Microwave Pulse Duration Research Articles

Dynamics and Reversible Control of the Bloch-Point Vortex Domain Wall in Short Cylindrical Magnetic Nanowires

Fast and efficient switching of nanomagnets is one of the main challenges in the development of future magnetic memories. We numerically investigate the evolution of static and dynamic spin-wave (SW) magnetization in short (50--400 nm in length and 120 nm in diameter) cylindrical ferromagnetic nanowires, where competing magnetization configurations of a single vortex (SV) and a Bloch-point vortex domain wall (BP-DW) can be formed. For a limited nanowire length range (between 150 and 300 nm), we demonstrate reversible transitions induced by a microwave field (forwards) and by opposite spin currents (backwards) between topologically different SV and BP-DW states. By tuning the nanowire length, the excitation frequency, the microwave pulse duration, and the spin-current value, we show that the optimum (low-power) manipulation of the BP-DW can be achieved with a microwave excitation tuned to the main SW mode for nanowire lengths around 230--250 nm, where single-vortex domain-wall magnetization reversal via nucleation and propagation of a SV-DW transition takes place. An analytical model of the dynamics of the Bloch point provides an estimate of the gyrotropic mode frequency close to that obtained via micromagnetic simulations. A practical implementation of the method in a device is proposed, involving microwave excitation and the generation of opposite spin currents via the spin-orbit torque. Our findings open up an alternative pathway for the creation of topological magnetic memories.

Influence of Short-Pulse Microwave Radiation on Thermochemical Properties Aluminum Micropowder.

The thermochemical properties of Al micropowder after exposure to microwave irradiation were investigated. The Al micropowder was exposed to microwave irradiation in air with a frequency of 2.85 GHz, a power density of 8 W/cm2, and a pulse duration of 25 ns and 3 µs. The thermochemical parameters of the irradiated metal powders were determined by the method of thermal analysis at the heating in air. It was found that an increase in the duration of microwave pulses and irradiation time leads to the thermal annealing of the metal particles, and the thermal processes of melting and sintering begin to dominate over non-thermal processes. The specific thermal effect of irradiated Al micropowder oxidation increases from 7744 J/g to 10,154 J/g in comparison with the unirradiated powder. The modeling of thermal heating processes of aluminum (Al) micropowder under the action of pulsed microwave radiation has been performed. It is shown that with an increase in the duration of microwave pulses and irradiation time, a significant heating of the Al micropowder occurs, leading to its melting and sintering. The results of modeling on the action of microwave radiation on the Al micropowder were compared with experimental results.

Atmospheric air plasma sustainment by semiconductor microwave for hydroxyl radical production and powder metal element analysis

A semiconductor microwave device that generates a series of burst microwaves at a sub-microsecond duration has been successfully used in a breakdown plasma spectrometer in atmospheric conditions. Microwave delivery has been changed to couple the microwave with laser sparks and electric sparks which are typical plasma ignition sources in laser-induced breakdown spectroscopy (LIBS) and spark-induced breakdown spectroscopy (SIBS). A helical antenna was used for the laser spark, while a coaxial antenna was considered more appropriate for the electric spark. The weak and transient sparks in LIBS and SIBS are enlarged by the microwaves which are stably sustained in the air. The microwave's output power and pulse duration are easily controllable, resulting in tunable plasma intensity and sustained production of hydroxyl radicals (OH radicals). Even in continuous-wave operation by microwave, the low-energy system prevented the formation of high-temperature thermal plasma (>10,000 K) without any mechanical cooling system. The microwave-enhanced LIBS (MW-LIBS) and microwave-enhanced SIBS (MW-SIBS) could be applied to optical emission spectroscopy analyses. In analytical applications, MW-SIBS produces no shockwave in contrast with MW-LIBS which is a great advantage in powdered samples. The MW-SIBS successfully analyzed the direct introduction of copper metal powders.

