Self consistent multi-fluid FDTD simulations of a nanosecond high power microwave discharge in air

FDTD simulations of the Maxwell equations are combined with the multi-fluid plasma equations to study the dynamics of a high power microwave discharge in air. The breakdown takes place in a short time of a few nanoseconds and the concentrations of electrons, ions, excited species, and the dissociation products are quickly enhanced. The breakdown time decreases with decreasing of the pressure and the pulse amplitude, while increases with increasing of the pulse width. N${{}_2}^{+}$ and O${}_2^{+}$ are the most important positive ions, whereas O^? is the most populated negative ion. For a single microwave pulse, the electron number density is large up to 1 μs, and the dissociation and excitation continue to increase the small radicals and excited species. Then the electron number density drops and the population of excited species declines. The ozone production becomes important after 1 μs when the three body association of O and O₂ dominates over the dissociation processes. The ozone number density continues to grow up to 5 ms, and then saturates at a value of 10²² m^?3. Quenching of electronically excited nitrogen molecules by O₂ molecules and the subsequent dissociation to atomic oxygen and generation of NO, are found to be important and can play a significant role in the ultrafast gas heating.     

Characteristics of a nanosecond discharge in atmospheric air with a liquid electrolytic electrode

The spatial, electric, and radiative characteristics of a pulse-periodic nanosecond discharge between an electrode based on a system of blades and the distilled water surface as well as an aqueous solution of zinc sulfate in atmospheric air are considered.     

Soft X-ray radiation due to a nanosecond diffuse discharge in atmospheric-pressure air

A source of soft X-rays with an effective photon energy of 9 keV and a subnanosecond pulse width is built around a gas diode filled with atmospheric-pressure air and a UAEB-150 generator. A collector placed behind a grounded mesh electrode detects an electron beam and a pulse with positive polarity, the latter being due to an electric field surrounding the mesh. It is shown that the intensity of soft X-rays from the gas-diode-based source depends on the material of a massive potential anode; namely, it grows with an increase in the atomic number of the cathode material. In the case of a tantalum anode, X-ray photons with an effective energy of 9 and 17 keV contribute to the exposure dose.     

Beam electron energy distribution at a volume nanosecond discharge in atmospheric-pressure air

The energy distributions of beam electrons and x-ray photons in a volume nanosecond discharge on atmospheric-pressure air are studied. Several groups of elevated-energy electrons are found. It is shown that electrons with an energy from several tens to several hundreds of kiloelectronvolts (which is lower than a maximal voltage across the gap) make a major contribution to the beam current measured behind thin foils. It is corroborated that fast electrons (with an energy from several kiloelectronvolts to several tens of kiloelectron-volts) arise 100–150 ps before the basic peak of the beam current, elongating the current pulse and significantly increasing its amplitude. The contribution from electrons with an anomalously high energy (exceeding a maximal voltage across the gap) to the beam current is shown to be insignificant (less than 5%). The x-ray spectra in gas-filled diodes of different design are studied. Techniques of measuring the subnanosecond electron beam current and mechanisms generating fast and runaway electrons in volume high-pressure gas discharges are analyzed.     

石英管对空气中锥-板结构纳秒脉冲放电的影响

报道了石英管对大气压下空气中锥-板电极结构纳秒脉冲放电的影响实验研究.实验过程中发现将石英管纵向移动位置,放电会出现四种明显不同的形貌状态,对其机理进行了初步的探讨.利用发射光谱诊断了其转动和振动温度(分别为295和2200 K),表明该等离子体具有高度非平衡度.还进一步提出大气压下空气中产生类辉光放电的有效途径. 关键词： 纳秒脉冲放电 空气 纵向介质 非平衡等离子体     

Emission characteristics of the cathode region of nanosecond discharge in atmospheric-pressure air

The emission spectra of a nanosecond discharge between copper electrodes in atmospheric-pressure air are studied at a high discharge gap overvoltage. The discharge was ignited between two cylindrical electrodes with a small radius of curvature of the working surface. Oscillograms of radiation pulses of spectral lines of copper atoms are also studied. The electron temperature averaged over the pulse is determined from the intensity distributions of the spectral lines of copper atoms.     

X-ray radiation and runaway electron beam spectra at a nanosecond discharge in atmospheric-pressure air

The dependences of the electron beam intensity and X-ray dose on the thickness of metal foils (Al, Cu) in a nanosecond discharge initiated in atmospheric-pressure air are studied theoretically and experimentally. Calculated curves of electron beam attenuation in aluminum and X-ray dose attenuation in copper agree well with experimental data. It is found that the amplitude of a super-short avalanche electron beam and the X-ray exposure dose reach maximal values at different values of the interelectrode gap. When the length of the cathode??s edge with a small radius of curvature increases, an interelectrode gap maximizing the amplitude of the runaway electron current shrinks.     

Pulsed microwave discharge in atmospheric air in the focus of a two-mirror resonator

Free-localized pulsed microwave discharge in atmospheric air in the focus of an open two-mirror high-Q resonator excited by linearly polarized electromagnetic radiation with a wavelength of 4.3 cm is described. This discharge is analogous to the previously studied streamer resonance microwave discharge ignited under similar conditions but with an electromagnetic radiation wavelength of 8.9 cm. Starting from a certain overcritical electric field, the discharge plasma channel has a high-temperature core.     

