuation by a continuous/pulsed, direct-current (DC) surface glow discharge is explored. The discharge comprises an array of pin electrode pairs flush mounted on a dielectric actuator surface that lies adjacent to stagnant air. Strong electrostatic fields produced in the cathode sheath region of the discharge provides a motive force on the ions which in turn drag the background gas resulting in directed air flow from the anode to the cathode. The induced flow velocity is estimated by particle image velocimetry (PIV) at 10 Hz with TiO₂ seeding. For a pulsed DC discharge with peak power of 5 W per electrode pair, the induced flow velocity reaches peak values of about 1.7 m/s which is comparable to dielectric-barrier discharge (DBD) or corona discharge actuators. The actuation effect quantified by the magnitude of induced velocity increases as the pulse frequency increases from 0 to 1 kHz. The actuation effect decreases for further increase in frequency above 1 kHz. Decreased actuation effect at high frequency is accompanied by structural change in the discharge. At fixed frequency of 1 kHz, flow actuation effect is highest for a square wave pulse with a duty cycle of 50% indicating that pulsed DC discharges produces better actuation than continuous DC with a corresponding reduction in energy consumption.     

Active plasma grid for on-demand airflow mixing increase

The present paper concerns the electromechanical characterization of an actuator composed of a ceramic plate perforated by 121 holes housing embedded and printed electrodes between which a high voltage is applied. The electrode arrangement is such that the holes where the gas flows are surrounded by surface discharges. Electrical measurements and iCCD images show that the discharge behaves as a typical surface dielectric barrier discharge with streamer and glow regimes during one period of the AC sine voltage. Particle image velocimetry has been used to measure the jet flow produced by the discharge. The plasma discharge is at the origin of a wall jet with mean velocity of about 2.2 m/s, oriented from the active electrode to the grounded one. The capability of this discharge for promoting mixing by reducing the length of the jet core is demonstrated for flow velocities from 20 up to 60 m/s. In all the tested cases, the actuator can improve the mixing downstream of the perforated plate, when periodic perturbations are imposed at the jet column mode (St_D = 0.3).     

Entwicklung leistungsfähiger Stickstofflaser mit abgeschmolzenem Laserrohr

For an increase of the laser pulse energy from longitudinally excited multiple electrodes tubes, experiments and model calculations have been accomplished. A variation of the electrode spacing in the laser tube has shown, that a maximum pulse energy could be achieved with about 40 mm electrode spacing. If barium titanat capacitors with a dielectric constant of 10000 in the pulse forming unit were applied, a voltage pulse transform occured enabling an increase of the laser power up to 50%. With a sealed off operated laser tube of 100 cm active length, a laser peak power of 600 kW and a pulse duration of 10 ns could be achieved. A tube with 200 cm active length generated a pulse power of 1.2 MW. The decrease of laser pulse energy with increasing pulse repetition rate can be explained by the decrease of impulse breakdown voltage. For an increase of the lifetime of laser tubes, appropriate technologies for the production of the tube have been developed.     

Electrohydrodynamic flow in a wire-plate non-thermal plasma reactor measured by 3D PIV method

This work was aimed at measurements of the electrohydrodynamic (EHD) secondary flow in a non-thermal plasma reactor using three-dimensional particle image velocimetry (3D PIV) method. The wide-type non-thermal plasma reactor used in this work was an acrylic box with a wire discharge electrode and two plate collecting electrodes. The positive DC voltage was applied to the wire electrode through a 10 MΩ resistor. The collecting electrodes were grounded. The voltage applied to the wire electrode was 28 kV. Air flow seeded with a cigarette smoke was blown along the reactor duct with an average velocity of 0.6 m/s. The 3D PIV velocity fields measurements were carried out in four parallel planes stretched along the reactor duct, perpendicularly to the wire electrode and plate electrodes. The measured flow velocity fields illustrate complex nature of the EHD induced secondary flow in the non-thermal plasma reactor.     

