Comparative investigation of the resistance and ability to trigger high voltage discharge for single and multiple femtosecond filaments in air

A comparative investigation of the resistance and ability to trigger high voltage(HV) discharge for a single filament(SF) and multiple filaments(MFs) has been carried out.The experimental results show that the trend of the breakdown threshold of the SF exactly follows that of its resistance,but this is not the case for the MF.The MF’s resistance is much smaller than the SF’s.However,the MF shows a slightly higher HV breakdown threshold than the SF.The underlying physics is that the measured resistance of the MF is collectively contributed by every filament in the MF while the HV breakdown threshold is determined by only one single discharging path.     

Testifying experiment of the multi-pulse phenomena of capillary discharge soft-X-ray laser

In a capillary discharge experiment for the neon-like argon lazing,we have proposed an experimental scheme to verify that the multi-spike of X-ray diode (XRD) signal is a multi-pulse laser or is a reflection of the laser pulse in the XRD.The ceramic capillary has an inner diameter of 3 mm and a length of 200 mm.At the gas pressure of 28 Pa and discharge current of 27 kA,stable lasing has been realized.The experimental results prove that the multi-spike of XRD signal is a reflection of the electromagnetic signal produced by the laser pulse in the XRD.The improved electrocircuit scheme of the XRD to minimize the reflection phenomena is also found.     

Sterilization of E. coli bacterium with an atmospheric pressure surface barrier discharge

The atmospheric pressure surface barrier discharge (APSBD) in air has been used in killing Escherichia coli (E.coli), There is almost no bacterial colony in the sample after treatment by discharge plasma for 2rain, A diagnostic technique based on mass spectrum has been applied to the discharge gas and the mechanism of killing is discussed. Ozone and monatomic oxide are considered to be the major antimicrobial active species. There is almost no harmful by-product. The experiment proves that APSBD plasma is a very simple, effective and innocuous tool for sterilization.     

Diagnostics of Atmospheric Pressure Surface Discharge Plasmas in Argon

Atmospheric pressure surface discharge is shown to have great prospects for a number of industrial applications.To acqu/re better results in application fields and considering that the study of the basic parameters includingelectron temperature and electron density is desirable, we develop an equivalent circuit model and the diagnostictechniques based on optical emission spectroscopy and electrical measurement in our laboratory. The electrontemperature has been determined to be about 0.7eV by a Fermi-Dirac model. The electron density has beencalculated to be near 10^10 cm^-3 from a time resolved electrical measurement (Ohmic heating method).     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

Bifurcations and observation of strange attractor in He-Ne lasers

We report the experimental observation of deterministic chaos in an He-Ne laserat 0.6328μm by varying the discharge current.Strange attractor of laser system in the recon-structed phase-space picture has been obtained.The mechanism of the phenomena was dis-cussed.     

A Novel Atmospheric Pressure Plasma Fluidized Bed and Its Application in Mutation of Plant Seeds

An atmospheric pressure plasma fluidized bed (APPFB) is designed to generate plasma using a dielectric barrier discharge (DBD) with one liquid electrode. In the APPFB system, the physical properties of DBD discharge and its application in plant-seed mutating are studied fundamentally. The results show that the generated plasma is a typical glow discharge free from filament and arc plasma, and the macro-temperature of the plasma fluidized bed is nearly at room temperature. There are no obvious changes in the pimientos when their seeds are treated by APPFB, but great changes are found for coxcombs.     

Influence of Wall Charges on Discharge Characteristics of Surface Discharge

The discharge current and the inception voltage in a surface discharge device have been measured to investigate the influence of wall charges on the discharge characteristics in argon. The results show that the inception voltage decreases as the amplitude of the applied voltage Up increases. However, the sum of the inception voltage and the applied voltage almost keeps constant when Up is changed, due to the fact that the wall charges are generated in the discharge process. This phenomenon suggests that net electric field between the electrodes almost keeps constant when the amplitude of the applied voltage is changed. The electron temperature calculated by the intensity ratio of emitted spectral lines has been estimated to be less than 0.3eV and almost keeps constant under different amplitudes of the applied voltage. The result is consistent with nearly constant net field. The duty ratio of the discharge current increases as the amplitude of the applied voltage increases.     

Transition of Discharge Mode of a Local Hollow Cathode Discharge

The discharge characteristics of hollow cathode discharge in argon in a cylindrical cavity are investigated experi- mentally. The voltage-current （V - I） characteristics and the light emission are measured. It is found that the discharge plasma is localized inside the hollow cavity, with an extensive Faraday dark space between the cathode and the anode. The discharge develops from predischarge to abnormal glow discharge, the hollow cathode effect （HCE） and a hybrid mode as the discharge current increases. The onset of the HCE is found for the first time by the transition from abnormal glow discharge together with a significant decrease in the slope of the V - I curve which shows a positive differential resistivity. The voltage increases proportionally with the current when the HCE is reached.     

