Optimized analysis of sharpening characteristics of a compact RF pulse source based on a gyro-magnetic nonlinear transmission line for ultrawideband electromagnetic pulse application

We constructed a compact high-power RF pulse generator based on a gyro-magnetic nonlinear transmission line (GNLTL) to produce a high-voltage pulse with a sub-nanosecond rise time and a relatively high repetition rate, which shows great potential for application in the high-power ultrawideband electromagnetic effect, etc. The influence of incident pulse parameters (rise time and voltage amplitude) and line length on the sharpening characteristics of the GNLTL were investigated experimentally to optimize the rising rate of the modulated pulse front. Based on the GNLTL equivalent circuit model consisting of an LC ladder network, the rise time, the voltage conversion coefficient and the rising rate properties of a modulated pulse were also numerically analyzed in a wider range. The results show that a>90 kV RF pulse with a rise time of 350 ps and a repetition rate of 1 kHz in burst mode is produced by the GNLTL at an axial biasing magnetic field of 22 kA m⁻¹ and a line length of 30 cm under the condition of a 70 kV incident pulse. Applying a faster and higher incident pulse is conducive to improving the sharpening effect of the GNLTL. Furthermore, within a certain range, increasing the line length of the GNLTL not only reduces the rise time, but increases the voltage conversion coefficient and the rising rate of a modulated pulse. Furthermore, considering the energy loss of ferrite rings, there is an optimal line length to obtain the fastest rising rate of a modulated pulse front edge.     

Influence of high-voltage pulse parameters on the propagation of a plasma synthetic jet

In this work, a typical pin-to-pin plasma synthetic jet in static air is excited by a pulsed DC power supply. The influences of the pulse rising time, the amplitude and the repetition frequency of the pulse voltage on the jet flow have been investigated. First, using a high-speed Schlieren imaging technique, the induced shock waves and the fast jet flow generated by the plasma synthetic jet are characterized. With a deposited energy of 44 mJ per pulse, the velocity of the shock wave and the maximum velocity of the jet flow reach 320 m s⁻¹ and 100 m s⁻¹, respectively. Second, when the applied voltage increases from 12.8 kV to 16 kV, the maximum jet velocity increases from 66 m s⁻¹ to 93 m s⁻¹. On the other hand, as the pulse rising time varies from 50 ns to 500 ns, or the pulse repetition frequency increases from 5 Hz to 40 Hz, the jet velocity induced by the plasma synthetic jet is weakly dependent. In addition, a comparative study of the plasma synthetic jets using three commercial pulsed power supplies (XJ-15, NPG- 18, and PG-30) is implemented. It reveals that the maximum jet velocity of 120 m s⁻¹ is obtained in the case of PG-30, with the longest pulse rising time and the lowest breakdown voltage, while the maximum velocity of 33 m s⁻¹ is detected in the case of NPG-18, even though it has the shortest pulse rising time and the highest breakdown voltage.     

Effects of the transverse electric field on nanosecond pulsed dielectric barrier discharge in atmospheric airflow

In this paper, an asymmetric electrode geometry(the misalignment between the ends of highvoltage and grounded electrodes) is proposed in order to investigate the effects of the transverse electric field on nanosecond pulsed dielectric barrier discharge(DBD). The results show that diffuse discharge manifests in the misaligned region and the micro-discharge channel in the aligned region moves directionally. Moreover, the diffuse discharge area increases with the decrease of the discharge gap and pulse repetition frequency, which is consistent with the variation of the moving velocity of the micro-discharge channel. When airflow is introduced into the discharge gap in the same direction as the transverse electric field, the dense filamentary discharge region at the airflow inlet of asymmetric electrode geometry is larger than that of symmetric electrode geometry. However, when the direction of the airflow is opposite to that of the transverse electric field, the dense filamentary discharge region of asymmetric electrode geometry is reduced. The above phenomena are mainly attributed to the redistribution of the space charges induced by the transverse electric field.     