Impacts of air pressure on the evolution of nanosecond pulse discharge products

Based on the nonequilibrium plasma dynamics of air discharge, a dynamic model of zero-dimensional plasma is established by combining the component density equation, the Boltzmann equation, and the energy transfer equation. The evolution properties of nanosecond pulse discharge (NPD) plasma under different air pressures are calculated. The results show that the air pressure has significant impacts on the NPD products and the peak values of particle number density for particles such as O atoms, O3 molecules, N2(A3) molecules in excited states, and NO molecules. It increases at first and then decreases with the increase of air pressure. On the other hand, the peak values of particle number density for N2(B3) and N2(C3) molecules in excited states are only slightly affected by the air pressure.     

Spectra of electrons and X-ray photons in a diffusive nanosecond discharge in air under atmospheric pressure

The spectra of electrons and X-ray photons generated in nanosecond discharges in air under atmospheric pressure are investigated theoretically and experimentally. Data for the discharge formation dynamics in a nonuniform electric field are gathered. It is confirmed that voltage pulses with an amplitude of more than 100 kV and a rise time of 1 ns or less causing breakdown of an electrode gap with a small-radius cathode generate runaway electrons, which can be divided into three groups in energy (their energy varies from several kiloelectronvolts to several hundreds of kiloelectronvolts). It is also borne out that the formation of the space charge is due to electrons appearing in the gap at the cathode and a major contribution to the electron beam behind the foil comes from electrons of the second group, the maximal energy of which roughly corresponds to the voltage across the gap during electron beam generation. X-ray radiation from the gas-filled diode results from beam electron slowdown both in the anode and in the gap. It is shown that the amount of group-3 electrons with an energy above the energy gained by runaway electrons (in the absence of losses) at a maximal voltage across the gap is much smaller than the amount of group-2 electrons.     

Thermal ionization instability of an air discharge plasma in a microwave field

Results are presented from experimental studies of the initial stage of an air discharge initiated in a linearly polarized quasi-optical microwave beam. The discharge was excited at an air pressure at which the electron-neutral collision frequency in the discharge plasma was considerably higher than the circular frequency of the electromagnetic field and at a microwave field amplitude close to the threshold field for air breakdown. The experiments revealed relatively bright plasma channels stretched along the microwave electric field. The development rate of these channels and their characteristic transverse dimensions are estimated. A comparison of the experimental data and theoretical estimates indicates that the channels observed arise due to the onset of thermal ionization instability in the microwave discharge plasma.     

Energy distribution of runaway and fast electrons upon nanosecond volume discharge in atmospheric-pressure air

Nanosecond space discharge in a gas-filled diode is promising for pumping of lasers and high-power lamps. The space charge formed in the absence of an additional preionization source has a few advantages. The energy distributions of the beam electrons and the X-ray spectrum are determined. It is demonstrated that several high-energy electron bunches are formed in such a discharge. The main contribution to the beam current measured behind the foil is related to the runaway electrons, which have energies of tens or hundreds of kiloelectronvolts (supershort avalanche electron beam (SAEB)). Fast electrons with energies of several or tens of kiloelectronvolts are responsible for the generation of the soft X rays in the discharge gap. Anomalous electrons whose energy is higher than the voltage across the gap provide for a minor (less than 5%) contribution to the beam current. The generation time of these electrons is equal to the SAEB generation time accurate to 0.1 ns. It is demonstrated that the anomalous electrons can be generated owing to the acceleration in the presence of the field in front of the moving background-electron multiplication wave. The spectra of the X-ray radiation generated by the fast electrons in the volume are calculated.     

Electric discharge in air in a deeply subcritical field of a quasi-optical microwave beam

We describe the results of experiments on initiation of an electric discharge in air in a quasi-optical microwave beam by an electromagnetic vibrator fixed above the screen. This method for initiating the electrical breakdown makes it possible to obtain a discharge at a level of the electric field component in the microwave, which two orders of magnitude lower than the minimal critical field of the electrodeless breakdown of air. In our experiments, the threshold value of the air pressure determining the low- and high-temperature forms of the microwave discharge are determined depending on the field level.     

Development of nanosecond combined volume discharge with plasma electrodes in an air flow

The space-time characteristics of a nanosecond combined volume discharge with preionization from a plasma sheet with a nanosecond duration in air (～200 ns) are investigated. The integral discharge radiation, radiation spectrum, and discharge current under conditions within the discharge volume, including gas-dynamic flow with a planar shock wave, are analyzed. It is shown that the volume discharge glow is homogeneous in the master phase. The glow in the area of the shock-wave front increases and its duration may be more than 2 μs.     

Investigation of the redistribution of the current in a nanosecond volume discharge after the appearance of a cathode spot

An investigation was made of the process of contraction of a nanosecond volume discharge in air with a homogeneous field distribution and a current density 500 A/cm² when the gap was 0.7 cm and the pressure 76 torr. It was found that the appearance of a cathode spot in this volume discharge resulted in an effective contraction of the current into a spot and formation of a diffuse channel. A high-conductivity channel emerging from the cathode spot appeared when the current density in the diffusion channel above this spot reached (1–3)·10⁵ A/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 16–18, January, 1981.     

Microstructure of the current channels in a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields

The microstructure of a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields is investigated. It is found that an 0.1-to 0.4-mm spark channel consists of a large number (from 100 to 1000) of 5-to 10-μm-diameter microchannels distributed nearly uniformly over the channel cross section. The current amplitude in the spark is 1.5–3 kA, and the current density in a microchannel is 10⁷ A/cm². It is shown that the formation of the microstructure cannot be attributed to ionization-heating instability. The instability of the ionization wave front is suggested as a mechanism for the microstructure formation.