Vol 60, No 8 (2017)

The data on the measured static breakdown voltages and pre-breakdown currents flowing in the main gap of a two-section, sealed-off thyratron with a cold-cathode TPI1-10k/50 are presented. It is shown that pre-breakdown current in the lower section appears at lower voltages than in the upper section. A few methods are proposed for increasing the thyratron breakdown voltage via re-distribution of the voltage between the sections using capacitive and resistive voltage dividers. It is demonstrated that either the self-capacitance of one of the sections or the resistance due to the pre-breakdown current flow could serve as one of the arms of the divider.

The method of magnetron sputtering was used to produce a-C and a-C:Ni films on substrates of monocrystalline silicon and thermoelectric material of n-type ((Bi₂Te₃)_0.94(Bi₂Se₃)_0.06) and p-type ((Bi₂Te₃)_0.20(Sb₂Te₃)_0.80) conductivity. The authors studied the effect of Ni concentration on specific electric resistance, hardness and adhesion of the produced films. It was demonstrated that specific resistance of a-C films deposited by graphite target sputtering when supplying high bias voltage onto the substrate can be reduced by increasing the share of graphitized carbon. Adding Ni to such films allows additionally reducing their specific resistance. The increase in Ni content is accompanied with the decrease in hardness and adhesion of a-C:Ni films. The acquired values of specific electric resistance and adhesion of a-C:Ni films to thermoelectric materials allow using them as barrier anti-diffusion coatings of thermoelectric modules.

Results of investigations of UV and VUV radiation sources (excilamps) based on inert gases and mixtures of inert gases with halides are presented. Conditions are described in which maximum radiation parameters are attained. Photoreactors and irradiators promising for application in medicine, agriculture, and gas industry have been developed based on the excilamps.

The experimental investigations of a subnanosecond breakdown initiated by runaway electrons in air and nitrogen at the pressures within 0.013–0.4 MPa are performed. The temporal development patterns of the discharge plasma glow in different regions of the discharge gap are registered with a fast photodiode, a four-channel ICCD-camera, and an ultrafast streak camera. It is shown that the breakdown occurs in the form of ionization waves propagating from the electrode with a small radius of curvature. It is found that a runaway electron beam behind the anode foil at the nitrogen and air pressure ~0.1 MPa is detected with the collector at the point of time corresponding to the maximum gap voltage.

Frequency-domain numerical modeling is performed and disc-shaped wide-aperture absorbing loads of calorimeters with a working liquid based on ethyl alcohol, which are designed to measure the energy of highpower microwave pulses, are optimized using the finite element method and the Nelder–Mead gradientless procedure. The possibility of using caprolon and polycarbonate as a load casing material with a corrugated inlet window together with low-pressure high-density polyethylene is investigated. The results of calculations significantly refined the results obtained earlier using a computational technique based on the finite-difference time-domain method for a simplified model of the absorbing load.

Highly effective mode of generation of a relativistic microwave oscillator of twistron type is calculated by numerical simulation using an axisymmetric version of completely electromagnetic PiC-code KARAT. With accelerating voltage of 180 kV, electron beam current of 1.2 kA, and 1.9 T guiding magnetic field strength, the simulated stationary generation power is 120 MW at an operating frequency of 10.7 GHz. The efficiency of electron beam power conversion to electromagnetic radiation is 55%, and the electronic efficiency of the oscillator calculated from the change of the initial electron energy is 64%.

Results of experiments on the study of dynamics of an overvoltage discharge at the low pressure р = 0.5–2.5 Torr up to its transition to the high-current low-voltage regime are presented, and the instability mechanism leading to a sharp voltage drop across the discharge is suggested.

The results of studies on the development of laser sources in the UV and IR spectral ranges for the lidar systems operating on the basis of a method of laser fragmentation with subsequent laser-induced fluorescence (LF/LIF) are presented. It is shown that LF/LIF is an effective method for the remote detection of harmful and dangerous substances including nitrogen oxides in the atmosphere. The formation of a high-quality sounding UV radiation in excimer lasers and its transportation along the atmospheric path is studied. The causes are revealed that lead to the formation of noise arising in the atmosphere and limiting the sensitivity of the applied method.

