Ignition of hydrocarbon films under the conditions of surface microwave discharge

In the present work, rapid plasma-stimulated ignition of liquid hydrocarbons was carried out in still air under conditions of surface microwave discharge. It was shown that, depending on the microwave input power, the breakdown time changed in a range from 5 to 30μs, ignition occurred on an antenna in the area of the surface microwave discharge at a temperature not exceeding 1000 K, and the speed of the front boundary propagation of the intense ignition region near the antenna was 300 m s^?1.     

Study of spatial characteristics of the incomplete surface discharge in atmospheric-pressure air

Spectra of plasma of the incomplete surface discharge in atmospheric-pressure air were measured. Bands of the systems 2⁺, 1⁺, and 1⁻ of nitrogen were identified. It was shown that periodic excitation at the C ³Π _u nitrogen level results in local equilibrium characterized by a vibrational temperature of 2250 K. For the 1⁺ band system of nitrogen, the structure of rotational transitions was resolved; it was found that the distribution in the 1⁺ band system is significantly nonequilibrium due to the A ³Σ _u ⁺ level metastability. Transverse and longitudinal sections of the plasma band were scanned. It was found that the transverse distribution of the emission intensity has a maximum at a distance of 1 mm from the electrode edge followed by an exponential decay. The plasma band width was ∼5 mm. The longitudinal discharge structure consisted of a system of microchannels (with a characteristic diameter of 0.4 mm) with diffuse overlapping. The average channel density was 10–15 cm⁻¹. It was shown that the spatial distribution of incomplete discharge plasma is characterized by high stability against wide-range variations of discharge electrical characteristics. For example, as the excitation voltage varies within 2–6 kV, the plasma band width changed by no more than 1 mm, and the vibrational temperature varied within 10–12%.     

Ignition of thin liquid hydrocarbon films via surface microwave discharge generated in a paired-pulse mode

The process of ignition and combustion of liquid hydrocarbon films under the conditions of a self-maintained surface microwave discharge in still air that is generated on a dielectric antenna in the paired pulse mode is investigated. The effect of the medium pre-excitation, which occurs during the first pulse on the period of induction and intensity of hydrocarbon combustion during the second microwave pulse, is demonstrated.     

X-ray radiation from the volume discharge in atmospheric-pressure air

X-ray radiation from the volume discharge in atmospheric-pressure air is studied under the conditions when the voltage pulse rise time varies from 0.5 to 100 ns and the open-circuit voltage amplitude of the generator varies from 20 to 750 kV. It is shown that a volume discharge from a needle-like cathode forms at a relatively wide voltage pulse (to ≈60 ns in this work). The volume character of the discharge is due to preionization by fast electrons, which arise when the electric field concentrates at the cathode and in the discharge gap. As the voltage pulse rise time grows, X-ray radiation comes largely from the discharge gap in accordance with previous experiments. Propagation of fast avalanche electrons in nitrogen subjected to a nonuniform unsteady electric field is simulated. It is demonstrated that the amount of hard X-ray photons grows not only with increasing voltage amplitude but also with shortening pulse rise time.     

Optico-spectroscopic diagnostics of large-scale plasma clouds created above A surface under the action of A single-pulse laser in atmospheric-pressure air

To whom correspondence should be addressd.     

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.     

Source of an atmospheric-pressure plasma jet formed in air or nitrogen under barrier discharge excitation

We analyzed the atmospheric pressure plasma jet excited in air and nitrogen by a barrier discharge. The source forming stable plasma jets of length up to 4 cm in air and nitrogen is constructed, and its energy and spectral characteristics are measured.     

High-temperature dynamics of surface magnetism in iron thin films

Finite-temperature magnetic properties of iron thin films are investigated by computer simulation over a broad range of temperatures up to the point of the ferromagnetic–paramagnetic phase transition. The coupled dynamics of atoms and magnetic moments is treated using the large-scale spin–lattice dynamics (SLD) algorithm. We investigate surface and bulk magnetic properties of iron, and how these properties vary as a function of temperature, film thickness and surface crystallography. We find that magnetization at surfaces is enhanced at low temperatures and suppressed at higher temperatures, in agreement with experimental observations. The effective Curie temperature of a film decreases as a function of thickness. Short-range magnetic order and non-vanishing spin–spin spatial correlations are found above the Curie temperature. The spin autocorrelation functions exhibit slower oscillations with longer decoherence times near the surface. We also find that the directional spin disorder has a significant effect on the surface strain.     

