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RSS2013 Vol. 25, No. 03
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2013,
25: 537-543.
doi: 10.3788/HPLPB20132503.0537
Abstract:
Power semiconductor devices have been used for repetitive pulsed power generation, due to their advantages in stability, reliability, and lifetime. On the other hand, their limitations in power capability and noise vulnerability are expected to be overcome by using various techniques in gate control and circuit configuration. In this paper, some pulse circuit methods to improve the working ability of semiconductor switches are introduced, including using a number of devices in series and parallel, voltage superposition, combination and complementation of switching devices.
Power semiconductor devices have been used for repetitive pulsed power generation, due to their advantages in stability, reliability, and lifetime. On the other hand, their limitations in power capability and noise vulnerability are expected to be overcome by using various techniques in gate control and circuit configuration. In this paper, some pulse circuit methods to improve the working ability of semiconductor switches are introduced, including using a number of devices in series and parallel, voltage superposition, combination and complementation of switching devices.
2013,
25: 544-548.
doi: 10.3788/HPLPB20132503.0544
Abstract:
According to the analysis of the characteristics of oscillation paths in the optical cavity of the unstable-resonator DF chemical laser, we point out that different oscillation paths of the upstream and downstream spectra will lead to certain difference in the emission spectrum. The emission spectrum of a continuous-wave DF laser was measured, and it is found that there is no significant difference in the major spectral line components, but some difference in the relative intensity of the upstream and downstream spectral lines. The spectral band of the upstream laser spot, namely the rotational quantum number at maximum relative intensity, tends to be in the direction of the low rotational quantum number for each spectral band. According to the measured spectrum, the cavity temperature and particle population ratio were estimated, and it is found that the average temperature of the upstream cavity is lower than that of the downstream cavity.
According to the analysis of the characteristics of oscillation paths in the optical cavity of the unstable-resonator DF chemical laser, we point out that different oscillation paths of the upstream and downstream spectra will lead to certain difference in the emission spectrum. The emission spectrum of a continuous-wave DF laser was measured, and it is found that there is no significant difference in the major spectral line components, but some difference in the relative intensity of the upstream and downstream spectral lines. The spectral band of the upstream laser spot, namely the rotational quantum number at maximum relative intensity, tends to be in the direction of the low rotational quantum number for each spectral band. According to the measured spectrum, the cavity temperature and particle population ratio were estimated, and it is found that the average temperature of the upstream cavity is lower than that of the downstream cavity.
2013,
25: 549-552.
doi: 10.3788/HPLPB20132503.0549
Abstract:
By analyzing the one-dimensional heat transfer equation under the heat flux boundary condition, a numerical method for inverse computation from the back surface temperature data to the front surface reflectivity data is proposed. The inversely computing program is verified by the positive and inverse computation of one-dimensional heat transfer. The inverse computation results are compared with the reflectivity measured in the experiments, showing that the variation trend of two reflectivity curves are in good agreement. Thus the inverse computation is feasible in one-dimensional temperature field.
By analyzing the one-dimensional heat transfer equation under the heat flux boundary condition, a numerical method for inverse computation from the back surface temperature data to the front surface reflectivity data is proposed. The inversely computing program is verified by the positive and inverse computation of one-dimensional heat transfer. The inverse computation results are compared with the reflectivity measured in the experiments, showing that the variation trend of two reflectivity curves are in good agreement. Thus the inverse computation is feasible in one-dimensional temperature field.
2013,
25: 553-556.
doi: 10.3788/HPLPB20132503.0553
Abstract:
Ultraviolet differential absorption lidar (UV DIAL) is a powerful instrument for measuring O3 vertical distribution in the troposphere. Using self-manufactured AML-2, O3 concentration distribution in the low troposphere in southern suburb of Beijing was surveyed in November, 2009. Partial results were compared with those from a radiosonde balloon, and they show the same distribution trend. Experiment data during fine weather were selected and analyzed in detail. The results indicate that, O3 concentration decreases while the height increases in the low troposphere; the average O3 concentration has obvious diurnal variations from ground level 500 meters to 1.5 kilometers; the maximum appears at about 2 p.m. and is less than 410-8 with a maximum variance of 3.210-9.
Ultraviolet differential absorption lidar (UV DIAL) is a powerful instrument for measuring O3 vertical distribution in the troposphere. Using self-manufactured AML-2, O3 concentration distribution in the low troposphere in southern suburb of Beijing was surveyed in November, 2009. Partial results were compared with those from a radiosonde balloon, and they show the same distribution trend. Experiment data during fine weather were selected and analyzed in detail. The results indicate that, O3 concentration decreases while the height increases in the low troposphere; the average O3 concentration has obvious diurnal variations from ground level 500 meters to 1.5 kilometers; the maximum appears at about 2 p.m. and is less than 410-8 with a maximum variance of 3.210-9.
2013,
25: 557-560.
doi: 10.3788/HPLPB20132503.0557
Abstract:
The laser-ablation threshold of aluminum alloy was accurately measured by dual-wavelength dual-pulse laser-ablation laser-induced breakdown spectroscopy under orthogonal geometric arrangement. For 532 nm laser wavelength and 12 ns laser pulse width, the laser-ablation energy threshold of aluminum alloy was determined to be 48 J, equivalent with a fluence threshold of 9.8 J/cm2, under tightly focused condition with an aspheric lens of 2 cm focal length. This is a new spectroscopic method to measure laser-ablation threshold. It is more sensitive and more accurate in comparison with established methods for laser-ablation threshold measurement, and the measurement is rapid and quite convenient. This method can be used for accurate laser-ablation threshold measurement for different materials.
The laser-ablation threshold of aluminum alloy was accurately measured by dual-wavelength dual-pulse laser-ablation laser-induced breakdown spectroscopy under orthogonal geometric arrangement. For 532 nm laser wavelength and 12 ns laser pulse width, the laser-ablation energy threshold of aluminum alloy was determined to be 48 J, equivalent with a fluence threshold of 9.8 J/cm2, under tightly focused condition with an aspheric lens of 2 cm focal length. This is a new spectroscopic method to measure laser-ablation threshold. It is more sensitive and more accurate in comparison with established methods for laser-ablation threshold measurement, and the measurement is rapid and quite convenient. This method can be used for accurate laser-ablation threshold measurement for different materials.
2013,
25: 561-564.
doi: 10.3788/HPLPB20132503.0561
Abstract:
Using the technique of short pulse generation by electro-optical (E-O) switches, a method for amplified spontaneous emission (ASE) suppression of the excimer lasers amplifier chain is established. An ultraviolet E-O switch with high contrast ratio was produced, and its principle experiments were carried out. The switch was applied to ASE suppression of the first pre-amplifier in excimer single-channel master oscillator power-amplifier (MOPA) system. The experimental result shows that the contrast ratio of the E-O switch is more than 2000, and the signal contrast ratio of output pulse of the pre-amplifier is promoted to over 20 000 after clipping. The result is beneficial to the following amplification of the laser pulse signal in the system, and provides a technical support for the system to obtain narrow-pulse signal output.
Using the technique of short pulse generation by electro-optical (E-O) switches, a method for amplified spontaneous emission (ASE) suppression of the excimer lasers amplifier chain is established. An ultraviolet E-O switch with high contrast ratio was produced, and its principle experiments were carried out. The switch was applied to ASE suppression of the first pre-amplifier in excimer single-channel master oscillator power-amplifier (MOPA) system. The experimental result shows that the contrast ratio of the E-O switch is more than 2000, and the signal contrast ratio of output pulse of the pre-amplifier is promoted to over 20 000 after clipping. The result is beneficial to the following amplification of the laser pulse signal in the system, and provides a technical support for the system to obtain narrow-pulse signal output.
2013,
25: 565-568.
doi: 10.3788/HPLPB20132503.0565
Abstract:
Based on the working principle of the satellite borne scanning imaging system, we expound traditional line scan and oversampling detection. The mathematical modeling and simulation imaging of the point target detection in two detection modes are presented. The oversampling system oversamples the image twice along the line direction while 2 to 10 times along the scanning direction(i.e. the time direction) with Matlab. The signal is analyzed by routinely scanning and oversampling the spot targets. The relationship between the power of the signal and the sizes, and the imaging effects brought by different locations of targets are given. At last, we obtain 4X oversampling images of different targets through two channels and analyze the multi-pixel phenomenon existing in the images.
Based on the working principle of the satellite borne scanning imaging system, we expound traditional line scan and oversampling detection. The mathematical modeling and simulation imaging of the point target detection in two detection modes are presented. The oversampling system oversamples the image twice along the line direction while 2 to 10 times along the scanning direction(i.e. the time direction) with Matlab. The signal is analyzed by routinely scanning and oversampling the spot targets. The relationship between the power of the signal and the sizes, and the imaging effects brought by different locations of targets are given. At last, we obtain 4X oversampling images of different targets through two channels and analyze the multi-pixel phenomenon existing in the images.
2013,
25: 569-573.
doi: 10.3788/HPLPB20132503.0569
Abstract:
The distribution of light field behind a vaulted axicon is calculated with diffraction theory and simulated with software, the result demonstrates that vaulted axicons can be used to generate bottle beams. The light intensity is simulated again after changing the base angle or the focal length, the result shows that the base angle of vaulted axicons has some effect on the period and the number of dark regions of bottle beams, while the focal length of plano-convex lenses on the top of vaulted axicons is related to the number of dark regions but has little effect on the period. The required bottle beam can be obtained according to adjusting the relevance parameters in practical application.
