Reflected Microwave Power Research Articles

A new 915 MHz coaxial-line-based microwave plasma source

Microwave plasma is known for its versatility in providing tailored operating conditions (pressure, working gas composition and residence time of reagents) for specific applications. Microwave plasma sources (MPSs) are vital in modern applications, demanding continuous improvement. This work introduces a coaxial-line-based nozzleless MPS that operates at atmospheric pressure at an unique frequency of 915 MHz. The measured electrodynamic characteristics in nitrogen of the MPS highlighted the need for improved energy efficiency of the device. The main novelty of this work lies in improving an energy efficiency of the presented MPS, which led to an advanced new version of the device. To achieve this, a dual strategy is employed. Firstly, numerical simulations are used to design a construction modifications to the MPS, which should increase the efficiency of transferring microwave energy from the microwave source to the generated plasma. In this step, a standard model for homogeneous plasma and a two-port equivalent method were used. Then, the theoretical results were experimentally validated by manufacturing a new energy improved version of the MPS. In the new MPS the achieved reflected microwave power (losses) was less than 3% of incident microwave power in the tested range of nitrogen flow rate (50–100 Nl/min). Compared to the MPS before improvement, this means a two-fold decreasing the reflected microwave power. To test the new MPS, the electrodynamic characteristics of the new device version and properties of the microwave plasma generated in nitrogen, using optical emission spectroscopy (OES), were investigated. The OES was used to determine the vibrational Tvib and rotational Trot temperatures of nitrogen molecules and molecular ions. In this work, the estimated Tvib and Trot temperatures for nitrogen molecules ranged from 4000 to 5300 K, depending on discharge conditions, while for nitrogen molecular ions, the temperatures changed between 4700 and 6100 K, respectively. Both the Tvib and Trot temperatures decrease linearly along the plasma flame.

Dielectric and physico-chemical behavior of single thermally thick wood blocks under microwave assisted pyrolysis

Pyrolysis of thermally thick beech wood blocks with a size of around 2.5 × 8 × 6 cm3 (width × length × height) was carried out in a lab scale microwave reactor with a frequency of 2.45 GHz, operated, both, at 300 W and 600 W under inert conditions, using N2 at around 400 mbar absolute pressure. The microwave cavity had a size of 20 × 20 × 20 cm3. The specific energy supply referred to the untreated wood block was 4–8 W/g, with slight variations depending on the initial water content. The mass loss and the reflected microwave power were in-situ monitored during the experiments. The sample surface and chamber temperatures were measured with a pyrometer and a thermocouple, respectively. Physico-chemical and dielectric properties of the produced solids were investigated and compared to those of chars produced under conventional pyrolysis using the same raw materials. It is shown that the complex dielectric permittivity of the solid products changed drastically during the pyrolysis process, with increasing heating properties as the conversion process evolved. This was easily achieved using 600 W without susceptors. However, 300 W was not enough to achieve a high conversion degree, independently of the irradiation time. This, together with the physico-chemical analyses of the solids, hinted to the importance of the transport kinetics in thermally thick materials, although further investigation is still required.

Energy deposition and plasma propulsion during the time window of microwave-assisted CH4-O2 spark ignition with Ar/N2/CO2 dilution

