Short Microwave Pulses Research Articles

Q-Band Loop-Gap Resonator for EPR Applications with Broadband-Shaped Pulses

Recently, Q-band-pulsed electron paramagnetic resonance spectroscopy has strongly advanced its performance by the introduction of high-power microwave amplifiers and the use of shaped pulses. For such applications, the resonator Q value has to be low enough to achieve sufficient bandwidth for short microwave pulses and to reduce the ring-down time after the pulses. However, a low Q value reduces the detection sensitivity as well as the conversion efficiency of the microwave input power to the magnetic field strength at the sample position. Therefore, the resonator Q value has to be optimized for a given microwave input power and specific application. We designed a three-loop/two-gap resonator using CST Microwave Studio for such applications, and tested its performance in comparison with a standard Bruker D2 Q-band microwave resonator by accomplishing broadband SIFTER experiments on a nitroxide model compound.

Microwave pulse shortening in plasma relativistic high-current microwave electronics

We describe mechanisms of microwave pulse shortening in radiation sources with the power of about 108 W based on interaction between relativistic electron beams of nanosecond duration and preformed plasma. The shortening is mainly due to the electron return flow through the plasma, which leads to a multiple decrease in the linear gain of the microwave by the relativistic electron beam and in the reflection coefficient of the plasma wave that provides the generator feedback. The ways to eliminate the effect of microwave pulse shortening are proposed.

Time-dependent numerical simulation of diffraction and absorption effects in diagnostics of short high-power microwave pulses using wide-aperture liquid calorimeters

Time-dependent numerical simulation of diffraction and absorption effects in diagnostics of short high-power microwave pulses using wide-aperture liquid calorimeters

An improve RCB method based on microwave induced thermo acoustic tomography

Thermoacoustic imaging (TAI) is a non-ionizing and non-invasive imaging method, which combines the merits of high ultrasound imaging resolution with high microwave imaging contrast. In TAI, a short non-ionizing microwave pulse irradiates tissues to induce a small temperature rise, which consequently causes thermoelastic expansion to generate TA signals. By using an image reconstruction algorithm, TAI can then recover the microwave absorption distribution inside the tissue and further distinguish abnormal areas from background normal tissues. TAI for breast cancer detection is the main purpose of this study, the basic theory of TAI was introduced at first, especially the RCB reconstruction algorithm for TAI. After that, in this dissertation, two sets of thermoacoustic imaging systems were developed, which named thermoacoustic tomography and ultra-short pulse based high resolution TAI. According to experiments and theoretical studies carried out in this dissertation, the feasibility of thermoacoustic imaging method by Robust Capon Beam-former (RCB) for breast cancer detection before its clinical investigation is fully validated. Due to high imaging performance needs for the early detection of breast cancer. Based on the advantages of TAI, the potential applications of TAI for finger joints and brain diseases diagnosing are explored, which is opening up a new field for TAI.

Kinetic study on non-thermal volumetric plasma decay in the early afterglow of air discharge generated by a short pulse microwave or laser

This paper reports a kinetic study on non-thermal plasma decay in the early afterglow of air discharge generated by short pulse microwave or laser. A global self-consistent model is based on the particle balance of complex plasma chemistry, electron energy equation, and gas thermal balance equation. Electron-ion Coulomb collision is included in the steady state Boltzmann equation solver to accurately describe the electron mobility and other transport coefficients. The model is used to simulate the afterglow of microsecond to nanosecond pulse microwave discharge in N2, O2, and air, as well as femtosecond laser filament discharge in dry and humid air. The simulated results for electron density decay are in quantitative agreement with the available measured ones. The evolution of plasma decay under an external electric field is also investigated, and the effect of gas heating is considered. The underlying mechanism of plasma density decay is unveiled through the above kinetic modeling.

ANALYSIS OF SHORT PULSE IMPACTING ON MICROWAVE INDUCED THERMO-ACOUSTIC TOMOGRAPHY

Microwave induced thermo-acoustic tomography (MITAT) is a developing technique for biomedical applications, especially for early breast cancer detection. In this paper, impacts of short microwave pulse on thermo-acoustic (TA) signals are analyzed and verified through some experimental comparisons. In these experiments, short microwave pulses with widths of 10 ns and 500 ns are employed as radiation resources. TA signals generated from a cubic sample are analyzed in both time- and frequency-domain. A trapezoid sample is also performed for experimental comparing. Different from previous literature, the effects of rising edge of radiation microwave pulse have been intensively studied. Experimental results demonstrate that shorter rising edge duration conducts broader bandwidth of TA signal, which give rise to better spatial resolution for tomography imaging.

