Pulse-periodic Regime Research Articles

In Situ Analyses of Surface-Layer Composition of CxNy Thin Films Using Methods Based on Penning Ionization Processes-Introductory Investigations.

Carbon nitride materials have received much attention due to their excellent tribological, mechanical and optical properties. It was found that these qualities depend on the N/C ratio; therefore, the possibility to control it in situ in the sputtered film is of high importance. The plasma-electron spectroscopy method based on the Penning ionization process analysis is developed here to control this ratio in CNx films produced by plasma-sputtering in a pulsed-periodic regime of glow discharge. The electron energy distribution function is determined by the means of a single Langmuir probe placed in the center of the discharge tube. The mixture N2:CH4:He was used in the process of sputtering. The applied concentrations of CH4 varied in the range of 2–8%, and He concentration was 80–90%. The gas pressure in the discharge tube used for sputtering varied between 1 and 10 Torr, and the current was between 10 and 50 mA. It was shown that the proposed method enables the extraction of information on the composition of the surface layer of the investigated film and the development of an on-line inspection, without extracting the film from the sputtering chamber.

Effect of the Erosion of Collector Surface on the Operation of a Pulse-Periodic Plasma Relativistic Microwave Generator

The pulse-periodic operation regime of a plasma relativistic microwave generator is considered. The reasons for the increase of average frequency of the output microwave radiation in a pulse train created in the same conditions of initial plasma generation are analyzed. The effect of solid body erosion that occurs due to the interaction of the relativistic electron beam with the collector surface on the output microwave radiation is studied. The analysis of experimental data is accompanied by numerical simulations by the electromagnetic PIC code KARAT. It is shown that increasing the amount of mater extracted from the surface of the collector during erosion leads not only to an increase of the plasma density, but it can also lead to qualitative changes of the parameters of the output radiation, which are caused by the change of microwave generation regime. In particular, the generation of an ion background that is inhomogeneous in the radial direction caused a change of conditions of plasma–beam interaction during the second half of the pulse and a change of the spectral density of radiation at the main frequency. Numerical simulations were used to demonstrate this for two different admixtures, iron and graphite.

Control of the Formation of a Transonic Region in an Axisymmetric Supersonic Flow with the Use of a Jet and a Near-Wall Energy Supply

The control action of a throttling air jet produced by a gas generator and of the energy sources positioned at the walls of an axisymmetric channel, which models the combustion-chamber unit of a ramjet in which a gas flow is decelerated, on the shock-wave structure of a supersonic gas flow in the channel was investigated for the purpose of formation of a lengthy transonic region in it. A heat energy was supplied to the gas flow in the indicated channel in the pulse-periodic regime, and the pressure in the gas generator was also changed periodically. The range of energies corresponding to a stable transonic flow regime in the channel was determined on the basis of solution of the nonstationary Euler equations in dimensionless variables. For comparison, the combustion of a gaseous fuel inflowing to the channel through a slot in the channel wall upstream of the control air jet was considered. A nonstationary gas flow in the channel was calculated on the basis of the Reynolds-averaged Navier–Stokes equations with the use of the k–ω SST model of turbulence. It is shown that a controllable energy release in the channel, providing the obtaining of a transonic flow regime in it analogous to that obtained on the basis of solution of the Euler equations, can be realized with the use of a control transverse air jet.

Measurements of Microwave Power Absorbed during ECR Plasma Heating at the L-2M Stellarator

The results are presented from measurements of the power absorbed in plasma of the L-2M stellarator in experiments on the electron cyclotron resonance (ECR) plasma heating at the second harmonic of the gyrofrequency. Hydrogen plasma was created and heated in the stellarator vacuum chamber as a result of the resonance absorption of microwave power supplied by the gyrotrons in the pulse-periodic operation regime. The total energy of the toroidal plasma column and the absorbed power were measured using the diamagnetic diagnostics. The shielding effect of the metal vacuum chamber on the diamagnetic signal measurements was taken into account. It was ascertained that, during the axial ECR heating, up to 90% of the inputted gyrotron power is absorbed in the plasma that is consistent with the theoretical estimates.