Microwave-assisted spark ignition (MAI) is a promising technology to achieve ultra-lean combustion and thus satisfy the requirements in the low-carbon age. However, the coupling process between microwave and the early flame kernel is still a complex and critical question before its wide application and further optimization. Therefore, this work explores the energy deposition and plasma propulsion effect during the MAI through the ignition experiments fueled by the methane-air mixture under a range of equivalence ratios with argon (Ar), nitrogen (N2), and carbon dioxide (CO2) as the carrier gas. Both the flame and plasma characteristics are recorded and analyzed through the high-speed shadowgraph method. The assessment of the time window and the microwave energy depositing process is based on the voltage of the spark electrode and the detected voltage of reflected microwave power. Results show that in Ar and N2 cases, the microwave enhancing effect on early flame development decreases as the equivalence ratio approaches the stoichiometric ratio, but this tendency is less obvious in the CO2 case. During the MAI, there is a time window for the microwave energy deposition, and once it ends, the following microwave pulses will no longer be absorbed. In successfully ignited cases, the time window in MAI mode is 1.8 ms in the N2 case and 1.5 ms in the CO2 case, while it is extended to 2.4 ms when adopting Ar as the carrier gas causing one more microwave pulse absorbed. Generally, changes in equivalence ratio show no obvious influence on the energy-absorbing process. Microwave pulses during the time window periodically generate a plasma jet that originates from the electrodes gap region towards the flame front. This plasma jet acts as the medium for the microwave influencing the flame development, and thus the relative position between the plasma jet and the flame front determines the final microwave enhancing effect.

Continuous microwave-assisted extrusion for high moisture texturized foods: A feasibility study

This study demonstrates a technical scale set-up for microwave-assisted extrusion of high moisture, texturized food systems. The set-up has been evaluated in a stationary study regarding the correlation of microwave tuner position and gap with the reflected microwave power and the product temperature, respectively. On that basis, a casein-based model food system has been plasticized by continuous microwave extrusion. The product outlet temperature and chemical composition have been investigated with varying screw speeds and power inputs.The stationary study resulted in specific tuner positions, giving an area of desired process parameters. This area was successfully transferred to a continuous product application, where desired gel-sol transition was achieved. With increasing power input, the product temperature increased, while it decreased with increasing screw speed and constant power input. The fat, protein, and dry matter content of the casein-based model system did not differ significantly throughout all trials. Industrial relevanceMicrowave heating is a promising alternative heating method for food applications. Within microwave extrusion it shows high potential for upscaling applications whilst maintaining a homogenous temperature distribution throughout the product. Moreover, microwave technology offers high efficiencies and the possibility to use green energy sources. Furthermore, the space saving design and easy application provide high potential for scale-up.

Cyclotron resonance in the high mobility GaAs/AlGaAs 2D electron system over the microwave, mm-wave, and terahertz- bands

The reflected microwave power from the photo-excited high mobility GaAs/AlGaAs 2D device has been measured over the wide frequency band spanning from 30 to 330 GHz simultaneously along with diagonal magnetoresistance as a function of the magnetic field. Easily distinguishable resonances in the reflected power signal are observed at the same magnetic fields as a reduced amplitude in the Shubnikov-de Haas (SdH) oscillations of the diagonal magnetoresistance. The reflection resonances with concurrent amplitude reduction in SdH oscillations are correlated with cyclotron resonance induced by microwave, mm-wave, and terahertz photoexcitation. The magnetoplasma effect was also investigated. The results suggest a finite frequency zero-magnetic-field intercept, providing an estimate for the plasma frequency. The experimentally measured plasma frequency appears to be somewhat lower than the estimated plasma frequency for these Hall bars. The results, in sum, are consistent with an effective mass ratio of m*/m = 0.067, the standard value, even in these high mobility GaAs/AlGaAs devices, at very large filling factors. Preliminary findings from this article have been published as conference proceedings, see Kriisa, A., et al., J. of Phys. Conf. Ser. 864, 012057 (2017).