Discharge characteristics in plasma display cell at high frequency

We present the experimental and simulation studies of the glow discharge plasma in a macroscopic AC plasma display panel cell operating at a high frequency. We find that at high frequencies the plasma from the previous pulse has a significant influence on the discharge properties. The xenon excitation efficiency is larger than that at low frequencies.The discharge mode and efficiency at high frequencies have been discussed in this paper.     

Electric ignition energy evaluation and the energy distribution structure of energy released in electrostatic discharge process

Ignition energy is one of the important parameters of flammable materials, and evaluating ignition energy precisely is essential to the safety of process industry and combustion science and technology. By using electric spark discharge test system, a series of electric spark discharge experiments were conducted with the capacitor-stored energy in the range of 10 J, 100 J, and 1000 J, respectively. The evaluation method for energy consumed by electric spark, wire, and switch during capacitor discharge process has been studied respectively. The resistance of wire, switch, and plasma between electrodes has been evaluated by different methods and an optimized evaluation method has been obtained. The electric energy consumed by wire, electric switch, and electric spark-induced plasma between electrodes were obtained and the energy structure of capacitor-released energy was analyzed. The dynamic process and the characteristic parameters(the maximum power, duration of discharge process) of electric spark discharge process have been analyzed. Experimental results showed that, electric spark-consumed energy only accounts for 8%–14% of the capacitor-released energy. With the increase of capacitor-released energy, the duration of discharge process becomes longer, and the energy of plasma accounts for more in the capacitor-released energy. The power of electric spark varies with time as a damped sinusoids function and the period and the maximum value increase with the capacitor-released energy.     

Controlling chaos by a delayed continuous feedback in a gas discharge plasma

Control of chaos by a delayed continuous feedback is studied experimentally in a gas discharge plasma. The power spectrum, the maximum of Lyapunov exponents and the time series of the signals all indicate that the period-1 unstable periodic orbit is controlled successfully. The dependence of the control on the delay time and the feedback gain as well as the strength of white noise is also investigated in detail. The experimental results show that the scaling index of the control versus the strength of white noise is 1.995, which is very close to that obtained from the simple logistic map.     

Scattering behaviours to the two-dimensional electron gasinduced by the Al composition fluctuation in AlxGa1-xNbarrier in AlxGa1-xN/GaN heterostructures

<正>This paper reports that cathodoluminescence（CL） measurements have been done to study the alloy fluctuation of the Al_0.3Ga_0.7N layer in Al_0.3Ga_0.7N/GaN heterostructures.The CL images and linescanning results demonstrate the existence of compositional fluctuation of Al in the Al_0.3Ga_0.7N barrier.A model using aδ-shape perturbation Hamilton function has been proposed to simulate the scattering probability of the two dimensional electron gases （2DEG） induced by Al composition fluctuation.Two factors,including conduction band fluctuation and polarization electric field variation,induced by the Al composition fluctuation have been taken into account.The scattering relaxation time induced by both factors has been estimated to be 0.31 ns and 0.0078 ns,respectively,indicating that the variation of the piezoelectric field is dominant in the scattering of the 2DEG induced by Al fluctuation.     

Effects of N2 and glow discharge on growth behaviours of carbon nanotubes prepared by the hot filament chemical vapour deposition method

Carbon nanotubes (CNTs) are prepared by the hot filament chemical vapour deposition method using CH4 and H2 as reaction gases, and the growth behaviours are investigated by scanning electron microscopy and transmission electron microscopy. The results indicate that the CNTs prepared in the absence of N2 or glow discharge are bent; however, they are in an aligned state after introducing N2 into the chamber or forming glow discharge under a negative biased voltage. The results also indicate that the CNTs are of a bamboo-like structure when N2 is intruded as a reaction gas and the alignment degree of the CNTs grown under glow discharge is higher than that grown in N2. This illustrates that the nitric environment and glow discharge play important roles in the growth of CNTs. Combining with the theory related to alloy thermodynamics and collisions, we have analysed the growth mechanism of the CNTs in nitric environment and glow discharge.     