Dependence of induction copper vapor laser radiation on the parameters of RF discharge and the optical cavity

New numerical simulation results are presented for the operation of a copper vapor laser pumped by an induction (electrodeless) RF discharge in a pulse-periodic regime. The influence of optical cavity parameters on the radiation pulse profile and energy characteristics of the output emission of such a laser is studied. Specific features of the laser emission are considered in the undervoltage regime, and low pulse repetition frequencies are considered. Note that such a laser has not been implemented experimentally yet. Hence, the numerical simulation is actual from the viewpoint of estimating the prospects for practical applications.

Modeling of the Structure of Heat-Insulating Semi-Transparent Materials and Coatings for Industrial and Transport Power Plants

The influence of optical characteristics of semitransparent thermal barrier coatings (TBC) on thermal regulation of heat-stressed elements of power plants is studied. There was used the developed by the authors methodology of physical and mathematical simulation of thermoradiational (in the range 1-2 μm) and conductive heat transmission in ceramic thermal insulation of chamber combustion inner walls of a diesel engine. The paper discusses temporal temperature regimes in model two-layer TBC-coatings in the form of a selectively scattering and absorbing (in the near infrared range) ceramic heat-insulating layer, deposited on the bonding sublayer (with boundary reflection) of the internal surface of the combustion chamber of Low Heat Rejection diesel. Spectrophotometric measurements of the optical parameters have ensured estimates of optical and thermal fields of the investigated ceramics, the structural composition of which was determined on the basis of ZrO2+8%Y2O3. For typical values of radiant-convective flux up to 1-2 MW/m2 (effecting on heat-stressed elements of heat-insulated combustion chamber of a diesel engine in pulse-periodic regime) optical and structural parameters TBC-coatings that ensure control and superintendence of the ceramic layer surface temperature and its temperature gradient were proposed.

Nanosecond room-temperature Fe : ZnSe laser pumped inside the resonator of a transversely diode-pumped Er : YLF laser

Lasing at room temperature is obtained in a Fe : ZnSe crystal installed as a passive Q-switch in a diode-pumped Er : YLF laser resonator. A miniresonator of the Fe : ZnSe laser is formed by mirrors pressed against the optical surfaces of the crystal with high reflectivities at λ = 4 − 4.5 μm and a high transmittance for the Er : YLF laser generation lines. In the Er : YLF laser resonator, short pulses of ∼100 ns duration at λ = 2.65 μm are generated, stipulated by passive Q-switching of the resonator by the Fe : ZnSe crystal. The absorption of the Er : YLF laser radiation in the Fe : ZnSe crystal leads to the pumping of Fe : ZnSe laser, which generates pulses with a duration of less than 50 ns and an energy of ∼0.5 μJ at λ = 4.2 μm. The laser operates in the pulse-periodic pump regime with a repetition rate up to 200 Hz.

Matrix multi-pass scheme disk amplifier

Two multi-pass optical schemes of a disk laser amplifier have been proposed. Different variants of both the schemes with the smallest amount of optical elements and a lens in the active element taken into account have been calculated. For 64 passes of radiation through the active element, the average power of ∼50 W with ∼10% optical-to-optical efficiency in a pulse-periodic regime with a repetition rate of 10kHz and a pulse duration of 2ns was obtained at the amplifier output. The small signal gain amounted to 200.

Pulse-Periodic Regimes of Kinetic Instabilities in the Non-Equilibrium Plasma of an Electron Cyclotron Resonance Discharge Maintained by Continuous-Wave Radiation of a 24 GHz Gyrotron

We have experimentally discovered an instability, which manifests itself as precipitations of hot electrons occurring synchronously with generation of bursts of electromagnetic radiation, in the plasma of an electron cyclotron resonance discharge maintained by a high-power, continuous-wave radiation of a 24 GHz gyrotron, for the first time. The observed instability has the kinetic nature and is determined by the formation of the non-equilibrium velocity distribution of hot particles. Two possible explanations are proposed for the mechanism of wave excitation in a two-component plasma with a stationary source of non-equilibrium particles. The results of the studies performed are of interest for modeling of the dynamics of magnetospheric cyclotron masers.