AERATED CONCRETE MICROWAVE REFLECTION AND TRANSMISSION PROPERTIES IN A WET ENVIRONMENT

Excess water content within the building material could greatly alter the interaction of the microwave with the material. In this study aerated concrete (AEROC) block walls (60x60cm) were investigated for their microwave (2.4 GHz) properties under wetting conditions. The spray wetting of the wall was conducted to simulate the environmental processes such as raining or water damage. 50ml/m² water dosage was applied on the surface of the concrete 21 times in 1 min intervals. The results show a noticeable decrease in microwave penetration power through the material after the sample gets enriched with water. As the water content on the surface of the material rises, so does the transmission loss. During the different water content of the aerated concrete, the reflection loss varied from -15.04 dB (dry wall) to -5.03 dB (wet wall’s surface). The transmission loss continues to rise during the entire length of the experiment, from -4.5 dB as a dry sample to -8.3 dB after 441 ml (1035 ml/m² during 23 min) of sprayed water. The variation of reflected microwave power was approximately ten times, which is quite a significant indicator of alteration of microwave propagation. The results demonstrate wetting process as an considerable factor in assessing microwave propagation in near the sources, such as mobile phone base station antennas, industrial microwave heaters and ovens etc.

Development of a microwave photoconductance measurement technique for the study of carrier dynamics in highly-excited 4H-SiC

The microwave conductance decay (MCD) technique combining an initially matched transmission line setup and picosecond optical excitation was developed and applied for the monitoring of transmitted and reflected microwave power transients in a 4H-SiC epilayer in a wide excitation range, from 2 × 1014 to 1018 cm−3. The excitation-dependent decrease in measurement sensitivity in the power-law relations of the transients was observed at excess carrier densities above 1016 cm−3 due to the line mismatches and decrease in the internal microwave field in the illuminated sample. The calibration procedure of MCD data on excess carrier density was applied for the correction of the MCD transients and resulted in nearly identical MCD kinetics in the reflection and transmission. In a 35 μm-thick n-type 4H-SiC epilayer, the tendencies of the gradual decrease of the initial decay time with an excitation increase and the excitation-enhanced carrier recombination rate in MCD tails were analyzed numerically. These tendencies were attributed to the excitation dependent surface recombination rate and the enhanced trap-related bulk recombination, correspondingly.

Toward broad-band x-ray detected ferromagnetic resonance in longitudinal geometry

An ultrahigh-vacuum-compatible setup for broad-band X-ray detected ferromagnetic resonance (XFMR) in longitudinal geometry is introduced which relies on a low-power, continuous-wave excitation of the ferromagnetic sample. A simultaneous detection of the conventional ferromagnetic resonance via measuring the reflected microwave power and the XFMR signal of the X-ray absorption is possible. First experiments on the Fe and Co L3-edges of a permalloy film covered with Co nanostripes as well as the Fe and Ni K-edges of a permalloy film are presented and discussed. Two different XFMR signals are found, one of which is independent of the photon energy and therefore does not provide element-selective information. The other much weaker signal is element-selective, and the dynamic magnetic properties could be detected for Fe and Co separately. The dependence of the latter XFMR signal on the photon helicity of the synchrotron light is found to be distinct from the usual x-ray magnetic circular dichroism effect.

Current-driven non-linear magnetodynamics in exchange-biased spin valves

This work investigates the excitation of parametric resonance in exchange-biased spin valves (EBSVs). Using a mechanical point contact, high density dc and microwave currents were injected into the EBSV sample. Observing the reflected microwave power and the small rectification voltage that develops across the contact allows detecting the current-driven magnetodynamics not only in the bulk sample but originating exclusively from the small contact region. In addition to ferromagnetic resonance (FMR), parametric resonance at twice the natural FMR frequency was observed. In contrast to FMR, this non-linear resonance was excited only in the vicinity of the point contact where current densities are high. Power-dependent measurements displayed a typical threshold-like behavior of parametric resonance and a broadening of the instability region with increasing power. Parametric resonance showed a linear shift as a function of applied dc bias which is consistent with the field-like spin-transfer torque induced by current on magnetic moments in EBSV.

