Microwave Output Power Research Articles

Efficient 58-GHz relativistic orotron with combined mode selection

This article presents time-dependent numerical modeling of V-band relativistic orotron: a high-power microwave generator with a TM03 oscillation mode in an oversized (D/λ ∼ 2.7) electrodynamic structure and diffractive output in the TM02 mode. Single-mode operation of the oscillator is ensured by the cyclotron selection of the working axisymmetric mode, with simultaneous depression of competing non-axisymmetric modes by means of cutting longitudinal slits in the wall of the slow-wave structure. The simulated output microwave power is 350 MW with a power conversion efficiency of 31% when using a 3.6 kA, 310 keV electron beam transported in the 3.9 T magnetic field. The simulation employed 2.5D axisymmetric and 3D Cartesian versions of the KARAT code.

Microwave Generation in Synchronized Semiconductor Superlattices

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Kinetic Mechanisms for Preparing Silymarin via Microwave-assisted Extraction

The kinetic mechanism for the preparation of silymarin from milk thistle seeds was studied during the microwave-assisted extraction (MAE) process. The results showed that the transfer rate of silymarin from milk thistle seeds increased with the microwave output power and temperature during MAE processing. The apparent extraction rate constant k (s−1) was 1.2028×10−2, 1.2248×10−2, and 1.2485×10−2, and diffusion coefficient D (m2 • s−1) was 4.21×10−10, 4.29×10−10, and 4.37×10−10 at the microwave temperatures of 383, 393, and 403 K, respectively, in the silymarin MAE process. With the help of scanning electron microscopy (SEM), the microstructures of the samples extracted by MAE were observed. The results revealed that the increased efficiency and rate of MAE of silymarin could be attributed mainly to the subsequent cell change resulting from superheating effects during MAE.

Microwave Drying Characteristics of Soybeans in Single and Variable Microwave Power Density

Abstract Drying characteristics of soybeans under different microwave (MW) processing schemes were studied in regard to energy aspects. For drying in a single microwave power density, the experiments were carried out at a constant MW output power throughout the process. It was found that the drying rate is enhanced together with reduced energy consumption at higher single power density, but the soybean cracking ratio is increased gradually from 2 % to 40 % when the power density was changed from 0.2 W/g to 0.8 W/g. Efforts were made to reach a compromise between the drying rate and dried soybean quality by varying the MW power density in several steps overall the drying process, but soybean cracking occurred at a higher ratio due to the fluctuation of drying rate. Microwave drying of soybeans under the power density less than 0.2 W/g with slow and stable drying rate can achieve the soybean cracking ratio controlled within 5 % and lower energy consumption.

Microwave, convective, and intermittent microwave-convective drying of pulsed vacuum osmodehydrated pumpkin slices

The drying of pumpkin slices (Cucurbita moschata Duch.) by microwave, convective, and intermittent microwave–convective techniques was studied. The samples were pretreated by blanching followed by pulsed vacuum osmotic dehydration. The microwave output power was 780 W. Air temperatures were 30 °C and 60 °C and air velocities were 2.22 and 4.44 m s−1. The final moisture content was set at 0.08 kg [H2O] kg−1 [dry matter]. The drying kinetics were obtained and fitted with the Fick's model and the Page's empirical correlation. Color, water activity, shrinkage, and rehydration were evaluated. Shorter drying times, higher diffusivity and better color quality were obtained by employing microwave. Convective experiments carried out to higher water activity and lower shrinkage. For describing the drying behavior, the better adjustment was reached by the Page's empirical correlation. Practical applications The pumpkin (Cucurbita moschata Duch.) is well known as an important source of carotenoids, vitamins and minerals. After the harvest, techniques are required to assist the preservation of fruits, once they present high moisture contents. Microwave drying is an alternative method to preserve temperature-sensitive compounds and to economize energy and time. Microwave energy combined with other drying methods can improve the drying efficiency as well as the quality of food products which is far better than that achievable by the application of each method alone. This study investigates the efficacy of convective, microwave and intermittent microwave–convective drying techniques for osmodehydrated pumpkin slices. The mathematical models provide information about the processes. It was also evaluated the changes on physical characteristics of the product after drying.

