Power Microwave Pulses Research Articles

ULTRASHORT MICROWAVE PULSE GENERATION BY PASSIVE PULSE COMPRESSION IN A COMPACT REVERBERANT CAVITY

In this paper, we demonstrate a device that is capable of generating an ultrashort (sub-nanosecond) high power microwave pulse by means of passive pulse compression in a compact reverberant cavity. The long duration input pulse into the cavity is created using time-reversal techniques, which allows the waveform to contain the inverse profile of the cavity phase distortion. When fed back into the cavity, the wave focusing at the output port results in a compressed ultrashort pulse with enhanced peak amplitude. We experimentally demonstrate a pulse compressor consisting of a 0.0074 m 3 cavity capable of generating a 130 picosecond pulse from an input waveform of 300 nanosecond duration with the peak gain of up to 19 dB.

Self-consistent fluid modeling and simulation on a pulsed microwave atmospheric-pressure argon plasma jet

In present study, a pulsed lower-power microwave-driven atmospheric-pressure argon plasma jet has been introduced with the type of coaxial transmission line resonator. The plasma jet plume is with room air temperature, even can be directly touched by human body without any hot harm. In order to study ionization process of the proposed plasma jet, a self-consistent hybrid fluid model is constructed in which Maxwell's equations are solved numerically by finite-difference time-domain method and a fluid model is used to study the characteristics of argon plasma evolution. With a Guass type input power function, the spatio-temporal distributions of the electron density, the electron temperature, the electric field, and the absorbed power density have been simulated, respectively. The simulation results suggest that the peak values of the electron temperature and the electric field are synchronous with the input pulsed microwave power but the maximum quantities of the electron density and the absorbed power density are lagged to the microwave power excitation. In addition, the pulsed plasma jet excited by the local enhanced electric field of surface plasmon polaritons should be the discharge mechanism of the proposed plasma jet.

Stepped-frequency continuous-wave microwave-induced thermoacoustic imaging

Microwave-induced thermoacoustic (TA) imaging combines the dielectric contrast of microwave imaging with the resolution of ultrasound imaging. Prior studies have only focused on time-domain techniques with short but powerful microwave pulses that require a peak output power in excess of several kilowatts to achieve sufficient signal-to-noise ratio (SNR). This poses safety concerns as well as to render the imager expensive and bulky with requiring a large vacuum radio frequency source. Here, we propose and demonstrate a coherent stepped-frequency continuous-wave (SFCW) technique for TA imaging which enables substantial improvements in SNR and consequently a reduction in peak power requirements for the imager. Constructive and destructive interferences between TA signals are observed and explained. Full coherency across microwave and acoustic domains, in the thermo-elastic response, is experimentally verified and this enables demonstration of coherent SFCW microwave-induced TA imaging. Compared to the pulsed technique, an improvement of 17 dB in SNR is demonstrated.

Synergistic effect of microwave plasma and ultrasonic wave on decomposition of organic compounds in water

To improve the decomposition efficiency of organic compounds dissolved in water, an ultrasonic wave was synchronously superposed to a microwave plasma produced under water. By changing the phase difference between the microwave plasma pulse and the ultrasonic wave, enhancement of OH emission in the plasma as well as enhancement of organic decomposition was observed when the pulsed plasma was ignited at a phase of instantaneous reduced pressure induced by the ultrasonic wave. This result suggests that the ultrasonic power can enhance OH radical formation by choosing the timing of the synchronously-injected pulsed microwave power. The decomposition rate from the viewpoint of energy efficiency was about four times larger with the ultrasonic wave superposition compared with the case of ordinary microwave plasma treatment without the ultrasonic superposition.