The distribution of light field behind a vaulted axicon is calculated with diffraction theory and simulated with software, the result demonstrates that vaulted axicons can be used to generate bottle beams. The light intensity is simulated again after changing the base angle or the focal length, the result shows that the base angle of vaulted axicons has some effect on the period and the number of dark regions of bottle beams, while the focal length of plano-convex lenses on the top of vaulted axicons is related to the number of dark regions but has little effect on the period. The required bottle beam can be obtained according to adjusting the relevance parameters in practical application.
2013,
25: 574-578.
doi: 10.3788/HPLPB20132503.0574
Abstract:
In order to obtain the image and the position information of the target, an azimuth measuring method for moving targets based on monocular vision system is investigated. According to the principle of projection imaging, the mapping relationship between the 3-D position and the 2-D image of the target is derived, and the target azimuth measuring mathematical model is established based on monocular vision system. A detection combination algorithm is proposed using image subtraction and KLT method for moving targets under still background and changing background, and the sub-pixel orientation method is applied to the positioning of target feature points. The experimental and simulation results show that the method can extract the target feature points effectively, achieving a relatively accurate target positioning, and the target azimuth information is measured through monocular vision method, with the error within 8%.
In order to obtain the image and the position information of the target, an azimuth measuring method for moving targets based on monocular vision system is investigated. According to the principle of projection imaging, the mapping relationship between the 3-D position and the 2-D image of the target is derived, and the target azimuth measuring mathematical model is established based on monocular vision system. A detection combination algorithm is proposed using image subtraction and KLT method for moving targets under still background and changing background, and the sub-pixel orientation method is applied to the positioning of target feature points. The experimental and simulation results show that the method can extract the target feature points effectively, achieving a relatively accurate target positioning, and the target azimuth information is measured through monocular vision method, with the error within 8%.
2013,
25: 579-582.
doi: 10.3788/HPLPB20132503.0579
Abstract:
An experiment system for laser-induced acoustic signal has been built. Acoustic signal is induced by pulse laser focused into water. A hydrophone is used to receive and convert acoustic signal into electric signal. The energy of the acoustic signal is calculated. Laser acoustic energy conversion efficiency is theoretically and experimentally analyzed. The results are as follows: as laser energy increases, energy conversion efficiency decreases; the longer distance the laser travels before being focused, the smaller the energy conversion efficiency is; energy conversion efficiency increases as absorbed energy by plasma increases; water salinity has little influence on energy conversion efficiency.
An experiment system for laser-induced acoustic signal has been built. Acoustic signal is induced by pulse laser focused into water. A hydrophone is used to receive and convert acoustic signal into electric signal. The energy of the acoustic signal is calculated. Laser acoustic energy conversion efficiency is theoretically and experimentally analyzed. The results are as follows: as laser energy increases, energy conversion efficiency decreases; the longer distance the laser travels before being focused, the smaller the energy conversion efficiency is; energy conversion efficiency increases as absorbed energy by plasma increases; water salinity has little influence on energy conversion efficiency.
2013,
25: 583-586.
doi: 10.3788/HPLPB20132503.0583
Abstract:
A novel single-shot method for the investigation of surface nonlinearity in materials with limited transparency is demonstrated. When a circular disk that blocks the beam is placed in the beam path before the detector in the conventional reflection 4F coherent imaging system, the surface nonlinear optical response of the sample can be determined by measuring the normalized reflectance under different conditions. The theoretical principle of the method is presented in detail and the influences of incidence-angle variation on the measurement sensitivity are analyzed using numerical simulation. This approach allows for sensitivity enhancement, one-laser-shot detection, determination of the nonlinear absorption coefficient and nonlinear refractive index simultaneously. The results show that the measurement sensitivity of this novel technique is two orders of magnitude larger than that of the original reflection Z-scan technique under the same condition.
A novel single-shot method for the investigation of surface nonlinearity in materials with limited transparency is demonstrated. When a circular disk that blocks the beam is placed in the beam path before the detector in the conventional reflection 4F coherent imaging system, the surface nonlinear optical response of the sample can be determined by measuring the normalized reflectance under different conditions. The theoretical principle of the method is presented in detail and the influences of incidence-angle variation on the measurement sensitivity are analyzed using numerical simulation. This approach allows for sensitivity enhancement, one-laser-shot detection, determination of the nonlinear absorption coefficient and nonlinear refractive index simultaneously. The results show that the measurement sensitivity of this novel technique is two orders of magnitude larger than that of the original reflection Z-scan technique under the same condition.
2013,
25: 587-592.
doi: 10.3788/HPLPB20132503.0587
Abstract:
According to photometric measurement, the orbit performance of space targets is judged and the identification catalog about space targets is carried out. The applications of optoelectronic telescopes in the domain of space target identification are summarized and discussed. A method for detecting dark space targets is tested, which is called the Hough transform. The factors which affect the limiting-magnitude of optoelectronic telescopes are analyzed by simulation and calculations. Based on the general signal-to-noise ratio formulation for the CCD detector, the theoretical results of limiting-magnitude of optoelectronic telescopes are derived and calculated, which are demonstrated and validated with the method of star-map comparisons. Experiment results show star-map comparison is a reliable method for detecting star magnitude.
According to photometric measurement, the orbit performance of space targets is judged and the identification catalog about space targets is carried out. The applications of optoelectronic telescopes in the domain of space target identification are summarized and discussed. A method for detecting dark space targets is tested, which is called the Hough transform. The factors which affect the limiting-magnitude of optoelectronic telescopes are analyzed by simulation and calculations. Based on the general signal-to-noise ratio formulation for the CCD detector, the theoretical results of limiting-magnitude of optoelectronic telescopes are derived and calculated, which are demonstrated and validated with the method of star-map comparisons. Experiment results show star-map comparison is a reliable method for detecting star magnitude.
2013,
25: 593-597.
doi: 10.3788/HPLPB20132503.0593
Abstract:
2024 aluminum alloy samples were treated by multiple laser shock processing(LSP). Residual stresses of impact zone were analyzed by XRD. The distribution of residual stress state by multiple LSP was studied. The quality of laser shock processing was evaluated by the distribution of residual stress state. Experimental results show that plastic deformation and plastic strain gradient decreased with the increasing shock times, and the measured points were in two-direction compressive stress state. Plastic strain gradient was increased by 4 LSP impacts. The point center was in one-direction compressive stress state and the others were in two-direction compressive stress state. The quality of LSP was optimum by the laser power density of 2.8 GW/cm2 and 3 LSP impacts, with measured points in two-direction compressive stress state. In this condition, the mean values of maximum principal residual stresses and stress intensity were maximum, the variances of them were minimum, the distribution of them was uniform, and the plastic strain gradient was minimum.
2024 aluminum alloy samples were treated by multiple laser shock processing(LSP). Residual stresses of impact zone were analyzed by XRD. The distribution of residual stress state by multiple LSP was studied. The quality of laser shock processing was evaluated by the distribution of residual stress state. Experimental results show that plastic deformation and plastic strain gradient decreased with the increasing shock times, and the measured points were in two-direction compressive stress state. Plastic strain gradient was increased by 4 LSP impacts. The point center was in one-direction compressive stress state and the others were in two-direction compressive stress state. The quality of LSP was optimum by the laser power density of 2.8 GW/cm2 and 3 LSP impacts, with measured points in two-direction compressive stress state. In this condition, the mean values of maximum principal residual stresses and stress intensity were maximum, the variances of them were minimum, the distribution of them was uniform, and the plastic strain gradient was minimum.
2013,
25: 598-602.
doi: 10.3788/HPLPB20132503.0598
Abstract:
A multi-tone narrow-linewidth fiber amplifier is demonstrated. In this kind of amplifier the seed laser is attained by combining several single frequency seed lasers which all have nearly the same wavelength (wavelength separation less than 1 nm) and with frequency difference larger than twice the Brillouin shift. An integrated theoretical model of this kind of amplifier is set up. Relationship between stimulated Brillouin scattering(SBS) and the number of the seeds is analyzed using this model. Two-tone and three-tone amplifiers are set up to study SBS threshold output power experimentally. The experimental results are in good agreement with theoretical results, revealing rationality of the theoretical model. Two-tone and three-tone amplifications can suppress SBS effectively and enhance output powers of narrow-linewidth fiber amplifiers greatly.
A multi-tone narrow-linewidth fiber amplifier is demonstrated. In this kind of amplifier the seed laser is attained by combining several single frequency seed lasers which all have nearly the same wavelength (wavelength separation less than 1 nm) and with frequency difference larger than twice the Brillouin shift. An integrated theoretical model of this kind of amplifier is set up. Relationship between stimulated Brillouin scattering(SBS) and the number of the seeds is analyzed using this model. Two-tone and three-tone amplifiers are set up to study SBS threshold output power experimentally. The experimental results are in good agreement with theoretical results, revealing rationality of the theoretical model. Two-tone and three-tone amplifications can suppress SBS effectively and enhance output powers of narrow-linewidth fiber amplifiers greatly.
2013,
25: 603-606.
doi: 10.3788/HPLPB20132503.0603
Abstract:
A nonuniformity correction algorithm based on image decomposition of nonuniformity in IRFPA was proposed, for the engineering application limitations of traditional adaptive nonuniformity for IRFPA. After analysis of the constructing factor of nonuniformity, the algorithm decomposes the high-frequency part of nonuniformity into bright pixels, row and column nonunniformity, and the low-frequency part of nonuniformity into gradient nonunifority, and then corrects every kind of nonunifority using a special algorithm, and completes the correction by combination. The algorithm was tested in a infrared imaging system and compared with other adaptive non-uniformity correction algorithms. According to the analysis of simulation results, this algorithm has the advantage of strong ability to eliminate the target fade-out, fast convergence speed and high precision.
