The thermal damage of mouse skin under different output power of 1064 nm laser was studied by experiment and theoretical analysis. The degree of thermal injury of mouse skin tissue was evaluated by dermoscope and optical coherence tomography (OCT), and the theoretical analysis was made according to Arrhenius thermal damage equation, and the results were compared with the experimental results. The results show that when the laser’s continuous irradiation time is 400 ms and the laser output power density is less than 958 W/cm2, there is no obvious damage; when the laser output power density is 958-1160 W/cm2, the damage is white blister-like; when the laser output power density is 1160-1370 W/cm2, the damage is a shallow pit-like macula with a circle of bulging white skin blisters around the damage spot; when the laser power density is at 1370-2190 W/cm2, the damage is a red pit-like spot, and there is black-yellow eschar around the injury spot.
In this paper, the finite element method is used to simulate the energy, electric field and effective refractive index changes of the modes in the microfiber before and after coating. The transmission characteristics of HE11, TE01, HE21 and TM01 modes in the microfiber and the interaction principle with the platinum film are analyzed. The microfiber was fabricated by etching the optical fiber with buffer oxide etchant, and the platinum film was coated by ion sputtering to obtain a microfiber device with a diameter of 13.2 μm and a platinum film thickness of 40 nm. Its saturable absorption properties were tested, the modulation depth and saturation intensity were 0.57% and 0.8 MW/cm2, respectively. An all-fiber mode-locked laser was fabricated, its mode-locked threshold is 180 mW. The mode-locked pulse repetition rate is 17.93 MHz, the pulse width is 103 ps, the center wavelength is 1031.6 nm, and the full width at half maximum is 3.5 nm.
High efficiency and high average power nanosecond pulsed solid state lasers are playing an increasingly important role in photoelectric countermeasures, lidar, material modification, laser processing and many other fields. However, at present, Yb:YAG or Nd-doped materials are adopted as gain medium in most nanosecond high average power lasers. The high saturation flux or low energy storage density of the materials lead to complex laser amplification link and large laser volume. In this paper, a disordered garnet crystal Yb:CNGG that is more suitable as a gain medium for high average power and high pulse energy lasers is studied and compared. The multi-pass gain characteristics of Yb:CNGG in the structure of active mirror are researched. The amplification process is analyzed and the multi-pass amplification model is established. The crystal parameters are optimized under certain pumping conditions to achieve better energy storage. The double-pass amplification experiment was carried out, and a gain of 1.53 was obtained at the pump power density of 15 kW/cm2. The multi-pass amplification capability of Yb:CNGG and Yb:YAG is compared. Under the same crystal parameters and pumping conditions, the pulse energy output of 2.11 J can be achieved by Yb:CNGG crystal at the incident energy of 1 mJ, which is better than the energy output of 1.41 J of Yb:YAG crystal.
To study the thermal effect of composite configuration grazing-incidence slab amplifier, a composite configuration was used, in which the Sapphire with high thermal conductivity was bonded on the pump face of the laser medium Nd:YVO4 crystal.The thermodynamic model of the grazing-incidence slab amplifier was built up. The temperature distribution, optical path difference and thermal lens effect were analyzed theoretically in detail. Also, the temperature distribution, thermal strain, optical path difference and thermal focal length were simulated by finite element analysis software COMSOL. The results indicate that compared with single medium structure, the maximum temperature of the composite configuration decreased by 60 K, and the maximum deformation of the pump face decreased by 1/3, while the pump power is 60 W, the thickness of the slab is 2 mm and the thickness of the bonding layer is 1mm. The novel composite configuration has reduced the thermal effect effectively.
The 2 μm laser with low repetition rate, high peak power and high beam quality has a wide application prospect for nonlinear frequency transformation of mid-infrared and long-infrared optical parameters. The Ho: YLF crystal is pumped by a 42W Tm doped fiber laser with an L-shaped resonator structure. Laser output with wavelength of 2.05 μm, repetition rate of 50 Hz, pulse width of 18 ns , pulse energy of 13.5 mJ, and peak power of 0.75 MW is realized based on the electro-optic Q-switching technology of rubidium titanate phosphate (RTP). And the beam quality M2 factors in the horizontal and vertical directions are 1.4 and 1.1, respectively. The Ho: YLF solid state laser is pumped by a fiber laser and has a compact structure, and it will build the foundation for Ho laser with higher pulse energy.