Enhanced loss of magnetic-mirror-trapped fast electrons by a shear Alfvén wave

Laboratory observations of enhanced loss of magnetic mirror trapped fast electrons irradiated by a shear Alfvén Wave (SAW) are reported. The experiment is performed in the quiescent after-glow plasma in the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62(12), 2875–2883 (1991)]. A trapped energetic electron population (>100 keV) is generated in a magnetic mirror section (mirror ratio ≈ 2, length = 3.5 m) by an X-mode high power microwave pulse, and forms a hot electron ring due to the grad-B and curvature drift. SAWs of arbitrary polarization are launched externally by a Rotating Magnetic Field source (δB/B0 ≈ 0.1%, λ∥ ≈ 9 m). Irradiated by a right-handed circularly polarized SAW, the loss of electrons, in both the radial and the axial direction of the mirror field, is significantly enhanced and is modulated at fAlfvén. The periodical loss continues even after the termination of the SAW. Experimental observations suggest that a spatial distortion of the ring is formed in the SAW field and creates a collective mode of the hot electron population that degrades its confinement and leads to electron loss from the magnetic mirror. The results could have implications on techniques of radiation belt remediation.

Afterglow of Argon Plasmas with H2, O2, N2, and CO2Admixtures Observed by Thomson Scattering

This study assess the decay of a power-pulsed microwave plasma in argon mixtures using time resolved Thomson scattering. In argon at intermediate pressure, i.e. tens of mbar, the electron temperature decays very fast after power off and approaches the heavy particle temperature, while the electron density decays slower, in the order of tens of microseconds. Argon with small admixtures of O2 or H2 exhibits a similar behavior. However, strikingly different is the result for small additions of N2 or CO2. For up to 100 μs after the power is switched off, the electron temperature does not decrease to the gas temperature but remains at values as high as 0.8 eV. Nitrogen and carbon dioxide exhibit relatively large cross-sections to excite vibrational states by electron collisions. Knowing that, the observed post-heating can be attributed to super-elastic energy transfer from vibrational excited N2(X) and CO2(X) molecules to electrons.

Applied research of stainless steel fiber-filled acrylonitrile–butadiene–styrene copolymer composites

Composite is prepared by filling acrylonitrile–butadiene–styrene copolymer with stainless steel fiber. Its physical properties and electromagnetic shielding effectiveness are tested. When the mass percent of stainless steel fiber is 15%, shielding effectiveness of composite is 21–59 dB in frequency band of 15 MHz to 3 GHz and cylindrical shell is made of it. Experiments under radiation of X-band high power microwave and ultra-wideband electromagnetic pulse are conducted in microwave anechoic chamber while magnetic fuzes connected with electrical fire head are placed at the center of cylindrical shell. Results show good agreement with test. Coupling effect is analyzed while shell has a hole or slot of millimeter level.

微波击穿大气电子温度与电子密度依存关系

微波击穿大气电子温度与电子密度依存关系

高功率微波脉冲大气击穿概率研究

高功率微波脉冲大气击穿概率研究

Characterization of High Power Microwave Radiation by an Axially Extracted Vircator

Characterization results of high-power microwave radiation, from an axial vircator driven by pulsed electron beam accelerator AMBICA-600 are reported in this paper. We present a study on variation in pulsed microwave power output and dominant frequency by discretely varying anode-cathode (A-C) gap. While keeping the cathode diameter fixed at 40 mm, for the A-C gap distance in the range 5-9 mm, dominant frequencies have been measured to be lying in the range 4.7-9.8 GHz. The trend of a subsequent increase in the dominant frequency at lower A-C gap distances (and vice versa) revealed that center frequency is mainly governed by the longitudinal size of the potential well. The highest microwave power of ~ 14 MW for ~ 75-ns pulse duration was obtained at A-C gap of 7 mm having the dominant frequency in C-band at ~ 6.9 GHz. The beam-to-microwave power conversion efficiency of ~ 1.2% has been demonstrated in our experiments. On the basis of power distribution pattern obtained by the gas breakdown technique, the dominant mode of emission is believed to be transverse magnetic mode. Relative analysis of frequency spectrums obtained for various A-C gap distances evidenced experimental recognition of optimum A-C spacing as a generation of narrowband distinct frequency peak of large magnitude with minimal mode hopping.

Inertial confinement fusion driven by long wavelength electromagnetic pulses

AbstractA method for inertial confinement fusion driven by powerful long wavelength electromagnetic pulses (EMPs), such as ${\rm CO}_{2}$ laser pulses or high power microwave pulses, is proposed. Due to the high efficiency of generating such long wavelength electromagnetic pulses, this method is especially important for the future fusion electricity power Special fuel targets are designed to overcome the shortcomings of the long wavelength electromagnetic pulses.