A nonuniformity correction algorithm based on image decomposition of nonuniformity in IRFPA was proposed, for the engineering application limitations of traditional adaptive nonuniformity for IRFPA. After analysis of the constructing factor of nonuniformity, the algorithm decomposes the high-frequency part of nonuniformity into bright pixels, row and column nonunniformity, and the low-frequency part of nonuniformity into gradient nonunifority, and then corrects every kind of nonunifority using a special algorithm, and completes the correction by combination. The algorithm was tested in a infrared imaging system and compared with other adaptive non-uniformity correction algorithms. According to the analysis of simulation results, this algorithm has the advantage of strong ability to eliminate the target fade-out, fast convergence speed and high precision.
2013,
25: 607-610.
doi: 10.3788/HPLPB20132503.0607
Abstract:
Self-organized passive coherent beam combination(CBC) of multichannel fiber laserswith all-fiber configuration is reported. The influence of pump power and the number of CBC channels on beam quality and combination efficiency is experimentally studied by combining 2, 3 and 4 channels of lasers. The efficiency of coherent combination is above 90%. The result demonstrates that the energy rate of central lobe is improved with the increasing of pump power and the number of lasers. At the same output power, the energy rate of central lobe declines when the number of CBC channels increases. The power scaling from a single channel is better than that from CBC of several channels for the beam quality at the same output power.
Self-organized passive coherent beam combination(CBC) of multichannel fiber laserswith all-fiber configuration is reported. The influence of pump power and the number of CBC channels on beam quality and combination efficiency is experimentally studied by combining 2, 3 and 4 channels of lasers. The efficiency of coherent combination is above 90%. The result demonstrates that the energy rate of central lobe is improved with the increasing of pump power and the number of lasers. At the same output power, the energy rate of central lobe declines when the number of CBC channels increases. The power scaling from a single channel is better than that from CBC of several channels for the beam quality at the same output power.
2013,
25: 611-614.
doi: 10.3788/HPLPB20132503.0611
Abstract:
For studying the polarization in laser beam coherent combination, ideal rectangular beams are used in simulation. With changing the degradation rate of polarization, the angles between beams axes and the number of combining beams, the synthetic effects of the laser fields are observed. And the far-field intensity distribution, the peak power and the beam quality of the combined beams are analyzed. Results show that if the degradation rate of polarization is less than 10% or the polarization angle is less than 18 degrees, the beam quality factor of combined beams could be controlled in 1.1 times the diffraction limit.
For studying the polarization in laser beam coherent combination, ideal rectangular beams are used in simulation. With changing the degradation rate of polarization, the angles between beams axes and the number of combining beams, the synthetic effects of the laser fields are observed. And the far-field intensity distribution, the peak power and the beam quality of the combined beams are analyzed. Results show that if the degradation rate of polarization is less than 10% or the polarization angle is less than 18 degrees, the beam quality factor of combined beams could be controlled in 1.1 times the diffraction limit.
2013,
25: 615-619.
doi: 10.3788/HPLPB20132503.0615
Abstract:
Poly[bis(6-carbazolhexyloxy)]phosphazene with good thermal stability(about 290 ℃) and a low glass transition temperature(about 36 ℃) was synthesized with N-(6-hydroxyhexyl)carbazole and linear polydichlorophosphazene. Steady state fluorescence spectra elucidated that the maximum fluorescence-emission wavelengths of carbazole, N-(6-hydroxyhexyl)carbazole and poly[bis(6-carbazolhexyloxy)]phosphazene are 420, 410, and 393 nm, respectively. The fluorescence intensity of N-(6-hydroxyhexyl)carbazole and poly[bis(6-carbazolhexyloxy)]phosphazene decreases orderly and has a blue-shift compared with carbazole. The transient fluorescence spectra show that the linear backbone and the isolated d-p hybrid orbital of polyphosphazene might increase steric hindrance and disrupt the previous conjugate system, leading to decrease of the fluorescence lifetime of poly[bis(6-carbazolhexyloxy)]phosphazene at several emission wavelengths.
Poly[bis(6-carbazolhexyloxy)]phosphazene with good thermal stability(about 290 ℃) and a low glass transition temperature(about 36 ℃) was synthesized with N-(6-hydroxyhexyl)carbazole and linear polydichlorophosphazene. Steady state fluorescence spectra elucidated that the maximum fluorescence-emission wavelengths of carbazole, N-(6-hydroxyhexyl)carbazole and poly[bis(6-carbazolhexyloxy)]phosphazene are 420, 410, and 393 nm, respectively. The fluorescence intensity of N-(6-hydroxyhexyl)carbazole and poly[bis(6-carbazolhexyloxy)]phosphazene decreases orderly and has a blue-shift compared with carbazole. The transient fluorescence spectra show that the linear backbone and the isolated d-p hybrid orbital of polyphosphazene might increase steric hindrance and disrupt the previous conjugate system, leading to decrease of the fluorescence lifetime of poly[bis(6-carbazolhexyloxy)]phosphazene at several emission wavelengths.
2013,
25: 620-626.
doi: 10.3788/HPLPB20132503.0620
Abstract:
A brief review of X-ray phase contrast imaging theory for spherical objects is given. Using Tracepro, simulated X-ray phase-contrast images of SiO2/DD cryogenic capsules are produced. The ability of phase contrast enhanced X-ray imaging for characterization of cryogenic fuel layers inside ablator shells has been proven with the simulation result. X-ray phase contrast imaging equipment has been established for characterization of cryogenic targets, with which imaging experiments for low density divinyl benzene foam shells and SiO2/DD cryogenic capsules have been carried out, and phase contrast images of cryogenic DD fuel layers in SiO2 capsules are obtained, with a resolution up to 1.5 m. Ablator and cryogenic fuel layers can be simultaneously characterized by X-ray phase contrast imaging.
A brief review of X-ray phase contrast imaging theory for spherical objects is given. Using Tracepro, simulated X-ray phase-contrast images of SiO2/DD cryogenic capsules are produced. The ability of phase contrast enhanced X-ray imaging for characterization of cryogenic fuel layers inside ablator shells has been proven with the simulation result. X-ray phase contrast imaging equipment has been established for characterization of cryogenic targets, with which imaging experiments for low density divinyl benzene foam shells and SiO2/DD cryogenic capsules have been carried out, and phase contrast images of cryogenic DD fuel layers in SiO2 capsules are obtained, with a resolution up to 1.5 m. Ablator and cryogenic fuel layers can be simultaneously characterized by X-ray phase contrast imaging.
2013,
25: 627-632.
doi: 10.3788/HPLPB20132503.0627
Abstract:
In the inertial confinement fusion (ICF) system, continuous phase plate (CPP) designed by improved G-S algorithm can be used to improve the far-field distribution of distorted wave-front. Since the smoothing results of CPP for different distorted wave-fronts are different, the smoothing tolerance ability of CPP is analyzed systematically. According to the transmission characteristics of the distorted laser beams, root mean square (RMS) gradient of wave-front is used to quantify the wave-front distortion. And the far-field intensity distributions of different distorted beams which have translated through CPP are calculated by the numerical method. By comparing the uniformity of far-field focal spot after smoothing by CPP, a conclusion can be gotten that when the RMS gradient is under 0.32 wave/mm, CPP has good beam smoothing ability.
In the inertial confinement fusion (ICF) system, continuous phase plate (CPP) designed by improved G-S algorithm can be used to improve the far-field distribution of distorted wave-front. Since the smoothing results of CPP for different distorted wave-fronts are different, the smoothing tolerance ability of CPP is analyzed systematically. According to the transmission characteristics of the distorted laser beams, root mean square (RMS) gradient of wave-front is used to quantify the wave-front distortion. And the far-field intensity distributions of different distorted beams which have translated through CPP are calculated by the numerical method. By comparing the uniformity of far-field focal spot after smoothing by CPP, a conclusion can be gotten that when the RMS gradient is under 0.32 wave/mm, CPP has good beam smoothing ability.
2013,
25: 633-636.
doi: 10.3788/HPLPB20132503.0633
Abstract:
Pd-doped carbon aerogel powder is prepared with radiation method in 100, 200 and 500 kGy radiation doses, which has been confirmed by XRD, SEM and energy spectrum test. The SEM images show that Pd is well distributed in all the surface of aerogel particles. The nitrogen adsorption data analysis shows that the specific surface area, average pore diameter and total pore volume of carbon aerogel powder all decrease obviously after Pd doping. As the reduced metal mostly decorates the outer surface of the carbon aerogel powder, the textural parameters of doped carbon aerogel powder in different radiation doses are close to each other.
Pd-doped carbon aerogel powder is prepared with radiation method in 100, 200 and 500 kGy radiation doses, which has been confirmed by XRD, SEM and energy spectrum test. The SEM images show that Pd is well distributed in all the surface of aerogel particles. The nitrogen adsorption data analysis shows that the specific surface area, average pore diameter and total pore volume of carbon aerogel powder all decrease obviously after Pd doping. As the reduced metal mostly decorates the outer surface of the carbon aerogel powder, the textural parameters of doped carbon aerogel powder in different radiation doses are close to each other.
