High Pulse Repetition Frequency Research Articles

Reverberation-based high-speed guided-wave ultrasonic propagation imager for structural inspection of thick composites

Increasing use of thick composite structures in many fields, including the aerospace and shipbuilding industries, necessitates appropriate inspection techniques to ensure structural integrity. Of many nondestructive testing techniques, laser ultrasonic testing is a non-contact method that yields accurate inspection results with high spatial resolution; this method was adopted in this study for the inspection of thick composite structures. We propose a novel guided-wave ultrasonic propagation imaging (UPI) method, called the reverberation wave UPI (R-wave UPI), which is capable of faster target scanning and clearer highlighting of confining waves. To prove the merits of the R-wave UPI, a relatively thin carbon/epoxy composite (thickness, 12 mm) was first tested using the proposed method and the conventional through-transmission UPI. The R-wave UPI modifies the guided-wave UPI with a faster pulse repetition frequency (PRF) laser and ultrasonic energy mapping with straightforward parameter determination. The through-transmission UPI consisting of a raster mechanical scanner, a Q-switched laser, and a laser Doppler vibrometer, was used as a reference, whereas the R-wave UPI consisting of an angular laser mirror scanner, a Q-switched laser, and a PZT sensor was operated at a high PRF in the kilohertz regime to induce reverberation. The reverberation phenomenon was used in the post-image processing, called ultrasonic energy mapping (UEM). UEM with a high PRF was then compared to the through-transmission inspection reference to verify its accuracy and reliability for mapping of structural anomalies. Finally, a thick and curved composite blade with a rough and curved surface that could not be inspected using the conventional through-transmission UPI was tested using 7 kHz PRF and its delamination was successfully visualized using the UEM.

Study on All-Solid High Repetition-Rate Pulse Generator Based on DSRD

The pulse generator based on drift step recovery diode (DSRD) has broad development prospects in the research of high pulse repetition frequency (PRF), high power pulse generators. In this letter, a pulse generator based on DSRD is successfully developed. The design of circuit topology and design principles are presented. The pulse generator can work stably outputting almost 30kV peak voltage at 100 kHz PRF or 140kV at 25kHz with a 50kΩ load. What's more, the influences of power supply voltage and load resistance are studied experimentally.

Elevated OH production from NPHFD and its effect on ignition

This work explores ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous OH planar laser-induced fluorescence (PLIF) and schlieren imaging were utilized at a frequency of 50 kHz to examine the time dependent radical generation during and after the plasma discharge. The results indicate that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of the pulse repetition frequency (PRF). In addition, it is demonstrated that the intensity of the OH-PLIF signal during and shortly after the discharge is elevated for PRF≥10 kHz, indicating that both the volume and concentration of OH are built up at high PRF. This accumulation of OH radicals in the inter-electrode region is directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. It is shown that in cases in which the PRF is below a certain threshold (<10 kHz), OH accumulation does not occur, and it is believed that this condition, along with lower discharge temperatures, is directly responsible for reduced ignition probability.

A numerical investigation of NH3/O2/He ignition limits in a non-thermal plasma

This numerical study of the ignition characteristics of ammonia in a pulsed plasma discharge includes the assembly of a kinetic model for the oxidation of ammonia/oxygen/helium mixtures under a plasma discharge. The model was used to perform a series of simulations under varying pulsed discharge frequencies and pulse numbers, at atmospheric pressure and moderate to high temperatures (600–1500 K). A zero-dimensional solver which combines the ZDPlasKin and CHEMKIN software is used to explore the effect of pulse number and frequency on ignition delay time. For a moderate amount of pulses, a reduction of 40–60% in ignition delay time is achieved, with higher pulse repetition frequencies (PRFs) yielding shorter ignition delay times. Analysis of OH radical time evolution reveals that high PRFs support an increasing radical pool at low temperatures, whereas at lower PRFs radicals recombine in between pulses. In the thermal runaway phase, the radicals formed in conventional chain branching events are prevalent, so that OH formed in later pulses has little effect. When looking low temperatures and high PRFs, higher pulse frequencies allow for lower initial temperatures which will result in ignition. At a high enough frequency, the hysteresis of ignition and extinction is altered due to a high amount of radicals supplied and sustained by the plasma, so that there is a smooth transition and reactions at all temperatures.