Evaluation of the membrane performance of ultra-smooth silicon organic coatings depending on the process energy density

Plasma-enhanced chemical vapor deposition (PECVD) can be used to apply thin layers to plastics with a wide range of customizable functionality. The selective permeation properties of the layers are particularly interesting for the application field of membrane gas separation processes. In order to be able to produce plasma polymerized membranes in an application specific manner, it is necessary to be able to correlate the plasma process parameters, such as the energy density introduced into the system, with the coating and thus separation properties. Within the scope of this work, different layers produced in a Plasma-enhanced chemical vapor deposition process were investigated with respect to their growth behavior, topographic and chemical properties as well as permeability to evaluate their performance as separation layers of a membrane. The results of the investigations show a strong dependence of the layer and separation properties on the pulsed microwave (MW) power coupled into the process. Energy input was varied over the MW power level as well as the MW pulse duration. Analysis shows the highest permeation and separation performance of the produced layers in the low energy density range of the process.

Optimization and Kinetics Modeling of Microwave-Assisted Subcritical n-Butane Extraction of Tigernut Oil.

In this study, tigernut oil was extracted from tigernut meal by subcritical n-butane extraction with the assistance of microwave pretreatment. Effects of microwave pulse duration, particle size of tigernut meal, and subcritical extraction variables (temperature, time, solid-liquid ratio, number of extraction cycles) on extraction efficiency were examined by single-factor experiments and Response Surface Methodology (RSM) modeling. The results indicate that microwaving (560 W, 6 min) significantly increased the subcritical extraction efficiency. The variation of extraction yield could be interpreted as a nonlinear function of extraction time, temperature and liquid-solid ratio. Changing the independent variables could affect the oil extraction efficiency. The subcritical extraction of tigernut oil with a liquid-solid ratio of 3.62 kg/(kg of tigernut meal) at a temperature of 52°C for 32 min after three extraction cycles produced the most oil, and a maximum yield (24.736%) of tigernut oil was achieved. The ratio of unsaturated to saturated fatty acids (4.68 UFA/SFA), low acid value (3.30 mg KOH/g oil), low peroxide value (0.28 meq.kg-1), and preponderance of oleic acid indicate a high-quality oil. To describe the extraction kinetics, a modified Brunner's mathematical model was used. The model fit the experimental data well over the entire operating range, and the explanation coefficient exceeds 96%. Our results can be used to develop an optimized method for subcritical fluid extraction of tigernut oil and can move industry further toward implementing microwave-assisted subcritical extraction in oil processing.

Microwave synthesis of single-phase nanoparticles made of multi-principal element alloys

Metal nanoparticles of multi-principal element alloys (MPEA) with a single crystalline phase have been synthesized by flash heating/cooling of nanosized metals encapsulated in micelle vesicles dispersed in an oil phase (e.g., cyclohexane). Flash heating is realized by selective absorption of a microwave pulse in metals to rapidly heat metals into uniform melts. The oil phase barely absorbs microwave and maintains the low temperature, which can rapidly quench the high-temperature metal melts to enable the flash cooling process. The precursor ions of four metals, including Au, Pt, Pd, and Cu, can be simultaneously reduced by hydrazine in the aqueous solution encapsulated in the micelle vesicles. The resulting metals efficiently absorb microwave energy to locally reach a temperature high enough to melt themselves into a uniform mixture. The duration of microwave pulse is crucial to ensure the reduced metals mix uniformly, while the temperature of oil phase is still low to rapidly quench the metals and freeze the single-phase crystalline lattices in alloy nanoparticles. The microwave-enabled flash heating/cooling provides a new method to synthesize single-phase MPEA nanoparticles of many metal combinations when the appropriate water-in-oil micelle systems and the appropriate reduction reactions of metal precursors are available.

The Effect of Removing the High Power Pulse Transformer on the Microwave Pulse Duration in the Preionization Phase in Taban Tokamak

In this study, a simple, economical and efficient preionization system (2.45 GHz, 1 kW) was constructed to assist the current startup in Taban tokamak ( $B_{T}= 0.7$ T, $R = 0.45$ m, a = 0.15 m). Power pulse transformer is a part of the preionization circuit, and its removal from the system can have impact on the performance of microwave duration and plasma preionization. In the present system, the magnetron was connected directly to the capacitor using a triggered spark gap. This new preionization system allows studying the electron cyclotron emission (ECR) preionization phenomenon at different toroidal magnetic fields. The toroidal magnetic field ripple was also used to determine the entrance moment of the ECR layer into the chamber. As a result, the exact moment of sending microwave can be determined using the moment of the ECR layer formation inside the chamber for maximum absorption of wave power.