To meet the needs of multi-way power distribution applied to high-power solid-state sources, a multi-way power distribution device based on coaxial waveguide is designed and studied. By analyzing the transmission characteristics of coaxial waveguides and by applying the theory of impedance matching, an S-band 1∶16 power divider is designed by electromagnetic simulation software. Moreover, the device is machined for testing. Experimental results show that the reflection coefficient S11 of the power divider is less than -15 dB in the range of 2.28 GHz to 2.86 GHz (the relative bandwidth is 23%). In the range of 2.37 GHz to 2.57 GHz (the relative bandwidth is 8.1%), the reflection coefficient S11 of the input port is less than -20 dB, with the amplitude imbalance ±0.1 dB and phase imbalance ±5° at the output ports. To sum up, the power divider meets the amplitude and phase consistency requirements at output ports and can be applied to S-band hundred-watt continuous wave power distribution.
To meet the power-handling capacity and compactness needs of high-power microwave system, a novel waveguide diplexer is proposed. Overmoded waveguides are introduced to improve power-handling capacity. The resonant cavity with triangular metal insert structure is introduced to design waveguide filters and waveguide bends are introduced to achieve compactness. The theoretical analysis of filter design is carried out by using microwave network method and two X-band filters are designed. Furthermore, the diplexer is formed after the size of T-junction is determined by using terminal short-circuit method. The results of simulation and practical test show that power-handling capacity of each channel of the diplexer is greater than 0.11 GW and 0.12 GW respectively, the transmission efficiency is higher than 83.9% and 82.4% respectively, and the isolation is above 20 dB. Besides, not only can the filter order be increased according to the demand, but also more waveguide bends can be introduced to improve the space utilization.
Multipactor is one of the bottlenecks for spaceborne high-power microwave components. In this work, based on 3D particle-in-cell simulation, we investigate the fluctuation of number of electrons, the instantaneous secondary electron yield, the normalized reflection wave voltage and the gap voltage between the dielectric surface and metallic plate in the evolution of multipactors with parallel-plate dielectric-loaded waveguide for three cases: just considering external microwave field (case 1), considering both external microwave field and space charge (case 2), and considering external microwave field, space charge and surface charge on the dielectric (case 3). The electron distribution and charge density on the surface of dielectric versus time for case 3 are given. Simulation results reveal the roles of space charge and surface charge on the dielectric: space charge results in the saturation state of the multipactor, while surface charge on the dielectric results in an unsustainability of the saturation, extinguishing the multipactor. The existence of surface charge on the dielectric is responsible for the special circumstances including the different decreasing rate of instantaneous secondary electron yield of dielectric and metal plate, the eye-like shape of envelope of the normalized reflective wave voltage amplitude, the gap-voltage DC-like bias, and the unsymmetrical distribution of the energy of electrons.
In this paper, a three-band monopole antenna for hand-held harmonic radar is designed. The antenna is fed by a coplanar waveguide. By loading L-shaped radiating branch on the main radiating body of the monopole antenna and cutting a triangular notch on the ground plane close to the main radiating body, the antenna resonates in three frequency bands: 2.4 GHz, 4.8 GHz and 7.2 GHz. At the same time, a metal baffle is loaded at a distance of 10 mm from the antenna to enhance the directionality of antenna radiation and to receive electromagnetic waves reflected in multiple directions. The size of antenna is 54 mm×53 mm×1.6 mm, and the bandwidth in the three operating frequency bands is 0.51 GHz (2.35-2.86 GHz), 1.39 GHz (4.17-5.56 GHz), 1.46 GHz (6.17-7.63 GHz), respectively. It can effectively cover all the working frequency band of harmonic radar. According to the peak gain and radiation pattern of the antenna, the gain performance and overall radiation performance of the antenna are good in operating frequency band.
High intensity radiated field construction system is a key equipment for electromagnetic irradiation effect test of various weapon systems. It can excite high intensity and evenly distributed electromagnetic field in a certain distance from the antenna. In this paper, an X-band offset Cassegrain multimode reflector antenna is designed for this system. Reflector antennas are used to obtain high gain so that the field strength in the desired region is as large as possible. The flat top narrow beam is realized by using the theory of multimode reflector, which makes the field in the desired area tend to be evenly distributed, while the field outside the area decreases rapidly. The measured results show that the gain of the proposed antenna is greater than 29.8 dB, and the 3 dB beamwidth is not less than 4.6°. In this range, the amplitude fluctuation of the pattern is less than 2 dB, and the flat top characteristic is obvious. In addition, the dual-bias reflector antenna has the advantages of small feed occlusion, low feeder loss and easy folding, which can be well applied to electromagnetic environment simulation test equipment.
