Insulation characteristics of triple mixtures of c-C4F8/N2/CO2 under lightning impulse voltage

This paper has researched the insulation characteristics of 10%c-C₄F₈/N₂/CO₂ mixtures under lightning impulse voltage by experiment. It is shown that the positive and negative lightning impulse breakdown voltages of 10%c-C₄F₈/N₂/CO₂ gas mixtures rise linearly as the electrode gap distance and gas pressure increase and under the same conditions, the positive lightning impulse breakdown voltage of the gas mixtures is always higher than the negative lightning impulse breakdown voltage. As the gas mixtures have a little higher liquefied temperature than SF₆ and the comprehensive GWP is about 5% of SF₆ , and the positive and negative lightning impulse breakdown voltages can both reach 60% of SF₆ , 10%c-C₄F₈/N₂/CO₂ gas mixtures can be applied as insulation gas in electrical equipment such as C-GIS, GIT, GIL and so on.     

Investigation on the Dielectric Properties of CO2 and CO2-Based Gases Based on the Boltzmann Equation Analysis

In this paper,the dielectric properties of CO_2,CO_2/air,CO_2/O2,CO_2/N_2,CO_2/CF_4,CO_2/CH_4,CO_2/He,CO_2/H_2,CO_2/NH_3 and CO_2/CO were investigated based on the Boltzmann equation analysis,in which the reduced critical electric field strength(E/N)crof the gases was derived from the calculated electron energy distribution function(EEDF) by solving the Boltzmann transport equation.In this work,it should be noted that the fundamental data were carefully selected by the published experimental results and calculations to ensure the validity of the calculation.The results indicate that if He,H_2,N_2 and CH_4,in which there are high ionization coefficients or a lack of attachment reactions,are added into CO_2,the dielectric properties will decrease.On the other hand,air,O2,NH_3 and CF_4(ranked in terms of(E/N)_(cr) value in increasing order) have the potential to improve the dielectric property of CO_2 at room temperature.     

Calculation and characteristic analysis on synergistic effect of CF3I gas mixtures

CF_3I is a potential SF_6 alternative gas.In order to study the insulation properties and synergistic effects of CF_3I/N_2 and CF_3I/CO_2 gas mixtures,two-term approximate Boltzmann equations were used to obtain the ionization coefficient α,attachment coefficient η and the critical equivalent electrical field strength(E/N)_(cr).The results show that the(E/N)_(cr)of CF_3I gas at 300 K is 1.2 times that of SF_6 gas,and CF_3I/N_2 and CF_3I/CO_2 gas mixtures both have synergistic effect occurred.The synergistic effect coefficient of CF_3I/CO_2 gas mixture was higher than that of CF_3I/N_2 gas mixture.But the(E/N)_(cr)of CF_3I/N_2 is higher than that of CF_3I/CO_2 under the same conditions.When the content of CF_3I exceeds 20%,the (E/N)_(cr) of CF_3I/N_2 and CF_3I/CO_2 gas mixture increase linearly with the increasing of CF_3I gas content.The breakdown voltage of CF_3I/N_2 gas mixture is also higher than that of CF_3I/CO_2 gas mixture in slightly non-uniform electrical field under power frequency voltage,but the synergistic effect coefficients of the two gas mixtures are basically the same.     

Breakdown Voltage Research of Penning Gas Mixture in Plasma Display Panel

Paschen law and equations, which ignore the influence of the Penning ionization on the electron ionization coefficient （α）, are always used as the approximation of the breakdown voltage criterion of the Penning gas mixture in current researches of discharge characteristics of the plasma display panel （PDP）. It is doubtful that whether their results match the facts. Based on the Townsend gas self-sustaining discharge condition and the chemical kinetics analysis of the Penning gas mixture discharging in PDP, the empirical equation to describe the breakdown of the Penning gas mixture is given. It is used to calculate the breakdown voltage curves of Ne-Xe/MgO and Ne-Ar/MgO in a testing macroscopic discharge cell of AC-PDP. The effective secondary electron emission coefficients （γeff） of the MgO protective layers are derived by comparing the breakdown voltage curves obtained from the empirical equation with the experimental data of breakdown voltages. In comparison with the results calculated by the Paschen law and the equation which ignore the influence of the Penning ionization on α , the results calculated by the empirical equation have better conformity with experimental data. The empirical equation characterizes the breakdown of the Penning gas mixture in PDP effectively, and gives a convenient way to study its breakdown characteristics and the secondary electron emission behaviors.     