A large-grain-size thick-film polycrystalline diamond detector for x-ray detection

A diamond film with a size of 6 × 6 × 0.5 mm³ is fabricated by electron-assisted chemical vapor deposition. Raman spectrum analysis, x-ray diffraction and scanning electron microscope images confirm the high purity and large grain size, which is larger than 300 μm. Its resistivity is higher than ${10}^{12}\,{\rm{\Omega }}\cdot {\rm{cm}}.$ Interlaced-finger electrodes are imprinted onto the diamond film to develop an x-ray detector. Ohmic contact is confirmed by checking the linearity of its current–voltage curve. The dark current is lower than 0.1 nA under an electric field of 30 kV cm⁻¹. The time response is 220 ps. The sensitivity is about 125 mA W⁻¹ under a biasing voltage of 100 V. A good linear radiation dose rate is also confirmed. This diamond detector is used to measure x-ray on a Z-pinch, which has a double-layer 'nested tungsten wire array'. The pronounced peaks in the measured waveform clearly characterize the x-ray bursts, which proves the performance of this diamond detector.     

A low-jitter self-triggered spark-discharge pre-ionization switch:primary research on its breakdown characteristics and working mechanisms

MV pulsed switch plays a key role as the transfer switch in large electromagnetic pulse simulators.To broaden the range of self-triggering time,a novel spark-discharge pre-ionization switch,in which the main gap electric field is superposed at the trigger gap to let the electrons in its spark channel also become initial electrons,is proposed and tested.The design idea is:as electrons in the spark channel of the trigger gap always exist after its breakdown,the injection time of pre-ionization should have a more negligible effect on reducing the switch jitter.The experiment results under pulses with a rise time of～100 ns support the above assumptions.When the operating voltage is from～300 to～800 kV and the self-triggering time is～45％to～75％of the peak time,the breakdown time delay jitter is less than 2 ns,and the breakdown voltage jitter is smaller than 1.25％.Under specific self-triggering time,the breakdown time delay jitter is less than 1.5 ns,and the breakdown voltage jitter is smaller than 0.8％.     

Simulational Investigation of a High-Efficiency X-Band Magnetically Insulated Line Oscillator

The magnetically insulated line oscillator(MILO) is a gigawatt-class, coaxial crossed-field microwave tube, which is at present a major hotspot in the field of high-power microwaves(HPM) research. In order to improve the power conversion efficiency and eliminate or at least minimize anode plasma formation in the load region and radio frequency(RF) breakdown in the slow wave structure(SWS) section, an X-band MILO is presented and investigated numerically with KARAT code. The design idea is briefly presented and the simulation results are given and discussed. In the simulation, HPM is generated with peak power of 3.4 GW, maximum electric field of about 1 MV/cm, and peak power conversion efficiency of 14.0%, when the voltage is 559.1 kV and the current is 43.2 kA. The microwave frequency is pure and falls in the X-band of 9.0 GHz. The theoretical investigation and the simulation results are given to prove that the anode plasma formation and the RF breakdown can be effectively avoided or at least minimized,respectively.     

A two-dimensional air streamer discharge modified model based on artificial stability term under non-uniform electric field at low temperature and sub-atmospheric pressure

In this paper, an improved air discharge fluid model under non-uniform electric field is constructed based on the plasma module COMSOL Multiphysics with artificial stability term, and the boundary conditions developed in the previous paper are applied to the calculation of photoionization rate. Based on the modified model, the characteristics of low temperature subatmospheric air discharge under 13 kV direct current voltage are discussed, including needle-plate and needle-needle electrode structures. Firstly, in order to verify the reliability of the model, a numerical example and an experimental verification were carried out for the modified model respectively. Both verification results show that the model can ensure the accuracy and repeatability of the calculation. Secondly, according to the calculation results of the modified model, under the same voltage and spacing, the reduced electric field under low temperature subatmosphere pressure is larger than that under normal temperature and atmospheric pressure. The high electric field leads to the air discharge at low temperature and sub atmospheric pressure entering the streamer initiation stage earlier, and has a faster propagation speed in the streamer development stage, which shortens the overall discharge time. Finally, the discharge characteristics of the two electrode structures are compared, and it is found that the biggest difference between them is that there is a pre-ionization region near the cathode in the needle-needle electrode structure. When the pre-ionization level reaches 10~(13) cm~(-3), the propagation speed of the positive streamer remains unchanged throughout the discharge process, and is no longer affected by the negative streamer. The peak value of electric field decreases with the increase of pre-ionization level, and tends to be constant during streamer propagation. Based on the previous paper, this paper constructs the air discharge model under non-uniform electric field, complements with the previous paper, and forms a relatively complete set of air discharge simulation system under low temperature and sub atmospheric pressure, which provides a certain reference for future research.     