High and low frequency relaxation oscillations in a capacitive discharge plasma

Both high and low frequency relaxation oscillations have been observed in an argon capacitive discharge connected to a peripheral grounded chamber through a slot with dielectric spacers. The oscillations, observed from time-varying optical emission of the main discharge chamber, show, for example, a high frequency （46 kHz） relaxation oscillation at 100 mTorr, with an absorbed power near the peripheral breakdown, and a low frequency （2.7-3.7 Hz） oscillation, at a higher absorbed power. The high frequency oscillation is found to ignite a plasma in the slot, but usually not in the periphery. The high frequency oscillation is interpreted by using an electromagnetic model of the slot impedance, combined with the circuit analysis of the system including a matching network. The model is further developed by using a parallel connection of variable peripheral capacitance to analyse the low frequency oscillation. The results obtained from the model are in agreement with the experimental observations and indicate that a variety of behaviours are dependent on the matching conditions.     

Temperature-dependent characteristics of 4H-SiC junction barrier Schottky diodes

The current-voltage characteristics of 4H-SiC junction barrier Schottky(JBS) diodes terminated by an offset field plate have been measured in the temperature range of 25-300 C.An experimental barrier height value of about 0.5 eV is obtained for the Ti/4H-SiC JBS diodes at room temperature.A decrease in the experimental barrier height and an increase in the ideality factor with decreasing temperature are shown.Reverse recovery testing also shows the temperature dependence of the peak recovery current density and the reverse recovery time.Finally,a discussion of reducing the reverse recovery time is presented.     

Experimental Scaling for Mass Ablation Rate in Planar Targets Irradiated by Smoothed 0.351μm Laser Beam

Layered planar targets, consists of al and Au layers, were irradiated by smoothed 0.351-μm-high power laser with relatively large focus spot on Xingguang Ⅱ laser facility. The burn-through time of laser ablation of Al sample has been obtained by measuring the delay time of Au N-Band x-ray emission with respect to the Al K-Band x-ray emission. The scaling law of mass ablation rate with the laser flux has been obtained by only varying the laser energy on the target surface on different shots. The experimental scaling law is greatly different from the results of Key et al. [Phys. Fluids 23 (1983) 2011] and is in good agreement with the analytical model.     

Temperature-dependent characteristics of 4Hben SiC junction barrier Schottky diodes

The current-voltage characteristics of 4H-SiC junction barrier Schottky (JBS) diodes terminated by an offset field plate have been measured in the temperature range of 25-300 ℃. An experimental barrier height value of about 0.5 eV is obtained for the Ti/4H-SiC JBS diodes at room temperature. A decrease in the experimental barrier height and an increase in the ideality factor with decreasing temperature are shown. Reverse recovery testing also shows the temperature dependence of the peak recovery current density and the reverse recovery time. Finally, a discussion of reducing the reverse recovery time is presented.     

Mode Transition of Vacuum Arc Discharge and Its Effect on Ion Current

The mode transition of a pulsed vacuum arc discharge, which is from vacuum surface flashover to non-surface vacuum breakdown or vice versa, is studied by simply adjusting a trigger resistor. This approach provides a possibility to research the transition discharge process. Since the transition process is smooth and controllable, the transition mechanism and its effect on the performance of an ion source can be investigated via various diagnosis experiments. The experimental results show that the mode transition occurs when the resistance is in the range of 0- 10Ω. With the mode transition from surface flashover to non-surface vacuum breakdown, the vacuum arc discharge becomes more intense and the ion current produced by the Ti cathode ion source increases by two times. The related physical mechanism is also discussed in detail.     

Forebody asymmetric vortex control with extended dielectric barrier discharge plasma actuators

Plasma control of forebody asymmetric vortices is mostly achieved by means of dielectric barrier discharge(DBD)plasma actuators. However, DBD actuators suffer from some disadvantages such as a weak induced body force, a singledirection induced jet, and an unclear control mechanism. We carry out wind tunnel experiments involving the forebody vortex control of a slender body at high angles of attack using an innovative extended DBD actuator, which has a stronger capacity to induce an electric wind than a DBD actuator. Through synchronous measurements of the pressure distribution and particle image velocimetry(PIV), the spatiotemporal evolution of the dynamic interactions between plasma-actuationinduced vortices and forebody asymmetric vortices is analyzed. The influence of plasma discharge on the boundary layer separation around a slender body and the spatial topological structures of asymmetric vortices are further surveyed, as the optimized actuation parameters. Extended DBD actuators are found to be more capable of controlling asymmetric vortices than DBD actuators, and a linear proportionality of the sectional lateral force versus the duty ratio is achieved.There exists an optimal normalized reduced frequency( f+= 2π fpd/U∞= 2.39) for asymmetric vortex control under the present experimental conditions. The research results can provide technical guidance for the control and reuse of forebody asymmetric vortices.