High-Power Ka-Band Transmission Line with a Frequency Bandwidth of 1 GHZ

We present experimental results on a high-power transmission line from the broadband pulsed Ka-band gyro-TWT to the phased antenna array. The transmission line is designed to operate in a pulse-periodic regime with a pulse width of up to 250 μs, a duty factor of 8, and an average output power of up to 15 kW. Amplitude–frequency and phase–frequency characteristics of the transmission line were measured at a low power level. It is shown that the nonlinearity of the phase–frequency characteristic does not exceed ±10° in the 34 ± 0.5 GHz frequency band.

Effect of gas heating on the generation of an ultrashort avalanche electron beam in the pulse-periodic regime

The generation of an ultrashort avalanche electron beam (UAEB) in nitrogen in the pulse-periodic regime is investigated. The gas temperature in the discharge gap of the atmospheric-pressure nitrogen is measured from the intensity distribution of unresolved rotational transitions (C3Πu, v′ = 0) → (B3Πg, v″ = 0) in the nitrogen molecule for an excitation pulse repetition rate of 2 kHz. It is shown that an increase in the UAEB current amplitude in the pulse-periodic regime is due to gas heating by a series of previous pulses, which leads to an increase in the reduced electric field strength as a result of a decrease in the gas density in the zone of the discharge formation. It is found that in the pulse-periodic regime and the formation of the diffuse discharge, the number of electrons in the beam increases by several times for a nitrogen pressure of 9 × 103 Pa. The dependences of the number of electrons in the UAEB on the time of operation of the generator are considered.

Pulse-Periodic Regime of Kinetic Instability of the ECR Discharge Plasma Under the Conditions of Double Plasma Resonance

UDC 533.9.01 We study the stability of a dense nonequilibrium plasma of the electron cyclotron resonance (ECR) discharge in an open magnetic trap immediately after the end of heating. The observed instability is accompanied by pulse-periodic generation of high-power electromagnetic radiation at a frequency that is close to the frequency of the upper hybrid resonance and the double gyrofrequency of electrons and by synchronous precipitations of fast electrons from the trap. It is shown that the observed instability is connected with excitation of plasma waves under the conditions of double plasma resonance in the decaying plasma of the ECR discharge.

Optical limiting of high-repetition-rate laser pulses by carbon nanofibers suspended in polydimethylsiloxane

The optical limiting (OL) behavior of carbon nanofibers (CNFs) in polydimethylsiloxane (PDMS) was studied and compared with that of CNFs in water, and polyhedral multi-shell fullerene-like nanostructures (PMFNs) also in water. It was shown that when switching from single-shot to pulse-periodic regime of laser pulses (10 Hz), the CNF in PDMS suspension retains its OL characteristics, while in the aqueous suspensions, considerable degradation of OL characteristics is observed. It was also observed that a powerful laser pulse causes the CNF in PDMS suspension to become opaque for at least three seconds, while such a pulse brings out a bleaching effect in aqueous PMFN and CNF suspensions. The processes of OL degradation in aqueous suspensions, bleaching and darkening of the studied materials are discussed herein.

Generation of high-voltage pulses with subnanosecond front rise times in open discharge

The investigation results for plasma switching devices of high-voltage pulses with pulse rise times less than 1 ns are presented. The approach is based on using conditions suitable for bringing a gas discharge chamber in a state with high conductivity due to generation of an electron beam owing to photoelectron emission from the device cathode. It is shown that in co-axial geometry pulses, switching time 0.45 ns on an active load RL = 50 Ω at voltage U = 20 kV can be achieved. It is shown with the method of doubled impulses that such a device can regenerate the acceptable electric strength during 10 μs. It is indicated of the principle possibility of working in the pulse-periodical regime to the repetition rate of 100 kHz.