Characterizing Microwave Power in a MPCVD System using Gaussian Mixture Modeling

Microwave Plasma Chemical Vapor Deposition (MPCVD) can be used to grow various kinds of diamond films and carbon nanotubes at various temperatures. Issues concerning plasma modeling and control play an important role in MPCVD systems. One crucial factor in controlling the plasma shape and position is the tunable reflected microwave power of the MPCVD system. However, modeling the tunable reflected power is highly complex and re mains a poorly developed. In this paper, the microwave power distribution corresponding to the adjustable electromagnetic field is mo deled by 2-D Gaussian Mixture Modeling (GMM). The simulations using the model show that microwave power data can be simplified to a linear combination of some Gaussian functions, allowing predictable control for tuning manufacturing parameters and plasma sharp in real-time. The experimental results show that each E-H tuner position can fabricate the Multiwall Carbon Nanotubes (MWCNTs) films with high reproducibility after GMM modeling.

Experimental investigation of microwave interaction with magnetoplasma in miniature multipolar configuration using impedance measurements

A miniature microwave plasma source employing both radial and axial magnetic fields for plasma confinement has been developed for micro-propulsion applications. Plasma is initiated by launching microwaves via a short monopole antenna to circumvent geometrical cutoff limitations. The amplitude and phase of the forward and reflected microwave power is measured to obtain the complex reflection coefficient from which the equivalent impedance of the plasma source is determined. Effect of critical plasma density condition is reflected in the measurements and provides insight into the working of the miniature plasma source. A basic impedance calculation model is developed to help in understanding the experimental observations. From experiment and theory, it is seen that the equivalent impedance magnitude is controlled by the coaxial discharge boundary conditions, and the phase is influenced primarily by the plasma immersed antenna impedance.

Beam extraction and high stability operation of high current electron cyclotron resonance proton ion source

A high current electron cyclotron resonance proton ion source is designed and developed for the low energy high intensity proton accelerator at Bhabha Atomic Research Centre. The plasma discharge in the ion source is stabilized by minimizing the reflected microwave power using four stub auto tuner and magnetic field. The optimization of extraction geometry is performed using PBGUNS code by varying the aperture, shape, accelerating gap, and the potential on the electrodes. While operating the source, it was found that the two layered microwave window (6 mm quartz plate and 2 mm boron nitride plate) was damaged (a fine hole was drilled) by the back-streaming electrons after continuous operation of the source for 3 h at beam current of 20-40 mA. The microwave window was then shifted from the line of sight of the back-streaming electrons and located after the water-cooled H-plane bend. In this configuration the stable operation of the high current ion source for several hours is achieved. The ion beam is extracted from the source by biasing plasma electrode, puller electrode, and ground electrode to +10 to +50 kV, -2 to -4 kV, and 0 kV, respectively. The total ion beam current of 30-40 mA is recorded on Faraday cup at 40 keV of beam energy at 600-1000 W of microwave power, 800-1000 G axial magnetic field and (1.2-3.9) × 10(-3) mbar of neutral hydrogen gas pressure in the plasma chamber. The dependence of beam current on extraction voltage, microwave power, and gas pressure is investigated in the range of operation of the ion source.

Temperature and density evolution during decay in a 2.45 GHz hydrogen electron cyclotron resonance plasma: Off-resonant and resonant cases

Time resolved electron temperature and density measurements during the decay stage in a hydrogen electron cyclotron resonance (ECR) plasma are presented for a resonance and off-resonance magnetic field configurations. The measurements are conducted on a ECR plasma generator excited at 2.45 GHz denominated test-bench for ion-sources plasma studies at ESS Bilbao. The plasma parameters evolution is studied by Langmuir probe diagnostic with synchronized sample technique developed for repetitive pulsed plasmas with a temporal resolution of 200 ns in typical decay processes of about 40 μs. An afterglow transient is clearly observed in the reflected microwave power signal from the plasma. Simultaneously, the electron temperature evolution shows rebounding peaks that may be related to the interplay between density drop and microwave coupling with deep impact on the Electron Energy Distribution Function. The correlation of such structures with the plasma absorbed power and the coupling quality is also reported.