White noise jammer mathematical modelling and simulation

The radar equation is the fundamental mathematical model of the basic function of a radar system. Moreover, there are many versions of the radar equation, which correspond to particular radar operations, like low pulse repetition frequency (PRF), high PRF, or surveillance mode. In many cases, all these expressions of the radar equation exist in their combined forms, giving little information to the actual physics and signal geometry between the radar and the target involved in the process. In this case study, we divide the radar equation into its major steps and present a descriptive mathematical modelling of the radar and other related equations utilizing the free space loss and target gain concepts to simulate the effect of a white noise jammer on an adversary radar. We believe that this work will be particularly beneficial to instructors of radar courses and to radar simulation engineers because of its analytical block approach to the main equations related to the fields of radar and electronic warfare. Finally, this work falls under the field of predictive dynamics for radar systems using mathematical modelling techniques.

A time-aligned plane wave compounding method for high frame rate shear wave elastography

Shear wave elastography (SWE) is an established ultrasonic imaging technique, that provides quantitative measurements of tissue mechanical properties. This technique relies on the combination of acoustic radiation force push beams to generate propagating shear waves and ultrafast imaging, which allows high two-dimensional imaging frame rates to capture the induced motion. Utilizing coherent plane wave compounding (PWC) can improve signal-to-noise ratio (SNR) of acquired data. While the image quality improves, the pulse repetition frequency (PRF) is reduced, reaching limiting bandwidths to perform SWE. To address these challenges, a method for making SWE measurements with high PRF and SNR is presented. The idea follows the Time Aligned Sequential Tracking method, proposed to allow SWE with enhanced SNR in conventional clinical ultrasound scanners. The time alignment process is applied here to the full field-of-view, acquired by using plane waves transmitted at different angles. In this way, the full PRF and SNR gains are obtained. Results are shown from experiments in tissue-mimicking phantoms, where motion and extracted shear wave velocity are compared for the traditional and proposed data processing schemes. Preliminary results on arterial phantoms are also discussed, to emphasize the beneficial effects of the method in dispersive geometries.

Effects of microwave radiation on laser induced plasma ignition of n-butane/air mixture under atmospheric conditions

This study presents the effect of the microwave radiation control parameters (i.e. pulse width and pulse repetition frequency (PRF)) on laser-induced plasma (LIP) ignition in a constant volume combustion chamber (CVCC) of the n-butane-air mixture (ϕ = 0.7) under atmospheric conditions. The ignition behaviors were investigated using ignition success rate, high-speed imaging, net heat release, and plasma temperatures. The result shows that the ignition success rate (ISR) defined in this study gets increased with an increase in the pulse width. Moreover, ISR was found insensitive to PRF at higher pulse width. This implies that the combustion advancement occurred at a faster rate on the coupling of high PRF and longer pulse width microwave radiation with the LIP. The ignition delay evaluated based on the net heat release gets reduced; consequently, the ignition advances much faster with an increase in the pulse width as compared to the small pulse width. The direct images of early combustion revealed an expansion of the plasma when the coupling between microwave radiation and the LIP occurs. This expansion of the plasma is the generation of active radicals due to the direct collisions of electrons with ionized molecules via interaction between the electromagnetic field and free electrons. Besides, the combustion advancement mechanism was investigated using plasma emission spectroscopy. The plasma temperatures which are closely related to the chemical reaction rates were calculated by fitting the homo-nuclear N2(2+) system. The results show that the ignition success rate enhances with an increase in the rotational temperature. This might be due to much faster rotational-transnational energy transfer as compare to vibrational-transnational energy transfer. This implies that the bulk gas temperature increases quickly since the rotational temperature is approximately equal to the gas temperature.

