Email alert
RSS2023 Vol. 35, No. 7
- Cover and Contents
- Laser Damage of Optical Elements
- High Power Laser Physics and Technology
- Inertial Confinement Fusion Physics and Technology
- High Power Microwave Technology
- Particle Beams and Accelerator Technology
- Pulsed Power Technology
- 核科学与工Nuclear Science and Engineering
- Nuclear Science and Engineering
- Advanced Interdisciplinary Science
KDP-family crystals are the only nonlinear optical crystal material according with the optical aperture of ICF laser drivers. As KDP-family crystals are grown by aqueous solution method, the macroscopic inclusions and microscopic lattice defects easily occur in the bulk of the crystals. The high density pinpoints damage phenomenon appears as they are irradiated by the high power laser. All the laser induced damage properties are different from the surface damage of crystals grown by other methods, which are only limited by optical processing. The laser induced damage by defects or precursors are related to the laser wavelengths and even the laser polarization direction, and the different samples from the same as-grown single crystal and applied to different optical functions in ICF laser drivers show different laser induced damage properties. Therefore, the damage mechanism is very complicated, and it is urgent to know the laser induced damage mechanism of KDP-family crystals. In this paper, the cooperated research of Shanghai Institute of Optics and Mechanics with Fujian Institute of Research on the Structure of Matter, Shandong University and other crystal research institutes is reviewed. The laser induced damage properties of KDP and DKDP crystals applied as optical switching, frequency doubling and frequency mixing optical elements were investigated. The optimization of crystal growth process and the control of key factors were guided and the existing problems and solutions were prospected. The research has reference value for the development of high-performance KDP-family crystals and their rational application in high-power laser systems
Under continuous laser irradiation, the damage threshold of fused silica and other optical materials will continue to decrease, showing the “fatigue effect”, which seriously affects the life and stability of the repetition frequency optical system. This paper introduces the performance under fatigue effect of optical materials mainly fused silica materials, supplemented by several other typical optical materials (bismuth niobate crystal, lithium triborate crystal and HfO2/SiO2 multilayer film). The effects of laser wavelength, spot diameter, laser frequency and material position on fatigue effect are summarized. Two modes of fatigue effect are introduced: statistical false fatigue and material modified true fatigue. Three main mechanisms of fatigue effect are introduced: absorption defect model, bond-breaking model and coloured center model. Two experimental modes in fatigue experiment are compared, and their advantages and disadvantages and applicable research objects are analyzed. Finally, the present research status of this field are summarized, and the future development trend and directions are prospected.
The surface damage characteristics in DKDP crystals under laser irradiation at 355 nm and 1064 nm were studied and compared by using a Nd: YAG laser. The damage precursors and mechanisms corresponding to each damage morphologies were analyzed. The damage results reveal that the surface damage in DKDP crystal is more complex than that in bulk damage. Under the irradiation laser corresponding to the pulse width of 10 ns and the damage probability at 0% − 50%, surface damage morphology in DKDP crystals mainly contained four typical damage morphology: crater with cavity, crater with flat bottom, surface damage crack, and surface ablation. Through the comparison and analysis of the imaging of optical microscope and scanning electron microscope, damage precursors that induced damage craters with bottom cavity and surface cracks were mainly bulk defects, which were the same as the precursors forming the internal damage points (pinpoints). The precursors that induced damage craters with flat bottom were relatively complex, which could be the surface contamination, surface cracks, machining defects, and shallow surface bulk defects. For surface ablation, it was mainly caused by surface contamination and surface absorption defects. Surface damage is still one of the important factors limiting the laser damage resistance of KDPD crystals.
The absorption front model for laser induced damage of optical materials is modified. Different from the original model, the impurity defect absorption term is introduced, and the one-dimensional model is extended to a three-dimensional model. Using the modified absorption front model, temperature distribution near impurity(taking metal iron as an example), damage radius and damage threshold of infrared optical material of monocrystalline silicon are numerically studied, which is irradiated by 1064nm picosecond laser. The influence of initial temperature of the optical material on damage threshold is also studied. Our results show that: (1) Different from the traditional heat thermal transport models, near the damage threshold, a small change of laser field energy density from below to equal to or beyond damage threshold leads to a great change of temperature field in the presently modified absorption front model; (2) The maximum temperature near impurity and damage radius characterized by the absorption front increase approximately linearly with the increase of the irradiation energy density as the laser energy density goes far beyond damage threshold; (3) The laser damage threshold decreases with the increase of the initial temperature of the material. Our results prove that the presently modified absorption front model can better describe the laser damage induced by impurity defects in optical materials. Compared with the traditional thermal transport models, the present absorption front model can represent the sudden change of temperature field near the damage threshold more reasonably, and can quantitatively analyze laser damage size of optical materials induced by impurities. In addition, our results also show that increasing the initial temperature of the material can effectively reduce its laser damage threshold, which provides a way to improve the laser damage efficiency of photodetectors in photoelectric countermeasures.