Preparation of Activated Carbon from Municipal Organic Solid Wastes

Microwave pretreatment is a promising and unsophisticated technique for energy recovery in solid waste management. The present investigation was focused on microwave activated carbonization of mixed solid waste (MSW). The effect of process parameters such as size of particles, temperature and duration of carbonization, microwave output power for pretreatment, irradiation time and amount of additives added during microwave heating were studied. The most important factors that influenced MW drying (pre-treatment) were found to be the sample mass and the Microwave output power. Microwave induced carbonization not only overcomes the disadvantages of conventional heating method such as low heat transfer rate but also improves the quality of final carbonization products.

Order of magnitude improvement of nano-contact spin torque nano-oscillator performance.

Spin torque nano-oscillators (STNO) represent a unique class of nano-scale microwave signal generators and offer a combination of intriguing properties, such as nano sized footprint, ultrafast modulation rates, and highly tunable microwave frequencies from 100 MHz to close to 100 GHz. However, their low output power and relatively high threshold current still limit their applicability and must be improved. In this study, we investigate the influence of the bottom Cu electrode thickness (tCu) in nano-contact STNOs based on Co/Cu/NiFe GMR stacks and with nano-contact diameters ranging from 60 to 500 nm. Increasing tCu from 10 to 70 nm results in a 40% reduction of the threshold current, an order of magnitude higher microwave output power, and close to two orders of magnitude better power conversion efficiency. Numerical simulations of the current distribution suggest that these dramatic improvements originate from a strongly reduced lateral current spread in the magneto-dynamically active region.

Freeze-thaw stability of oil-in-water emulsions stabilized by soy protein isolate-dextran conjugates

The soy protein isolate-dextran (SPI-D) conjugates were prepared by ultrasound (power output 500 W for 40 min) and microwave (power output 800 W for 2 min) assisted glycosylation to improve the freeze-thaw stability of soy protein isolate stabilized oil-in-water (o/w) emulsions. Fourier transform infrared and fluorescence emission spectroscopy analyses confirmed that covalent bonds were formed between soy protein isolate (SPI) and dextran molecules through the Maillard reaction. The stability of SPI, SPI+D mixture, ultrasound SPI-D conjugates (SPI-DU) and microwave SPI-D conjugates (SPI-DM) emulsions subjected to from one to three freeze-thaw cycles was investigated. In comparison with SPI and SPI+D emulsions, SPI-DU and SPI-DM emulsions exhibited smaller creaming index, oiling off, droplet diameters, flocculation degree (FD) and coalescence degree (CD) after each freeze-thaw cycle. In addition, the zeta potential and specific surface area (SSA) were greater. Appearance and microstructure indicated that SPI-DU and SPI-DM emulsions exhibited a relative stable state after three freeze-thaw cycles.

Device for timing and power level setting for microwave applications

Nowadays, the microwaves are widely used for various technological processes. The microwaves are emitted by magnetrons, which have strict requirements concerning power supplies for anode and filament cathodes, intensity of magnetic field, cooling and electromagnetic shielding. The magnetrons do not tolerate any alteration of their required voltages, currents and magnetic fields, which means that their output microwave power is fixed, so the only way to alter the power level is to use time-division, by turning the magnetron on and off by repetitive time patterns. In order to attain accurate and reproducible results, as well as correct and safe operation of the microwave device, all these requirements must be fulfilled. Safe, correct and reproducible operation of the microwave appliance can be achieved by means of a specially built electronic device, which ensures accurate and reproducible exposure times, interlocking of the commands and automatic switch off when abnormal operating conditions occur. This driving device, designed and realized during the completion of Mr.Ursu's doctoral thesis, consists of a quartz time-base, several programmable frequency and duration dividers, LED displays, sensors and interlocking gates. The active and passive electronic components are placed on custom-made PCB's, designed and made by means of computer-aided applications and machines. The driving commands of the electronic device are delivered to the magnetron power supplies by means of optic zero-passing relays. The inputs of the electronic driving device can sense the status of the microwave appliance. The user is able to enter the total exposure time, the division factor that sets the output power level and, as a novelty, the clock frequency of the time divider.