2013,
25: 637-640.
doi: 10.3788/HPLPB20132503.0637
Abstract:
A method is proposed to measure the cryocooler vibration via the Doppler effect. Both axial and radial vibration of a typical G-M RDK-408S cryocooler is measured on the experimental setup. The experimental results indicate that the cryocooler has a working frequency of about 1 Hz. Compared with other directions, the axial direction vibration is the largest, which is about 30 m, while the radial direction also has pulse vibration，equivalent to that of the axial vibration. The experimental results are much larger than the vibration requirements in inertial confinement fusion research and therefore vibration reduction methods must be adopted.
A method is proposed to measure the cryocooler vibration via the Doppler effect. Both axial and radial vibration of a typical G-M RDK-408S cryocooler is measured on the experimental setup. The experimental results indicate that the cryocooler has a working frequency of about 1 Hz. Compared with other directions, the axial direction vibration is the largest, which is about 30 m, while the radial direction also has pulse vibration，equivalent to that of the axial vibration. The experimental results are much larger than the vibration requirements in inertial confinement fusion research and therefore vibration reduction methods must be adopted.
2013,
25: 641-645.
doi: 10.3788/HPLPB20132503.0641
Abstract:
By analyzing the monochromatic X-ray backlighting system with spherically bent crystal, the expressions for the spatial resolution and the spectral resolution of this system are deduced, and the relationship between these key performance parameters and the design parameters of the system is analyzed. Analysis results of the imaging system with different imaging modalities are obtained, which coincide with the ray tracing results. A demo test of the monochromatic imaging system was operated on the X-ray source, and the results are consonant with the simulation and the calculation. This diagnosis could have a broad prospect of application in the measurements on Shenguang laser facility.
By analyzing the monochromatic X-ray backlighting system with spherically bent crystal, the expressions for the spatial resolution and the spectral resolution of this system are deduced, and the relationship between these key performance parameters and the design parameters of the system is analyzed. Analysis results of the imaging system with different imaging modalities are obtained, which coincide with the ray tracing results. A demo test of the monochromatic imaging system was operated on the X-ray source, and the results are consonant with the simulation and the calculation. This diagnosis could have a broad prospect of application in the measurements on Shenguang laser facility.
2013,
25: 646-650.
doi: 10.3788/HPLPB20132503.0646
Abstract:
The X-ray spatial distribution of implosion core zone is calculated with Multi1D and the spatial radiation transport equation. The results indicate that the spatial zone of higher energy X-ray emission is reduced with X-ray photon energy increasing, and the spatial zone of lower energy X-ray emission is reduced with X-ray photon energy decreasing. The spatial distribution of core temperature is extracted by the genetic algorithm, and the results are compared with simulation, with consideration of the opacity of materials in core zone. Their uncertainty is no more than 4%.
The X-ray spatial distribution of implosion core zone is calculated with Multi1D and the spatial radiation transport equation. The results indicate that the spatial zone of higher energy X-ray emission is reduced with X-ray photon energy increasing, and the spatial zone of lower energy X-ray emission is reduced with X-ray photon energy decreasing. The spatial distribution of core temperature is extracted by the genetic algorithm, and the results are compared with simulation, with consideration of the opacity of materials in core zone. Their uncertainty is no more than 4%.
2013,
25: 651-656.
doi: 10.3788/HPLPB20132503.0651
Abstract:
With a simulation method for backlighter radiography established in the paper, we study the effect of many non-ideal factors on the Abel density inversion of plastic target shell frequently used in inertial confinement fusion. These non-ideal factors include the pixel size of the recording screen, the spatial distribution of backlit intensity, the non-monochromatic spectra of backlighter source, the diffusion of pinholes and the shape of pinholes. The details of the density profile can be covered up by large size of pixel; the Gaussian backlighter intensity can result in lower density relative to real density profile and unphysical negative density; the density shows a certain deviation from real profile if the L-band of Mo is used as the backlighter source; the peak density can be decreased significantly by the diffusion effect if the thickness of the shell is smaller than the size of the pinhole, and the results can be improved if the Wiener filter is used to process the radiography. With the same area of pinholes, the peak density for the elliptical pinhole is smaller than that for the circular pinhole.
With a simulation method for backlighter radiography established in the paper, we study the effect of many non-ideal factors on the Abel density inversion of plastic target shell frequently used in inertial confinement fusion. These non-ideal factors include the pixel size of the recording screen, the spatial distribution of backlit intensity, the non-monochromatic spectra of backlighter source, the diffusion of pinholes and the shape of pinholes. The details of the density profile can be covered up by large size of pixel; the Gaussian backlighter intensity can result in lower density relative to real density profile and unphysical negative density; the density shows a certain deviation from real profile if the L-band of Mo is used as the backlighter source; the peak density can be decreased significantly by the diffusion effect if the thickness of the shell is smaller than the size of the pinhole, and the results can be improved if the Wiener filter is used to process the radiography. With the same area of pinholes, the peak density for the elliptical pinhole is smaller than that for the circular pinhole.
2013,
25: 657-661.
doi: 10.3788/HPLPB20132503.0657
Abstract:
The relationship of the roughness degree of ablation shell and the two peaks of fusion reaction rate was analyzed. We developed a system for fusion reaction history measurement with high temporal resolution, and applied this system to the SG-Ⅲ prototype for measuring fusion reaction rate. Fusion reaction rate history was measured for the first time with deuterium-tritium filled targets under neutron yields of about 1010. We anatomized possible influence factors during fusion reaction rates measuring. The two peaks of reaction rate can be obtained from the simulation results, and have been confirmed with the experimental results. It is found that neutrons in our experiment are generated by the first peak which is called the shock peak. The simulation and experimental results can provide strong technical support for the research on fusion reaction rate in inertial confinement fusion.
The relationship of the roughness degree of ablation shell and the two peaks of fusion reaction rate was analyzed. We developed a system for fusion reaction history measurement with high temporal resolution, and applied this system to the SG-Ⅲ prototype for measuring fusion reaction rate. Fusion reaction rate history was measured for the first time with deuterium-tritium filled targets under neutron yields of about 1010. We anatomized possible influence factors during fusion reaction rates measuring. The two peaks of reaction rate can be obtained from the simulation results, and have been confirmed with the experimental results. It is found that neutrons in our experiment are generated by the first peak which is called the shock peak. The simulation and experimental results can provide strong technical support for the research on fusion reaction rate in inertial confinement fusion.
2013,
25: 662-666.
doi: 10.3788/HPLPB20132503.0662
Abstract:
Design and simulations of the CAEP high power THz free electron laser(FEL) have been carried out using our 3D-Osifel code. The main work focuses on the optimization of different schemes through physical analysis. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. The gain and power of resonator are calculated, which are corresponding to eleven frequencies. It can be seen that when the facility works at 2.6 THz, the gain and output power are high. So the experiment may be conducted at this frequency first. When the device works at 1 THz, the slippage effect between the electron beam and radiation increases due to the increase of the radiation wavelength, which results in lack of overlapping between the electron and the optical pulses, then, the gain and output power lower. So experiment running at 1 THz could be challenged and further probe work should been put forward to solve the problem.
Design and simulations of the CAEP high power THz free electron laser(FEL) have been carried out using our 3D-Osifel code. The main work focuses on the optimization of different schemes through physical analysis. The wiggler peak field strength and electron beam energy have been selected with eleven frequencies ranging from 1 THz to 3 THz. The gain and power of resonator are calculated, which are corresponding to eleven frequencies. It can be seen that when the facility works at 2.6 THz, the gain and output power are high. So the experiment may be conducted at this frequency first. When the device works at 1 THz, the slippage effect between the electron beam and radiation increases due to the increase of the radiation wavelength, which results in lack of overlapping between the electron and the optical pulses, then, the gain and output power lower. So experiment running at 1 THz could be challenged and further probe work should been put forward to solve the problem.
2013,
25: 667-670.
doi: 10.3788/HPLPB20132503.0667
Abstract:
For the rigorous analysis of the propagation properties of dielectric-lined hollow cylindrical metallic waveguides operating in the THz range of frequencies, the characteristic equation for propagation constants is derived from the electromagnetic field equations and the boundary conditions of the dielectric-lined waveguides. The propagation constant of the dominant hybrid HE11 mode can be obtained by solving the characteristic equation with the improved Muller method. The classical relaxation-effect model for the conductivity is adopted to describe the frequency dispersive behavior of normal metals for the metallic waveguide wall. For a 1.8 mm bore diameter silver waveguide with the inner surface coated with a 17 m-thick layer of polystyrene(PS) film, the transmission losses of HE11 mode can be reduced to the level below 1 dB/m at 1.5~3.0 THz, and the dispersion is relatively small for HE11 mode. In addition, with the PS film thickness increasing, the transmission losses of HE11 mode increase first and then decrease for a 2.2 mm bore diameter silver waveguide at 2.5 THz, and the minimum loss can be achieved by adopting the optimum dielectric layer thickness.
