The effects of initial spin orientation on the final electron beam polarization in laser wakefield acceleration in a pre-polarized plasma are investigated theoretically and numerically.From the results of variation of...The effects of initial spin orientation on the final electron beam polarization in laser wakefield acceleration in a pre-polarized plasma are investigated theoretically and numerically.From the results of variation of the initial spin direction,the spin dynamics of the electron beam are found to depend on the self-injection mechanism.The effects of wakefields and laser fields are studied using test particle dynamics and particle-in-cell simulations based on the Thomas-Bargmann-Michel-Telegdi equation.Compared with transverse injection,longitudinal injection is found to be preferable for obtaining a highly polarized electron beam.展开更多
Laser-accelerated ion beams(LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here w...Laser-accelerated ion beams(LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here we explore an application of using laser-accelerated ion beams to prepare graphene. The pulsed LIBs produced a great instantaneous beam current and thermal effect on the SiC samples with a shooting frequency of 1 Hz. In the experiment, we controlled the deposition dose by adjusting the number of shootings and the irradiating current by adjusting the distance between the sample and the ion source. During annealing at 1100℃, we found that the 190 shots ion beams allowed more carbon atoms to self-assemble into graphene than the 10 shots case. By comparing with the controlled experiment based on ion beams from a traditional ion accelerator, we found that the laser-accelerated ion beams could cause greater damage in a very short time. Significant thermal effect was induced when the irradiation distance was reduced to less than 1 cm, which could make partial SiC self-annealing to prepare graphene dots directly. The special effects of LIBs indicate their vital role to change the structure of the irradiation sample.展开更多
Betatron radiation from laser wakefield accelerated electrons and X-rays scattered off a counter-propagating relativistic electron bunch arecollimated and hold the potential to extend the energy range to hard X-ray or...Betatron radiation from laser wakefield accelerated electrons and X-rays scattered off a counter-propagating relativistic electron bunch arecollimated and hold the potential to extend the energy range to hard X-ray or gamma ray band. The peak brightness of these incoherent radiations could reach the level of the brightest synchrotron light sources in the world due to their femtosecond pulse duration and source sizedown to a few micrometers. In this article, the principle and properties of these radiation sources are briefly reviewed and compared. Then wepresent our recent progress in betatron radiation enhancement in the perspective of both photon energy and photon number. The enhancement istriggered by using a clustering gas target, arousing a second injection of a fiercely oscillating electron bunch with large charge or stimulating aresonantly enhanced oscillation of the ionization injected electrons. By adopting these methods, bright photon source with energy over 100 keVis generated which would greatly impact applications such as nuclear physics, diagnostic radiology, laboratory astrophysics and high-energydensity science.展开更多
In laser wakefield acceleration,injecting an external electron beam at a certain energy is a promising approach for achieving a high-quality electron beam with low energy spread and low emittance.In this paper,the pro...In laser wakefield acceleration,injecting an external electron beam at a certain energy is a promising approach for achieving a high-quality electron beam with low energy spread and low emittance.In this paper,the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations.A Bayesian optimization method is used to optimize the key laser and plasma parameters so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth.The effect of the rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth.Finally,a high-quality electron beam with an energy of 1.5 GeV,a normalized transverse emittance of 0.5 mm·mrad and a relative energy spread of 0.5%at 10 pC is obtained.展开更多
AIM:To assess the visual outcomes and corneal biomechanical properties of myopia patients between laser in situ keratomileusis(LASIK)and LASIK combined with accelerated corneal crosslinking(LASIK Xtra).METHODS:This pr...AIM:To assess the visual outcomes and corneal biomechanical properties of myopia patients between laser in situ keratomileusis(LASIK)and LASIK combined with accelerated corneal crosslinking(LASIK Xtra).METHODS:This prospective study analyzed 52 consecutive myopia patients treated with LASIK Xtra and 45 consecutive myopia patients treated with LASIK.Only the right eyes in the two groups were analyzed.The uncorrected distance visual acuity(UDVA),keratometry values,postoperative central corneal thickness(CCT),corneal demarcation line depth,the corneal compensated intraocular pressure(IOPcc),Goldmann-correlated IOP(IOPg),corneal resistance factor(CRF)and corneal hysteresis(CH)from Ocular Response Analyzer(ORA)were analyzed.Further,the correlation between the demarcation line depth and ORA-related biomechanical parameters were analyzed.RESULTS:No significant differences in UDVA,postoperative CCT,or mean K values were found between the 2 groups at 1 to 12mo postoperative follow-up(all P>0.05).The changes of CRF