波导等离子体限幅器中气体的选择与触发条件计算

 为保护电子设备不受高功率微波损坏,在矩形波导中嵌入等离子体限幅器。计算了不同气体的微波击穿场强随气体压强以及微波频率的变化规律。在高气压条件下（1 333~133 320 Pa）,气体击穿场强随气压增大而增大,在计算的4种气体中Ne的击穿场强最小;低气压条件下（1.333 2~133.32 Pa）,气体击穿场强随气压增大而减小,且Xe具有最小击穿场强。高气压条件下气体的击穿场强明显高于低气压下的击穿场强。计算结果表明：当填充133.32 Pa的Xe时,限幅器能够在约30 km范围内,有效地防护10 GW级高功率微波对电子设备的损坏。     

Circuit modeling of a vacuum gap during breakdown

In this paper several aspects of circuit modeling of a vacuum gap during breakdown are improved or introduced for the first time. More accurate perveance formulas are derived by the method of tracing electron trajectories in the self-consistent electric field calculated by the finite element method. The formula for maximum anode current density is also derived by the same method. A practical model of anode heating is proposed, by which transient anode temperature is calculated coupled with gap voltage and current, providing more accurate modeling of anode plasma initiation. The circuit model of a vacuum gap during breakdown incorporating all these features is implemented as a subcircuit element in the PSPICE     

Application of Mn nano-flower sculptured thin films produced on interdigitated pattern as cathode and anode electrodes in field ionization gas sensor

The photolitography method was used for producing interdigitated configurations for cathode and anode electrodes of a field ionization gas sensor in which Mn helical nano-flowers with 3-fold symmetry were deposited using oblique angle deposition together with rotation of the substrate about its surface normal, with each rotation divided into six sections. These sections were alternately rotated at high and low speeds. Three different distances were chosen in the design between anode and cathode electrodes, namely 40, 100 and 200 μm. Physical structure and morphology of electrodes were studied by field emission scanning electron microscope and atomic force microscope analyses.The breakdown voltage of the system was studied for nitrogen, oxygen, argon, air and carbon mono-oxide gases. Investigations with these gases at different distances between anode and cathode and different gas pressures confirmed Paschen's Law. Results showed that at low pressures, decreasing the gap between electrodes increases the breakdown voltage. With fewer gas molecules between the electrodes the number of interactions between particles is reduced and higher energies are required for ionization of gas molecules. At high pressures, the breakdown voltage is decreased because of an increased number of molecular interactions. The sensor demonstrated good selectivity between the different gases and selectivity was enhanced with increasing gas pressure. A direct relationship was found at low pressures (e.g., 0.1 mbar) between the breakdown voltage and the gas ionization energy while at high pressures (e.g., 1000 mbar) this relationship was reversed.     

Optimum conditions of the runaway electron beam generation during a uniform gas breakdown

It was shown that a uniform breakdown of a gas gap must be accompanied by the Stoletov effect, i.e., a current maximum in pressure at a given voltage pulse. Analogues of the Stoletov constant were calculated for the pulsed non-self-sustained discharge in various gases. These constants also determine the minimum breakdown delay time. It was shown that the maximum current of the electron beam generated in the gas-filled diode is reached at a pressure corresponding to the maximum current and for an electrode spacing corresponding to the electron drift length during the pulse.     

Theoretical Studies of the Breakdown Characteristics at Microwave Frequencies

This paper describes several theoretical approaches for studies of the breakdown characteristics for a few atomic and molecular gases at microwave frequencies in small gaps. Numerical studies have been carried out extensively over a wide range of pressures for a microwave frequency of 2.45 GHz using both the kinetic and fluid approaches and compared with the results obtained by an analytical-experimental approach. The obtained results illustrate the physical phenomena that occur during breakdown with emphasis on the determination of the breakdown voltage and its dependence on the gap size and gas pressure at microwave frequency. Good agreement between the results achieved by using different approaches suggests that they can be applied even to gaps of a few hundred micrometers and high pressures.     