Strengthening effect of microwave on spark ignited spherical expanding flames of methane-air mixture

The Microwave Assisted Spark Ignition system is a novel and potential ignition method to improve efficiency of spark ignition engines. To better understand the coupling mechanisms/characteristics between microwaves and spark plasma, experiments were conducted to visualize spark discharge and flame development with microwave emitted in a Constant Volume Combustion Chamber. The operation parameters varied included equivalence ratio, ambient pressure, microwave pulse repetition frequency, and peak power. To clearly illuminate the coupling mechanism between microwave and spark plasma, a small-diameter antenna device was specifically designed to minimize heat loss. The microwave assisted spark ignition and flame propagation characteristics of methane-air premixed spherical flames were studied first. Subsequently, non-reactive discharge was experimented to exclude the effects from combustion. Results showed that microwave increased the development speed of an early flame kernel by 60% with 1 kHz microwave pulse repetition frequency and 1000 W peak power under the condition of equivalence ratio 0.6 and ambient pressure 0.1 MPa. The enhancement was believed to be the result of enhanced chemical reaction and the induced flame deformation by collisions between high energy electrons and other particles. As the ambient pressure increased, the enhancement effect of microwaves on flame kernels was found to diminish. In this situation, non-reactive discharge and related MAI experiments showed that a higher pulse repetition frequency or peak power was beneficial to strengthen the effect of microwave as much higher energy absorption efficiency was achieved.

SIMULATION-BASED DATA AUGMENTATION USING PHYSICAL PRIORS FOR NOISE FILTERING DEEP NEURAL NETWORK

Abstract. LiDAR (Light Detection and Ranging) mounted with static and mobile vehicles has been rapidly adopted as a primary sensor for mapping natural and built environments for a range of civil and military applications. Recently, technology advancement in electro-optical engineering enables acquiring laser returns at high pulse repetition frequency (PRF) from 100Hz to 2MHz for airborne LiDAR, which leads to an increase in the density of 3D point cloud significantly. Traditional systems with lower PRF had a single pulse-in-air zone (PIA) big enough to avoid a mismatch between pulse pair at the receiver. Modern multiple pulses-in-air (MPIA) technology ensures multiple windows of operational ranges for single flight line and no blind-zones; downside of the technology is projection of atmospheric returns closer to same PIA zone of neighbouring ground points and more likely to be overlapping with objects of interest. These characteristics of noise compromise the quality of the scene and encourage usage of noise filtering neural network as existing filters are not effective. A noise filtering deep neural network requires a considerable volume of the diverse annotated dataset, which is expensive. We developed simulation for data augmentation based on physical priors and Gaussian generative function. Our study compares deep learning networks for noise filtering and shows performance gain on 3D U-Net. Then, we evaluate 3D U-Net for simulation-based data augmentation, which shows an increase in precision and F1-score. We also provide an analysis of the underline spatial distribution of points and their impact on data augmentation, and noise filtering.

Investigation of the characteristics and mechanism of subnanosecond switching of a new type of plasma switches. I. Devices with counter-propagating electron beams—kivotrons

Novel opportunities of waveform tailoring for controlling plasma parameters based on the development of a high-voltage gas-discharge switch with a subnanosecond breakdown time and high pulse repetition frequency are discussed. The studies of characteristics and breakdown development mechanisms of the switch based on an ‘open’ discharge—kivotron are summarized. The discharge in the switch is carried out in conditions when counter-propagating electron beams in high electric field are generated. In this case, when using helium as an operating medium, firstly, atoms are effectively excited into the resonance state by fast particles. Secondly, due to the Doppler effect, resonant photons without reabsorption reach the cathode surface, maintaining the discharge current due to photoemission. Thirdly, fast heavy particles modify the cathode surface, thereby significantly (up to an order of magnitude) increasing the photoemission coefficient. The combination of these processes leads to an increase in the switching rate with an increase in the operating voltage U and helium pressure pHe. At U > 20 kV and pHe > 10 Torr, the switching time becomes less than 100 ps both in the experiment and according to the simulation. It is preferable to use planar geometry without a drift space as a switching device, in which, on the one hand, the most complete use of EB energy is realized in creating a plasma with a high charge density, on the other hand, a small wave impedance of the switch is realized. As a result, currents of tens of kiloamperes are achieved at voltages up to 100 kV. In an interpulse period plasma in the discharge gaps recombinates fast. As a result, switches can operate up to pulse repetition frequency 100 kHz. Together these achievements open new opportunities to control plasma parameters.