was significantly lower in the LASIK Xtra group compared to the LASIK group(all P<0.05)at all the postoperative visits.The changes of CH were significantly higher in the LASIK Xtra group(all P<0.05).No significant differences were discovered regarding the changes of IOPcc and IOPg posperatively(all P>0.05).Out of 52 cases in the LASIK Xtra group,the demarcation line was present in 40 eyes(77%).The average depth of the demarcation was 220.73±42.70μm(136 to 288μm).No significant correlation was observed between the depth of the demarcation line and any of the ORA-related biomechanical parameters such as IOPcc,IOPg,CRF and CH at 12mo(all P>0.05).CONCLUSION:Both procedures demonstrate comparable outcomes in terms of visual acuity,refraction and ablation predictability.This study confirms that corneal biomechanical properties of the included patients weakened after both procedures,but the cornea after LASIK Xtra are stiffer than conventional LASIK.展开更多
Laser wakefield accelerators(LWFAs)offer acceleration gradients up to 1000 times higher than those of conventional radio-frequency accelerators,offering a pathway to significantly more compact and cost-effective accel...Laser wakefield accelerators(LWFAs)offer acceleration gradients up to 1000 times higher than those of conventional radio-frequency accelerators,offering a pathway to significantly more compact and cost-effective accelerator systems.This breakthrough opens up new possibilities for laboratory-scale light sources.All-optical inverse Compton scattering(AOCS)sources driven by LWFAs produce high-brightness,quasimonochromatic X rays with micrometer-scale source sizes,delivering the spatial coherence and resolution required for X-ray phase-contrast imaging(XPCI).These features position AOCS X-ray sources as promising tools for applications in biology,medicine,physics,and materials science.However,previous AOCS-based imaging studies have primarily focused on X-ray absorption imaging.In this work,we report successful experimental demonstrations of edge-enhanced in-line XPCI using energy-tunable,quasi-monochromatic AOCS X rays.With a spatial resolution of~20μm,our results clearly show the potential of high-resolution,AOCS-based XPCI applications.展开更多
The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and...The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and lower than critical densities with plasmas extending over few micrometers,i.e.multiple wavelengths.The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam.Experiments at the Glass Hybrid OPCPA Scaled Test-bed(GHOST)laser system at University of Texas,Austin using such targets measured non-Maxwellian,peaked electron distribution with large bunch charge and high electron density in the laser propagation direction.These results are reproduced in 2D PIC simulations using the EPOCH code,identifying direct laser acceleration(DLA)[1]as the responsible mechanism.This is the first time that DLA has been observed to produce peaked spectra as opposed to broad,Maxwellian spectra observed in earlier experiments[2].This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.展开更多
In this study,we theoretically investigate the feasibility of using laser-wakefield accelerated(LWFA)electrons for the photonuclear measurement of nuclear isomers according to the characteristics of the electrons obta...In this study,we theoretically investigate the feasibility of using laser-wakefield accelerated(LWFA)electrons for the photonuclear measurement of nuclear isomers according to the characteristics of the electrons obtained from LWFA experiments conducted at the Compact Laser-Plasma Accelerator(CLAPA)laboratory.The experiments at the CLAPA show that a stable electron beam with an energy of 78-135 MeV and a charge of 300-600 pC can be obtained.The bremsstrahlung spectra were simulated using Geant4,which suggests that a bremsstrahlung source with a peak intensity of 10^(19) photons/s can be generated.Theoretical calculations of isomer production cross sections from the photonuclear reactions on six target nuclei,^(197)Au,^(180)Hf,^(159)Tb,^(115)In,^(103)Rh,and ^(90)Zr,were performed and compared with the available experimental data in EXFOR,which suggest that further experiments are required for a series of photonuclear reaction channels.Flux-averaged cross sections and isomer ratios(IR)resulting from such bremsstrahlung sources are theoretically deduced.The results suggest that IR measurements can be used to constrain nuclear components,such asγstrength function and optical model potential.In addition,the detection of the decay characteristics was evaluated with Geant4 simulations.The use of the LWFA electron beam and its bremsstrahlung for photonuclear studies involving nuclear isomers is anticipated.展开更多
We study a laser wakefield acceleration driven by mid-infrared （mid-IR） laser pulses through two-dimensional particle-in-cell simulations. Since a mid-IR laser pulse can deliver a larger ponderomotive force as compa...We study a laser wakefield acceleration driven by mid-infrared （mid-IR） laser pulses through two-dimensional particle-in-cell simulations. Since a mid-IR laser pulse can deliver a larger ponderomotive force as compared with the usual 0.8 μm wavelength laser pulse, it is found that electron self-injection into the wake wave occurs at an earlier time, the plasma density threshold for injection becomes lower, and the electron beam charge is substantially enhanced. Meanwhile, our study also shows that quasimonoenergetic electron beams with a narrow energy-spread can be generated by using mid-IR laser pulses. Such a mid-IR laser pulse can provide a feasible method for obtaining a high quality and high charge electron beam. Therefore, the current efforts on constructing mid-IR terawatt laser systems can greatly benefit the laser wakefield acceleration research.展开更多