球形不锈钢电离室响应实验研究

针对球形不锈钢电离室，开展了一系列实验研究，包括不同充气气体及其不同气压条件下的电离室的能量响应及单位压力灵敏度等。实验气体包括氦气、氮气、氩气、氪气和氙气，原子序数从2到54；充气气压从4kPa到1000kPa；照射射线能量从33keV到1．25MeV。研究结果表明：1）对不锈钢电离室，在充氙气时最灵敏，在相同充气压力下，对137^Cs源和低能射线（83keV）照射，充氙电离室的灵敏度约分别为充氩电离室的5．6倍和55倍；但在实验中发现，对充氙气电离室的灵敏度随压力变化曲线，当射线能量超过205keV时，在计示O．2个大气压处存在拐点，还有待进一步分析。2）从能量响应来看，充氩的最好，充氙的最差，充氦和氮的居中且两者大致相同。     

Effect of Addition of Nitrogen to a Capacitively Radio-Frequency Hydrogen Discharge

A hybrid PIC/MC model is developed in this work for H2-xN2 capacitively coupled radio-frequency （CCRF） discharges in which we take into account 43 kinds of collisions reaction processes between charged particles （e-, H3＋, H＋, H＋, N＋, N＋） and ground-state molecules （H2, H＋ N2）. In addition, the mean energies and densities of electrons and ions （ 3, H＋, H＋）, and electric field distributions in the H2-N2 CCRF discharge are simulated by this model. Furthermore, the effects of addition of a variable percentage of nitrogen （0-30%） into the H2 discharge on the plasma processes and discharge characteristics are studied. It is shown that by increasing the percentage of nitrogen added to the system, the RF sheath thickness will narrow, the sheath electric field will be enhanced, and the mean energy of hydrogen ions impacting the electrodes will be increased. Because the electron impact ionization and dissociative ionization rates increase when N2 is added to the system, the electron mean density will increase while the electron mean energy and hydrogen ion density near the electrodes will decrease. This work aims to provide a theoretical basis for experimental studies and technological developments with regard to H2-N2 CCRF plasmas.     

OH and O radicals production in atmospheric pressure air/Ar/H2O gliding arc discharge plasma jet

Atmospheric pressure air/Ar/H_2O gliding arc discharge plasma is produced by a pulsed dc power supply. An optical emission spectroscopic(OES) diagnostic technique is used for the characterization of plasmas and for identifications of OH and O radicals along with other species in the plasmas. The OES diagnostic technique reveals the excitation Tx?≈?5550–9000 K, rotational Tr?≈?1350–2700 K and gas Tg?≈?850–1600 K temperatures, and electron density n?(1.1-1.9) ′101 4 cm~(-3) e under different experimental conditions. The production and destruction of OH and O radicals are investigated as functions of applied voltage and air flow rate. Relative intensities of OH and O radicals indicate that their production rates are increased with increasing Ar content in the gas mixture and applied voltage. nereveals that the higher densities of OH and O radicals are produced in the discharge due to more effective electron impact dissociation of H_2O and O_2 molecules caused by higher kinetic energies as gained by electrons from the enhanced electric field as well as by enhanced n e.The productions of OH and O are decreasing with increasing air flow rate due to removal of Joule heat from the discharge region but enhanced air flow rate significantly modifies discharge maintenance properties. Besides, Tgsignificantly reduces with the enhanced air flow rate. This investigation reveals that Ar plays a significant role in the production of OH and O radicals.     