For the rigorous analysis of the propagation properties of dielectric-lined hollow cylindrical metallic waveguides operating in the THz range of frequencies, the characteristic equation for propagation constants is derived from the electromagnetic field equations and the boundary conditions of the dielectric-lined waveguides. The propagation constant of the dominant hybrid HE11 mode can be obtained by solving the characteristic equation with the improved Muller method. The classical relaxation-effect model for the conductivity is adopted to describe the frequency dispersive behavior of normal metals for the metallic waveguide wall. For a 1.8 mm bore diameter silver waveguide with the inner surface coated with a 17 m-thick layer of polystyrene(PS) film, the transmission losses of HE11 mode can be reduced to the level below 1 dB/m at 1.5~3.0 THz, and the dispersion is relatively small for HE11 mode. In addition, with the PS film thickness increasing, the transmission losses of HE11 mode increase first and then decrease for a 2.2 mm bore diameter silver waveguide at 2.5 THz, and the minimum loss can be achieved by adopting the optimum dielectric layer thickness.
2013,
25: 671-674.
doi: 10.3788/HPLPB20132503.0671
Abstract:
A double-anode magnetic injection gun (MIG) was designed to satisfy demands of the 0.14 THz confocal gyro-travelling-wave tube (TWT). According to analytic trade-off equations, the initial parameters were determined first, then simulations and refinement were done with the PIC software. As a result, the horizontal-to-vertical velocity ratio of this MIG is 0.75, the axial velocity spread is 1.33%. The effect of some parameters, such as electrode shape, cathode magnetic field, mod-anode voltage and beam current on electron beam performance was discussed. The results indicate, all the parameters should be optimized sufficiently.
A double-anode magnetic injection gun (MIG) was designed to satisfy demands of the 0.14 THz confocal gyro-travelling-wave tube (TWT). According to analytic trade-off equations, the initial parameters were determined first, then simulations and refinement were done with the PIC software. As a result, the horizontal-to-vertical velocity ratio of this MIG is 0.75, the axial velocity spread is 1.33%. The effect of some parameters, such as electrode shape, cathode magnetic field, mod-anode voltage and beam current on electron beam performance was discussed. The results indicate, all the parameters should be optimized sufficiently.
2013,
25: 675-679.
doi: 10.3788/HPLPB20132503.0675
Abstract:
Based on microwave net theory, a cold test simulation model of double-gap output circuit with double coupling apertures was developed from that of output circuit with single coupling aperture. A computation method for gap impedance was also developed based on this model, by which the real part and imaginary part of the gap impedance can be both obtained. Meanwhile, a double-gap cavity with double coupling apertures for an S band klystron was simulated to show how to compute the gap impedance and external quality factor of the output cavity with this simulation method, and the computed result is verified to be reasonable in theory. The computed result was analyzed and compared with those obtained with the field analysis method and the scattering parameter of output circuit, confirming the reliability of this cold test simulation method.
Based on microwave net theory, a cold test simulation model of double-gap output circuit with double coupling apertures was developed from that of output circuit with single coupling aperture. A computation method for gap impedance was also developed based on this model, by which the real part and imaginary part of the gap impedance can be both obtained. Meanwhile, a double-gap cavity with double coupling apertures for an S band klystron was simulated to show how to compute the gap impedance and external quality factor of the output cavity with this simulation method, and the computed result is verified to be reasonable in theory. The computed result was analyzed and compared with those obtained with the field analysis method and the scattering parameter of output circuit, confirming the reliability of this cold test simulation method.
2013,
25: 680-684.
doi: 10.3788/HPLPB20132503.0680
Abstract:
The pulse source is the hard-core in ground penetrating radar(GPR), whose high voltage and ultra-wideband assure surveying depth and accuracy. Besides that, the smoothness and stability of pulse waveforms also affect imaging quality. Avalanche transistors are chosen in the Marx circuit to produce unipolar pulses. Improving methods are put forward to remove the unsmoothness of pulse waveforms. Microstrip line is placed at the end of the circuit, and delay time occurred in it makes signals to be bipolar. Comblike PCB is available to minimize the pulse source. The measured results show that the width between minus peak and plus peak is 1.64 ns, the voltage value is 812 V (50 load resistance), and the trailer is less than 10% of the whole voltage value, which match the simulation results very well. The pulse source could meet the requirements of GPR.
The pulse source is the hard-core in ground penetrating radar(GPR), whose high voltage and ultra-wideband assure surveying depth and accuracy. Besides that, the smoothness and stability of pulse waveforms also affect imaging quality. Avalanche transistors are chosen in the Marx circuit to produce unipolar pulses. Improving methods are put forward to remove the unsmoothness of pulse waveforms. Microstrip line is placed at the end of the circuit, and delay time occurred in it makes signals to be bipolar. Comblike PCB is available to minimize the pulse source. The measured results show that the width between minus peak and plus peak is 1.64 ns, the voltage value is 812 V (50 load resistance), and the trailer is less than 10% of the whole voltage value, which match the simulation results very well. The pulse source could meet the requirements of GPR.
2013,
25: 685-688.
doi: 10.3788/HPLPB20132503.0685
Abstract:
A circularly polarized (CP) crossed antenna array with 22 elements based on a novel antipodal Vivaldi antenna is proposed. The Vivaldi antenna with tapering serrated structure at the edges, which is a ultra-wideband (UWB) linearly polarized antenna, has a high gain at 4.7-7.0 GHz, and a wide impedance bandwidth of 2.4-11.0 GHz when the reflection coefficient is smaller than -10 dB. The measured data of the antenna array show that the axial ratio is lower than 3 dB between 4.5 GHz and 7.0 GHz. A high antenna gain from 11 dBi to 13 dBi is also achieved at the whole bandwidth of the array. The proposed antenna element and array both own UWB and high-gain characteristics.
A circularly polarized (CP) crossed antenna array with 22 elements based on a novel antipodal Vivaldi antenna is proposed. The Vivaldi antenna with tapering serrated structure at the edges, which is a ultra-wideband (UWB) linearly polarized antenna, has a high gain at 4.7-7.0 GHz, and a wide impedance bandwidth of 2.4-11.0 GHz when the reflection coefficient is smaller than -10 dB. The measured data of the antenna array show that the axial ratio is lower than 3 dB between 4.5 GHz and 7.0 GHz. A high antenna gain from 11 dBi to 13 dBi is also achieved at the whole bandwidth of the array. The proposed antenna element and array both own UWB and high-gain characteristics.
2013,
25: 689-692.
doi: 10.3788/HPLPB20132503.0689
Abstract:
The opinion has been existing for a long time that the bandwidth of mode converters will increase as the converter length is shortened when the conversion efficiency remains the same at the central frequency. However this paper indicates that there is an optimum length of TE01-TE11 mode converter. When the length of the mode converter is longer than the optimum length, the bandwidth decreases as the length increases; when the length of the mode converter is shorter than the optimum length, the bandwidth increases as the length increases. It is verified by the design of the TE01-TE11 mode converter whose central frequency and radius are 35 GHz, 13.6 mm respectively. The optimum length is 550 mm, and a bandwidth reaches a maximum of 6.28%.
The opinion has been existing for a long time that the bandwidth of mode converters will increase as the converter length is shortened when the conversion efficiency remains the same at the central frequency. However this paper indicates that there is an optimum length of TE01-TE11 mode converter. When the length of the mode converter is longer than the optimum length, the bandwidth decreases as the length increases; when the length of the mode converter is shorter than the optimum length, the bandwidth increases as the length increases. It is verified by the design of the TE01-TE11 mode converter whose central frequency and radius are 35 GHz, 13.6 mm respectively. The optimum length is 550 mm, and a bandwidth reaches a maximum of 6.28%.
2013,
25: 693-698.
doi: 10.3788/HPLPB20132503.0693
Abstract:
An improved input coupler with tapered transitional section and output structure adopting cascaded Chebyshev waveguide, working in TE01 mode, has been designed for W band gyro-TWT. By means of theoretic analysis and numerical computation in Matlab,the outer radius, the length of input couplers cavity and two contours of the section of coupling output are obtained. According to the optimized results in HFSS, the bandwidth of the improved input coupler which meets the demand of S11 less than -20 dB and S21 greater than -0.1 dB is 4.06 GHz. The cascaded Chebyshev structure has good performance on transmitting TE01 mode and suppressing spurious modes with acceptable length.
An improved input coupler with tapered transitional section and output structure adopting cascaded Chebyshev waveguide, working in TE01 mode, has been designed for W band gyro-TWT. By means of theoretic analysis and numerical computation in Matlab,the outer radius, the length of input couplers cavity and two contours of the section of coupling output are obtained. According to the optimized results in HFSS, the bandwidth of the improved input coupler which meets the demand of S11 less than -20 dB and S21 greater than -0.1 dB is 4.06 GHz. The cascaded Chebyshev structure has good performance on transmitting TE01 mode and suppressing spurious modes with acceptable length.
2013,
25: 699-703.
doi: 10.3788/HPLPB20132503.0699
Abstract:
In order to make the calculation of lightning electromagnetic field closer to the actual status and simulate the lightning electromagnetic field in far zone, the analytical expressions of lightning electromagnetic field generated by oblique channel were derived with the dipole method, and the approximate relationships of lightning electromagnetic field and its derivative to channel-base current were obtained after approximate treatment of the exact analytical expressions for lightning electromagnetic field in far area. The waveforms of lightning electromagnetic field and its derivative from the exact and approximate results were compared based on the specified channel-base current described by a pulse function. The result shows that the difference between the exact and approximate waveforms decreases with the increase of the distance, and the applicable range for the approximate expressions of lightning electromagnetic field is stricter than that of the electromagnetic field derivative. The above result verifies the credibility of the approximate expressions, and provides the theoretical basis for the simulation of lightning electromagnetic fields in far zone.