The residual zonal flow in tokamak plasmas with a poloidal electric field

In a tokamak plasma with auxiliary heating by cyclotron waves, a poloidal electric field will be produced, and as a consequence influence the residual zonal flow(RZF) level. The poloidal electric field can also be induced through biasing electrodes at the edge region of tokamaks.Numerical evaluation for a large aspect ratio circular cross section tokamak for the electron cyclotron wave heating indicates that the RZF level decreases significantly when the poloidal electric field increases. Qualitatively, the ion cyclotron wave heating is able to increase the RZF level. It is difficult to apply the calculation to the real cyclotron wave heating experiments since we need to know factors such as the plasma profiles, the exact power deposition and the cross section geometry, etc. It is possible to use the cyclotron wave heating to control the zonal flow and then to control the turbulence level in tokamak experiments.     

重复频率亚纳秒硬X射线源研制及应用

为获得一种多用途X射线源,满足核辐射探测器研制、脉冲成像等多任务应用需要,本文研制出具备重复频率、超快、高稳定性、便于连续调节脉冲电压和脉宽等的多用途X射线源,其上升沿达98.6 ps,运行电压达425 kV,10 cm处峰值注量率高达2.07×10¹⁸ cm^-2•s^-1,为开展相关应用研究提供了理想的辐射条件。     

800kV高压传输装置的电场数值计算

介绍了800kV高压传输装置的组成和结构安排；对在SF6绝缘气体环境中的电缆终端进行了电场数值计算，并对计算结果进行了分析；最后得出结论，电缆终端的场强设计是可靠的。     

Study on the influence of the discharge voltage on the ignition process of Hall thrusters

A high-speed charge-coupled device camera was used to capture images of the plume and acceleration channel of a Hall effect thruster during ignition at different discharge voltages. To better understand the influence of changes in the discharge voltage on the plasma parameters during thruster ignition, a particle-in-cell numerical model was used to calculate the distribution characteristics of the ion density and electric potential at different ignition moments under different discharge voltages. The results show that when the discharge voltage is high, the ion densities in the plume and acceleration channel are significantly higher at the initial phase of thruster ignition; with the gradual strengthening of the ignition process, the propellant avalanche ionization during thruster ignition occurs earlier and the pulse current peak increases. The main reason for these phenomena is that the change in the discharge voltage results in different energy acquisitions of the emitted electrons entering the thruster channel.     

The regulation of memory effect and its influence on discharge properties of a dielectric barrier discharge driven by bipolar pulse at atmospheric-pressure nitrogen

The regulation of memory effect that the residual charges generated during and after discharge act on the initiation and development of subsequent discharge is explored by adjusting the pulse parameters,which have an influence on the discharge characteristics.The memory effect is quantified by the measurement of 'wall voltage'through a series of reference capacitors.The influences of memory effect on the discharge properties corresponding to rising/falling time 50-500 ns,pulse width 0.5-1.5 μs,and frequency 200-600 Hz are analyzed.It is found that the'wall voltage'increases from 1.4 kV to 2.4 kV with rising/falling time from 50 ns to 500 ns,it varies in the range of 0.18 kV with frequency of 200-600 Hz,and 0.17 kV with pulse width of 0.5-1.5μs.The propagation velocity of wavelike ionization under the negative pulse slows down from 2184 km s-1 to 1026km s-1 as the rising/falling time increases from 50ns to 500ns due to the weakening of the electric field by the surface memory effect.More intense and uniform emission can be achieved through faster rising/falling time and higher frequency based on the volume memory effect,while pulse width has less influence on the emission uniformity.Furthermore,similar laws are obtained for spectral and discharge intensity.Therefore,the memory effect is most effectively regulated by rising/falling time,and the discharge properties are affected by the surface and volume memory effect.     

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a0, which scales as ${L}_{\gamma }\propto {a}_{0}^{4}$. A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10⁻¹⁴ kg m² s⁻¹ is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 10²⁴ photons s⁻¹ mm⁻² mrad⁻² per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.     