The green laser can be used for the processing of highly reflective metals such as copper. Compared with the 1 μm laser which is broadly used now, green laser has the absorption efficiency nearly an order of magnitude higher, which can better meet the needs of various fields for the precision processing of highly reflective metals. Thus, the application prospect of high power green laser is very broad. In this paper, recent progress of high power green laser based on frequency doubling technology for fiber laser is investigated in detail. The power of green laser has increased from 100 W to 1 kW, the beam quality is close to the diffraction limit, and the output power is expected to be further improved. There are two technical routes to obtain high power green laser by using fiber laser frequency doubling technology. One is to use high power single beam fiber laser as the fundamental frequency light source and cascade single-pass frequency doubling technology. The other is to use multiple-beam fiber lasers as the fundamental frequency light source, realize beam combining and frequency doubling respectively, or beam combining and frequency doubling at the same time. The former route is simpler than the latter, but the latter has the potential of higher output power. The weak absorption of frequency doubling crystal is the common problem faced by the two technical routes.
Nanosecond (ns) pulsed laser with high average power and high repetition rate is a potential solution for laser cutting, laser welding and many other processing applications. With the increase of pulse repetition rate, especially when it is higher than 50 kHz, it is difficult to accumulate enough upper state population in a limited time, and the stability of laser pulse becomes a challenge in the design of laser. The main oscillator power amplifier (MOPA) system is the main method currently used. It is difficult to obtain ns pulsed laser with high average power, high repetition rate and high beam quality through direct oscillation. In this paper, the quantitative relationship between the intensity stability of laser pulse and the pump rate is analyzed according to the simulation of high repetition rate Q-switched laser. Combined with the use of plane-concave lenses to make the cavity a thermal-near-unstable cavity with a large-volume fundamental mode, the balanced design of Nd:YAG acousto-optic (AO) Q-switched laser oscillator with high repetition rate, high power and high beam quality was realized. A direct oscillation of 100 kHz high power high beam quality ns pulsed laser by using side pumped module was realized for the first time, the discrete coefficient of pulse intensity was only 0.041, the output power exceeded 142 W, the pulse width was 165 ns, and the beam quality factor M2 was 1.5.
In this paper, we report a simple structure, room-temperature operation, LD end-pumped 2.94 μm Er:YAG continuous wave laser. The laser uses double-ended bonded YAG end caps to reduce the end-surface temperature of the crystal. The pump source uses a small core diameter output fiber and an aspheric mirror coupling system, which reduces the dispersion rate of small pump spots in the crystal and therefore improves pumping uniformity. When the pump light wavelength is 969.7 nm, the absorption of pump light in the front section of Er:YAG crystal is weak, accordingly the thermal aggregation effect of the front end of the laser gain medium is mitigated. We observed and compared the temperature of the end faces of bonded and non-bonded Er:YAG crystals with a thermal imaging camera, simulated the thermal distribution using COMSOL software, and proved the effectiveness of the above measures in reducing the thermal effect of highly doped Er:YAG crystal. We finally succeeded in achieving a continuous laser output of 2.94 μm at 155 mW. We also observed the output wavelength’s red-shift phenomenon with the pump power increase and explained it theoretically at the energy transfer level.
To achieve an optoelectronic tracking platform based on biprisms, it is necessary to accurately calculate the angle of the Risley prisms based on the beam direction. This article uses non-paraxial ray tracing method and two-step method to fit the relationship between the angle difference and deflection angle of achromatic rotating biprism using the Neural Network. In solving the azimuth angle and the angle of achromatic rotating biprism, we separate the nonlinear relationship from the linear relationship, which is also fitted by using neural network. Finally, the mapping relationship between the angle of achromatic rotating biprism and the direction of the outgoing beam is obtained. The experiment shows that the exact inverse calculation of angle value of 0.000 1° is obtained under the condition of using only three layers of neural network with 20 neurons.