A Moderately Relativistic Subgigawatt Microwave Oscillator of Twistron Type Operating with 50% Efficiency

A regime of quasi-stationary microwave generation in moderately relativistic microwave oscillator of the twistron type with 50 ± 20% efficiency of electron beam power conversion into electromagnetic radiation has been obtained in experiment through optimization of the electron–wave interaction. For the selected electron beam parameters (diode voltage, 210 kV; beam current. 1.36 kA) the output microwave power at 10.63 GHz frequency was 140 ± 40 MW in a guiding magnetic field of about 1.9 T. The duration of microwave pulses was about 16 ns.

Умеренно релятивистский генератор микроволнового излучения субгигаваттного уровня типа твистрон с эффективностью 50%

AbstractA regime of quasi-stationary microwave generation in moderately relativistic microwave oscillator of the twistron type with 50 ± 20% efficiency of electron beam power conversion into electromagnetic radiation has been obtained in experiment through optimization of the electron–wave interaction. For the selected electron beam parameters (diode voltage, 210 kV; beam current. 1.36 kA) the output microwave power at 10.63 GHz frequency was 140 ± 40 MW in a guiding magnetic field of about 1.9 T. The duration of microwave pulses was about 16 ns.

Studies of the Combined Modes of Microwave Grain Drying

Abstract. To ensure the high quality of the materials being dried, one of the most promising ways to dehydrate vegetable raw materials is drying under the action of electromagnetic radiation (microwave drying). Microwave drying is widely used in various industries, in particular, in the food and woodworking industries. (Research purpose) Calculation of the microwave pulse and pause duration; their experimental determination, as well as the determination of the moisture removal rate at the pulse moment and the depth of pulse penetration into the layer. (Materials and methods) When drying in a microwave field, the gradient of moisture content in the material prevents the moisture movement towards the surface, internal cracks can be formed as well. Therefore, the combined methods of drying can yield the best results. The pulse duration has been calculated by the permissible increment of the grain temperature, the pause duration has been determined by assuming that during the pulse, moisture from the caryopsis kernel is pushed out and cooled under isothermal conditions by an air flow. (Results and discussion) It has been confirmed that at a microwave pulse energy duration of 4, 6, and 10 s, the speed of blow-off and blowdown of the layer with external air was 0.5 m per second. When grain is cooled by natural convection, the pulse time is 10 s, the pause time is 1, 2, 3, and 5 min. For the blow-off mode, the pulse exposure time was 6 and 10 s, that of blow-off - 0.5, 1.0, and 1.5 minutes. The maximum duration of the microwave pulse was determined by the flow density of allowable grain heating and the fraction of heat required for the evaporation of moisture when heated to 20-25 degrees. (Conclusions) The pause duration is determined by the grain cooling time to the temperature preceding the pulse. It has been experimentally established that the calculated values of the pulse and pause duration with an accuracy of 15 percent for grain with a moisture content of 20-24 percent at a microwave flow density of 0.7 kilowatts per square meter, at a depth of the microwave energy penetration into the grain by 70 percent, do not exceed 20-22 mm, and the moisture removal rate is 0.1-0.15 percent.

On Transient Time Reduction of a 2.4 GHz Relativistic Traveling Wave Oscillator with a Hollow Slow Wave System

The decrease in the transient time in a relativistic microwave traveling wave oscillator, in which the interaction is carried out between the relativistic electron beam and the fundamental harmonic of the forward-traveling electromagnetic wave slowed down to the speed of light in the hollow slow wave system system, has been numerically and experimentally demonstrated. It is shown that in this mode a high beam-to-wave coupling impedance up to ≈10 Ω is achieved, which ensures reduction of the transient time. In the experiment, a microwave peak power of 210 ± 30 MW at a frequency of 2.45 GHz in a guiding magnetic field of 0.16 T was obtained. The efficiency of the oscillator to convert the power of the electron beam into microwave power was 17 ± 3%. At the beam current pulse duration of about 50 ns the microwave pulse duration was about 20 ns and the transient time was about 22 ns.