Synthesis and reaction evolution of high-purity Ti2SC powder by microwave hybrid heating at low temperature

Highly pure Ti2SC was synthesised by microwave hybrid heating. The effects of composition and processing parameters, such as microwave output power, sintering temperature and dwell time on the formation process and morphology of Ti2SC were studied. The purity of Ti2SC is sensitive to the content of sulphur and sintering temperature. The results indicated that the optimum condition for synthesising Ti2SC was using a mixture of 1.15TiS2/2.85Ti/2C powders irradiated under a 100% microwave output power sintering at 1100°C for 3 min at Ar atmosphere. In addition, the reaction evolution of Ti2SC was discussed in details.

Axial motion of collector plasma in a relativistic backward wave oscillator

In this paper, it is proposed that plasma formed at the collector may drift back to the cathode and cause pulse shortening of the relativistic backward wave oscillator. Theoretical analysis shows that the axial drift velocity of plasma ions can be up to 5 mm/ns due to the presence of space charge potential provided by an intense relativistic electron beam. Particle-in-cell simulations indicate that the plasma electrons are initially trapped around the collector surface. With the accumulation of the plasma ions, a large electrostatic field forms and drives the plasma electrons to overcome the space charge potential and enter the beam-wave interaction region along the magnetic field lines. As a result, the beam current modulation is disturbed and the output microwave power falls rapidly. The plasma ions move in the beam-wave interaction region with an average axial velocity of 5–8 mm/ns. After the plasma ions reach the diode region, the emitted current at the cathode rises due to the charge neutralizations by the ions. The impedance collapse leads to further decrease of the microwave power. In experiments, when the diode voltage and beam current were 850 kV and 9.2 kA, and the collector radius was 2.15 cm, the output microwave power was 2.4 GW with a pulse width of less than 20 ns. The ion drift velocity was estimated to be about 5 mm/ns. After an improved collector with 3.35 cm radius was adopted, the pulse width was prolonged to more than 30 ns.

Optimization of Microwave-assisted Extraction of Syringin and Oleuropein from Syringa reticulata

We used microwave-assisted extraction to extract syringin and oleuropein from Syringa reticulate,and optimized the extraction process by the Box-Behnken response surface method. The order of the effect of factors on the yield of syringin and oleuropein was the concentration of ethanol the microwave time microwave power. The optimum conditions for the extraction were as follows: soaking time of 1 h,ethanol concentration of45%,ratio of solid to liquid of 1 ∶ 20 g ·m L~(-1),microwave output power of 600 W,and extraction time of 5min. With the optimum extraction conditions,the yield of syringin and oleuropein from Syringa reticulate was5. 28% ± 0. 102%. The microwave-assisted extraction was efficient and rapid for extraction syringin and oleuropein from Syringa reticulate,and was benefit for the comprehensive utilization of Syringa reticulate.

High power microwaves generation from intense relativistic electron beam using Kali-1000 pulse power system

Intense relativistic electron beam (IREB) is generated using Kali-1000 (Kilo Ampere Linear Injector) pulse power system, providing input electric power of 4 GW and stored energy of 1000 J. IREB is used for generating Non Ionizing (High Power Microwaves, HPM) and Ionizing (Flash X-rays) radiation. The present manuscript discusses generation of HPM by Kali-1000 system using an axial Virtual cathode oscillator. Kali-1000 pulse power system is a low impedance (~4 Ω) high voltage generator, consisting of Radial Tesla transformer to step up the voltage from 28 to 300 kV in 5 μs, which energizes the line, pulse discharged by a self triggered pressurized SF6 closing switch across a Relativistic Electron Beam (REB) diode load. The typical electron beam parameters are 300 kV, 15 kA and 100 ns with a current density of 3–4 hundreds of amperes per square centimeter. Experiments are carried with a REB diode consisting of graphite cathode and SS anode mesh. Measured pulse duration of HPM pulse (FWHM) varies from 70 to 110 ns on shot-to-shot basis. The major operating output frequencies have been measured to be 5.8 and 6.2 GHz within a range of 4.8–6.7 GHz using B-dot magnetic field sensor coupled to high frequency digital oscilloscope. Microwave output power from the virtual cathode oscillator has been measured to be 50 MW using double ridged waveguide horn antenna coupled with Schottky detector diode. The estimated beam to microwave power conversion efficiency is thereby found to be 1.1%. Particle-in-Cell (PIC) simulation has been performed for a set of Vircator experiments being driven by the Kali-1000 system. Experimentally measured cathode current, dominant output frequency, radiation pattern, etc., have been compared with simulation results and are in good agreement. The dominant emission mode from the virtual cathode oscillator is shown to be the TE01 mode based on the emission pattern of microwave induced air breakdown in this experiment.