In this paper,we propose a novel stacked laser dielectric acceleration structure.This structure is based on the inverse Cherenkov effect and represented by a parametric design formulation.Compared to existing dielectr...In this paper,we propose a novel stacked laser dielectric acceleration structure.This structure is based on the inverse Cherenkov effect and represented by a parametric design formulation.Compared to existing dielectric laser accelerators relying on the inverse Smith–Purcell effect,the proposed structure provides an extended-duration synchronous acceleration field without requiring the pulse front tilting technique.This advantage significantly reduces the required pulse duration.In addition,the easy to integrate layered structure facilitates cascade acceleration,and simulations have shown that low-energy electron beams can be cascaded through high gradients over extended distances.These practical advantages demonstrate the potential of this new structure for future chip accelerators.展开更多
Forward fast protons are generated by the moderate-intensity laser-foil interaction. Protons with maximum energy 190 keV are measured by using magnetic spectrometer and CR-39 solid state track detectors along the dire...Forward fast protons are generated by the moderate-intensity laser-foil interaction. Protons with maximum energy 190 keV are measured by using magnetic spectrometer and CR-39 solid state track detectors along the direction normal to the rear surface. The experimental results are also modeled by the paxticle-in-cell method, investigating the timevarying electron temperature and the rear sheath field. The temporal and spatial structure of the sheath electrical field, revealed in the simulation, suggests that these protons are accelerated by target normal sheath acceleration （TNSA） mechanism.展开更多
Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. T...Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.展开更多
Laser wakefield accelerators（LWFAs） are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators. It is attributed to th...Laser wakefield accelerators（LWFAs） are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators. It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones. The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators. To achieve the LWFAs suitable for applications,more and more attention has been paid to optimize the LWFAs for high-quality electron beams. A single-staged LWFA does not favor generating controllable electron beams beyond 1 Ge V since electron injection and acceleration are coupled and cannot be independently controlled. Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved.This paper provides an overview of the physical conceptions of the LWFA, as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.展开更多
A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical...A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44MeV, energy spread within ±5% and 10^6-8 protons per pulse. It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 10^9 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0-±5%0.展开更多
We present a first on-chip positron accelerator based on dielectric laser acceleration.This innovative approach significantly reduces the physical dimensions of the positron acceleration apparatus,enhancing its feasib...We present a first on-chip positron accelerator based on dielectric laser acceleration.This innovative approach significantly reduces the physical dimensions of the positron acceleration apparatus,enhancing its feasibility for diverse applications.By utilizing a stacked acceleration structure and far-infrared laser technology,we are able to achieve a seven-stage acceleration structure that surpasses the distance and energy gain of using the previous dielectric laser acceleration methods.Additionally,we are able to compress the positron beam to an ultrafast sub-femtosecond scale during the acceleration process,compared with the traditional methods,the positron beam is compressed to a greater extent.We also demonstrate the robustness of the stacked acceleration structure through the successful acceleration of the positron beam.展开更多
Dielectric laser accelerators(DLAs)are considered promising candidates for on-chip particle accelerators that can achieve high acceleration gradients.This study explores various combinations of dielectric materials an...Dielectric laser accelerators(DLAs)are considered promising candidates for on-chip particle accelerators that can achieve high acceleration gradients.This study explores various combinations of dielectric materials and accelerated structures based on the inverse Cherenkov effect.The designs utilize conventional processing methods and laser parameters currently in use.We optimize the structural model to enhance the gradient of acceleration and the electron energy gain.To achieve higher acceleration gradients and energy gains,the selection of materials and structures should be based on the initial electron energy.Furthermore,we observed that the variation of the acceleration gradient of the material is different at different initial electron energies.These findings suggest that on-chip accelerators are feasible with the help of these structures and materials.展开更多