Nonequilibrium plasma channel in gaseous media formed by powerful UV laser as a waveguide for transportation and amplification of short microwave pulses

The evolution of strongly nonequilibrium plasma in a channel created in gaseous media by powerful UV femtosecond laser pulse is studied. It is demonstrated that such a plasma channel can be used as a waveguide for both transportation and amplification of the microwave radiation. The specific features of such a plasma waveguide are studied on the basis of the self-consistent solution of the kinetic Boltzmann equation for the electron energy distribution function in different spatial points of the gas media and the wave equation in paraxial approximation for the microwave radiation guided and amplified in the channel. The results of modeling for rare gases (xenon) and air are compared. The amplification factor in xenon plasma in dependence on channel radius, intensity and frequency of the input RF radiation is analyzed.

Short-pulse high-power microwave breakdown at high pressures**Project supported by the National Basic Research Program of China (Grant No. 2013CB328904), the NSAF of China (Grant No. U1330109), and 2012 Doctoral Innovation Funds of Southwest Jiaotong University.

The fluid model is proposed to investigate the gas breakdown driven by a short-pulse (such as a Gaussian pulse) high-power microwave at high pressures. However, the fluid model requires specification of the electron energy distribution function (EEDF); the common assumption of a Maxwellian EEDF can result in the inaccurate breakdown prediction when the electrons are not in equilibrium. We confirm that the influence of the incident pulse shape on the EEDF is tiny at high pressures by using the particle-in-cell Monte Carlo collision (PIC-MCC) model. As a result, the EEDF for a rectangular microwave pulse directly derived from the Boltzmann equation solver Bolsig+ is introduced into the fluid model for predicting the breakdown threshold of the non-rectangular pulse over a wide range of pressures, and the obtained results are very well matched with those of the PIC-MCC simulations. The time evolution of a non-rectangular pulse breakdown in gas, obtained by the fluid model with the EEDF from Bolsig+, is presented and analyzed at different pressures. In addition, the effect of the incident pulse shape on the gas breakdown is discussed.

Investigation of a metallic photonic crystal high power microwave mode converter

It is demonstrated that an L band metallic photonic crystal TEM-TE11 mode converter is suitable for narrow band high power microwave application. The proposed mode converter is realized by partially filling metallic photonic crystals along azimuthal direction in a coaxial transmission line for phase-shifting. A three rows structure is designed and simulated by commercial software CST Microwave Studio. Simulation results show that its conversion efficiency is 99% at the center frequency 1.58 GHz. Over the frequency range of 1.56-1.625 GHz, the conversion efficiency exceeds 90 %, with a corresponding bandwidth of 4.1 %. This mode converter has a gigawatt level power handling capability which is suitable for narrow band high power microwave application. Using magnetically insulated transmission line oscillator(MILO) as a high power microwave source, particle-in-cell simulation is carried out to test the performance of the mode converter. The expected TE11 mode microwave output is obtained and the MILO works well. Mode conversion performance of the converter is tested by far-field measurement method. And the experimental result confirms the validity of our design. Then, high power microwave experiment is carried out on a Marx-driven Blumlein water line pulsed power accelerator. Microwave frequency, radiated pattern and power are measured in the far-field region and the results agree well with simulation results. The experiment also reveals that no microwave breakdown or pulse shortening took place in the experimental setup.

Thermal stress FEM analysis of rock with microwave energy

This paper presents a study of a thermal breakage process used to analyse the thermal stress and crack development that occur when rock is exposed to short-pulse microwave energy. A two-dimensional circular plate, containing two-phase minerals, was used in finite element simulation to calculate thermal stress for the purpose of better understanding thermal fracture behaviour in comminution. It is found that the thermal mismatch between a microwave-absorbing inclusion and a low-absorbing matrix mineral can generate large localized thermal stresses around the inclusion. Fracture initially occurs, not around the grain boundaries between the two minerals, but some distance away, as a result of thermal expansion stress on the matrix mineral. The results also indicate that though grain size is one of the factors causing cracks during heating of granular materials, it is not the only reason. The size and the thermal properties of the matrix mineral can also affect the results of thermal stresses.

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.

Elimination of emission quenching of plasma relativistic microwave generator

The mechanism of the development of the effect of microwave pulse shortening in a plasma relativistic microwave generator with a power of 108 W is determined using numerical simulation. Methods for eliminating the causes of microwave generation quenching are proposed. It is shown that the emission time can be significantly increased.