Quantitative evaluation of interfacial adhesion between fiber and resin in carbon fiber/epoxy composite cured by semiconductor microwave device

Interfacial adhesion between carbon fiber (CF) and epoxy resin in carbon fiber-reinforced epoxy composite, which was prepared by different heating process such as semiconductor microwave (MW) device and conventional electric oven, has been evaluated quantitatively. The interfacial shear strength (IFSS) between CF and epoxy resin, which was an indicator of adhesion on the interface, was measured by a single fiber fragmentation test. The single fiber fragmentation test showed that the IFSSs of the prepared specimens were different by heating methods. In the case of MW process, the curing reaction of epoxy resin on the CF interface would be progressed preferentially due to the selective heating of CF, resulting that the IFSSs of specimens prepared by MW irradiation were increased by enhancing the output power of MW. However, the IFSSs of the specimens were decreased by excessively high output power because the matrix resin on the CF interface was thermally degraded. As results, by optimizing the MW conditions of output power and irradiation time, the IFSS of the sample cured by MW was increased by 21% as compared to oven-heated one. It was found that the interfacial adhesion between CF and epoxy resin would be improved by the MW-assisted curing reaction on the surface of CF.

Determination of drying kinetics of Tunceli garlic with microwave drying technique

In this study, drying kinetics of 4 mm-thick slices of Tunceli garlic (Allium tuncelianum) was investigated by using microwave technique at an interval of 90-600 W microwave output power. Ten mathematical models were used to represent the experimental data in this study. The determination of coefficient (R2), root mean square error (RMSE), chi-square (χ2) and percentage error (E%) values that show compatibility to these models were determined. The determination of coefficient (R2) values were found to vary at an interval of 0.9440-0.9993. The Midilli et al model was determined to be the best model that represents the experimental data. It was seen that drying rate increases with the increase in microwave output power (P) and decreases with the increase in sample mass (m). Effective diffusion coefficients (Deff) and drying rate constants (k) were determined for different sample masses at microwave output power of 90 W. Activation energies for both effective diffusion coefficients and drying rate constants were determined by using an Arrhenius type exponential equation. Activation energies from effective diffusion coefficients and drying rate constants were found as 3.85 W g-1 and 3.99 W g-1 respectively.

A smart repetitive-rate wideband high power microwave source

A smart repetitive-rate wideband High Power Microwave (HPM) source based on the A6 Magnetron with Diffraction Output is described in this paper. The length of the HPM source is 30 cm and its weight is 35 kg. Computer simulations show that the source can produce microwave with central frequency of 1.91 GHz and bandwidth of about 11%. Experimental measurements show that the output microwave power from the source reaches in maximum 110 MW when the input electric power from the pulsed driver is ∼500 MW, which gives the power conversion efficiency 22%. Central frequency of the output HPM in the experiment is 1.94 GHz with the bandwidth ranging from 1.82 GHz to 2.02 GHz. The jitter of the output HPM power is lower than 3 dB when the source operates in the repetition mode with 50 Hz rate.