UV Emission from N2/O2 Pulse Microwave Plasma

The emission spectrum from pulse microwave discharge of N2/O2 gas mixture in the UV range (from 200 – 400 nm) was studied in a cylindrical quartz tube aiming to apply it as a mercury free UV light source. The UV light in the 200-280 nm range has germicidal effect which can be used for water purification. In our previous work we used continuous microwave power to produce UV emission from N2/O2 discharge. In the present work we used pulse microwave power instead of continuous microwave power. We studied the dependence of pulse width and pulse interval on the intensity of the UV emission from N2/O2 discharge. Experimental results showed that the UV emission intensity emitted from N2/O2 gas discharge in 200- 400 nm region does not vary with pulse width and pulse interval but depends on applied average power. DOI: http://dx.doi.org/10.3329/rujs.v38i0.16543 Rajshahi University J. of Sci. 38, 01-08 (2010)

Generation of beating wave by multi-coaxial relativistic backward wave oscillator

AbstractMulti-coaxial relativistic backward wave oscillator that generates the beating wave of high power microwave pulse driven by a single accelerator and a single guiding magnet system is presented. Making use of the coaxial annular cathodes that can synchronously produce three annular beams at one shot, the average power of 5.88 GW consisting of two frequency components 9.0 and 9.7 GHz is obtained under the diode voltage and current 724 and 19.57 kA, corresponding to the conversion efficiency 41.5%. The conversion efficiency and the beating frequency are considerably stable with the diode voltage. The coaxial transmission supporter developed from our previous experimental research is employed to conductively connect the coaxial structure and to incoherently combine the microwave pulse of two frequencies with little reflection. It is found that the equipotential connection of the coaxial structure modifies the field distribution in the diode structure to facilitate the operation of the coaxial annular cathodes. The coaxial cathodes of different lengths are proved to be efficient at depressing the space charge effect in order to prevent the explosive emission of the inner cathode from being shielded by the outer annular beams.

Plasma relaxation mechanics of pulsed high power microwave surface flashover

Microwave transmission and reflection characteristics of pulsed radio frequency field generated plasmas are elucidated for air, N2, and He environments under pressure conditions ranging from 10 to 600 torr. The pulsed, low temperature plasma is generated along the atmospheric side of the dielectric boundary between the source (under vacuum) and the radiating environment with a thickness on the order of 5 mm and a cross sectional area just smaller than that of the waveguide. Utilizing custom multi-standard waveguide couplers and a continuous low power probing source, the scattering parameters were measured before, during, and after the high power microwave pulse with emphasis on the latter. From these scattering parameters, temporal electron density estimations (specifically the longitudinal integral of the density) were calculated using a 1D plane wave-excited model for analysis of the relaxation processes associated. These relaxation characteristics ultimately determine the maximum repetition rate for many pulsed electric field applications and thus are applicable to a much larger scope in the plasma community than just those related to high power microwaves. This manuscript discusses the diagnostic setup for acquiring the power measurements along with a detailed description of the kinematic and chemical behavior of the plasma as it decays down to its undisturbed state under various gas type and pressure conditions.

The pulsed microwave damage trend of a bipolar transistor as a function of pulse parameters

In the present paper we conduct a theoretical study of the thermal accumulation effect of a typical bipolar transistor caused by high power pulsed microwaves (HPMs), and investigate the thermal accumulation effect as a function of pulse repetition frequency (PRF) and duty cycle. A study of the damage mechanism of the device is carried out from the variation analysis of the distribution of the electric field and the current density. The result shows that the accumulation temperature increases with PRF increasing and the threshold for the transistor is about 2 kHz. The response of the peak temperature induced by the injected single pulses indicates that the falling time is much longer than the rising time. Adopting the fitting method, the relationship between the peak temperature and the time during the rising edge and that between the peak temperature and the time during the falling edge are obtained. Moreover, the accumulation temperature decreases with duty cycle increasing for a certain mean power.