The dielectric laser accelerator(DLA) is a promising technology for achieving high-gradient acceleration in a compact design. Its advantages include ease of cascading and an energy gain per unit distance which can exc...The dielectric laser accelerator(DLA) is a promising technology for achieving high-gradient acceleration in a compact design. Its advantages include ease of cascading and an energy gain per unit distance which can exceed that of conventional accelerators by two orders of magnitude. This paper establishes rules for efficient particle acceleration using dielectric structures based on basic equations, proposes a design principle for DLA structures with clear physical images and verifies the accuracy of the corresponding formula for energy gain. DLA structures with different specifications, materials and geometric shapes are constructed, and the achievable acceleration gradient is calculated. Our results demonstrate that effective acceleration can be achieved when the electric field sensed by particles in the acceleration cavity has zero frequency,which provides a powerful method for designing such devices. Furthermore, we demonstrate that the simplified formula for calculating energy gain presented in this paper can accurately determine the energy gain of particles during the design of acceleration structures using a dielectric accelerator.展开更多
The energy and trajectory of the electron, which is irradiated by a high-power laser pulse in a cylindrical plasma channel with a uniform positive charge and a uniform negative current, have been analyzed in terms of ...The energy and trajectory of the electron, which is irradiated by a high-power laser pulse in a cylindrical plasma channel with a uniform positive charge and a uniform negative current, have been analyzed in terms of a single-electron model of direct laser acceleration. We find that the energy and trajectory of the electron strongly depend on the positive charge density, the negative current density, and the intensity of the laser pulse. The electron can be accelerated significantly only when the positive charge density, the negative current density, and the intensity of the laser pulse are in suitable ranges due to the dephasing rate between the wave and electron motion. Particularly, when their values satisfy a critical condition. the electron can stay in phase with the laser and gain the largest energy from the laser. With the enhancement of the electron energy, strong modulations of the relativistic factor cause a considerable enhancement of the electron transverse oscillations across the channel, which makes the electron trajectory become essentially three-dimensional, even if it is flat at the early stage of the acceleration.展开更多
The supersonic gas-jet target is an important experimental target for laser wakefield acceleration(LWFA),which has great potential for driving novel radiation sources such as betatron radiation and Compton scattering ...The supersonic gas-jet target is an important experimental target for laser wakefield acceleration(LWFA),which has great potential for driving novel radiation sources such as betatron radiation and Compton scattering gamma rays.According to different electron acceleration requirements,it is necessary to provide specific supersonic gas jets with different density profiles to generate electron beams with high quality and high repetition rates.In this study,the interference images and density profiles of different gas-jet targets were obtained through a modified Nomarski interference diagnosis system.The relationships between the gas density and back pressure,nozzle structure,and other key parameters were studied.Targets with different characteristics are conducive to meeting the various requirements of LWFA.展开更多
We suggest a scheme of electron acceleration by use of two tightly focused ultra-short intense laser pulses at a IOOTW level. Electroas obtain a preliminary acceleration with a small angular spread by the longitudinal...We suggest a scheme of electron acceleration by use of two tightly focused ultra-short intense laser pulses at a IOOTW level. Electroas obtain a preliminary acceleration with a small angular spread by the longitudinal ponderomotive force of the first pulse. They are then injected and further accelerated to hundreds of MeV by the second laser pulse.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.11804348,11775056,11975154,12225505,and 12405281)the Science Challenge(Project No.TZ2018005)+2 种基金supported by the Shanghai Pujiang Program(Grant No.23PJ1414600)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB0890203)supported by the Accelerator Technology Helmholtz Infrastructure consortium ATHENA.
文摘The effects of initial spin orientation on the final electron beam polarization in laser wakefield acceleration in a pre-polarized plasma are investigated theoretically and numerically.From the results of variation of the initial spin direction,the spin dynamics of the electron beam are found to depend on the self-injection mechanism.The effects of wakefields and laser fields are studied using test particle dynamics and particle-in-cell simulations based on the Thomas-Bargmann-Michel-Telegdi equation.Compared with transverse injection,longitudinal injection is found to be preferable for obtaining a highly polarized electron beam.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11875077,11975037,and 11921006)the National Grand Instrument Project of China(Grant Nos.2019YFF01014400 and 2019YFF01014404).