Pulse Shortening in Recirculating Planar Magnetrons

Recirculating planarmagnetron (RPM) experiments at the University of Michigan have utilized a 12-frame ultrafast intensified CCD camera to analyze the effect of plasma formation on microwave pulse duration. The RPM was driven by a −300-kV voltage pulse for 0.3–1.0 $\mu $ s, with a 0.08–0.27-T axial magnetic field. The RPM has previously demonstrated peakmicrowave powers of over 150 MW, with typical microwave pulse durations of 50–150 ns, far shorter than the available voltage pulse. To investigate possible causes of pulse shortening, the RPM was imaged with both S-band (2 GHz) and L-band (1 GHz) anodes, as well as two different cathodes with substantially different anode–cathode (AK) gap widths and end caps. It was found that a smaller AK gap produced brighter plasma and allowed for improvements in temporal resolution of the imaging configuration at the expense of larger end-loss current. Many shotsdemonstrated a direct correlationbetween intervane anode plasma formation and a sharp reduction in RF power. Anode vane plasma is formed at the axial ends of the anode, indicating field enhancement effects. Contact resistance plasma was also observed at the back of anode vane cavities. Details of plasma formation are illustrated and methods for remediating these plasmas are proposed.

Influence of surface microstructure on explosive electron emission properties of graphite cathode doped by silicon carbide whiskers

Explosive emission cathode (EEC) is a pivotal component in high power microwave source (HPMS), of which the ultimate properties are significantly dependent on the quality of electron beams generated from the cathode. Short lifetime and poor emission uniformity are the persistent drawbacks of conventional field EEC. Improvement of cathode material by changing its compositions and modifying surface micromorphology, is a feasible way to solve this problem. Graphite is one of the frequently used materials for EECs due to its long life-time and sturdy performance under high voltage and repetition frequency. Meanwhile silicon carbide (SiC) whiskers are distinguished by high aspect ratio (ratio of height to diameter) and low work function which is in favor of the fast onset of electron emission. In this work, the novel composites, composed of SiC whiskers, pitch and major graphite powders, are prepared by the conventional mingling and sintering. The cathodes are installed on TPG1000 system with a parameterized pulse of 970 kV, 9.2 kA, and 50 ns. By analyzing the changes of the rise edge of measured diode current and output microwave pulse duration, the effects of material composition and surface micromorphology on electron emission properties for the cathode are disclosed in detail. The results, based on the comparison of emission properties between graphite cathodes with and without SiC whiskers doped, reveal that SiC whiskers play an important role in accelerating the field emission of cathode, which is demonstrated by the eclipse of displacement current peak on the rise edge of measured current waveform after doping. Meanwhile, the duration of output microwave pulse is enhanced by about 11% after doping, which could be explained by the lower expansion speed of Si plasma. Moreover, the surface micro-protrusions of graphite cathode doped by SiC whiskers are constantly “polished” by heating effect and cathode plasma as the number of emission pulses increases to 11000. This is in quite good agreement with the appearance of the displacement current peak on the rise edge of measured current curves after 6000 and 11000 pulses treatment. These changes imply that the initial speed of field emission from cathode is slowed down gradually. The output microwave pulse starts early, which is benefited from the homogeneous surface micromorphology of the cathode due to “polishing” effect. The quantity of releasing absorbed gases, including water and vacuum pump oil vapor, decreases with increasing emission pulses. Then the pulse shortening phenomenon is restrained and the falling edge of output microwave pulse is extended. The duration of output microwave pulse is increased by about 5%, for graphite cathode doped by SiC whiskers after experiencing 11000 pulses. In conclusion, the reaction mechanism of SiC whiskers in the process of explosive electron emission (EEE) is considered as being due to accelerating the onset of felid emission and reducing the expansion speed of cathode plasma. Therefore, combination with SiC whiskers is an effective way to improve the electron emission properties of graphite EECs, especially in the output microwave pulse width and energy conversion efficiency of HPMS.