Modification of Enamel and Dentin Surfaces by Non‐Thermal Atmospheric Plasma

AbstractTooth material surface chemical and micromechanical properties are important for success and sustainability in restorative dentistry. Modification by cold atmospheric plasma jet treatment may help to improve presently existing shortcomings as a part of future restorative therapies. The present study has been focused on changes of surface properties of tooth substances by this approach. Polished and etched enamel and dentin slices from the vestibular face of bovine incisors are used. A plasma jet modifies these surfaces at a distance of 2 mm and with a scan velocity of 5.5 mm · s−1. The treatment parameters are: helium process gas flow 3.525 slm, plasma excitation by 2.45 GHz with an average pulsed microwave power of 2 W, single pulse power of 250 W, pulse width 5 µs. The surface modifications are subjected to laser scanning microscope roughness‐measurement, contact angle measurements, X‐ray photoelectron spectroscopy (XPS) analysis and morphological visualization by scanning electron microscopy (SEM). Plasma treatment causes changes of surface roughness and morphological alterations on dentin, while no such changes are measureable on etched dentin and polished and etched enamel surfaces. Plasma causes contact angle reductions for both, water and ethylene glycol. XPS analyses show significant carbon reduction on enamel surfaces after plasma treatment. Admixture of 30% of hydrogen peroxide or deionized water, respectively, as precursors in the plasma jet processing gas flow generates measureable amounts of CO‐bindings at the surfaces assigned to hydrophilic groups. Surface modifications by plasma treatment might enable interesting new options for dental procedures in all areas of interaction between tooth substance and restorative materials. magnified image

Wide band waveguide sensor for microwave pulse measurement

A wide band resistive sensor, the performance of which is based on electron heating effect in semiconductors, has been implemented in a double ridge waveguide WRD250 in a frequency range of 2.6–7.8 GHz. The dependences of the output signal on microwave pulse power and frequency response of the sensor were measured. The measured results of frequency response have been compared with the calculated ones obtained using a finite-difference time-domain method, and a reasonable agreement between them was found.

Wide band waveguide sensor for microwave pulse measurement

A wide band resistive sensor, the performance of which is based on electron heating effect in semiconductors, has been implemented in a double ridge waveguide WRD250 in a frequency range of 2.6–7.8 GHz. The dependences of the output signal on microwave pulse power and frequency response of the sensor were measured. The measured results of frequency response have been compared with the calculated ones obtained using a finite-difference time-domain method, and a reasonable agreement between them was found.

Transition from equilibrium to nonequilibrium combustion of premixed burner flame by microwave irradiation

We found that the burning velocity in premixed burner flame was enhanced by heated electrons in the flame. The electron heating was realized by irradiating microwave power onto the flame, and was confirmed by observing optical emission intensity of molecular nitrogen at the second positive system. Since the increase in the observed gas temperature was negligible, the enhancement of the burning velocity can be attributed to the nonequilibrium combustion chemistry which is driven by energetic electrons. We examined the time constants for the transition between equilibrium and nonequilibrium combustion states by irradiating pulsed microwave power. As a result, we found >2 × 104 s−1 for the frequency of electron heating, ∼1 × 103 s−1 for the loss frequency of heated electrons, and ∼(0.5–1) × 103 s−1 for the loss frequencies of OH and CH radicals.

Ultraminiature Pulsed Periodic Generator of Powerful Microwave Pulses, Based on Gas Discharge in Hollow Cathode

An ultraminiature pulsed periodic generator of powerful microwave pulses based on gas discharge, involving a hollow cathode, is developed and tested. The hollow cathode is presented by a cylindrical tube of 15-mm diameter and 15-mm height. The cavity was filled with air under 0.2-torr pressure. The periodic sequence of high-voltage pulses of 3.5-kV amplitude was sent onto gas-discharge gap. Coherent microwave pulses, featuring ~ 1-MW power and ~ 200-ns duration, were generated in a chamber from the frequency band Lp. The pulse repetition rate of 1 kHz was achieved in pulsed periodic mode with no noticeable degradation of microwave pulses.