Peculiarities of Dielectric Barrier Discharges

The dielectric barrier discharge (DBD) is a highly transient, non‐thermal discharge form, which exists in a broad pressure range. It occurs in arrangements, where a dielectric layer covers at least one electrode. The dielectric quenches the current and distributes the discharge over the whole surface. Depending on the geometrical conditions three basic types of DBD arrangements are distinguished. In arrangements with a gas gap a filamentary or a homogeneous‐diffuse discharge mode appears. The (stable) filamentary mode consists of a multitude of microdischarges, which in some extent can be rather easily tailored for e. g. plasma‐chemical applications. In arrangements with a long electrode (or several in parallel) on a dielectric surface and a plane counter‐electrode on the reverse side of the dielectric, pure surface discharges can be observed. They are characterised by low ignition voltages. The extension of the discharge on the surface depends on the voltage amplitude. If pairs of long electrodes are within the bulk of a dielectric, discharge phenomena appear on the surface of the dielectric. As these devices can be produced with small and precise electrode gaps, high mean field strengths in the discharge region can be realised. The properties of the discharges in these arrangements as well as their dynamics are described in detail and compared with one another. The advantages of each type are highlighted. Some aspects, which may be of interest for plasma‐chemical reactions on surfaces and in the gas space are discussed.     

大气压氦气多路低温等离子体射流

为了研究大气压低温等离子体多路射流阵列的放电特性,设计一个实现7路低温等离子体射流的放电装置,采用单电极放电结构,在开放的大气环境下通入氦气。采用高压窄脉冲重复频率电源激励驱动该放电装置,电源脉冲宽度约230 ns,脉冲上升沿约为120 ns。在重复频率为500 Hz的条件下,通过高速摄影初步发现放电电流脉宽约为110 ns,且无反向放电。试验结果表明:平均射流长度随电压幅值增加而增加,在一定电压幅值时射流长度有达到饱和的趋势,这是由于射流通道尾部有空气进入,电压幅值已不再是主要原因;只有在合适的气体流量值时,才能够获得较长的平均射流长度,这是由于气体流量过大或过小时射流均不足以维持形成的放电通道;此外,中心电极放电射流长度受气体流量影响较大,气体流量在一定值时可以观察到中心电极有较长的射流,射流放电强度较弱,气体流量过大或过小时中心电极几乎无放电,这是由于四周电极更易形成放电射流,削弱了中心电极放电。 ,     

Optimization of a Dielectric Barrier Discharge for Pulsed UV Emission of XeCl at 308 nm

A pulsed dielectric barrier discharge (DBD) has been investigated in a wide range of experimental conditions with the purpose of optimization for XeCl excimer radiation. For that the following operation parameters had been considered: four different lamps of coaxial geometry with gas gaps in the range of 1.3 ‐ 6.5 mm; gas mixtures of xenon and chlorine containing admixtures of 1%, 2% and 4% Cl₂ at total filling pressures between 5 mbar and 600 mbar; voltage rise times of 20 ‐ 50 ns and voltage amplitudes of up to 12 kV. A maximum radiation pulse energy of 1.8 µ J has been detected at 310 ± 10 nm with an estimated radiation decay by three orders of magnitude within about 5 µ s. It was shown that the minimization of the voltage rise time is essential for enhancing the radiation pulse energy. Furthermore a correlation between the discharge geometry and the optimum pressure for maximum radiation output was observed. The decay characteristics of the excimer emission provides evidence of the harpoon reaction being the main channel of XeCl formation under our operation conditions (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of atmospheric pressure dielectric barrier discharge air plasma on electrode surface

In order to study the influence of plasma on electrode, atmospheric pressure dielectric barrier discharge (DBD) air plasma is employed here to treat copper electrode surface. Plasma is generated between the parallel plate electrodes by means of high voltage produced by a high-frequency power supply with transformer. Electrode surface alterations induced by air plasma are investigated by using field emission scanning electron microscope (FE-SEM), X-ray energy dispersion spectroscopy (EDS) and contact angle measurement. The results show that DBD air plasma removes the organic contaminant on surface and causes electrode surface roughness, oxidization and nitridation. In addition, surface wettability is also improved, as concluded from contact angle measurements.     