In order to make the calculation of lightning electromagnetic field closer to the actual status and simulate the lightning electromagnetic field in far zone, the analytical expressions of lightning electromagnetic field generated by oblique channel were derived with the dipole method, and the approximate relationships of lightning electromagnetic field and its derivative to channel-base current were obtained after approximate treatment of the exact analytical expressions for lightning electromagnetic field in far area. The waveforms of lightning electromagnetic field and its derivative from the exact and approximate results were compared based on the specified channel-base current described by a pulse function. The result shows that the difference between the exact and approximate waveforms decreases with the increase of the distance, and the applicable range for the approximate expressions of lightning electromagnetic field is stricter than that of the electromagnetic field derivative. The above result verifies the credibility of the approximate expressions, and provides the theoretical basis for the simulation of lightning electromagnetic fields in far zone.
2013,
25: 704-708.
doi: 10.3788/HPLPB20132503.0704
Abstract:
The quality of the ultra-wide spectrum short-pulse electric field generated by standard devices is extremely dependent on the field structure at the feed. We studied the discontinuity of coaxial feed. The results show that the length of cone pin is important for impedance matching and obtaining good performance of time domain response. Equivalent circuit analysis, simulation and experiment were performed to analyze the response of the feed. The voltage reflectivity of the feed could be reduced to 4% by adjusting the cone pin when incident pulse width ranges from 0.3 ns to 1.5 ns, while the pin length is held at 1.0 mm. Furthermore, a D-dot sensor was used to measure the field pulse and the result shows a high-fidelity replica of the incident pulse waveform in the time window of the cone.
The quality of the ultra-wide spectrum short-pulse electric field generated by standard devices is extremely dependent on the field structure at the feed. We studied the discontinuity of coaxial feed. The results show that the length of cone pin is important for impedance matching and obtaining good performance of time domain response. Equivalent circuit analysis, simulation and experiment were performed to analyze the response of the feed. The voltage reflectivity of the feed could be reduced to 4% by adjusting the cone pin when incident pulse width ranges from 0.3 ns to 1.5 ns, while the pin length is held at 1.0 mm. Furthermore, a D-dot sensor was used to measure the field pulse and the result shows a high-fidelity replica of the incident pulse waveform in the time window of the cone.
2013,
25: 709-714.
doi: 10.3788/HPLPB20132503.0709
Abstract:
In the case of the reverberation chamber (RC) with reflectors in it, the effect of the change of the reflectors on the characteristics of the field in RC is researched. On the basis of the microwave chaotic theory, the distribution of the field in unregular cavities is studied. Then the methods to judge the field uniformity and isotropy in the RC are established according to the statistical electromagnetics. The influences of the size, the number and the distribution of the reflectors in the RC on the field uniformity are studied. Simulation results show that the bigger the reflector is, the greater the influence on the field will be. The uniformity and isotropy are both improved as a result. Moreover, increasing the number and enlarging the range of the reflectors are helpful to decrease the standard deviation of the field, which means the improvement of the uniformity of the field and the harmonicity of the energy.
In the case of the reverberation chamber (RC) with reflectors in it, the effect of the change of the reflectors on the characteristics of the field in RC is researched. On the basis of the microwave chaotic theory, the distribution of the field in unregular cavities is studied. Then the methods to judge the field uniformity and isotropy in the RC are established according to the statistical electromagnetics. The influences of the size, the number and the distribution of the reflectors in the RC on the field uniformity are studied. Simulation results show that the bigger the reflector is, the greater the influence on the field will be. The uniformity and isotropy are both improved as a result. Moreover, increasing the number and enlarging the range of the reflectors are helpful to decrease the standard deviation of the field, which means the improvement of the uniformity of the field and the harmonicity of the energy.
2013,
25: 715-720.
doi: 10.3788/HPLPB20132503.0715
Abstract:
By making use of an electromagnetic simulation tool Ansoft HFSS, transmission characteristics of an example defected coaxial Bragg waveguide operating at Ka band (centred in 37.5 GHz) have been numerically studied. Results show that, if one conductor or both of outer and inner conductors are corrugated with periodical ripples, one or more passbands can be effectively generated by introducing a phase-shift defect to each corrugated surface; for the latter case, if the initial phase difference between the outer-wall ripple and inner-rod ripple is set to and the defects are both in the middle of the waveguide, a passband with extremely narrow bandwidth and wide stop-band rejection frequency range can be achieved; a slight misalignment of the defects between the outer-wall and inner-rod defects can be used to perform a fine adjustment on the character of the passband; however, if these defects are separated with a larger gap, several passbands can be generated, which makes the structure be acted as a multichannel filter. These numerical results are expected to provide engineering references for different applications of defected coaxial Bragg waveguides.
By making use of an electromagnetic simulation tool Ansoft HFSS, transmission characteristics of an example defected coaxial Bragg waveguide operating at Ka band (centred in 37.5 GHz) have been numerically studied. Results show that, if one conductor or both of outer and inner conductors are corrugated with periodical ripples, one or more passbands can be effectively generated by introducing a phase-shift defect to each corrugated surface; for the latter case, if the initial phase difference between the outer-wall ripple and inner-rod ripple is set to and the defects are both in the middle of the waveguide, a passband with extremely narrow bandwidth and wide stop-band rejection frequency range can be achieved; a slight misalignment of the defects between the outer-wall and inner-rod defects can be used to perform a fine adjustment on the character of the passband; however, if these defects are separated with a larger gap, several passbands can be generated, which makes the structure be acted as a multichannel filter. These numerical results are expected to provide engineering references for different applications of defected coaxial Bragg waveguides.
2013,
25: 721-726.
doi: 10.3788/HPLPB20132503.0721
Abstract:
In order to solve cooling problem of the gyrotron traveling-wave tube(TWT) collector and guarantee the gyrotron TWTs reliability and stability, the electron trajectories in the gyrotron TWT are simulated using CST electron simulation software. Thermal analysis of the collector with finite element software ANSYS is performed. The ways of applying boundary that affects the distribution of collector temperature are compared. The influence of the water temperature and flow rate on collector temperature distribution under actual heat fluxes (boundary condition) is researched. The size and number of collector fins are optimized, and a relatively perfect structure is obtained finally. The result estimated by simulation is consistent with the experiment and proves that the model and method employed in this work are suitable.
In order to solve cooling problem of the gyrotron traveling-wave tube(TWT) collector and guarantee the gyrotron TWTs reliability and stability, the electron trajectories in the gyrotron TWT are simulated using CST electron simulation software. Thermal analysis of the collector with finite element software ANSYS is performed. The ways of applying boundary that affects the distribution of collector temperature are compared. The influence of the water temperature and flow rate on collector temperature distribution under actual heat fluxes (boundary condition) is researched. The size and number of collector fins are optimized, and a relatively perfect structure is obtained finally. The result estimated by simulation is consistent with the experiment and proves that the model and method employed in this work are suitable.
2013,
25: 727-731.
doi: 10.3788/HPLPB20132503.0727
Abstract:
Using the geometrical optics theory, the working principle of high-order overmoded Denisov launcher is analyzed. Based on the mode coupling theory, the design method of the Denisov launcher is put forward. The TE6,2 Denisov launcher is studied using the concrete parameters of the 3 mm whispering-gallery mode gyrotron. By adopting developed mathematical programme, reliable optimal geometrical parameters of wall distortion are obtained. The 96.4 GHz TE6,2 Denisov launcher with compact structure is designed, and is verified by an electromagnetic field emulator. Results show that the launcher wall current is well focused,the current of cut edges decreases to 10% of the central focused current, and the power conversion efficiency of the output quasi-Gaussian bundle is 96.51%.
Using the geometrical optics theory, the working principle of high-order overmoded Denisov launcher is analyzed. Based on the mode coupling theory, the design method of the Denisov launcher is put forward. The TE6,2 Denisov launcher is studied using the concrete parameters of the 3 mm whispering-gallery mode gyrotron. By adopting developed mathematical programme, reliable optimal geometrical parameters of wall distortion are obtained. The 96.4 GHz TE6,2 Denisov launcher with compact structure is designed, and is verified by an electromagnetic field emulator. Results show that the launcher wall current is well focused,the current of cut edges decreases to 10% of the central focused current, and the power conversion efficiency of the output quasi-Gaussian bundle is 96.51%.
2013,
25: 732-736.
doi: 10.3788/HPLPB20132503.0732
Abstract:
In order to realize simulation of the large scale device, validate and solve the influence of load in the design and experiment of high power microwave system, the electromagnetic PIC parallel algorithm has been improved using MPI message passing function. This paper puts forward a whole modeling, section analytic parallel algorithm, and increase the calculation efficiency by reducing a synchronous. Finally the improved algorithm has been verified using a magnetic insulated line oscillator, and when calculating process number from 4 to 16, the computing speed can improve about 2.5 times.
In order to realize simulation of the large scale device, validate and solve the influence of load in the design and experiment of high power microwave system, the electromagnetic PIC parallel algorithm has been improved using MPI message passing function. This paper puts forward a whole modeling, section analytic parallel algorithm, and increase the calculation efficiency by reducing a synchronous. Finally the improved algorithm has been verified using a magnetic insulated line oscillator, and when calculating process number from 4 to 16, the computing speed can improve about 2.5 times.
