A compact C-band high brightness photoinjector for ultra-fast electron diffraction(UED)is under development at Tsinghua University.The C-band photoinjector,operating at 5.172 GHz,uses a 1.45-cell C-band RF gun as the ...A compact C-band high brightness photoinjector for ultra-fast electron diffraction(UED)is under development at Tsinghua University.The C-band photoinjector,operating at 5.172 GHz,uses a 1.45-cell C-band RF gun as the electron source.The design of the RF gun has addressed the physics,beam dynamics and mechanical aspects,and based on these design studies,a prototype has been fabricated.In order to obtain optimized performance for the UED,beam dynamics simulation has been carried out using the particle tracking code Astra for the photoinjector consisting of an alternative multi-cell RF gun design and a C-band compressor downstream.The beam dynamics parameters that are optimized include the bunch length,peak current,horizontal emittance,and position of the minimum bunch length.展开更多
The geometric characteristics of fractures within a rock mass can be inferred by the data sampling from boreholes or exposed surfaces.Recently,the universal elliptical disc(UED)model was developed to represent natural...The geometric characteristics of fractures within a rock mass can be inferred by the data sampling from boreholes or exposed surfaces.Recently,the universal elliptical disc(UED)model was developed to represent natural fractures,where the fracture is assumed to be an elliptical disc and the fracture orientation,rotation angle,length of the long axis and ratio of short-long axis lengths are considered as variables.This paper aims to estimate the fracture size-and azimuth-related parameters in the UED model based on the trace information from sampling windows.The stereological relationship between the trace length,size-and azimuth-related parameters of the UED model was established,and the formulae of the mean value and standard deviation of trace length were proposed.The proposed formulae were validated via the Monte Carlo simulations with less than 5%of error rate between the calculated and true values.With respect to the estimation of the size-and azimuth-related parameters using the trace length,an optimization method was developed based on the pre-assumed size and azimuth distribution forms.A hypothetical case study was designed to illustrate and verify the parameter estimation method,where three combinations of the sampling windows were used to estimate the parameters,and the results showed that the estimated values could agree well with the true values.Furthermore,a hypothetical three-dimensional(3D)elliptical fracture network was constructed,and the circular disc,non-UED and UED models were used to represent it.The simulated trace information from different models was compared,and the results clearly illustrated the superiority of the proposed UED model over the existing circular disc and non-UED models。展开更多
In order to improve the video transmission performance in Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing （MIMO-OFDM） system, a new scheme, which integrates Multiple Description Coding （MD...In order to improve the video transmission performance in Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing （MIMO-OFDM） system, a new scheme, which integrates Multiple Description Coding （MDC）, Low Density Parity Check （LDPC） coding and hybrid space time coding, is proposed in this letter. In particular, a hybrid space time coding algorithm is combined with LDPC codes to perform Unequal Error Protection （UEP） of MDC encoded video streams. Comparing with the UEP transmission with only LDPC codes, the proposed scheme achieves more than ldB gain in terms of Signal to Noise Ratio （SNR） when the Peak Signal to Noise Ratio （PSNR） of reconstructed video is above 30dB.展开更多
文摘A compact C-band high brightness photoinjector for ultra-fast electron diffraction(UED)is under development at Tsinghua University.The C-band photoinjector,operating at 5.172 GHz,uses a 1.45-cell C-band RF gun as the electron source.The design of the RF gun has addressed the physics,beam dynamics and mechanical aspects,and based on these design studies,a prototype has been fabricated.In order to obtain optimized performance for the UED,beam dynamics simulation has been carried out using the particle tracking code Astra for the photoinjector consisting of an alternative multi-cell RF gun design and a C-band compressor downstream.The beam dynamics parameters that are optimized include the bunch length,peak current,horizontal emittance,and position of the minimum bunch length.
基金funded by National Natural Science Foundation of China(Grant No.41972264)Zhejiang Provincial Natural Science Foundation of China(Grant No.LR22E080002)the Observation and Research Station of Geohazards in Zhejiang,Ministry of Natural Resources,China(Grant No.ZJDZGCZ-2021).
文摘The geometric characteristics of fractures within a rock mass can be inferred by the data sampling from boreholes or exposed surfaces.Recently,the universal elliptical disc(UED)model was developed to represent natural fractures,where the fracture is assumed to be an elliptical disc and the fracture orientation,rotation angle,length of the long axis and ratio of short-long axis lengths are considered as variables.This paper aims to estimate the fracture size-and azimuth-related parameters in the UED model based on the trace information from sampling windows.The stereological relationship between the trace length,size-and azimuth-related parameters of the UED model was established,and the formulae of the mean value and standard deviation of trace length were proposed.The proposed formulae were validated via the Monte Carlo simulations with less than 5%of error rate between the calculated and true values.With respect to the estimation of the size-and azimuth-related parameters using the trace length,an optimization method was developed based on the pre-assumed size and azimuth distribution forms.A hypothetical case study was designed to illustrate and verify the parameter estimation method,where three combinations of the sampling windows were used to estimate the parameters,and the results showed that the estimated values could agree well with the true values.Furthermore,a hypothetical three-dimensional(3D)elliptical fracture network was constructed,and the circular disc,non-UED and UED models were used to represent it.The simulated trace information from different models was compared,and the results clearly illustrated the superiority of the proposed UED model over the existing circular disc and non-UED models。
基金the National Natural Science Foundation of China (No. 60328103).
文摘In order to improve the video transmission performance in Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing （MIMO-OFDM） system, a new scheme, which integrates Multiple Description Coding （MDC）, Low Density Parity Check （LDPC） coding and hybrid space time coding, is proposed in this letter. In particular, a hybrid space time coding algorithm is combined with LDPC codes to perform Unequal Error Protection （UEP） of MDC encoded video streams. Comparing with the UEP transmission with only LDPC codes, the proposed scheme achieves more than ldB gain in terms of Signal to Noise Ratio （SNR） when the Peak Signal to Noise Ratio （PSNR） of reconstructed video is above 30dB.