Power density control in microwave assisted air drying to improve quality of food

Currently, the new trend is the development of microwave hot air drying control in order to solve the problem of hot spots on the product. So in this study, a microwave/hot air drying system with the ability of automatic power density control was developed for tomato drying. Three power densities (3, 5 and 7W/g) at 40°C air temperature were applied and the output, reflected microwave power and the mass of the sample were registered online. Two different drying strategies were implemented. The first consisted of a microwave drying without specific power control. The second one introduced power control. A feedback control based on specific power related to the actual sample mass resulted in the best power control and the improvement of product quality compared to the dry sample without power control.

Microwave Plasma Enhancement of Various Flame Geometries at Atmospheric Pressure

A plasma-coupled methane-air flame is produced at atmospheric pressure by a microwave plasma source utilizing a tunable waveguide. Laser diagnostics are used to examine the direct-coupled, plasma-ignited, and sustained flame, for multiple flame types and nozzle geometries. OH radical number densities are quantified using planar laser-induced fluorescence and temperature measured by Rayleigh scattering thermometry. Premixed and nonpremixed flames are studied using both solid and hollow inner conductors in the plasma-applicating nozzle. The plasma source is powered by a continuous 2.45-GHz magnetron producing 360 W of power. Plasma power is controlled by adjusting the reflected microwave power, measured at a dummy load attached to a circulator. Maximum OH radical number densities were quantified as approximately (3 - 5) × 1016 cm-3 for plasma powers around 100 W, with small variation between configurations. The maximum temperatures occurred in the nonpremixed flame, where the plasma is generated in air, reaching values of 3500 K. Temperatures are lower, peaking at 2000 K, when the plasma is generated at the air-fuel boundary or the air-premixed boundary through use of the hollow inner conductor. Additional parameters are adjusted, including flow rates, power level, and equivalence ratio, and the effects are discussed. Nonpremixed configurations are ill suited for flame enhancement, whereas a premixed flow through the hollow electrode best demonstrates nonthermal plasma-assisted combustion.

Electric field measurement in microwave discharge ion thruster with electro-optic probe

In order to understand the internal phenomena in a microwave discharge ion thruster, it is important to measure the distribution of the microwave electric field inside the discharge chamber, which is directly related to the plasma production. In this study, we proposed a novel method of measuring a microwave electric field with an electro-optic (EO) probe based on the Pockels effect. The probe, including a cooling system, contains no metal and can be accessed in the discharge chamber with less disruption to the microwave distribution. This method enables measurement of the electric field profile under ion beam acceleration. We first verified the measurement with the EO probe by a comparison with a finite-difference time domain numerical simulation of the microwave electric field in atmosphere. Second, we showed that the deviations of the reflected microwave power and the beam current were less than 8% due to inserting the EO probe into the ion thruster under ion beam acceleration. Finally, we successfully demonstrated the measurement of the electric-field profile in the ion thruster under ion beam acceleration. These measurements show that the electric field distribution in the thruster dramatically changes in the ion thruster under ion beam acceleration as the propellant mass flow rate increases. These results indicate that this new method using an EO probe can provide a useful guide for improving the propulsion of microwave discharge ion thrusters.

Investigations of a microwave plasma source operating with air, N2, CO2 and argon gases

The present contribution will report on the development of a Microwave Plasma Torch (MPT) at the von Karman institute and on the characterization of subsonic plasma flow of various plasmagene gases (air, N2, CO2 and Ar) at atmospheric pressure. The aim of this work is to explore plasma flow regimes for a large envelope of operating conditions. Using a conventional arrangement for MPT, including cylindrical open-ended dielectric quartz tube, plasma generated with molecular gases have been found to be homogeneous while with argon filamentary structures have been observed. Electrical characterization of the source has been performed for some operating conditions using forwarded and reflected microwave power monitoring. Comprehensive characterization of the flow by means of high speed imaging has been performed to estimate the effect of the oscillation of the delivered power on plasma jet unsteadiness. Analysis in time and frequency domains of the light intensity recorded at 1 kHz rate is presented. Low resolution Optical Emission Spectroscopy (OES) diagnostic was performed in order to identify typical radiative signature of air, N2 and CO2 plasma jet. High resolution OES was performed on pure N2 plasma suitable for thermodynamic characterization using N2+ First Negative and N2 2nd Positive systems. Making the common assumption that the internal energy levels of molecular species are populated according to a Boltzmann distribution and making no assumption concerning the total species concentration similarly to [1], comparisons between measured and calculated radiative signature of molecular systems evidence for the achievement of thermal equilibrium and slight departure to Saha equilibrium.