An X-band high-impedance relativistic klystron amplifier with an annular explosive cathode

The feasibility of employing an annular beam instead of a solid one in the X-band high-impedance relativistic klystron amplifier (RKA) is investigated in theory and simulation. Small-signal theory analysis indicates that the optimum bunching distance, fundamental current modulation depth, beam-coupling coefficient, and beam-loaded quality factor of annular beams are all larger than the corresponding parameters of solid beams at the same beam voltage and current. An annular beam RKA and a solid beam RKA with almost the same geometric parameters are compared in particle-in-cell simulation. Output microwave power of 100 MW, gain of 50 dB, and power conversion efficiency of 42% are obtained in an annular beam RKA. The annular beam needs a 15% lower uniform guiding magnetic field than the solid beam. Our investigations demonstrate that we are able to use a simple annular explosive cathode immersed in a lower uniform magnetic field instead of a solid thermionic cathode in a complicated partially shielding magnetic field for designing high-impedance RKA, which avoids high temperature requirement, complicated electron-optical system, large area convergence, high current density, and emission uniformity for the solid beam. An equivalent method for the annular beam and the solid beam on bunching features is proposed and agrees with the simulation. The annular beam has the primary advantages over the solid beam that it can employ the immersing uniform magnetic field avoiding the complicated shielding magnetic field system and needs a lower optimum guiding field due to the smaller space charge effect.

A reliable, compact, and repetitive-rate high power microwave generation system.

A compact high power microwave (HPM) generation system is described in this paper. The main parts of the HPM system are a Marx generator with a pulse forming line and a magnetron with diffraction output. The total weight and length of the system are 250 kg and 120 cm, respectively. The output microwave power of the HPM system at 550 kV of applied voltage and 0.33 T of magnetic field reaches 1 GW at 2.32 GHz of central frequency with 38 ns of pulse duration, 23% of power conversion efficiency, and Gaussian radiation pattern. In the bursts operation, both time and amplitude jitters are less than 4 ns and lower than 1.5 dB, respectively.

Statistical analysis and optimization of ammonia removal from landfill leachate by sequential microwave/aeration process using factorial design and response surface methodology

The application of microwave (MW) radiation followed by aeration (A) for the purpose of ammonia removal from both synthetic solutions and landfill leachate was investigated in this study. 100mL of synthetic solution or landfill leachate was subjected to MW radiation for 30, 45, 60, 90 and 120s under 50 and 100% power output level and a pH of 10, 10.5 and 11. The samples were then aerated for 10min. The initial, after MW application and final total ammonia nitrogen (TAN) were measured. Results confirmed that the sequential microwave/aeration process was an effective approach for removal of ammonia from aqueous systems. Maximum ammonia removal of 81.7% for 100mL synthetic solution and 70% for 100mL landfill leachate was achieved by applying 78KJ microwave energy output and 10min aeration. Factorial design and response surface methodology were applied to evaluate and optimize the effects of pH, MW energy level and microwave power output. When applying the same energy output to the batch tests, the effect of varying MW power output is negligible. For 100mL synthetic ammonia solution, the optimum pH and MW energy output level for ammonia removal were 11 and 78KJ and the maximum ammonia removal efficiency predicted for the synthetic solution is 76.3%. R2 of 0.941 indicates that the observed results fitted well with the model prediction.

Comparison of two alternatives of combined drying to process blueberries (O'Neal): Evaluation of the final quality

ABSTRACTIn this work, we examined and compared two combined alternatives for the drying of blueberries (O’Neal). Pretreatments of osmotic dehydration (60°Brix sucrose solution at 40°C for 6 h) and hot air drying (HAD) (60°C, 2.5 m/s for 90 min) were performed to reach the same water content. Pretreated blueberries were then dried by microwave at different microwave output power values: 562.5, 622.5, and 750 W. The combined drying processes were also compared with HAD alone (control). The effects of the processes over blueberries were studied in terms of decrease in water content, drying rate (DR), mechanical properties (firmness and stiffness), optical properties (L*, a*, and hue angle (h)), antioxidant capacity, and rehydration capacity. The hot air–microwave drying decreased the process time and presented a high drying rate compared with the osmotic dehydration–microwave processes and the control drying. In terms of quality, the antioxidant and rehydration capacities were the most affected. The results showed that the best drying method to obtain the desired final product was the hot air–microwave drying (750 W).