Time-resolved microplasma electron dynamics in a pulsed microwave discharge

Microwave-driven microplasmas are typically operated in a steady-state mode in which the electron temperature is constant in time. Transient measurements of excitation temperature and helium emission lines, however, suggest that short microwave pulses can be used to increase the electron energy by 20–30%. Time-resolved optical emission spectrometry reveals an initial burst of light emission from the igniting microplasma. This emission overshoot is also correlated with a measured increase in excitation temperature. Excimer emission lags atomic emission, however, and does not overshoot. A simple model shows that an increase in electron temperature is responsible for the overshoot of atomic optical emission at the beginning of each microwave pulse. The formation of dimers and subsequent excimer emission requires slower three-body collisions with the excited rare gas atoms, which is why excimer emission does not overshoot the steady-state values. Similar results are observed in argon gas. The overshoot in electron temperature may be used to manipulate the collisional production of species in microplasmas using short, low-duty cycle microwave pulses.

The effect of strong microwave electric field radiation on: (1) vegetable seed germination and seedling growth rate

The effect of high power microwave (HPM) irradiation on seed germination and seedlings was evaluated. Vegetable seeds were subjected to HPM irradiation. The main focus was on the thermal heating elimination during seeds irradiation. For that reason short high frequency microwave pulses were used. The study object was seeds of different harvest years (2000, 2003 and 2008) of radish (Raphanus sativus L.) variety ‘Babtų žara’, tomato (Lycopersicon esculentum Mill.) variety ‘Viltis’, carrot (Daucus sativus Rohl.) variety ‘Vaiguva’ and tomato (Lycopersicon esculentum Mill.) variety ‘Red Cherry’ (seeds harvested in 1980). In the first experiment, all seeds were exposed to microwaves at 9.3 GHz frequency for 10 min. In the second experiment, carrots ‘Vaiguva’ seeds (11 years-old) were exposed to microwaves at 9.3 GHz frequency for 5 and 20 min and exposed to 2.6 and 5.7 GHz microwaves for 10 min. To establish microwave effect on seedling growth rate, tomato, carrot and radish seeds harvested in 2008 were exposed to microwaves at 9.3 GHz frequency for 10 min. It was established that 11 year-old radish seeds exposed to 9.3 GHz microwaves had higher germination as compared with non-irradiated seeds. HPM increased radish germination energy by 6% in seeds harvested in 2003. HPM exposure significantly increased the germination energy and germination of 8 year-old carrot seeds. The highest carrot seed germination was established at 9.3 GHz microwave frequency under 5 min exposure. Seed exposure to HPM (9.3 GHz) had a significant positive effect on dry weight of tomato seedling shoots and on tomato and radish seedling height, but it had negative influence on carrot seedling height. Seed irradiation with HPM (9.3 GHz) had a significant positive effect on the amount of chlorophyll a, chlorophyll b, chlorophylls a + b and carotenoids in tomato seedlings’ fresh mass, but the opposite effect was determined in carrot seedlings’ fresh mass.

Sustained propagation of ultra-lean methane/air flames with pulsed microwave energy deposition

An investigation of pulsed microwave energy addition to laminar methane/air flame fronts is undertaken. Microwave coupling efficiencies of up to 60% are measured in a microwave resonator with very low average power requirements (<30W). Pulsed microwave energy addition to laminar flame fronts is quantified using power absorption measurements and compared with filtered Rayleigh scattering temperature measurements of a laminar stagnation flame. Relative increases in nitric oxide concentrations are measured using a selective radar REMPI laser technique. For outwardly propagating methane/air flames, short microwave pulse bursts increase the effective flame speed up to 25%. With microwave pulse bursts, results show the ability to sustain deflagration fronts at ultra-lean equivalence ratios of 0.3 (compared to the 0.6 normal lean limit) with microwave energy deposition of 10% of the available thermal energy content of the fuel.

Gas breakdown driven by L band short-pulse high-power microwave

High power microwave (HPM) driven gas breakdown is a major factor in limiting the radiation and transmission of HPM. A method that HPM driven gas breakdown could be obtained by changing the aperture of horn antenna is studied in this paper. Changing the effective aperture of horn antenna can adjust the electric field in near field zone, leading to gas breakdown. With this method, measurements of air and SF6 breakdowns are carried out on a magnetically insulated transmission-line oscillators, which is capable of generating HPM with pulse duration of 30 ns, and frequency of 1.74 GHz. The typical breakdown waveforms of air and SF6 are presented. Besides, the breakdown field strengths of the two gases are derived at different pressures. It is found that the effects of air and SF6 breakdown on the transmission of HPM are different: air breakdown mainly shortens the pulse width of HPM while SF6 breakdown mainly reduces the peak output power of HPM. The electric field threshold of SF6 is about 2.4 times larger than that of air. These differences suggest that gas properties have a great effect on the transmission characteristic of HPM in gases.