Carbothermal Reduction of Iron Ore Applying Microwave Energy

AbstractThis paper presents the results of a study on carbothermal reduction of iron ore made under the microwave field in equipment specially developed for this purpose. The equipment allows the control of radiated and reflected microwave power, and therefore measures the microwave energy actually applied to the load in the reduction process. It also allows performing energy balances and determining the reaction rate with high levels of confidence by simultaneously measuring temperature and mass of the material upon reduction with high reproducibility. We used a microwave generator of 2.45 GHz with variable power up to 3000 W. Self‐reducing pellets under argon atmosphere, containing iron ore and petroleum coke, with 3.5 g of mass and 15 mm of diameter were declined. We obtained the kinetic curves of reduction of iron ore and of energy consumption to the process in the maximum electric field, in the maximum magnetic field and at different values of power/mass. The data allow analyzing how the microwave energy was actually consumed in the reduction of ore.

Effect of microwave and hot air drying on flax straw at controlled temperatures

Flax stems were subjected to microwave drying at controlled temperatures. The rate of drying was then compared with conventional hot air drying. The product temperature was maintained at 40°C, 60°C and 80°C for both microwave and hot air drying. The moisture content of flax stem was about 70% (wet basis). The microwave drying was conducted in a microwave apparatus which recorded mass, product temperature, incident microwave power, reflected microwave power and inlet/outlet air temperature. The final moisture content for experiment was set to 9% (wet basis). Microwave-convective drying ensured about 30% to 70% reduction of drying time for drying flax straw as compared to hot air drying. Curve fitting with different mathematical models were carried out and all the models were fitted well for both hot air and microwave drying. The tensile strength of flax straw, measured with an Instron apparatus, increased with an increase in the processing temperature of both processes. Hot air dried flax straw showed the greatest tensile strength and modulus of elasticity at processing temperatures of 60°C and 80°C with a significant difference.

Electron density measurements in a pulse-repetitive microwave discharge in air

We have developed a technique for absolute measurements of electron density in pulse-repetitive microwave discharges in air. The technique is based on the time-resolved absolute intensity of a nitrogen spectral band belonging to the Second Positive System, the kinetic model and the detailed particle balance of the N2C3Πu (ν = 0) state. This new approach bridges the gap between two existing electron density measurement methods (Langmuir probe and Stark broadening). The electron density is obtained from the time-dependent rate equation for the population of N2C3Πu (ν = 0) using recorded waveforms of the absolute C3Πu → B3Πg (0-0) band intensity, the forward and reflected microwave power density. Measured electron density waveforms using numerical and approximated analytical methods are presented for the case of pulse repetitive planar surface microwave discharge at the aperture of a horn antenna covered with alumina ceramic plate. The discharge was generated in air at 11.8 Torr with a X-band microwave generator using 3.5 μs microwave pulses at peak power of 210 kW. In this case, we were able to time resolve the electron density within a single 3.5 μs pulse. We obtained (9.0 ± 0.6) × 1013 cm–3 for the peak and (5.0 ± 0.6) × 1013 cm–3 for the pulse-average electron density. The technique presents a convenient, non-intrusive diagnostic method for local, time-defined measurements of electron density in short duration discharges near atmospheric pressures.