This paper is focused on a clarification and analysis of the regularities of formation of the structure and properties of samples of the titanium-based alloy VT6, obtained by methods of conventional metallurgy and formed by layered selective electron-beam sintering in vacuum (using the Arcam A2X (3D printer) system (Arcam, Sweden)) of VT6 titanium powder with particle size 40–100 μm. Additional modification of the samples was realized by irradiating the surface with an intense pulsed electron beam (15 keV, 45 J/cm², 200 μs, 10 pulses, 0.3 s^–1, 3.5·10^–2 Pa). It is shown that the action of a pulsed electron beam on the surface of samples formed by layered selective electron-beam sintering leads to a significant reduction in the porosity of the surface layer of the material and formation in the surface layer of a polycrystalline structure (grain size 15–60 μm) with a substructure in the form of crystallization cells (cell size 0.5–1.2 μm). Electron-beam processing of samples prepared by methods of conventional metallurgy for the indicated electron-beam parameters leads to the formation in the surface layer of a polycrystalline structure (grain size 50–800 μm) with a laminar intragrain substructure. Mechanical tests, performed by stretching flat samples, showed that the highest combination of mechanical strength and plasticity is possessed by samples obtained by layered selective electron-beam sintering with subsequent irradiation by an intense pulsed electron beam.

The electrodynamic characteristics of a biperiodic slow-wave waveguide have been investigated numerically. Conditions have been found for the existence of an anomalous dispersion of the TM₀₁ mode at frequencies approaching the π-type oscillation frequencies. The slow TM₀₁ modes propagating in a periodic and in a biperiodic waveguide have been compared in field structure. A method for calculating the eigenfields (resonances) in a biperiodic waveguide section of finite length is proposed.

Generation of high charge state metal ions in arc plasma is investigated and methods of elevation of the average ion charge state are analyzed. To realize multiple ionization of the arc plasma, a strong magnetic field in the arc cathode area, an ark current spike, an injection of a high-current electron beam or a high-power microwave radiation flow into the plasma, and a creation of a short high-current arc are realized. For each method, the main features are considered and the maximum ion charge states are determined.

The paper deals with the soft X-ray backlighting based on the X-pinch as a powerful tool for physical studies of fast processes. Proposed are the unique small-size pulsed power generators operating as a low-inductance capacitor bank. These pulse generators provide the X-pinch-based soft X-ray source (hν = 1–10 keV) of micron size at 2–3 ns pulse duration. The small size and weight of pulse generators allow them to be transported to any laboratory for conducting X-ray backlighting of test objects with micron space resolution and nanosecond exposure time. These generators also allow creating synchronized multi-frame radiographic complexes with frame delay variation in a broad range.

Characteristics of Z-pinch plasma radiation in the form of a double shell neon gas puff with outer plasma shell are investigated in the microsecond implosion mode. Experiments are performed using a GIT-12 mega-joule generator with load current doubler having a ferromagnetic core at implosion currents up to 5 MA. Conditions for matching of the nonlinear load with the mega-ampere current multiplier circuit are determined. The load parameters (plasma shell characteristics and mass and geometry of gas puff shells) are optimized on the energy supplied to the gas puff and n energy characteristics of radiation. It is established that the best modes of K-shell radiation in neon are realized for such radial distribution of the gas-puff material at which the compression velocity of the shell is close to a constant and amounts to 27–30 cm/μs. In these modes, up to 40% of energy supplied to the gas puff is converted into K-shell radiation. The reasons limiting the efficiency of the radiation source with increasing implosion current are analyzed. A modernized version of the energy supply from the current doubler to the Z-pinch is proposed.

Results of a study of the short-pulse dielectric strength of millimeter plane vacuum gaps with electrodes that have been treated with an electron beam are presented. It is shown that the electric field strength of the first breakdown of vacuum gaps with pure metal electrodes is determined to a significant extent by the crystal structure of the metal. The development of the first short-pulse breakdown is accompanied by a very abrupt growth of the electric current. The short duration of the test pulses rules out the influence of all well-known inertial mechanisms of breakdown with characteristic action times greater than 20 ns. Some general assumptions regarding the nature of the factors stimulating the short-pulse breakdown of vacuum gaps are considered.

The paper deals with the formation of the stainless steel-copper surface alloy which occurs during the single vacuum cycle. Deposition of the stainless steel film onto a copper substrate is performed via successive magnetron sputtering followed by its liquid-phase mixing with copper using the low-energy, high-current electron beam of microsecond length. Numerical calculations are used to identify the optimum irradiation modes for the surface alloying. It is found that the optimum irradiation modes provide the homogeneous surface alloying, and the copper concentration in the obtained layer increases with the increase in energy density of the low-energy, high-current electron beam. The formation of the stainless steel-copper surface alloy increases the wear resistance of copper specimens by 7.5 times as compared to the original specimens. In addition, it is ascertained that the wear resistance of the surface alloy exceeds that of the common film coating (1 μm) more than 5 times.