Preparation of hydrophobic coating on glass surface by dielectric barrier discharge using a 16 kHz power supply

A 16 kHz power supply was used to investigate the preparation of hydrophobic film on glass surface by means of atmospheric pressure dielectric barrier discharge (DBD). Air nonthermal plasma was induced between the two parallel electrodes with a glass plate as dielectric barrier. The process for hydrophobic film includes two parts: one is plasma pretreatment to produce active layer on glass surface, another is to form hydrophobic film on glass surface by means of the interaction between air plasma and polydimethylsiloxane oil. The surface changes were observed using contact angle measurement and atomic force microscope. The results show DBD can increase surface roughness, and effectively improve glass surface activation and form a hydrophobic coating on glass surface, and it is possibility to prepare hydrophobic glass with middle frequency power supply.     

Control of a turbulent flow separated at mid-chord along an airfoil with DBD plasma actuators

The goal of the present experimental study is to investigate the ability of surface DBD plasma actuators to delay flow separation along the suction side of a NACA0015 airfoil. Three single surface DBD actuators that can operate separately are mounted on the suction side of the profile, at 18%, 27% and 37% of the chord length. The boundary layer is transitioned by a tripper to be sure that the flow control is not due to the laminar-to-turbulent transition. The angle of attack is equal to 11.5° and the free-stream velocity to U₀ = 40 m/s, resulting in a chord-based Reynolds number of Re_c = 1.33 × 10⁶. The flow is studied with a high-resolution PIV system. In such conditions, the baseline flow separation occurs at 50% of chord. Then, the different single DBD have been switched on separately, in order to investigate the actuator location effect. One highlights that the DBD located at x_c/c = 18% is more effective than the two others ones, with a separation delay up to 64% of chord. When the three DBDs operate simultaneously, the separation point moves progressively toward the trailing edge when the high voltage is increased, up to 76% of chord at 20 kV. Finally, the effect of the actuation frequency on the control authority has been investigated, by varying the value of the operating frequency and by burst-modulation. For frequencies equal to 50 Hz and 500 Hz (reduced frequency F⁺ = 0.31 and 3.1), the separation has been delayed at 76 and 80% of chord, respectively.     

2D Simulations of Short‐Pulsed Dielectric Barrier Discharge Xenon Excimer Lamp

Self‐consistent two‐dimensional (2D) simulations of short‐pulsed dielectric barrier discharge (DBD) in pure xenon have been performed. It is shown that during short current pulse the traversal inhomogeneity of the plasma parameters can be important only at the end of the current pulse as an edge effect close to the side walls. During the current pulse, the gap voltage drops until the ionization wave reaches the cathode so the current in the cathode sheath is the displacement current. This means that almost all of the absorbed power is deposited into excitation of xenon atoms and not to the ion heating in the cathode sheath as in the traditional glow discharges. This fact is one of the reasons of high efficiency of short‐pulsed DBD. The developed model allows one to estimate the temporal position of the plasma‐sheath boundary. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于薄膜介质线的紧凑型脉冲功率源设计与实验研究

介绍了一种基于薄膜介质线的紧凑型脉冲功率源的设计与实验,脉冲功率源系统体积约0.5 m3,输出功率大于4 GW、脉宽约150 ns。该脉冲功率源采用模块化设计,系统主要包括充电组件、薄膜介质线模块和气体火花间隙开关组三个部分。薄膜介质线储能介质为聚酰亚胺薄膜,为抑制电磁耦合以及电晕现象,匝间距选为30 mm。优化设计的三电极场畸变开关直径150 mm、高45 mm、电感值16.2 nH。为降低系统电感,薄膜介质线模块与开关间采用传输线的布线方式,中间绝缘采用聚酰亚胺膜,在2 mm间距下实现了100 kV耐压。