文摘Laser-accelerated ion beams(LIBs) have been increasingly applied in the field of material irradiation in recent years due to the unique properties of ultra-short beam duration, extremely high beam current, etc. Here we explore an application of using laser-accelerated ion beams to prepare graphene. The pulsed LIBs produced a great instantaneous beam current and thermal effect on the SiC samples with a shooting frequency of 1 Hz. In the experiment, we controlled the deposition dose by adjusting the number of shootings and the irradiating current by adjusting the distance between the sample and the ion source. During annealing at 1100℃, we found that the 190 shots ion beams allowed more carbon atoms to self-assemble into graphene than the 10 shots case. By comparing with the controlled experiment based on ion beams from a traditional ion accelerator, we found that the laser-accelerated ion beams could cause greater damage in a very short time. Significant thermal effect was induced when the irradiation distance was reduced to less than 1 cm, which could make partial SiC self-annealing to prepare graphene dots directly. The special effects of LIBs indicate their vital role to change the structure of the irradiation sample.
基金This work was supported by the National Basic Research Program of China(2013CBA01500)the National Key Scientific Instrument and Equipment Development Project(2012YQ120047)+1 种基金the National Natural Science Foundation of China(11334013,11421064,11374210,and 11305185)the CAS Key Program(KGZD-EW-T05).
文摘Betatron radiation from laser wakefield accelerated electrons and X-rays scattered off a counter-propagating relativistic electron bunch arecollimated and hold the potential to extend the energy range to hard X-ray or gamma ray band. The peak brightness of these incoherent radiations could reach the level of the brightest synchrotron light sources in the world due to their femtosecond pulse duration and source sizedown to a few micrometers. In this article, the principle and properties of these radiation sources are briefly reviewed and compared. Then wepresent our recent progress in betatron radiation enhancement in the perspective of both photon energy and photon number. The enhancement istriggered by using a clustering gas target, arousing a second injection of a fiercely oscillating electron bunch with large charge or stimulating aresonantly enhanced oscillation of the ionization injected electrons. By adopting these methods, bright photon source with energy over 100 keVis generated which would greatly impact applications such as nuclear physics, diagnostic radiology, laboratory astrophysics and high-energydensity science.
基金supported by Science and Technology Major Project of Hubei Province in China(No.2021AFB001)。
文摘In laser wakefield acceleration,injecting an external electron beam at a certain energy is a promising approach for achieving a high-quality electron beam with low energy spread and low emittance.In this paper,the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations.A Bayesian optimization method is used to optimize the key laser and plasma parameters so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth.The effect of the rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth.Finally,a high-quality electron beam with an energy of 1.5 GeV,a normalized transverse emittance of 0.5 mm·mrad and a relative energy spread of 0.5%at 10 pC is obtained.
基金Supported by Wu Jieping Medical Foundation(No.320.6750.2021-04-15).
文摘AIM:To assess the visual outcomes and corneal biomechanical properties of myopia patients between laser in situ keratomileusis(LASIK)and LASIK combined with accelerated corneal crosslinking(LASIK Xtra).METHODS:This prospective study analyzed 52 consecutive myopia patients treated with LASIK Xtra and 45 consecutive myopia patients treated with LASIK.Only the right eyes in the two groups were analyzed.The uncorrected distance visual acuity(UDVA),keratometry values,postoperative central corneal thickness(CCT),corneal demarcation line depth,the corneal compensated intraocular pressure(IOPcc),Goldmann-correlated IOP(IOPg),corneal resistance factor(CRF)and corneal hysteresis(CH)from Ocular Response Analyzer(ORA)were analyzed.Further,the correlation between the demarcation line depth and ORA-related biomechanical parameters were analyzed.RESULTS:No significant differences in UDVA,postoperative CCT,or mean K values were found between the 2 groups at 1 to 12mo postoperative follow-up(all P>0.05).The changes of CRF was significantly lower in the LASIK Xtra group compared to the LASIK group(all P<0.05)at all the postoperative visits.The changes of CH were significantly higher in the LASIK Xtra group(all P<0.05).No significant differences were discovered regarding the changes of IOPcc and IOPg posperatively(all P>0.05).Out of 52 cases in the LASIK Xtra group,the demarcation line was present in 40 eyes(77%).The average depth of the demarcation was 220.73±42.70μm(136 to 288μm).No significant correlation was observed between the depth of the demarcation line and any of the ORA-related biomechanical parameters such as IOPcc,IOPg,CRF and CH at 12mo(all P>0.05).CONCLUSION:Both procedures demonstrate comparable outcomes in terms of visual acuity,refraction and ablation predictability.This study confirms that corneal biomechanical properties of the included patients weakened after both procedures,but the cornea after LASIK Xtra are stiffer than conventional LASIK.