High power and high pulse repetition frequency transistorized pulser by time base stability improvement and power synthesis technique.

Output power of a transistorized pulser is usually limited by the power capacity of avalanche transistors. To improve the total output power, the power synthesis method is widely used, in which a single pulser with high output power and high time base stability is required. However, the time base stability tends to deteriorate as the output power increases. To improve the output power under the premise of high time base stability, from the perspective of carrier movement, the mechanisms of pulse jitter and pulse drift are investigated. It is found that the pulse jitter is caused by time dispersion of the ionization process in the collector depletion region, while the pulse drift is due to the decrement of the diffusion coefficient Dn and the electron mobility μn, which are both temperature-dependent. Based on the microscopic theoretical study, some macroscopic improvements on the time base stability are made. Some parameters of the trigger pulse and the circuit (e.g., charging capacitance) are optimized experimentally. Consequently, we achieved a pulser with an amplitude of 1.8 kV, pulse jitter of 25 ps, pulse drift of 100 ps/min at a pulse repetition frequency (PRF) of 100 kHz. Additionally, a new parameter k, the product of the highest PRF f and the peak power Ep, is defined to evaluate the output power. With almost the same time base stability, the proposed pulser has a k of 6.48 GHz W, which is improved significantly. Finally, a synthesized pulser with an amplitude of 2.5 kV and highest PRF of 100 kHz is achieved.

Conventional spark versus nanosecond repetitively pulsed discharge for a turbulence facilitated ignition phenomenon

This work applies both conventional-single-spark-discharge (CSSD) at 500-µs pulse duration time and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency PRF = 5–70 kHz to explore a turbulence facilitated ignition (TFI) phenomenon using a pair of pin-to-pin electrodes at an inter-electrode gap of 0.8 mm in randomly-stirred lean n-butane/air mixture with Lewis number ≫ 1. For CSSD, measured laminar and turbulent minimum ignition energies (MIEL and MIET) at 50% ignitability show that MIEL≈ 23 mJ > the smallest MIET≈ 19.7 mJ at u′ = 0.9 m/s (TFI) and then MIET≈ 28.6/30.8/36.8 mJ at u′ = 1.4/2.1/2.8 m/s (no TFI), where u′ is the r.m.s turbulent fluctuating velocity. For comparison, all NRPD experiments apply the same total ignition energy Etot≈ 23 mJ via a fixed train of 11 pulses, each pulse with 2.2 mJ except for the first pulse with 1 mJ. NRPD results show a cumulatively synergistic effect depending on the coherence between PRF and an inward reactant flow recirculation frequency (fRC) inside the torus-like kernel induced by the discharge that could enhance ignition. When PRF is approximately synchronizing with fRC, the synergistic effect is most profound at PRF = 20-kHz/40-kHz with very high ignition probability Pig = 90%/85% > 50% in quiescence, whereas lower values of Pig = 42%/34% are found at PRF = 10-kHz/60-kHz. Further, Pig = 0 at PRF = 5-kHz even when 5000 pulses (Etot≈ 10 J) are applied. We discover that Pig decreases significantly with increasing u′ for most PRFs (no TFI) except at higher PRF ≥ 60 kHz showing possible TFI. These results are attributed to the interactions between turbulent dissipation, differential diffusion, and synergistic influence, which are substantiated by Schlieren images of initial kernel development and the ignition time determined at one half of the flame critical radius that leads to a self-sustained spherical flame propagation.

Single-frequency Q-switched Er:YAG laser with high frequency and energy stability via the Pound-Drever-Hall locking method.