Atmospheric Pressure Plasma Jet in Ar and O2/Ar Mixtures： Properties and High Performance for Surface Cleaning＊

An atmospheric pressure plasma jet generated in Ar and O2/Ar mixtures has been investigated by specially designed equipment with double power electrodes at 20~32 kHz, and their effects on the cleaning of surfaces have been studied. Properties of the jet discharge are studied by electrical diagnostics, including the waveform of discharge voltage, discharge current and the Q-V Lissajous figures. The optical emission spectroscopy is used to measure the plasma parameters, such as the excitation temperature and the gas temperature. It is found that the consumed power and the excitation temperature increase with increase of the discharge frequency. On the other hand, at the same discharge frequency, these parameters in O2/Ar mixture plasma are found to be much larger. The effect on surface cleaning is studied from the changes in the contact angle. For Ar plasma jet, the contact angle decreases with increase of the discharge frequency. For O2/Ar mixture plasma jet, the contact angle decreases with increase of discharge frequency up to 26 kHz, however, further increase of discharge frequency does not show further decrease in the contact angle. At the same discharge frequency, the contact angle after O2/Ar mixture plasma cleaning is found to be much lower compared to the case of pure Ar. From the results of quadrupole mass-spectrum analysis, we can identify more fragment molecules of CO and H2O in the emitted gases after O2/Ar plasma jet treatment compared with Ar plasma jet treatment, which are produced by the decomposition of surface organic contaminants during the cleaning process.     

Two-dimensional simulation and experiments of helium discharge between parallel-plate electrodes in short gaps at atmospheric pressure

ABSTRACT

Helium gas is used as the coolant in high temperature gas-cooled reactor while its insulation property is not good and brings big challenges to insulation design work. In this paper, breakdown voltages of two parallel-plate electrodes in millimeter-scale gaps at atmospheric pressure are calculated numerically with a two-dimensional hydrodynamic model. The Finite-Difference Flux-Corrected Transport method (FD-FCT) is used to avoid spurious oscillation caused by convection and diffusion restrictions. The simulation results are in good agreement with our experimental work. Different discharge characteristics are presented for a breakdown or a non-breakdown voltage. Importantly, when a breakdown is applied, ground-state ionization will make up the most in early stages while later accumulation of metastable atoms and molecules will prompt fractions of penning ionization to increase rapidly. It is predicted that penning ionization will eventually become comparable with ground-state ionization, especially near the anode, as the current density keeps increasing to order of 1 A. Some other simulation results are presented such as time evolutions of the distribution of electrons, ions, field strength and axial potential, etc. Attainment of these breakdown voltages and characteristics of helium discharge is going to help with insulation design work of electric equipment in reactor engineering.     

A Simplified Numerical Study of the Kr/Cl2 Plasma Chemistry in Dielectric Barrier Discharge

In this paper, the generation of excimers and exciplexe radiation in mixtures of rare gas with halogen by homogeneous dielectric barrier discharge （DBD） is investigated. The typical characteristics of an excilamp based on KrCl exciplexe molecules and the kinetic processes for the formation and the decay of this molecules in the Kr/Cl₂ mixture are studied. The computer model developed is based on the Kr/Cl₂ mixture chemistry, the equivalent electric circuit and the Boltzmann equations. The importance in the kinetic processes of some species such as the metastable state of Krypton （Kr（3P0,2）） and the negative ion of chloride （Cl） is considered. The results illustrate the time variations of charged species （ne,Kr+,Cl,Cl+,Cl+2,Kr+2）, excited atoms and molecules （Kr（3P0,2）, Kr（3P1）, Cl, Cl2）, the excimers （Kr2,KrCl（B）,KrCl（C）,Kr2Cl） and the UV photon concentrations （in 222nm,235nm,258nm and 325nm range）. The effects of chlorine concentration and the total gas pressure in the Kr-Cl₂ discharge on the electric parameters and radiation emissions are investigated.     

He+ (np) cross sections for 2, 3, 4 and 5 MeV C4+ ions colliding with He atoms

The cross sections of ionization plus excitation of He are measured by the C⁴⁺ + He prototype reaction with energies ranging from 2 to 5 MeV. Theoretically the independent electron approximation is used to calculate the HeII (np) ionization plus excitation cross sections. The results of the calculations are compared with our experimental data.     

Understanding CO_2 decomposition by thermal plasma with supersonic expansion quench

CO_2 pyrolysis by thermal plasma was investigated,and a high conversion rate of 33% and energy efficiency of 17% were obtained.The high performance benefited from a novel quenching method,which synergizes the converging nozzle and cooling tube.To understand the synergy effect,a computational fluid dynamics simulation was carried out.A quick quenching rate of 10~7Ks(-1) could be expected when the pyrolysis gas temperature decreased from more than 3000 to 1000 K.According to the simulation results,the quenching mechanism was discussed as follows: first,the compressible fluid was adiabatically expanded in the converging nozzle and accelerated to sonic speed,and parts of the heat energy converted to convective kinetic energy; second,the sonic fluid jet into the cooling tube formed a strong eddy,which greatly enhanced the heat transfer between the inverse-flowing fluid and cooling tube.These two mechanisms ensure a quick quenching to prevent the reverse reaction of CO_2 pyrolysis gas when it flows out from the thermal plasma reactor.     