基金supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB0530000)the Discipline Construction Foundation of“Double World-class Project”.
文摘Laser wakefield accelerators(LWFAs)offer acceleration gradients up to 1000 times higher than those of conventional radio-frequency accelerators,offering a pathway to significantly more compact and cost-effective accelerator systems.This breakthrough opens up new possibilities for laboratory-scale light sources.All-optical inverse Compton scattering(AOCS)sources driven by LWFAs produce high-brightness,quasimonochromatic X rays with micrometer-scale source sizes,delivering the spatial coherence and resolution required for X-ray phase-contrast imaging(XPCI).These features position AOCS X-ray sources as promising tools for applications in biology,medicine,physics,and materials science.However,previous AOCS-based imaging studies have primarily focused on X-ray absorption imaging.In this work,we report successful experimental demonstrations of edge-enhanced in-line XPCI using energy-tunable,quasi-monochromatic AOCS X rays.With a spatial resolution of~20μm,our results clearly show the potential of high-resolution,AOCS-based XPCI applications.
基金supported by NNSA cooperative agreement DE-NA0002008the Defense Advanced Research Projects Agency's PULSE program(12-63-PULSE-FP014)the Air Force Office of Scientific Research(FA9550-14-1-0045).
文摘The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and lower than critical densities with plasmas extending over few micrometers,i.e.multiple wavelengths.The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam.Experiments at the Glass Hybrid OPCPA Scaled Test-bed(GHOST)laser system at University of Texas,Austin using such targets measured non-Maxwellian,peaked electron distribution with large bunch charge and high electron density in the laser propagation direction.These results are reproduced in 2D PIC simulations using the EPOCH code,identifying direct laser acceleration(DLA)[1]as the responsible mechanism.This is the first time that DLA has been observed to produce peaked spectra as opposed to broad,Maxwellian spectra observed in earlier experiments[2].This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.
基金supported by the National Natural Science Foundation of China (Nos.11921006,U2230133)Beijing Outstanding Young Scientists Program+2 种基金National Grand Instrument Project (No.2019YFF01014400)National Key R&D Program of China (No.2022YFA1603303)Open Foundation of Key Laboratory of High Power Laser and Physics,Chinese Academy of Sciences (No.SGKF202104)。
文摘In this study,we theoretically investigate the feasibility of using laser-wakefield accelerated(LWFA)electrons for the photonuclear measurement of nuclear isomers according to the characteristics of the electrons obtained from LWFA experiments conducted at the Compact Laser-Plasma Accelerator(CLAPA)laboratory.The experiments at the CLAPA show that a stable electron beam with an energy of 78-135 MeV and a charge of 300-600 pC can be obtained.The bremsstrahlung spectra were simulated using Geant4,which suggests that a bremsstrahlung source with a peak intensity of 10^(19) photons/s can be generated.Theoretical calculations of isomer production cross sections from the photonuclear reactions on six target nuclei,^(197)Au,^(180)Hf,^(159)Tb,^(115)In,^(103)Rh,and ^(90)Zr,were performed and compared with the available experimental data in EXFOR,which suggest that further experiments are required for a series of photonuclear reaction channels.Flux-averaged cross sections and isomer ratios(IR)resulting from such bremsstrahlung sources are theoretically deduced.The results suggest that IR measurements can be used to constrain nuclear components,such asγstrength function and optical model potential.In addition,the detection of the decay characteristics was evaluated with Geant4 simulations.The use of the LWFA electron beam and its bremsstrahlung for photonuclear studies involving nuclear isomers is anticipated.