Experimental and simulated investigation of microdischarge characteristics in a pin-to-pin dielectric barrier discharge (DBD) reactor

Both experimental and simulated studies of microdischarge (MD) are carried out in a dielectric barrier discharge with a pin-to-pin gap of 3.5 mm, ignited by a sinusoidal voltage with a peak voltage of 10 kV and a driving frequency of 5 kHz. Statistical results have shown that the probability of the single current pulse in the positive half-period (HP) reaches 73.6% under these conditions. Experimental results show that great luminous intensity is concentrated on the dielectric surface and the tip of the metal electrode. A 1D plasma fluid model is implemented by coupling the species continuity equations, electron energy density equations, Poisson equation, and Helmholtz equations to analyze the MD dynamics on the microscale. The simulated results are in good qualitative agreement with the experimental results. The simulated results show that the MD dynamics can be divided into three phases: the Townsend phase, the streamer propagation phase, and the discharge decay phase. During the streamer propagation phase, the electric field and electron density increase with the streamer propagation from the anode to the cathode, and their maximal values reach 625.48 Td and 2.31 × 1019 m−3, as well as 790.13 Td and 3.58 × 1019 m−3 in the positive and negative HP, respectively. Furthermore, a transient glow-like discharge is detected around the anode during the same period of streamer propagation. The formation of transient glow-like discharge is attributed to electrons drifting back to the anode, which is driven by the residual voltage in the air gap.     

400 kV强流中子发生器的物理设计

对400 kV强流中子发生器进行了物理设计。采用Poisson/Superfish软件对中子发生器高压电极和加速管的电场分布进行了模拟,结果显示,各关键区域的空间电场最大值远低于击穿电场限值。以强流束旁轴包络方程为基本模型,发展了强流束传输系统束包络的计算机模拟程序IONB1.0,模拟了中子发生器传输系统中40 mA的D束流包络。结果显示,设计方案中所采取的两间隙高梯度加速结构有较强的聚焦性能,能有效抵消强流束空间电荷效应造成的束流发散,加速管出口处的束包络半径约3 cm,由加速管出口处的空间电荷透镜和三重四极磁透镜组成的传输系统能将束流聚焦在约140 cm处的靶上,且束斑直径小于2 cm。     

X-Ray-Induced Photoconductivity in Dielectric Films

We have measured the conductivity induced in films of polyethylene, epoxy, polytetrafluoroethylene, polyethylene terephthalate, polyimide, and glass by x rays at dose rates between 109 and 1010 rad/sec (dose in air). The films were 0.05 to 1.25 mm thick. The x-ray spectrum peaked in the vicinity of 10 keV, and the x-ray pulse width was about 40 nsec FWHM. X-ray induced photocurrents were found to obey Ohm's law at low bias voltages (less than 1 kV). Above 1 kV, however, we observed that the peak photoconductivity signals from some of the 0.05-mm-thick materials began to increase at a slightly faster than linear rate with bias voltage. The glass samples exhibited no apparent delayed conductivity, while the other sample materials showed various amounts. The magnitude of the delayed conductivity in polytetrafluoroethylene, polyethylene terephthalate, and polyimide depended on the electric field, an effect that is consistent with Poole-Frenkel field assisted carrier generation. We have qualitatively described the magnitude and time dependence of the conductivity signals by a simple trapping model, using reasonable values for mobility, trap density, capture cross sections and trap depths.     

Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge).OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame,and the experimental apparatus consists of dump burner,plasma-generating system,gas supply system and OH-PLIF system.Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes:regime Ⅰ for voltage lower than 6.6 kV;regime Ⅱ for voltage between 6.6 and 11.1 kV;and regime Ⅲ for voltage between 11.1 and 12.5 kV.In regime Ⅰ,aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role,while in regime Ⅲ,the temperature rising effect will probably superimpose on the chemical effect and amplify it.For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field,the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape.With regard to in situ plasma discharge in flames,the discharge pattern changes from streamer type to glow type.Compared with the case of reactants pretreatment,the flame propagates further in the upstream direction.In the discharge region,the OH intensity is highest for in situ plasma assisted combustion,indicating that the plasma energy is coupled into flame reaction zone.     