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.     

120 kV下常压空气纳秒脉冲电晕放电特性

使用上升沿15 ns、脉宽30~40 ns的重复频率纳秒脉冲电源对120 kV下大气压空气中管-板电极结构电晕放电进行了实验研究,通过电压电流测量、放电图像拍摄和X射线探测分析了纳秒脉冲电晕放电特性。结果表明:纳秒脉冲电晕放电中存在X射线辐射,但辐射强度较弱,X射线辐射计数随着气隙距离的增大而减少,随着脉冲重复频率的增大而增多;放电空间的残余电荷加强了下一个脉冲到来时的局部电场,从而导致高重复频率下易于出现分散的电晕通道。     

Microstructure of the current channel of an atmospheric-pressure diffuse discharge in a rod-plane air gap

The multichannel structure of the current channel of an atmospheric-pressure diffuse discharge excited in a 10-cm rod-plane air gap was investigated using the imprint technique. A voltage pulse with an amplitude of 240 kV, a duration of 180 ns, and a rise time of 14 ns was applied to a 1-cm-diameter bullet-shaped cathode with a tip curvature radius of 0.2 mm; the discharge current reached 350 A. It is found that the diameter of the discharge channel in the anode plane varies in the range 2.5–9.7 mm from shot to shot. The overall imprint of the current channel is formed of 170–9500 imprints of microchannels with an average diameter of 5–20 μm. The parameters of the observed microstructure do not correlate with variations in the main electric characteristics of the discharge and the parameters of the generated X-ray pulse. It is shown that the formation of the microstructure is related to the onset of cathode-directed plasma structures developing from the anode. It is suggested that the same mechanism is responsible for both the formation of the current channels structure and the anode microstructure of diffuse nanosecond discharges developing in atmospheric-pressure air gaps with a highly nonuniform electric field.     

120 kV下常压空气纳秒脉冲电晕放电特性

使用上升沿15 ns、脉宽30~40 ns的重复频率纳秒脉冲电源对120 kV下大气压空气中管-板电极结构电晕放电进行了实验研究,通过电压电流测量、放电图像拍摄和X射线探测分析了纳秒脉冲电晕放电特性。结果表明：纳秒脉冲电晕放电中存在X射线辐射,但辐射强度较弱,X射线辐射计数随着气隙距离的增大而减少,随着脉冲重复频率的增大而增多;放电空间的残余电荷加强了下一个脉冲到来时的局部电场,从而导致高重复频率下易于出现分散的电晕通道。     

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.     

Peak effect in surface resistance at microwave frequencies in Dy-123 thin films

A pronounced peak in the microwave (at frequency 9.55 GHz) surface resistance, R _s vs. T plot (where T is the temperature) has been observed in epitaxial DyBa₂Cu₃O_7−y superconducting thin films in magnetic fields (parallel to c-axis) in the range 2 to 8 kOe, and temperatures close to the superconducting transition temperature T _c(H). Our data suggest that the nature of peaks observed in the two films is different, thereby indicating different defect structures in the films.     

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.     

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.     

Present status of thin oxide films creation in a microwave plasma

This paper summarizes present knowledge of the creation of thin films in isotropic and magnetoactive plasma. It analyses conditions under which films in the microwave plasma can be created and shows how the growth rate and properties of films depend on microparameters of plasma. On the basis of plasma floating potential measurements it is shown why the creation of thin films in microwave discharges takes place at high electron plasma densities (N 10¹² cm^–3) only. Besides this it describes properties of created films, underlies negative role of fast electrons in forming of good quality films and gives recommendation how to avoid their generation. Considerable attention is devoted also to a comparison of films creation in pulse and continuous plasma. At the end possibility of films creation at low temperatures by a deposition technique utilizing a microwave excitation of molecular gases is given.The authors gratefully acknowledge many stimulating and fruitful discussions with Dr. F. áek of Institute of Plasma Physics of Czechoslovak Academy of Sciences. We also thank Dr. V. Malina of Institute of Radioelectronics of Czechoslovak Academy of Sciences for measurements of C—V curves of SiO₂ films.     

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.     

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.