With the prosperity and progress of anti-jamming technology, traditional jamming technologies represented by barrage and deception jamming are facing challenges. Therefore, a distributed ultra-wideband (UWB) electromagnetic pulse jamming technology based on low-orbit satellites is proposed in this paper. Compared with the traditional jammers, UWB electromagnetic pulse jamming is a new type of electromagnetic attack system. Initially, the power spectrum of repetitive UWB electromagnetic pulse is derived. Furthermore, the feasibility of distributed jamming technology is evaluated, and the transmit power required for distributed jamming based on low-orbit satellites is calculated. Finally, the effect of a low noise amplifier (LNA) in the navigation receiver is investigated in the UWB electromagnetic pulse jamming experiment, and the constellation layout of the low-orbit satellite for carrying jammers at mid-low latitudes is designed by Satellite Tool Kit (STK). The experimental results show that temporary gain compression occurs in the LNA under the jamming of the UWB electromagnetic pulse. UWB single pulse with a width of 0.7 ns can suppress the navigation signal by nearly 400 ns after through the LNA, and the signal can be completely suppressed under repetitive frequency. Consequently, the distributed UWB electromagnetic pulse jamming system based on low-orbit satellites can effectively enhance the jamming effect, which has the potential to achieve full coverage of the target area.
Continuous phase plate (CPP) is a typical phase optical element. It will form a speckle field focused by a lens. The statistical characteristics of the speckle field affect the beam smoothing result. When a lens with large number aperture is used, scalar diffraction theory is unsuitable to analyze the distribution character of the focal spot because the paraxial approximation is no longer valid. In this paper, the Richar-Wolf vector diffraction theory is employed to calculate the focal spot of CPP under the strong focusing condition. Then both the profile of the focal spot and its statistical characteristics are discussed in detail. The results show that the spot size that calculated by the vector method is larger than which calculated by scalar method due to the non-paraxial effect. According to the feature of the vector method, the z-component of the light field can be obtained. The amplitude distribution of the speckle meets the Rayleigh distribution character, and its intensity distribution meets the negative exponential distribution character. Influenced by the z-component, the intensity distribution in the vectorial resultant direction will slightly deviate from the negative exponential distribution character .
The core emission was observed by Kirkpatrick-Baez (KB) microscope with about 5 μm space resolution. It is important to evaluate the uncertainty of core image thus to tune the implosion asymmetry. This article evaluates the spatial resolution and SNR of the backlight image got by KB microscope in the experiment, and calculates the uncertainty for the Legendre moments of core emission. The results show that the uncertainty of P2 is about 6% that of and P4 is about 8%.
In this paper, based on the SALOME platform, Open CASCADE works as the geometric modeling engine and adopts the idea of replacing a surface with an entity and Cell hierarchical multinomial tree structure to carry out the research on the Inversion of the computing model and 3D visualization method. Based on this method of this paper, the inversion visualization program module SALOME-MC is developed, and the module can realize the functions of Monte Carlo computational model geometric modeling, material modeling, and model 3D visualization. Three typical reactor models were selected to test and validate the method and procedure, the test results show that the method and program can properly handle the complex geometry and large-scale repeated structure of the Monte Carlo calculation model, and accurately realize the CAD three-dimensional inversion visualization of the Monte Carlo calculation model, which proves the correctness and reliability of the inversion ability and visualization effect of the SALOME-MC Monte Carlo calculation model, and improves the geometric modeling efficiency and display degree of the Monte Carlo calculation model.
Shenzhen Superconducting Soft X-ray Free Electron Laser (S3FEL) will be the only high refrequency free electron laser in soft X-ray band in the world. Dump, playing an important role in system beam tuning, is an important equipment of S3FEL device. As an important component of dump, dump beam window is used to isolate and protect the ultra-high vacuum environment of the accelerator. In this paper, several commonly used materials for dump beam window are compared and analyzed, beryllium is finally chosen as the material. A dump beam window with water-cooled structure is designed using to beryllium. The deposition power of beam window with different thickness is calculated by Monte Carlo method. The thermal-mechanical simulations based on the finite element analysis method show that the water-cooled beryllium window with a thickness of 1.6 mm is the best and meet the application requirements. Its maximum temperature is 121.6 ℃. The maximum stress and central deformation at low vacuum of 1 Pa are 198.7 MPa and 0.000 82 mm respectively. The maximum stress and central deformation at low vacuum of 101 325 Pa are 204.2 MPa and 0.097 mm respectively. The present study provides a critical theoretical basis for the design of dump beam window in S3FEL.