An Efficient Procedure to Identify and Quantify New Molecules for Insulating Gas Mixtures

In this contribution, a new procedure to systematically identify and quantify novel molecular gases with low global warming potential for application in high voltage insulation as gas mixtures is presented. The attention is focused on highly efficient procedures to be able to scan a large number of candidate gases. To identify new molecules, we derived an empirical correlation between the electric strength of a gas and certain molecular properties, like polarizability or dipole moment, which can be calculated by means of density functional theory. The swarm parameters of these pre‐selected molecules in mixtures with buffer gases is then quantified, using a newly set‐up Pulsed Townsend experiment. The setup operates with a high degree of automation to enable systematic evaluation of gas mixtures not to miss possible synergistic effects. Key element of this PT setup is a new photocathode that works with a high quantum efficiency and long lifetime even when exposed to reactive species during the measurements. Moreover, for an automated operation it is important to know precisely in which range the experiment can be operated, i.e. for example to know up to which electron density space charge effects can be neglected. Finally, the measured swarm parameters need to be translated into breakdown voltage strengths of different electrode arrangements and different applied voltage wave shapes. For this, a model of the the streamer to leader transition in SF6 will be applied to other strong electronegative gases in future studies to test if the model is universally valid. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement of laser-induced breakdown threshold intensities of high-pressure gasses and water droplets to determine the number density of an aerosol

The laser-induced breakdown threshold intensities of high-pressure gasses as a function of pressure were measured. The breakdown threshold intensity of N₂ gas was found to be much higher than those of He and Ar gasses at a gas pressure of 2.0 MPa. It was also 7 to 11 times higher than that of aerosol droplets, and is sufficiently high to allow the number density of droplets to be measured in a high-pressure aerosol by laser-induced breakdown.     

Finiteness of the triple gauge-ghost vertices in $${{\mathcal {N}}}=1$$ supersymmetric gauge theories: the two-loop verification

The European Physical Journal C - Dielectric breakdown strength is one of the critical performance metrics for pure gases and gas mixtures used in large, high pressure gas time projection chambers....     

Similarity law for a homogeneous discharge in the presence of a strong field and analogues of the Stoletov constant for the pulsed regime

It is demonstrated that the similarity relationships (breakdown curves), which establish a dependence of the field strength divided by the pressure on the product of the pressure and the delay time of the breakdown, are realized upon the uniform breakdown of the gas gap in the presence of both rectangular and triangular voltage pulses, which is interesting for the physics of gas and plasma discharges, and remain valid for strong fields. The breakdown criterion is described with a two-valued curve such that the effective multiplication of electrons in gas becomes possible in the presence of both weak and strong fields and at small products of the pressure and the pulse time. An analogue of the Stoletov effect, which corresponds to a maximum in the current with respect to pressure at a given voltage pulse, is demonstrated for the pulsed discharge. The analogues of the Stoletov constant are calculated for non-self-sustained pulsed discharges in various gases. The minimum delay time of the breakdown is also determined by these constants.     

双极结型晶体管的集电结反向雪崩击穿特性

通过气体放电产生更高浓度的低温等离子体要求具有纳秒上升沿和纳秒脉宽的高重频快脉冲,而目前被广泛使用的MOSFET和IGBT都无法满足这些参数要求,而双极结型晶体管(BJT）的集电极与发射极之间的雪崩击穿过程具有快导通、快恢复、高稳定性等优点,适合作为小型Marx发生器的自击穿开关。文中对用多种型号的BJT进行击穿特性比较测试实验,发现可以通过改变BJT的门极和发射极的并联电阻来调节其雪崩击穿电压,实现一定范围的工作电压。雪崩击穿恢复特性实验表明,当击穿电流衰减到低于维持电流时,BJT就会开始恢复绝缘而关断,通过改变电路中的参数以控制击穿电流的变化就可以控制BJT的雪崩击穿导通时间（即导通脉宽）。将这些结论应用到实际电路中,可获得上升沿5 ns、脉宽为10 ns、幅值2 kV、重复频率高达100 kHz的纳秒快脉冲,可用于激发高浓度低温等离子体。     

The Effective Cross Section

The effection cross section concept for low intensity light illumination of gas discharges is shown to be analogous to high intensity breakdown of gases as interpreted by Panarella's "effective photon" concept. This mathematical result is suggested as a possible interpretation of strikingly similar nonlinearities of gas breakdown by light measured in both cases at their respectively different intensity levels.     

Generation of microcellular poly(methyl methacrylate) by compressed gases

Abstract

Generation of microcellular poly(methy1 methacrylate) (PMMA) was studied in CO₂ and N₂O at pressures from 2 to 15MPa at three temperatures, 293.2K, 308.2K, and 323.2 K. The average diameter d and average number density N of voids generated by a rapid expansion of compressed gases in PMMA were measured by use of an optical microscope. Effects of gases, temperature, and pressure on the d and N values were examined. Even at pressure below glass transition pressure of PMMA with both gases, voids of diameter being as small as those found at high pressure, 15MPa, were obtained at each temperature. However, the void density of PMMA at lower pressure by both gases was not so good as those obtained at high pressures.     