We present an injection-seeding Q-switched 1645 nm Er:YAG ceramic laser with high frequency stability and energy stability by combining the injection-seeding technique and Pound-Drever-Hall technique. 10.31 mJ single-frequency pulses with optimal frequency stability (525 kHz) and relative energy stability (0.52%) at a high pulse repetition rate of 1 kHz are obtained. To the best of our knowledge, this is the highest frequency stability and energy stability so far in a high pulse repetition frequency, large pulse energy, single-frequency Q-switched Er-doped solid laser. This single frequency laser with high stability provides an excellent light source for a coherent lidar system.

Experiments on the Pulse Repetition Frequency Optimization of 1.3-GHz, 100-kW Microwave Pulse Compressor

A microwave pulse compressor (MPC) system formed by an inductive iris, a straight rectangular waveguide, and an H-plane T-junction is experimentally studied. The MPC design, which is based on an equivalent circuit simulation model, ensures the optimum cavity-storage gain at 1.3 GHz. A low-pressure gas discharge tube (GDT) that is placed at the side arm of the T-junction implements the switching trigger for the cavity energy extraction. The experimental results show that high pulse repetition frequency (PRF) operation together with optimal cavity-extraction gain is feasible by carefully selecting the GDT gas type, pressure level, and flow rate. Under optimal conditions, the MPC system approaches the theoretically predicted cavity-extraction gain of 19 dB and a PRF up to 300 Hz.

Multi-PRF and multi-frame track-before-detect algorithm in multiple PRF radar system

This paper considers the detection and tracking of dim targets for multiple pulse repetition frequency (PRF) pulsed Doppler radar systems through the use of multi-frame track-before-detect (TBD) methods. The main challenge is that each measurement frame is a mixture with measurements recorded, periodically, at different PRF settings, thus with different range and Doppler ambiguities. To achieve this goal, we propose variable step size multi-PRF and multi-frame (or scan) TBD (MP-MF-TBD) algorithm. Specifically, by incorporating the ambiguity factors related to both the range and velocity ambiguities, MP-MF-TBD first accomplishes the target SNR integration directly using multiple ambiguous measurement frames. After analyzing the properties of the over-threshold tracks, we further propose an efficient detection procedure, which can effectively eliminate the false tracks caused by the measurement ambiguities and accomplish the target kinematic state estimation and multi-PRF ambiguity resolution simultaneously. The proposed MP-MF-TBD does not require certain limitations on the PRF type (i.e., low, medium and high PRF) or the number of PRFs in pulsed Doppler radar systems, and appears to be robust to the target loss when target fall into a blind frequency band of certain PRFs. Extensive numerical simulations demonstrated the efficacy of the proposed MP-MF-TBD by comparing with classical techniques.

Unambiguous velocity estimation method based on intra‐pulse cross‐correlation

In this study, by employing the intra-pulse cross-correlation (IPCC) operation, an unambiguous velocity estimation method is proposed for narrow-band long-range radars with high carrier frequency and low pulse repetition frequency. This estimation algorithm is simple and could be easily implemented in existing radar systems without changing the radar hardware or the pulse transmitting scheme. Comparing with the slow time dimension correlation algorithm, the accuracy of the proposed intra-pulse frequency domain method is greatly improved, and the brute-force search for the unknown motion parameters is also unnecessary. By first setting a small frequency offset of the IPCC operation, the unambiguous velocity region could be significantly enlarged. Using the relatively coarse but unambiguous estimates and increasing the frequency offset step by step, the IPCC is repeatedly applied to obtain more accurate estimates. Note that the estimation results of the IPCC algorithm could be used in the maximum-likelihood estimator for ambiguity resolution. The Cramer-Rao bound for the proposed algorithm is derived, and the optimal frequency offset in the sense of estimation accuracy is also analysed. Through numerical simulations for both synthetic and real radar data, the effectiveness of the proposed estimation algorithm is verified.