Accurate measurement of the intense pulse electron beam transverse position is not only related to the technology of beam position monitor (BPM) design, machining and assembly, but also related to calibration of BPM. This paper describes the physical design of the calibrated device based on the measuring principle of intense pulse electron beam position monitor in linear induction accelerator. A coaxial line structure is used as the calibrated device. The axial length that affects the electromagnetic field at a given location and the position measurement affected by coaxial line with displaced inner conductor are analyzed in theory, the results determine the length and the characteristic impedance of the coaxial line. The extent of inner conductor displacement is determined by the calculated errors of beam position measurement. Pulse signal mismatch transmission in calibration, the PSpice simulation and experimental measurement are performed, and results show that a required pulse waveform is obtained.
The simulated results have shown that the direction of Cherenkov Radiation (CR) light in very thin layer can be used to measure electron beam divergence and its distribution directly. The measurement results are reliable if the parameters of devices used in the system are suited for the electron beam. This method is easy in data processing because it has no need to assume electron beam phasic space, beam divergence distribution, charge density distribution model and so on. The electron beam divergence distribution measurement system can be established by way of taking a thin enough quartz slice as the convertor and letting the electron beam incidence enter the convertor with Cherenkov radiation angle. Focus plane imaging method is required to obtain the divergence image of space distribution of electron at the same time. The beam divergence measurement technology and equipment development achieved on high current pulsed linear induction accelerator have proved that the system has the characteristics of simple structure, low difficulty and fast speed of data processing.
The China Spallation Neutron Source (CSNS), located in Dongguan city, is the country's first and only spallation neutron source. It is also the fourth spallation neutron source in the world. It achieved the design index of 100 kW power in February 2020, and its operation is reliable and efficient. The total beam power will be increased to 500 kW in the CSNS phase II upgrade design, where the linac beam energy will be boosted from 80 MeV to 300 MeV and the peak current intensity will be increased from 15 mA to 50 mA by incorporating a superconducting linear accelerator. The double spoke resonator (DSR) will be used in the energy range of 80 MeV to 165 MeV. In the energy range of 165 MeV to 300 MeV, 6-cell ellipsoidal cavity will be used. DSR has many advantages, such as large velocity acceptance, which allows the spoke cavity to be used for a wider range of velocities, small size, high shunt impedance and a high coupling degree, which allows the production error requirements to be relaxed and the frequency bandwidth to be broadened from neighboring modes, among others. The DSR performance, including electromagnetic properties and machine parameters, was simulated and optimized using CST, COMSOL and other simulation software, and it achieved the requirement of CSNS phase II upgrade project. To improve the operating stability of the cavity, the design focused on minimizing Ep/Eacc.
With the increasing power density of aerospace microwave devices, the possibility of multipactor effect in microwave devices is greatly increased. In view of the potential threat that space electronics may induce the multipactor discharge in microwave devices, a special waveguide structure was designed to study the multipactor discharge. The electron gun provided electron into special waveguide when high power microwave signal were introduced into the waveguide. The power detector and oscillograph were used to detect transmission waveforms and reflection waveform respectively. The continuous process of multipactor phenomenon was obviously observed. This work verifies that electron beam can induce multipactor phenomenon and provides an important means to study multipactor effect of microwave devices.
In view of the demand for repetitive charging of compact high-power pulse drive source, the constant current charging technology based on LC full bridge series resonant principle is studied, and the key parameters of the power supply are designed according to the working mode of compact Marx pulse power source. A compact high-voltage power supply with positive and negative bipolar charging is developed, charging the equivalent load capacitance of 0.15 μF to ±45 kV within 20 ms, and the average charging power is greater than 15.5 kW. The power supply uses a single high-frequency high-voltage transformer to achieve positive and negative bipolar high voltage synchronous output; the integrated insulation packaging design of transformer, rectifier circuit, isolation protection circuit and voltage detection circuit is adopted, which not only reduces the volume of the device but also reduces the risk of high-voltage insulation; through the design of isolation protection and electromagnetic shielding, the interference and damage of the instantaneous high-voltage signal to the power control system during the discharge of the Marx generator are effectively solved.