The history and development about the study of positrons generated by intense laser interaction with the plasmas are introduced. The mechanisms of the positron generation are presented. Specially, the typical experiment scheme (direct and indirect) of the positron generation, including important results about the experiment and computer simulation are described systemically. Eventually, the study of positron is reviewed and summarized. At present, the conclusions obtained from theoretical research and experimental research are quite different, and a lot of detailed work needs to be done in terms of laser equipment, experimental scheme design, and theoretical and simulation research.
To solve the problem of reconstructing two-dimensional shock wave fringe from compressed image obtained from Compressed Ultrafast Photography (CUP) and two-dimensional Velocity Interferometer System for Any Reflector (VISAR), a compressed image reconstruction algorithm based on low-rank constraint and total-variation regularization is proposed. The algorithm uses the similarity and smoothness of the spatial structure of the fringe image to transform the reconstruction problem into an optimization problem of kernel norm minimization and total-variation regularization, and splits the optimization problem into multiple sub-problems using the plug-and-play alternate direction multiplier method to solve the optimization problem, thus realizing accurate reconstruction of the CUP-VISAR compressed image. The simulation results show that under the condition of high noise, the peak signal-to-noise ratio of the reconstructed image is increased by 8.45 dB, and the structural similarity is increased by 8.52%. The reconstruction effect is better than that of the mainstream reconstruction algorithm. The experimental results show that the relative error of the maximum velocity of the shock wave fringe is reduced from 13.5% to 3.46% (reduced by nearly 10%), which verifies the effectiveness of the algorithm.
To meet the requirement of high-power nanosecond pulse emission, an ultra-wideband combined antenna for high-power microwave is designed. Voltage standing wave ratio (VSWR) and electric field distribution of high-power ultra-wideband combined antenna with point-fed balun and tapered-slot balun are compared. The relationship between characteristic impedance and dimensions of the combined antenna is analyzed. Klopfenstein impedance taper is used to reduce reflection. The size of the magnetic dipole is optimized by adjustable plate to improve the low frequency performance of the antenna, and validated by experiment for VSWR testing. On this basis, high-power microwave experiment is performed. With excitation of the high-power bipolar pulse whose bottom width is 3 ns, peak-to-peak value is 121 kV and center frequency is 329 MHz, the power handling capacity of the antenna is 73 MW, effective potential gain is 1.97.
The theoretical analysis and simulation on the permanent magnet guidance system of the E-type waveguide oscillator are developed. Firstly, the theoretical analysis of the modified paraxial ray equation, the minimum magnetic field under relativistic conditions and the transmission conditions of an intense relativistic electron beam in ideal reverse guidance magnetic field are presented. Then the reverse permanent magnet guidance system is designed according to the structure characteristics of the high-frequency interaction zone of the C-band E-type waveguide oscillator, and the expression of each magnetic field component is given. The reverse permanent magnet guidance system produces reverse guidance magnetic field by combining axial and radial magnetized cylindrical permanent magnets, and the total weight of the magnets is about 2.5 kg. The transmission characteristics of the intense relativistic electron beam in the guiding magnetic field are shown. The annular intense relativistic electron beam is produced by an explosive emission cathode. The results show that the designed reverse permanent magnet guidance system can guide the annular electron beam, with voltage of 400 kV, current of 580 A, to pass through the drift tube with a radius of 6mm. In addition, the E-type waveguide oscillator can stably generate 4.8 GHz microwave with power of 112.5 MW and efficiency of 48.49%, and the technical possibility of the reverse permanent magnet guidance system applied to the E-type waveguide oscillator is determined.
Perturbation theory of multi-cell superconducting radio-frequency (SRF) cavities, which is widely used in analysis of field flatness adjustment, is briefly described, and theoretical calculation method of field flatness is deduced. Multi-field coupled simulation calculations are performed for resonant cavities with minor longitudinal deformation, the field flatness and its trend with deformation quantity are obtained. For two widely adopted methods for SRF cavity development, including ‘first-single-then-the-whole pre-tuning’ and ‘whole-cavity stretching/squeezing’ during tuning process, theoretical analysis is carried out based on the capacitively coupled LC oscillator model of perturbation theory for multi-cell cavities, and meanwhile further verifying multi-field coupled simulation calculations are performed by combination of CST MWS and ANSYS for three typical SRF 9-cell cavities - for main Linac of XFEL, TRIUMF e-Linac, and for injector of SHINE as a candidate. It is shown that theoretical analysis results and simulation results are perfectly consistent with each other. The validity and feasibility of the two widely adopted methods above are verified and proved to be correct in the scope of perturbation. Cavity shape structure with excellent acceleration performance and field flatness can be obtained just by adding multi-physics field analysis to the optimization design of single-cell, which makes the design process of cavity shape more efficient. It is illustrated that the tuning sensitivity factors of optimized single-cells including end-cells and mid-cell should be as close or equal as possible to maintain field-flatness during cavity deformation.