Control of the Operation Mode of a Relativistic <italic>Ka</italic>-Band Backward-Wave Oscillator

Variations in the transient time and in the microwave pulse duration and peak power have been demonstrated for a relativistic subgigawatt Ka -band backward-wave oscillator (BWO) with the slow-wave structure geometry, the accelerating voltage, and the electron beam current remained unchanged. This was done taking into account that the BWO operation mode depends on the starting-current-to-beam-current ratio. Efficient variation of the starting current was attained by changing the beam path in view of a significant difference between the times of the guide pulsed magnetic field diffusion into the electron injection region and beam-wave interaction space.

Полупроводниковый генератор импульсного действия с электронным переключением частот Ka-диапазона

Transmitting devices for small radars in the millimeter wavelength range with high resolution on range and noise immunity. The work presents the results of research and development of compact pulse oscillators with digital frequency switching from pulse to pulse. The oscillator consists of a frequency synthesizer and a synchronized amplifier on the IMPATT diode. Reference oscillator of synthesizer is synchronized by crystal oscillator with digital PLL system and contains a frequency multiplier and an amplifier operating in pulse mode. Small-sized frequency synthesizer of 8 mm wave lengths provides an output power of ~1.2 W per pulse with a frequency stability of no worse than 2•10–6. Radiation frequency is controlled by three-digit binary code in OOL levels. Synchronized amplifier made on IMPATT diodes provides microwave power up to 20 W in oscillator output with microwave pulse duration of 100—300 ns in an operating band. The oscillator can be used as a driving source for the synchronization of semiconductor and electro-vacuum devices of pulsed mode, and also as a transmitting device for small-sized radar of millimeter wave range.

On the radiation phase stability of a relativistic coaxial backward-wave oscillator at decimeter wavelengths

The stability of the microwave radiation phase of A relativistic coaxial backward-wave oscillator with a modulating reflector relative to a fixed voltage at a rising edge of the feeding high-voltage pulse is shown. At a carrying frequency of 1.3 Ghz, the standard phase deviation in a series of 50 consecutive pulses was not more than 20 ps for the microwave pulse duration of 80 ns.

Research on the Emission Uniformity of Explosive Emission Cathodes in Foilless Diodes

Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator.

Output power variations in a plasma relativistic microwave amplifier during a 500-ns relativistic electron beam current pulse

A relativistic plasma microwave amplifier with a gain of about 30 dB and an output power of about 60–100 MW in the frequency range from 2.4 to 3.2 GHz is studied experimentally. The total duration of the output microwave pulse is equal to the duration of the current pulse of the driving relativistic electron beam (500 ns); however, the maximum output power is observed only within 200 ns. It is shown that variations in the output microwave power during the current pulse of the annular relativistic electron beam are caused by variations in the beam radius and thickness. Analysis of the experimental data and results of numerical simulations show that the thickness of the electron beam is determined by the density of the cathode emission current.

Radiation Enhancement of a Relativistic Backward-Wave Oscillator Driven by Two Annular Electron Beams

The radiation enhancement of a relativistic backward-wave oscillator (RBWO) driven by two annular electron beams is demonstrated in this paper. The slow-wave structure of RBWO is composed of a cylindrical grating with rectangular corrugation. The concentric electron beams are maintained with a velocity difference to initiate two-stream instability (TSI). The dispersion equations of the RBWO are derived and solved numerically for the growth rate of microwave signal. Compared with the single beam device, an enhancement in the growth rate is observed for the two beam case and that is attributed by the TSI. This theoretical observation is verified by 2-D particle in cell simulation and experiment. The experimental results show that the output power of 150 MW for a signal-beam device is enhanced to 200 MW for the two beam case and duration of microwave pulse is also enlarged from 10.6 to 16.8 ns. The efficiency of the devices is ~ 6%.

Experimental Investigation of the Relativistic Millimeter-Wave Gyroklystron

We present the results of experimental studies of the gyroklystron with cavities operated with the sequence of TE7.1.1—TE 7.3.1 volume modes and a radiation frequency of 35.4 GHz. At an accelerating voltage of 320 kV, we obtained an output radiation power of 15 MW for an efficiency over 30%, an amplification coefficient of 30 dB, an amplification band of 50 MHz, and a microwave pulse duration of 0.5 μs.