A Bethe theory for the directional dependence of stopping by molecules

We derive a Bethe formula for the stopping of swift ions by target molecules oriented with respect to the beam. The theory is characterized by a directional mean excitation energy formed from transition energies and dipole matrix elements. The expression for the mean excitation energy is similar to the one for an isotropic sample but the dipole matrix elements corresponding to the various transition moment directions carry unequal weights which depend on the orientation of the molecule with respect to the beam. For a large class of molecules, the logarithm of the mean excitation energy is linear in sin² θ, where θ denotes the angle between the beam and the principal molecular axis. We illustrate the properties of the theory in the simple case of a diatomic molecule.     

Plasma discharge characteristics of segmented diverter strips subject to lightning strike

In order to research segmented diverters for aircraft lightning protection, a transient 2 D multiphysics model based on magnetohydrodynamics theory is proposed to predict the location of the arc plasma discharge and lightning channel, and to simulate the electrothermal behavior.Based on numerical calculation and preliminary analysis, factors that affect the breakdown voltage of the segmented diverter are discussed. The results show that the voltage increase rate of the voltage source, the width of the air gap between metal segments and the geometry of these segments influence the breakdown voltage of the strip. High-voltage tests of the segmented diverter are performed to reveal air breakdown of the strip and redirect the lightning current.Experimental and numerical results are compared to verify the correctness of the numerical model. The ionization of the air gap between metal segments and the breakdown voltage of the strip calculated by the model are qualitatively consistent with experimental results. The breakdown voltage of the segmented diverter is far lower than the lightning voltage. When a lightning strike occurs, the segmented diverter can be quickly ionized to form a plasma channel which can guide the lightning current well.     

空心玻璃微球D2/Ne混合气体充气工艺

本工作主要研究空心玻璃微球对D₂和Ne气体渗透系数的差异，以及研究采用热扩散法在高压充气系统上向空心玻璃微球充入D₂/Ne混合气体的充气工艺。利用干涉条纹法测量了在充气和保气时Ne的气体渗透系数，它们分别为K_Ne,350℃=2.6×10^-18和K_Ne,25℃=8.0×10^-22mol•m^－1•s^－1•Pa^－1。根据D₂的气体渗透系数确定了玻璃微球充D₂/Ne混合气体的充气方法和充气平衡时间，平衡时间以充纯Ne时间为准。此外，还研究了空心玻璃微球充入混合气体后的保气性能。     

Silicon detector response to heavy ions at energies of 1–2 MeV/amu

The response of silicon detectors has been measured for He, O, S, Cl, Br, Ag and Pb ions in the energy range 1–2 MeV/amu. Following deliberate, long exposures of the detector, a transient effect was observed for 140 MeV Br ions, in which the pulse height decreased with increasing ion dose and then partially recovered within an hour of the final exposure. Using brief, consecutive exposures, the effective energy for creating a detectable electron–hole pair was determined using the pulse height difference method. The energy deposited by ions in the ‘dead-layer' at the detector surface and energy loss via non-ionizing events was taken into account. For ions with atomic numbers 2Z17 and energies above the Bragg peak, the effective energy was found to decrease linearly with increasing electronic stopping power at first, and then to level off at 3.52 eV/electron–hole pair. For intermediate mass ions (17<Z40), at energies close to the Bragg peak, increases slightly (2%) with increasing stopping power. For the heaviest ions studied (Z40), whose energies are below the Bragg peak of the stopping power curve, increases strongly (10–20%), even though the electronic stopping power is approximately constant.     