2013,
25: 737-740.
doi: 10.3788/HPLPB20132503.0737
Abstract:
In order to construct compact free-electron laser facility, and to study on free-electron laser technology, it is necessary to develop a microwave RF gun as high-quality electron beam source. Therefore, an independently-tunable-cell(ITC) thermionic-cathode RF gun is manufactured in CAEP. Based on the preliminary power test results obtained before, more experiments are launched and experimental results are examined in depth by improving experimental environment and optimizing experimental parameters. The experimental results show that, a beam intensity over 500 mA can be measured at exit of the RF gun, and the back electron bombardment can be changed with adjustment of phase and amplitude of the electric field in the first cavity. At present, the thermionic-cathode RF gun has had applications in compact free-electron laser facility already.
In order to construct compact free-electron laser facility, and to study on free-electron laser technology, it is necessary to develop a microwave RF gun as high-quality electron beam source. Therefore, an independently-tunable-cell(ITC) thermionic-cathode RF gun is manufactured in CAEP. Based on the preliminary power test results obtained before, more experiments are launched and experimental results are examined in depth by improving experimental environment and optimizing experimental parameters. The experimental results show that, a beam intensity over 500 mA can be measured at exit of the RF gun, and the back electron bombardment can be changed with adjustment of phase and amplitude of the electric field in the first cavity. At present, the thermionic-cathode RF gun has had applications in compact free-electron laser facility already.
2013,
25: 741-745.
doi: 10.3788/HPLPB20132503.0741
Abstract:
In order to meet special requirements of the high intensity accelerator for collimators with good heat dissipation performance, radiation protection performance, ultra-high vacuum, high positioning accuracy, high stability, and online real-time adjustments, researches have been done for the primary collimator of rapid cycling synchrotron of China Spallation Neutron Source (CSNS/RCS). To ensure the reliability of design for the CSNS/RCS primary collimator, the technical difficulties of the key components, such as the scraper, vacuum chamber, driving system and remote clamp are also discussed. Thermal analysis of the scraper and structure analysis of support parts have been completed by using ANSYS code. Radiation shielding is simulated with FLUKA program.
In order to meet special requirements of the high intensity accelerator for collimators with good heat dissipation performance, radiation protection performance, ultra-high vacuum, high positioning accuracy, high stability, and online real-time adjustments, researches have been done for the primary collimator of rapid cycling synchrotron of China Spallation Neutron Source (CSNS/RCS). To ensure the reliability of design for the CSNS/RCS primary collimator, the technical difficulties of the key components, such as the scraper, vacuum chamber, driving system and remote clamp are also discussed. Thermal analysis of the scraper and structure analysis of support parts have been completed by using ANSYS code. Radiation shielding is simulated with FLUKA program.
2013,
25: 746-750.
doi: 10.3788/HPLPB20132503.0746
Abstract:
The rapid cycling synchrotron (RCS) of the China Spallation Neutron Source (CSNS) accumulates an 80 MeV injected beam, and accelerates 1.561013 particles to the design energy of 1.6 GeV, and extracts the high energy beam to the target. In order to keep low beam losses in the ring, it is necessary to control the chromaticity and to minimize the dependence of the optical properties on beam momentum spread. Three chromatic correction schemes are proposed and compared for CSNS/RCS in this paper. The combined effects of sextupole field and space charge are investigated by using the code SIMPSONS. The tune spread produced by chromaticity can be apparently reduced by using chromaticity sextupoles. However, the chromaticity sextupoles are non-linear elements, and can introduce an amplitude-dependent tuneshift. The combined effects of sextupole field and space charge can induce some beam loss.
The rapid cycling synchrotron (RCS) of the China Spallation Neutron Source (CSNS) accumulates an 80 MeV injected beam, and accelerates 1.561013 particles to the design energy of 1.6 GeV, and extracts the high energy beam to the target. In order to keep low beam losses in the ring, it is necessary to control the chromaticity and to minimize the dependence of the optical properties on beam momentum spread. Three chromatic correction schemes are proposed and compared for CSNS/RCS in this paper. The combined effects of sextupole field and space charge are investigated by using the code SIMPSONS. The tune spread produced by chromaticity can be apparently reduced by using chromaticity sextupoles. However, the chromaticity sextupoles are non-linear elements, and can introduce an amplitude-dependent tuneshift. The combined effects of sextupole field and space charge can induce some beam loss.
2013,
25: 751-754.
doi: 10.3788/HPLPB20132503.0751
Abstract:
To solve the problem of flash radiographic anti-scatter grid image inpainting, a radial basis function (RBF) neural network based image inpainting algorithm is proposed. First the anti-scatter grid image is divided into a series of blocked images. Then the weights of the RBF network are estimated and a continuous function is constructed in each blocked image, and with them the pixels of missing information can be filled in. The experimental results show that the new algorithm has better general performance in inpainting quality and boundary maintenance compared with the linear interpolation and spline interpolation method.
To solve the problem of flash radiographic anti-scatter grid image inpainting, a radial basis function (RBF) neural network based image inpainting algorithm is proposed. First the anti-scatter grid image is divided into a series of blocked images. Then the weights of the RBF network are estimated and a continuous function is constructed in each blocked image, and with them the pixels of missing information can be filled in. The experimental results show that the new algorithm has better general performance in inpainting quality and boundary maintenance compared with the linear interpolation and spline interpolation method.
2013,
25: 755-761.
doi: 10.3788/HPLPB20132503.0755
Abstract:
To eliminate the edge artifacts of industrial CT images, and improve the identification ability of the image and the precision of the dimension measurement, a coefficient adjusting method for reducing crosstalk noise is proposed. It is concluded from theoretical analysis that crosstalk generated from adjacent detectors by Compton scattering is the major reason for the edge artifacts. According to the mathematic model of the detector crosstalk, we design a special detector system configuration and stair-step phantom for estimating the quantity of crosstalk noise. The relationship between crosstalk ratio and intensity of the incident X-ray is acquired by regressing experimental data with least square method. The experimental result shows that the first-order crosstalk ratio between detectors is about 9.0%, and the second-order crosstalk ratio is about 1.2%. Thus the first-order crosstalk is the main factor causing edge artifacts. The proposed method can reduce the edge artifacts significantly, and meanwhile maintain the detail and edge of CT images.
To eliminate the edge artifacts of industrial CT images, and improve the identification ability of the image and the precision of the dimension measurement, a coefficient adjusting method for reducing crosstalk noise is proposed. It is concluded from theoretical analysis that crosstalk generated from adjacent detectors by Compton scattering is the major reason for the edge artifacts. According to the mathematic model of the detector crosstalk, we design a special detector system configuration and stair-step phantom for estimating the quantity of crosstalk noise. The relationship between crosstalk ratio and intensity of the incident X-ray is acquired by regressing experimental data with least square method. The experimental result shows that the first-order crosstalk ratio between detectors is about 9.0%, and the second-order crosstalk ratio is about 1.2%. Thus the first-order crosstalk is the main factor causing edge artifacts. The proposed method can reduce the edge artifacts significantly, and meanwhile maintain the detail and edge of CT images.
2013,
25: 762-766.
doi: 10.3788/HPLPB20132503.0762
Abstract:
The photoionization of krypton atomic clusters by 25 ns laser pulses with wavelengths of 266, 355, 532 and 1064 nm was studied using time-of-flight mass spectrometry at intensities of 1010-1011 Wcm-2. The distinguished highest charge of the multi-charged ions is +2,+4,+11 and +17, while the lasers with the above wavelengths illuminate the Kr atomic clusters in turn. It is indicated that the charge states of Kr ions significantly depend on laser wavelength, rather than depend on laser intensity. The results further support the three stage model of multiphoton ionization inverse bremsstrahlung heating and electron impact ionization.
The photoionization of krypton atomic clusters by 25 ns laser pulses with wavelengths of 266, 355, 532 and 1064 nm was studied using time-of-flight mass spectrometry at intensities of 1010-1011 Wcm-2. The distinguished highest charge of the multi-charged ions is +2,+4,+11 and +17, while the lasers with the above wavelengths illuminate the Kr atomic clusters in turn. It is indicated that the charge states of Kr ions significantly depend on laser wavelength, rather than depend on laser intensity. The results further support the three stage model of multiphoton ionization inverse bremsstrahlung heating and electron impact ionization.
2013,
25: 767-772.
doi: 10.3788/HPLPB20132503.0767
Abstract:
In order to study the effect of the cathode microprotrusion shape on its thermal instability, the evolution process of the thermal instability has been numerically investigated with the microprotrusion shape chosen as column, truncated cone and cone under different applied electric field. There is an obvious difference in temperature distribution in the microprotrusion with different shapes when the temperature at the microprotrusion peak reaches melting point of the cathode material. While the shape of microprotrusion is changed from column, truncated cone to cone, the region where temperature is close to the melting point reduces. At a given applied electric field, the smaller the ratio of top radius to bottom radius of the microprotrusion is, the larger the time delay of explosive electron emission is; the time delay of explosive electron emission increases exponentially with the decrease of the ratio of top radius to bottom radius of the microprotrusion or strength of the cathode surface electric field when the cathode surface electric field is larger than 11 GV/m.
In order to study the effect of the cathode microprotrusion shape on its thermal instability, the evolution process of the thermal instability has been numerically investigated with the microprotrusion shape chosen as column, truncated cone and cone under different applied electric field. There is an obvious difference in temperature distribution in the microprotrusion with different shapes when the temperature at the microprotrusion peak reaches melting point of the cathode material. While the shape of microprotrusion is changed from column, truncated cone to cone, the region where temperature is close to the melting point reduces. At a given applied electric field, the smaller the ratio of top radius to bottom radius of the microprotrusion is, the larger the time delay of explosive electron emission is; the time delay of explosive electron emission increases exponentially with the decrease of the ratio of top radius to bottom radius of the microprotrusion or strength of the cathode surface electric field when the cathode surface electric field is larger than 11 GV/m.