High Energy Photon Source (HEPS), as a 4th generation synchrotron radiation light source, has stringent requirement for beam orbit stability: the orbit fluctuations should be below 10% of the beam RMS sizes in both horizontal and vertical directions with a bandwidth around 500 Hz. Overall, the latency of the Fast Orbit FeedBack (FOFB) system is the key factor to achieve the requirement. Data transmission of the beam positions from the beam position monitor (BPM) electronics to all of the FOFB sub-stations is the key to achieve very low latency, as which contributes the largest part of the total FOFB system latency. Preliminary test results of prototype showed that the total latency of data transmission is less than 10 μs with no bit errors during data transmission, satisfying the requirements on the FOFB system of HEPS.
Based on the AC-LinkTM energy conversion method, this paper improves the topology and working mode of the converter, so that the resonant circuit can work in bipolar mode, and designs the corresponding commutation timing and control algorithm based on the new topology, which enhances the adaptability of the converter to the three-phase input voltage fluctuation, so that it has the theoretical boost capability and high input power factor. Using Matlab/Simulink, a 30 kW simulation model was built, and the simulation results show that the output voltage reached 3kV and the boost capacity reached 6 times under the conditions of three-phase grid input, transformer ratio 1∶1 and resistive load, indicating that the converter has the ability to operate over a wide range of output voltages
According to the requirements of high robustness and positive and negative polarity controllability of the sampling circuit for bipolar charging power supply designed for a large capacity energy storage device, a positive and negative bipolar DC high-voltage isolation sampling circuit with isolation voltage ≥ 30 kV and conversion voltage fractional error ≤ 0.1% has been developed. The voltage/frequency and frequency/voltage conversion methods were adopted, integrated with optical fibers, transformer isolation and other measures, the positive and negative bipolar DC high voltage were isolated and sampled independently at the same time. The problems of the bipolar DC high voltage power supply, such as the voltage imbalance of positive and negative polarity, the incomplete isolation of ground wires between the control signal system and the high power system, and so on were solved. The EMI resistance of the power supply was improved. As the bipolar charging voltage output achieved ± 10 kV, the deviation of positive and negative polarity voltage is less than 0.1%. More than 100 chargers operated reliably and stably under the complex electromagnetic interference environment generated by discharge of the 18.3 MJ pulsed device.
It is important to reduce the jitter of gas switch under nanosecond pulse voltage for the output stability of electromagnetic pulse simulator. Especially under the condition that external triggering is inconvenient, the reduction of the jitter of self-triggered switch is worth paying attention to. In this paper, two types of self-triggered switches are designed, including the cathode grooved switch and the preionization switch. A nanosecond pulse experimental platform was built and the breakdown voltage, time delay and other parameters of the two types of switches are measured at three different pulse rise time of 40 ns, 70 ns and 120 ns respectively. With the use of data statistics and processing, the breakdown voltage and time jitter of the two switches are obtained. The experimental results show that the breakdown jitter of the switch can be effectively reduced by cathode grooved control emission or cathode preionization injection; The jitters of the two switches are between 1～1.8 ns under three rise time pulse voltages; Under the rise time of 40 ns and 70 ns pulses, the jitter of cathode grooved switch is shorter, which can be less than 1.2 ns, and the breakdown voltage dispersion is less than 1.29%; Under rise time of 120 ns pulse, the jitter of preionization switch is shorter than 1.6 ns.
The helium radiation damage of copper was simulated by using the molecular dynamics method, and the change process of copper microstructure under helium irradiation was observed on the atomic scale. The changes of the microstructure and mechanical properties of copper induced by helium radiation were analyzed, and the single crystal copper and the polycrystalline copper were compared. It is found that, the number of defect pairs in single crystal copper increases first and then decreases with the increasing number of helium atoms, and the peak value increases continuously. The number of defect pairs of polycrystalline copper continues to increase, but the fluctuation rule is not obvious. The tensile property test shows that the yield strength of copper is reduced because of the helium radiation. When the helium atom content reaches 0.54%, the yield strength of single crystal copper and polycrystalline copper is reduced by 46.94% and 49.2% respectively.