Transition from interpulse to afterglow plasmas driven by repetitive short-pulse microwaves in a multicusp magnetic field

In the power-off phase, plasmas generated by repetitive short-pulse microwaves in a multicusp magnetic field show a transitive nature from interpulse to afterglow as a function of pulse duration tw = 20–200 μs. The ionized medium can be driven from a highly non equilibrium to an equilibrium state inside the pulses, thereby dictating the behavior of the plasma in the power-off phase. Compared to afterglows, interpulse plasmas observed for tw &amp;lt; 50 μs are characterized by a quasi-steady-state in electron density that persists for ∼ 20–40 μs even after the end of the pulse and has a relatively slower decay rate (∼ 4.3 × 104 s−1) of the electron temperature, as corroborated by optical measurements. The associated electron energy probability function indicates depletion in low energy electrons which appear at higher energies just after the end of the pulse. The transition occurs at tw ∼ 50 μs as confirmed by time evolution of integrated electron numbers densities obtained from the distribution function.

Spin dynamics of paramagnetic centers with anisotropic g tensor and spin of 1/2

The influence of g tensor anisotropy on spin dynamics of paramagnetic centers having real or effective spin of 1/2 is studied. The g anisotropy affects both the excitation and the detection of EPR signals, producing noticeable differences between conventional continuous-wave (cw) EPR and pulsed EPR spectra. The magnitudes and directions of the spin and magnetic moment vectors are generally not proportional to each other, but are related to each other through the g tensor. The equilibrium magnetic moment direction is generally parallel to neither the magnetic field nor the spin quantization axis due to the g anisotropy. After excitation with short microwave pulses, the spin vector precesses around its quantization axis, in a plane that is generally not perpendicular to the applied magnetic field. Paradoxically, the magnetic moment vector precesses around its equilibrium direction in a plane exactly perpendicular to the external magnetic field. In the general case, the oscillating part of the magnetic moment is elliptically polarized and the direction of precession is determined by the sign of the g tensor determinant (g tensor signature). Conventional pulsed and cw EPR spectrometers do not allow determination of the g tensor signature or the ellipticity of the magnetic moment trajectory. It is generally impossible to set a uniform spin turning angle for simple pulses in an unoriented or ‘powder’ sample when g tensor anisotropy is significant.

Electrodynamics of a split-ring Josephson resonator in a microwave line

We consider the coupling of an electromagnetic wave to a split-ring Josephson oscillator or radio-frequency superconducting quantum interference device in the hysteretic regime. This device is similar to an atomic system in that it has a number of steady states. We show that one can switch between these with a suitable short external microwave pulse. The steady states can be characterized by their resonant lines which are of the Fano type. Using a static magnetic field, we can shift these spectral lines and lift their degeneracy.

Frequency switching in a relativistic magnetron with diffraction output

Symmetric axial extraction of radiation from a relativistic magnetron with diffraction output (MDO) facilitates the use of any eigenmode as the operating one. As a consequence, a relatively small input RF signal can be used for mode switching, unlike the case for asymmetric extraction when only non-degenerate modes (the π- mode or the 2π- mode) can be used as the operating one. Using the MAGIC particle-in-cell code we demonstrate that about 180 MW is required to switch these non-degenerate modes in the well-known 400 kV A6 magnetron with extraction of radiation from one of its cavities when driven by a solid cathode, and about 30 MW is required for the same device when driven by a transparent cathode. For the gigawatt A6 MDO with a transparent cathode, however, only 200–300 kW is sufficient for mode switching and the switched mode continues to be generated after elimination of the input short RF signal when the amplitude of the applied axial magnetic field is near the critical value corresponding to the boundary between synchronous regions for neighboring modes. In repetitively pulsed systems, in order to switch each subsequent pulse independent of the previous one, the time between voltage pulses must be chosen to be not less than 20–30 ns (the time for the stored electromagnetic energy to flow out of the cavity) so that decreasing the output power of the previous pulse cannot switch the subsequent pulse. Finally, using this mode switching technique, we demonstrate the possibility of generating short gigawatt microwave pulses with different frequency and polarization by using a short, weak, single frequency signal that is very attractive for radar applications.