Datation and Range Calibration of Radar Altimeter Exploiting Fully Focused SAR Processing

External calibration of satellite altimeters is commonly used to assess the errors introduced by the instrument when performing its measurements. The on-ground processing of the altimeter acquisitions over an external calibrator is a key step to assess these errors starting from the knowledge of the exact location of the external calibrator and its characteristics. In this letter, a novel on-ground processing is proposed for high pulse repetition frequency altimeters based on the fully focused synthetic-aperture radar (FF-SAR) concept, which relies on the capability of the instrument to preserve the coherency of the transmitted pulses for sufficiently long time. Starting from the FF-SAR processing, methods are here proposed to perform both absolute range calibration and datation calibration. The effectiveness and performance of the proposed approach are verified by processing the transponder acquisitions of CryoSat.

Frequency Characteristics of a Subnanosecond Plasma Switch

Frequency characteristics of a subnanosecond plasma switch based on a combination of an open discharge or a discharge in a hollow cathode and a capillary discharge removed from the internal cavity of the plasma cathode are presented. It is shown that in the presence of an external screen that provides a capacitive discharge and hence, exclusion of the period of weak electron multiplication in the capillary from the delay mechanism of the discharge development, a high pulse repetition frequency is reached for different designs of the plasma cathode and the capillary section. It is demonstrated that at the voltage U = 30 kV, the pulse repetition frequency f = 100 kHz is reached for the capillary with round cross section at pHe ≤ 2.6 Torr and for the capillary with rectangular cross section at pHe ≤ 3.05 Torr.

레이다 모션에 따른 STAP 위상 보상

The conventional space-time adaptive processing (STAP) using a short coherent processing interval (CPI) is generally applied to an environment, where the variations in angle and Doppler information caused by stationary clutter patches during the CPI are negligible. The direct application of the conventional STAP to a relatively long CPI environment causes a performance degradation. This study proposes a method of applying the conventional STAP to the data acquired for a medium CPI scenario. In the proposed method, we compensate for the received signal phase by considering the change in the angle and Doppler information during the CPI, which results in the reduction of the phase variation due to the radar motion. Furthermore, we propose a method of improving the angular resolution by forming a virtual aperture when a high PRF radar with a non-negligible motion that satisfies the DPCA condition. Finally, we demonstrate the proposed method’s performance through a simulation.

Real-Time Transcranial Histotripsy Treatment Localization and Mapping Using Acoustic Cavitation Emission Feedback.

Cavitation events generated during histotripsy therapy generate large acoustic cavitation emission (ACE) signals that can be detected through the skull. This article investigates the feasibility of using these ACE signals, acquired using the elements of a 500-kHz, 256-element hemispherical histotripsy transducer as receivers, to localize and map the cavitation activity in real time through the human skullcap during transcranial histotripsy therapy. The locations of the generated cavitation events predicted using the ACE feedback signals in this study were found to be accurate to within <1.5 mm of the centers of masses detected by optical imaging and found to lie to within the measured volumes of the generated cavitation events in >~80 % of cases. Localization results were observed to be biased in the prefocal direction of the histotripsy array and toward its transverse origin but were only weakly affected by focal steering location. The choice of skullcap and treatment pulse repetition frequency (PRF) were both observed to affect the accuracy of the localization results in the low PRF regime (1-10 Hz), but the localization accuracy was seen to stabilize at higher PRFs (≥10 Hz). Tests of the localization algorithm in vitro, for treatment delivered to a bovine brain sample mounted within the skullcap, revealed good agreement between the ACE feedback-generated treatment map and the morphological characteristics of the treated volume of the brain sample. Localization during experiments was achieved in real time for pulses delivered at rates up to 70 Hz, but benchmark tests indicate that the localization algorithm is scalable, indicating that higher rates are possible with more powerful hardware. The results of this article demonstrate the feasibility of using ACE feedback signals to localize and map transcranially generated cavitation events during histotripsy. Such capability has the potential to greatly simplify transcranial histotripsy treatments, as it may provide a non-MRI-based method for monitoring and localizing transcranial histotripsy treatments in real time.