Experimental study on toluene removal by a two-stage plasma-biofilter system

Volatile organic compounds (VOCs) are typical pollutants that affect air quality. Discharge plasma is thought to be a potential method that can remove VOCs from flue gas. In this experiment, pulsed corona discharge plasma combined with a biological tower was carried out to remove the benzene series, and toluene was selected as the typical VOC. The results indicated that the removal efficiency of toluene by pulsed corona plasma was slightly higher than that of direct current (DC) corona plasma, while its energy efficiency was much higher than DC corona plasma. Under the optimal experimental conditions of pulse voltage 8.5 kV, initial toluene concentration 1400 mg m−3, and toluene flow rate of 12 l h−1, the toluene removal efficiency reached 77.11% by the single method of pulsed corona discharge plasma, and the energy efficiency was up to 1.515 g/(kW·h) under the pulse voltage of 4.0 kV. The trickling biofilter was constructed by using the screened and domesticated Acinetobacter baumannii, and the highest toluene removal efficiency by the pulsed corona discharge plasma combined with the trickling biofilter rose up to 97.84%. Part of the toluene was degraded into CO2, H2O, and some intermediate products such as o-diphenol under the influence of Acinetobacter baumannii. When the remaining waste gas passed through the discharge plasma reactor, the benzene ring structure could be directly destroyed by the collision between toluene and plasma. Meanwhile, O·, OH·, and some other oxidizing radicals generated by the discharge also join into the oxidative decomposition of toluene and its intermediate products, thereby further improving the removal efficiency of toluene. Therefore, the two-stage plasma-biofilter system not only showed a high toluene removal efficiency, but also had a good energy efficiency. The results of this study will provide theoretical support and technical reference for industrial VOC treatment.     

Experimental investigation on DBD plasma reforming hydrocarbon blends

To improve the'detonation-supporting'performance of fuel-rich catalytic combustion products,DBD plasma,stimulated by adjustable nanosecond pulse power supply,was used to further regulate the components and concentrations of the hydrocarbon blends.In this paper,the parameters including load voltage,frequency,rising(falling)edge,pulse width and feeding flow rate were changed respectively,and the corresponding concentration and proportion change of the components in blend gas were investigated.According to the experiment result,it was found that when the discharge frequency is low,the plasma mainly promotes the transformation of light gaseous substances,while it mainly promotes the conversion to heavy hydrocarbons when the frequency is larger.Increasing load voltage will strengthen this trend.The controlling and reforming effect of plasma on the blend gas will decrease with the increase of voltage rising(falling)edge and the feeding flow rate.The regulation effect will be strengthened with the increase of pulse width under 200 ns.With the increase of discharge intensity,the'carbon'settles on the walls of the reactor,which will change the dielectric constant,leading to the loss of control of the discharge.     

Degradation of tiamulin by a packed bed dielectric barrier plasma combined with TiO2 catalyst

Recently, a plasma catalyst was employed to efficiently degrade antibiotic residues in the environment. In this study, the plasma generated in a packed bed dielectric barrier reactor combined with TiO2 catalyst is used to degrade the antibiotic tiamulin (TIA) loaded on the surface of simulated soil particles. The effects of applied voltage, composition of the working gas, gas flow rate and presence or absence of catalyst on the degradation effect were studied. It was found that plasma and catalyst can produce a synergistic effect under optimal conditions (applied voltage 25 kV, oxygen ratio 1%, gas flow rate 0.6 l min−1, treatment time 5 min). The degradation efficiency of the plasma combined with catalyst can reach 78.6%, which is 18.4% higher than that of plasma without catalyst. When the applied voltage is 30 kV, the gas flow rate is 1 l min−1, the oxygen ratio is 1% and the plasma combined with TiO2 catalyst treats the sample for 5 min the degradation efficiency of TIA reached 97%. It can be concluded that a higher applied voltage and longer processing times not only lead to more degradation but also result in a lower energy efficiency. Decreasing the oxygen ratio and gas flow rate could improve the degradation efficiency. The relative distribution and identity of the major TIA degradation product generated was determined by high-performance liquid chromatography–mass spectrometry analysis. The mechanism of TIA removal by plasma and TiO2 catalyst was analyzed, and the possible degradation path is discussed.