A modified Paschen law for the initiation of a dc glow discharge in inert gases

Breakdown of inert gases in a homogeneous dc electric field is studied experimentally and theoretically at various distances L between the electrodes and radii R of the discharge tubes. It is shown that, for arbitrary geometric dimensions of the discharge chamber and cathode materials, the ratio of the breakdown electric field strength to the gas pressure holds constant at the breakdown curve minimum. A modified Paschen law is obtained, according to which the breakdown voltage is a function of both the product of the gas pressure by the distance L and the ratio L/R.     

Microwave breakdown in slots

The properties of microwave-induced breakdown of air in narrow metallic slots are investigated, both theoretically and experimentally, with emphasis on factors important for protection against transmission of incident high-power microwave radiation. The key factors investigated are breakdown power threshold, breakdown time, peak-leakage power, and total transmitted energy, as functions of incident pulse shape and power density. The theoretical investigation includes estimates of the electric field intensification in narrow slots and basic breakdown plasma modeling. New results important for application to the high-power microwave field, such as the influence of pulse shape on breakdown time and peak-leakage power, are presented. The experimental investigation comprises a set of slot breakdown experiments at atmospheric pressure, which are analyzed to extract key parameters, such as transmission cross section, breakdown time, peak leakage power, and transmitted energy. The experimental data is compared and shown to be in good agreement with results obtained in the theoretical investigation.     

Mayer and Virial Series at Low Temperature

We analyze the Mayer and virial series (pressure as a function of the activity resp. the density) for a classical system of particles in continuous configuration space at low temperature. Particles interact via a finite range potential with an attractive tail. We propose physical interpretations of the Mayer and virial series’ radii of convergence, valid independently of the question of phase transition: the Mayer radius corresponds to a fast increase from very small to finite density, and the virial radius corresponds to a cross-over from monatomic to polyatomic gas. Our results are consistent with the Lee-Yang theorem for lattice gases and with the continuum Widom-Rowlinson model.     

An axisymmetric hypersingular boundary integral formulation for simulating acoustic wave propagation in supercavitating flows

Flow supercavitation begins when fluid is accelerated over a sharp edge, usually at the nose of an underwater vehicle, where a phase change occurs and causes a low density gaseous cavity to gradually envelop the whole object (supercavity) thereby allowing for higher speeds of underwater vehicles. The supercavity may be maintained through ventilated cavitation caused by injection of gases into the cavity, which causes fluctuations at the vapor–water interface. A major issue that concerns the efficient operation of an underwater object’s guidance system (which is achieved by high frequency acoustic sensors mounted within the nose region), is the hydrodynamic noise produced due to the fluctuating vapor–water interface. It is important to carry out a detailed study on the effect of self-noise at the vehicle’s nose that is generated by the ventilating gas jet impingement on the supercavity wall. For this purpose, the present study uses a boundary element method which is more versatile compared to other numerical techniques such as the finite element/finite difference methods. The variation of acoustic pressure at the vehicle nose for various shapes of cavitators, boundary conditions and jet impact diameters are presented. Comparisons are made with the semi-analytical procedure of Howe et al. (Howe et al., On self-noise at the nose of a supercavitating vehicle. Journal of Sound and Vibration, 322 (2009a), 772–784) and finite element based COMSOL commercial package. Several issues pertaining to the behaviour of analytical and numerical results are highlighted. Finally, the proposed boundary element technique is used to study arbitrary shapes of supercavities which may encountered at various stages of supercavity development.     

Effect of aperture field distribution on the maximum radiated power at atmospheric pressure

The air breakdown in the high-power antenna near-field region limits the enhancement of the radiated power. A model coupling the field equivalent principle and the electron number density equation is presented to study the breakdown process in the near-field region of the circular aperture antenna at atmospheric pressure. Simulation results show that, although the electric field in the near-field region is nonuniform, the electron diffusion has small influence on the breakdown process when the initial electron number density is uniform in space. The field magnitude distribution on the aperture plays an important role in the maximum radiated power above which the air breakdown occurs. The maximum radiated power also depends on the phase difference of the fields at the center and edge of the aperture, especially for the uniform field magnitude distribution.     

Definition and measurement of global thermodynamic variables for laser-cooled trapped gas

We have used the definition of global thermodynamic variables like pressure and volume for atoms trapped in a nonuniform potential to measure the state equation for a sample of cold Na atoms kept trapped in a quadrupole magnetic field. The results show that, for low atomic density, the system behaves like an ideal gas where pressure and volume are inversely proportional. At high density values (compressed system), the deviation from an ideal gas is clear. A model based on virial expansion shows that the measured deviation is larger than the expected first-order correction. Employing the concept of global variables may be an important procedure to describe the thermodynamic of gases in the ultracold regime eventually crossing the values where critical phenomena like Bose condensation, among others, take place.