Nonequilibrium modeling study on plasma flow features in a low-power nitrogen/hydrogen arcjet thruster

Gasdynamic flow features in an electrothermal arcjet thruster with a mixture of 1:2 nitrogen/ hydrogen as the working gas have been studied by a two-temperature numerical simulation.Seven species and 17 kinetic processes are included in the chemical kinetic model used to represent dissociation,ionization,and the corresponding recombination reactions in this nitrogen/hydrogen mixture system.Based on the gas flow characteristics inside the arcjet nozzle,a new method is introduced to define the edge of the cold boundary layer,which is more convenient to analyze the evolution and development of plasma flow in an arcjet thruster.The results show that the arcjet thruster performance is determined largely by the exchange of energy and momentum between the low-density,high-temperature arc region and the high-density,coolttow region near the nozzle wall.A significant thermal nonequilibrium is found in the cold boundary layer in the expansion portion of the nozzle.The important chemical kinetic processes determining the distribution of hydrogen and nitrogen species in different flow regions are presented.It has been shown that the reaction rate of hydrogen species ionization impacted by electrons is much higher than that of nitrogen species ionization in the center of the constrictor of the arcjet thruster.This indicates that hydrogen species is very important in the conversion of applied electric energy into thermal energy in the constrictor region of the arcjet thruster.     

Ionization process and distinctive characteristic of atmospheric pressure cold plasma jet driven resonantly by microwave pulses

In the present study, a coaxial transmission line resonator is constructed, which is always capable of generating cold microwave plasma jet plumes in ambient air in spite of using argon, nitrogen, or even air, respectively. Although the different kinds of working gas induce the different discharge performance, their ionization processes all indicate that the ionization enhancement has taken place twice in each pulsed periods, and the electron densities measured by the method of microwave Rayleigh scattering are higher than the amplitude order of 10¹⁸ m⁻³. The tail region of plasma jets all contain a large number of active particles, like NO, O, emitted photons, etc, but without O3. The formation mechanism and the distinctive characteristics are attributed to the resonance excitation of the locally enhanced electric fields, the ionization wave propulsion, and the temporal and spatial distribution of different particles in the pulsed microwave plasma jets. The parameters of plasma jet could be modulated by adjusting microwave power, modulation pulse parameters (modulation frequency and duty ratio), gas type and its flow rate, according to the requirements of application scenarios.     

Nonlinearity of pulse height of recoil helium in 3Heproportional counters

The pulse height distribution in ³He proportional counters has been investigated to accurately quantify the response function to be used in fast neutron spectroscopy. The measurements were performed as a function of the operating voltage, the counting gas mixture, and the incident neutron energy. The special emphasis of this study has been put on the nonlinearity of the pulse height of a recoil helium induced by the ³He(n,n)³He reaction. A distinct deviation of 14% at maximum to lower energies was found in the position of the recoil edge with the energy axis calibrated by the full energy peaks of the ³He(n,p)T reaction produced with neutrons of thermal energy and 2.413 MeV. It was confirmed that the nonlinear effects, such as an initial recombination and a space-charge effect, take place more significantly in the energy loss process of a recoil helium because of its higher linear energy transfer (LET) in the filling gas compared to that of a proton and triton pair emitted from the ³ He(n,p)T reaction. The deviation becomes larger with an increase of the average LET in the gas mixture. The experimental results suggested that the nonlinear effect caused by a higher LET recoil helium is inevitable for the filling gas, even if the operating voltage is changed to an acceptable range     

Reconstruction of energy spectrum of runaway electrons in nanosecond-pulse discharges in atmospheric air

This paper presents an experimental investigation into the runaway electron spectrum with a gas diode composed of a rough spherical cathode and plane anode under the excitation of a nanosecond-pulse generator in atmospheric air. The runaway electron beams are measured by a collector covered with aluminum foil with a thickness from 0 μm(mesh grid) to 50 μm. The energy spectrum is calculated by an improved Tikhonov regularization called the maximum entropy method. The experimental results show that the transition state of the discharge consisted of multiple streamer channels stretched from the cathode with glow-like plasma uniformly distributed over the anode. The number of runaway electrons measured by the collector is in the order of 1010 in atmospheric pressure air with a gap spacing of 5 mm and applied voltages of70–130 kV. The cathode with a rough surface creates a more inhomogeneous electric field and larger emission site for the runaway electrons around the cathode, providing conditions for the coexistence of filamentary streamer and diffuse discharge. The reconstructed spectra show that the energy distribution of the runaway electrons presents a single-peak profile with energies from eU_m/2–2 eU_m/3(U_m is maximal voltage across the gap).