2013,
25: 773-777.
doi: 10.3788/HPLPB20132503.0773
Abstract:
It has been observed that pixellated CdZnTe detectors fabricated from crystals experience a further polarization effect. In this case, in some areas of the CdZnTe detector, polarization occurs without high incident flux. Results from these studies reveal that at very high photon flux rates, a space charge region with high density develops and extends, consistent with the accumulation of positive space charge due to the trapping of free-carrier holes created by the X-ray irradiation. The further polarization is caused by the extending of space charge region. The extending of space charge region observed without high X-ray has a direct influence on the irradiation hardness and charge collection efficiency of the devices.
It has been observed that pixellated CdZnTe detectors fabricated from crystals experience a further polarization effect. In this case, in some areas of the CdZnTe detector, polarization occurs without high incident flux. Results from these studies reveal that at very high photon flux rates, a space charge region with high density develops and extends, consistent with the accumulation of positive space charge due to the trapping of free-carrier holes created by the X-ray irradiation. The further polarization is caused by the extending of space charge region. The extending of space charge region observed without high X-ray has a direct influence on the irradiation hardness and charge collection efficiency of the devices.
2013,
25: 778-782.
doi: 10.3788/HPLPB20132503.0778
Abstract:
The process of emittance increase in proton radiography is analyzed. A radiographic system is designed, and based on it the factors affecting emittance increase are studied with theoretical analysis and Monte Carlo simulation. The results show that the extracted emittance from the accelerator is neglectable for high energy proton radiography, and cannot be neglected for medium energy proton radiography. The extracted emittance could be reduced through decreasing the thickness of the diffuser and enhancing the focusing ability of the accelerators extraction system.
The process of emittance increase in proton radiography is analyzed. A radiographic system is designed, and based on it the factors affecting emittance increase are studied with theoretical analysis and Monte Carlo simulation. The results show that the extracted emittance from the accelerator is neglectable for high energy proton radiography, and cannot be neglected for medium energy proton radiography. The extracted emittance could be reduced through decreasing the thickness of the diffuser and enhancing the focusing ability of the accelerators extraction system.
2013,
25: 783-786.
doi: 10.3788/HPLPB20132503.0783
Abstract:
A transducer with a fixed aperture is proposed as a virtual array detector to collect data. The transducer only needs to rotationally scan the sample 180 degrees. In the experiment, a Q-switched Nd: YAG laser operating at 532 nm was used as the light source, with a pulse width of 7 ns and a repetition frequency of 10 Hz. An unfocused transducer with a central frequency of 2.25 MHz and a diameter of 6 mm was applied to detecting photoacoustic signals. And the image was reconstructed by processing the received photoacoustic signals with the inverse Radon transform. An in vitro claw of a rat was used as the imaging object in the experiment and the data were obtained by rotating the transducer 180 degrees with a step of 2 degrees. The reconstructed image from the inverse Radon transform clearly displays the bones within the claw. The results indicate that the inverse Radon transform can be used for photoacoustic reconstruction and limited-view photoacoustic imaging is achieved with this method.
A transducer with a fixed aperture is proposed as a virtual array detector to collect data. The transducer only needs to rotationally scan the sample 180 degrees. In the experiment, a Q-switched Nd: YAG laser operating at 532 nm was used as the light source, with a pulse width of 7 ns and a repetition frequency of 10 Hz. An unfocused transducer with a central frequency of 2.25 MHz and a diameter of 6 mm was applied to detecting photoacoustic signals. And the image was reconstructed by processing the received photoacoustic signals with the inverse Radon transform. An in vitro claw of a rat was used as the imaging object in the experiment and the data were obtained by rotating the transducer 180 degrees with a step of 2 degrees. The reconstructed image from the inverse Radon transform clearly displays the bones within the claw. The results indicate that the inverse Radon transform can be used for photoacoustic reconstruction and limited-view photoacoustic imaging is achieved with this method.
2013,
25: 787-790.
doi: 10.3788/HPLPB20132503.0787
Abstract:
An organic phototransistor was fabricated by an innovative spray technology suited to large-area, low-cost and room temperature processing for organic electronic and thin-film devices. Top contact organic thin film transistors (OTFTs) were fabricated with poly(3-hexylthiophene) (P3HT) active layer and low cost polymerthylmethacrylate (PMMA) insulator，which exhibit a good output and the transfer characteristics of P3HT transistors. The OTFT device based on P3HT was measured under a standard solar simulator circumstance of 100 mW/cm2. The responses of the OTFT are remarkable in the absence or the presence of light, with good reversibility and repeatability. It was found that the threshold voltage shift was obvious, after comparing the characteristic curves of the OTFT exposed to darkness and light for 1 min, 2 min, and 4 min.
An organic phototransistor was fabricated by an innovative spray technology suited to large-area, low-cost and room temperature processing for organic electronic and thin-film devices. Top contact organic thin film transistors (OTFTs) were fabricated with poly(3-hexylthiophene) (P3HT) active layer and low cost polymerthylmethacrylate (PMMA) insulator，which exhibit a good output and the transfer characteristics of P3HT transistors. The OTFT device based on P3HT was measured under a standard solar simulator circumstance of 100 mW/cm2. The responses of the OTFT are remarkable in the absence or the presence of light, with good reversibility and repeatability. It was found that the threshold voltage shift was obvious, after comparing the characteristic curves of the OTFT exposed to darkness and light for 1 min, 2 min, and 4 min.
2013,
25: 791-793.
doi: 10.3788/HPLPB20132503.0791
Abstract:
Insulator surface flashover is one of the bottlenecks restricting the development of pulsed power technology. To obtain a deeper understanding of flashover mechanism, UV illumination was introduced into surface flashover process. The flashover performance of different illumination positions on the insulator sample was studied. The experimental results show that the effect is larger when illumination position is close to the cathode triple junction where the flashover initiates, reflecting that different flashover stages play different roles in the flashover process.
Insulator surface flashover is one of the bottlenecks restricting the development of pulsed power technology. To obtain a deeper understanding of flashover mechanism, UV illumination was introduced into surface flashover process. The flashover performance of different illumination positions on the insulator sample was studied. The experimental results show that the effect is larger when illumination position is close to the cathode triple junction where the flashover initiates, reflecting that different flashover stages play different roles in the flashover process.
2013,
25: 794-798.
doi: 10.3788/HPLPB20132503.0794
Abstract:
A Blumlein-PFN (BPFN) was fabricated and the factors that affect energy transfer efficiency were investigated. The BPFN is composed of ceramic capacitor, aluminum strip and GaAs-PCSS. Air-gap was selected as the BPFN pulse forming switch first, then expected pulse width and BPFNs impedance were obtained, which confirms the effectiveness of our design. Experiments were conducted with GaAs-PCSS under different electric field and trigger laser energy. Results show that the loss induced by maintaining electric field is negligible when electric field is greater than 25 kV/cm and trigger laser energy of PCSS is between 3.12 kV/cm and 4.94 kV/cm. When trigger laser energy equals to 30.4mJ and 3.5 mJ, the energy efficiency of matched load is 62.9% and 55%, and the equivalent connective resistance is 2.3 and 3.13 respectively. The connective resistance has been proved to be the most important factor which limits energy efficiency.
A Blumlein-PFN (BPFN) was fabricated and the factors that affect energy transfer efficiency were investigated. The BPFN is composed of ceramic capacitor, aluminum strip and GaAs-PCSS. Air-gap was selected as the BPFN pulse forming switch first, then expected pulse width and BPFNs impedance were obtained, which confirms the effectiveness of our design. Experiments were conducted with GaAs-PCSS under different electric field and trigger laser energy. Results show that the loss induced by maintaining electric field is negligible when electric field is greater than 25 kV/cm and trigger laser energy of PCSS is between 3.12 kV/cm and 4.94 kV/cm. When trigger laser energy equals to 30.4mJ and 3.5 mJ, the energy efficiency of matched load is 62.9% and 55%, and the equivalent connective resistance is 2.3 and 3.13 respectively. The connective resistance has been proved to be the most important factor which limits energy efficiency.
2013,
25: 799-804.
doi: 10.3788/HPLPB20132503.0799
Abstract:
In order to build the electromagnetic pulse response models for surge protectors, a time-domain model based on the NARX neural network is given. The theoretical basis and design steps of modeling with the NARX neural network are presented. The transmission-line pulse system and electrostatic discharge system are built for proving the method. The measurement results obtained in transient voltage suppressor experiment is taken to set up the NARX neural network model. When the input signal is given as a square pulse, machine-model or body-metal-model electrostatic discharge electromagnetic pulse, the established model can forecast performance parameters such as response time, pulse peak and clamping voltage, which verifies the validity of the model.
In order to build the electromagnetic pulse response models for surge protectors, a time-domain model based on the NARX neural network is given. The theoretical basis and design steps of modeling with the NARX neural network are presented. The transmission-line pulse system and electrostatic discharge system are built for proving the method. The measurement results obtained in transient voltage suppressor experiment is taken to set up the NARX neural network model. When the input signal is given as a square pulse, machine-model or body-metal-model electrostatic discharge electromagnetic pulse, the established model can forecast performance parameters such as response time, pulse peak and clamping voltage, which verifies the validity of the model.