Stability and optical limiting properties of a single wall carbon nanotubes dispersion in a binary water-glycerol solvent

We report the temporal stability of the dispersion of single-wall carbon nanotubes in a binary solvent “water + glycerol” having eutectic composition (ca. 67% wt.) with sodium dodecylbenzene sulfonate as a dispersant. The system procures good temporal and thermal stability: its absorption spectra demonstrate no changes during one-year storage with temperature spanning −40 to +40 °C. The system provides non-linear optical power limiting of the incident laser radiation (532 nm) in a one-shot and pulse-periodic regimes of its applying.

Synthesis of superhard nanodispersed titanium compounds in a magnetoplasma accelerator operating in pulse-periodic regime

It is demonstrated that a system based on a coaxial magnetoplasma accelerator with multiple use of the titanium barrel for a consumable provides a dynamic synthesis of nanodispersed titanium-based crystal phases such as cubic titanium nitride (cTiN) and TiO2. The efficiency of material yield from the surface of the accelerating channel of the titanium barrel increases for cyclic regime of accelerator operation with dead time smaller than 1.0 s.

CARS diagnostics of the burning of H2 — O2 and CH4 — O2 mixtures at high temperatures and pressures

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to determine the parameters of gaseous combustion products of hydrogen and hydrocarbon fuels with oxygen at high temperatures and pressures. The methodical aspects of CARS thermometry, which are related to the optimal choice of molecules (diagnostic references) and specific features of their spectra, dependent on temperature and pressure, are analysed. Burning is modelled under the conditions similar to those of real spacecraft propulsion systems using a specially designed laboratory combustion chamber, operating in the pulse-periodic regime at high temperatures (to 3500 K) and pressures (to 20 MPa) of combustion products.

Strontium vapour laser with a pulse repetition rate of up to 1 MHz

For the first time it has been shown that the pulse repetition rate of the lasers on the self-terminating transitions of metal atoms may be as high as ∼1 MHz. The highest pulse repetition rate equal to ∼830 kHz was realised on self-terminating IR transitions in Sr I atoms (λ = 6.456 μm and ∼3 μm) and in Sr II ions (λ ∼ 1 μm) in a strontium vapour laser operating in a self-heating pulse periodic regime. The energy yield of a Sr laser was found to be proportional to the energy input into the active medium in a wide range of excitation pulse repetition frequencies; in this case, the average total specific output laser power is equal to 30 — 40 mW cm-3.

Intracavity laser spectroscopy of CH4 and NH3 gases by using a pulse-periodic Cr2+:ZnSe laser

The results of studying the characteristics of an intracavity laser spectrometer based on a polycrystalline Cr2+:ZnSe laser operating in a pulse-periodic regime with the pulse-repetition rate of 3 kHz and pulse duration of ∼50 ns are presented. Intracavity spectra of absorption of NH3 and CH4 gases in the vicinity of 2.35 μm are measured. The estimate of the spectrometer sensitivity is provided.

Emission of cyan upon excitation of nitrogen, air, and N2-CH4 mixture by discharge pulses in an inhomogeneous electric field

We have studied spectral and amplitude-time characteristics of the emission by nitrogen, air, and the N2-CH4 mixture upon excitation by nanosecond and microsecond high-voltage discharge pulses in an inhomogeneous electric field in the pressure range of 0.01–3 atm. In the pulsed and pulse-periodic discharge regimes, we have observed emission bands of the violet system of cyan, the transition B2Σ+ → X2Σ+, which were comparable in intensity with bands of the (2+) system of nitrogen. We show that, in the pulsed regime, the highest intensity of the violet system of cyan is observed in the N2-CH4 mixture. We show that, in the pulse-periodic discharge in nitrogen with a small amount of a carbon-containing admixture, upon contracting the discharge, the efficiencies of formation and emission of the violet system of cyan considerably increase. We confirm that admixtures of oxygen to nitrogen lead to suppression of the emission of the violet system of cyan molecules.