The accident source term and radiological consequence evaluation of small heating reactor at site boundary is the key content of nuclear and radiation safety review. According to the design characteristics of the advanced small reactor, the accident source term calculation model is established for fuel handling accident to study the release of radionuclides after the accident. Based on the experience of accident radiological consequence analysis of small reactor abroad and ARCON methodology in RG4.28, the atmospheric dispersion factor and individual dose at site boundary in fuel handling accident are analyzed. The results show that two hours after the accident, the radionuclides in the fuel cladding gap release into the environment, and the release amount of radionuclide in the environment reaches the radioactivity level of 1014 Bq. The release amount of inert gas is higher than that of iodine, and that of 133Xe is the largest. The individual effective dose and thyroid dose at the site boundary after 30 days of the fuel handling accident are within the dose limits and the maximum dose occurs at the east-north-east direction. The results of the accident source term and radiological consequence could provide technical support for offsite dose assessment and review of the advanced small reactor.
To solve the problem that multidimensional computing code needs huge computing resources when it simulates the lead-bismuth reactor for a long time, based on the self-developed one-dimensional CFD code, integrating the zero-dimensional point reactor dynamics model and the two-dimensional fuel rod heat transfer model, and carring out the multi-physical field coupling, we have developed a system safety analysis code for lead-bismuth pool reactor. The international benchmark for Accelerator Driven Sub-critical System (ADS) beams transient accident, which was published by OECD/NEA, was used to process the steady-state and transient validation, and to ensure the accuracy of the model. The verification results show that the code developed is in good agreement with the published results in terms of key parameters, and the computational resources required are obviously smaller than those of the multi-dimensional computing code. It is proved that this code can be used for preliminary thermo-hydraulic and safety analysis of lead-bismuth reactor.
With the frequent appearance of UAVs in several recent local wars and armed conflicts, the study of UAV detection and tracking technology has become a research hotspot in imagery and other fields. Due to the characteristics of low altitude UAV targets such as large mobility, small size, low contrast and complex background, their capture and tracking is a major challenge in the field of photoelectric detection. To address these difficulties, this paper proposes a real-time long tracking method based on YOLOv5 and CSRT algorithm optimization to achieve stable tracking of UAVs in clear sky, urban and forest scenes. First, two capture networks with different resolutions are established for different stages of tracking, and the two networks are optimized for small target detection and performance optimization respectively, and positive and negative samples are added to the UAV data set according to its characteristics to achieve data enhancement. Then, the CSRT algorithm is optimized using GPU and combined with feature point extraction to construct a low-altitude UAV detection and tracking model. Finally, the algorithm is deployed using Tensorrt and experimented on a self-built dataset. The experimental results show that the proposed method achieves a tracking performance of 400FPS on RTX 2080Ti and 70FPS on NVIDIA Jetson NX. Stable long-time tracking is also achieved in real field experiments.
Utilizing the muzzle voltage of the electromagnetic railgun, the contact resistance between the sliding armature and the copper rail surface during the launch process can be calculated to analyze the contact characteristics. However, the muzzle voltage signal contains a large amplitude of reverse induced electromotive force due to the complex augmented rails structure of the launcher. Meanwhile, the firing sequence of the pulse forming network disturb the detected muzzle voltage signal as system noise interference. Therefore, it is difficult to accurately calculate the contact resistance. To solve this problem, a noise suppression method of muzzle voltage system based on VMD-OptShrink is creatively utilized to suppress jagged noise. In this method, variational mode decomposition (VMD) can decompose the muzzle voltage signal in time-frequency domain according to the frequency characteristics. Then OptShrink is used to extract the low-rank components of the decomposed signal to obtain the denoised muzzle voltage. Finally, the contact resistance is calculated to analyze the armature-rail contact characteristics. The test results show that this method can suppress the muzzle voltage system noise well. The calculated armature-rail contact resistance waveform is smooth, which is conducive to the analysis of the armature-rail contact characteristics. The armature-rail contact resistance decreases rapidly at the initial stage of launching, then it fluctuates slowly until the armature slides out of the muzzle and the contact resistance increases sharply. The method proposed in this paper provides a new and reliable reference for the launching performance monitoring of electromagnetic railgun.