Pseudo-spark switch, with the advantages of large pulse current, wide operating voltage, long life expectancy and high reliability, is one of the key devices in the field of pulse power technology. To meet the miniaturized development requirement in the complete machine, the miniature pseudo-spark switch has been developed. The electron source is injected by introducing the trigger needle into the hollow cathode innovatively in order to achieve glow discharge. Through this physical design the volume of the pseudo-spark switch has been decreased as well as the design rationality has been verified by producing and testing the pseudo-spark switch. The test results indicate that the anode working voltage is ranged from 500 V to 10 kV and the anode pulse current is 40 kA. Under the condition that the anode pulse current is 36 kA, no sign of performance decline has been found after more than 20000 times of work. The sample has passed the reliability test of high temperature, low temperature, temperature cycle and vibration.
Studying the influencing factors of insulator flashover under nanosecond pulse has important reference significance for the design of insulation structure of electromagnetic pulse simulation device. By building an experimental platform for insulator flashover, the effects of pulse voltage waveform, insulating material and surface field strength distribution on the insulator surface flashover voltage were experimentally studied in 0.5 MPa SF6 gas. The results show that the flashover voltage of the insulator tends to increase with the decreasing of the pulse front time; Compared with the full pulse voltage wave, the flashover voltage of insulator is higher under the front waveform of pulse voltage; The insulation performance of the polyimide material is optimal; By reducing the maximum field strength along the insulator surface and improving the electric field distribution, the flashover voltage of the insulator can be effectively increased.
Aiming at the insulation failure caused by surface flashover phenomenon in high voltage equipment, this paper summarizes the key issues such as basic characteristic measurement means, influencing factors and occurrence mechanism of surface flashover phenomenon. In this paper, the main research progress of surface flashover observation methods and their influencing factors are introduced, and the specific mechanism of surface flashover process and the role of surface charge in the process of surface flashover are discussed. Among them, the external factors, the electrode - medium interface layer factor and the vacuum - medium surface layer factor affect the surface flashover as well as the surface charge accumulation and dissipation, and the specific mechanism is different. In the mainstream mechanism of surface flashover, the SEEA theory has a complete description of the initial process of surface flashover, while the ETPR theory has a better explanation of the development process of surface flashover. In addition, surface charge provides the necessary charge for the generation of flashover along the surface, and its accumulation and dissipation play a decisive role in the development of flashover along the surface. Developing insulating materials with low secondary electron emission coefficient and high surface conductance and surface modification technology will be the key research direction in this field in the future.
The pulsed discharge process in water is complex and there is no clear functional relationship between the discharge parameters and the discharge deposition energy. To obtain the optimum deposition energy, clarify the influence of different discharge parameters on the deposition energy and obtain the best combination of discharge parameters, this paper builds a high-voltage pulse discharge test platform in water and investigates the influence of three discharge parameters, namely voltage, electrode spacing and conductivity, on the deposition energy of discharge in water by combining with the Kriging agent model. The optimal combination of discharge parameters was determined by using a genetic algorithm. The results of the study show that: the root mean square error of the model is 6.95%, which satisfies the accuracy requirement through cross-validation; the deposition energy varies with multiple peaks under the synergistic effect of electrode spacing and conductivity at a certain applied voltage; the best combination of voltage, electrode spacing and conductivity is 17 kV, 2.28 mm and 0.8 mS/cm respectively, which produces the highest deposition energy. The relative deviation between predicted and actual values at the optimum point were experimentally verified to be within 8%.
Aiming at the requirement of large-scale switch synchronous triggering in all-solid-state linear transformer driver (LTD), this paper designs a new dual-channel synchronous high-resolution digital-to-time converter (DTC) based on the vernier method and pre-phase shift technology. On the basis of the original vernier DTC, the relationship between the overlapping positions of different generated pulse phases is calculated in advance, and the clock signal meets the phase relationship in advance through phase shift and phase detection, so as to achieve the purpose of triggering multiple pulse signals of different widths at the same time. The realization principle and circuit design module of DTC are expounded in detail, and the simulation and Field Programmable Gate Array (FPGA) realization are carried out. At the same time, the realization result is tested, analyzed and discussed. On the Xilinx ARTIX-7 FPGA development board, the resolution of the first pulse signal is 0.85 ps, and the differential nonlinearity (DNL) and integral nonlinearity (INL) are -2.55～2.17 LSB and -7.33～7.05 LSB, respectively. The resolution of the second pulse signal is 17.1131 ps, DNL and INL are -0.005～0.0105 LSB and -0.299～0.288 LSB respectively, and the performance of DTC can still be guaranteed in the ambient temperature of 0～80 ℃. The results show that this DTC has the advantages of simple implementation, low cost and high performance.