基金Supported by the National Basic Research Program of China under Grant Nos 2013CBA01504the National Natural Science Foundation of China under Grant Nos 11475260,11374209 and 11375265
文摘We study a laser wakefield acceleration driven by mid-infrared （mid-IR） laser pulses through two-dimensional particle-in-cell simulations. Since a mid-IR laser pulse can deliver a larger ponderomotive force as compared with the usual 0.8 μm wavelength laser pulse, it is found that electron self-injection into the wake wave occurs at an earlier time, the plasma density threshold for injection becomes lower, and the electron beam charge is substantially enhanced. Meanwhile, our study also shows that quasimonoenergetic electron beams with a narrow energy-spread can be generated by using mid-IR laser pulses. Such a mid-IR laser pulse can provide a feasible method for obtaining a high quality and high charge electron beam. Therefore, the current efforts on constructing mid-IR terawatt laser systems can greatly benefit the laser wakefield acceleration research.
基金the National Natural Science Foundation of China(Nos.12004353,11975214,11991071,11905202,and 12174350)Key Laboratory Foundation of the Sciences and Technology on Plasma Physics Laboratory(No.6142A04200103)Independent Scientific Research(No.JCKYS2021212011).
文摘In this paper,we propose a novel stacked laser dielectric acceleration structure.This structure is based on the inverse Cherenkov effect and represented by a parametric design formulation.Compared to existing dielectric laser accelerators relying on the inverse Smith–Purcell effect,the proposed structure provides an extended-duration synchronous acceleration field without requiring the pulse front tilting technique.This advantage significantly reduces the required pulse duration.In addition,the easy to integrate layered structure facilitates cascade acceleration,and simulations have shown that low-energy electron beams can be cascaded through high gradients over extended distances.These practical advantages demonstrate the potential of this new structure for future chip accelerators.
基金Project supported by the National Natural Science Foundation of China (Grant No. 10834008)the State Key Development Program for Basic Research of China (Grant No. 2006CB806004)
文摘Forward fast protons are generated by the moderate-intensity laser-foil interaction. Protons with maximum energy 190 keV are measured by using magnetic spectrometer and CR-39 solid state track detectors along the direction normal to the rear surface. The experimental results are also modeled by the paxticle-in-cell method, investigating the timevarying electron temperature and the rear sheath field. The temporal and spatial structure of the sheath electrical field, revealed in the simulation, suggests that these protons are accelerated by target normal sheath acceleration （TNSA） mechanism.
基金supported by the National Natural Science Foundation of China (Grant Nos.10734130,10935002,and 11075105)the National Basic Research Program of China (Grant No.2009GB105002)
文摘Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11127901,11425418,and 61221064)the National Basic Research Program of China(Grant No.2011CB808100)the Science and Technology Talent Project of Shanghai City,China(Grant Nos.12XD1405200 and 12ZR1451700)
文摘Laser wakefield accelerators（LWFAs） are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators. It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones. The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators. To achieve the LWFAs suitable for applications,more and more attention has been paid to optimize the LWFAs for high-quality electron beams. A single-staged LWFA does not favor generating controllable electron beams beyond 1 Ge V since electron injection and acceleration are coupled and cannot be independently controlled. Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved.This paper provides an overview of the physical conceptions of the LWFA, as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.
基金Supported by the National Natural Science Foundation of China under Grant No 11575011the National Grand Instrument Project under Grant No 2012YQ030142
文摘A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44MeV, energy spread within ±5% and 10^6-8 protons per pulse. It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 10^9 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0-±5%0.
基金supported by the National Natural Science Foundation of China(Grant No.11975214).
文摘We present a first on-chip positron accelerator based on dielectric laser acceleration.This innovative approach significantly reduces the physical dimensions of the positron acceleration apparatus,enhancing its feasibility for diverse applications.By utilizing a stacked acceleration structure and far-infrared laser technology,we are able to achieve a seven-stage acceleration structure that surpasses the distance and energy gain of using the previous dielectric laser acceleration methods.Additionally,we are able to compress the positron beam to an ultrafast sub-femtosecond scale during the acceleration process,compared with the traditional methods,the positron beam is compressed to a greater extent.We also demonstrate the robustness of the stacked acceleration structure through the successful acceleration of the positron beam.
基金the National Natural Science Foundation of China(Grant No.11975214)。
文摘Dielectric laser accelerators(DLAs)are considered promising candidates for on-chip particle accelerators that can achieve high acceleration gradients.This study explores various combinations of dielectric materials and accelerated structures based on the inverse Cherenkov effect.The designs utilize conventional processing methods and laser parameters currently in use.We optimize the structural model to enhance the gradient of acceleration and the electron energy gain.To achieve higher acceleration gradients and energy gains,the selection of materials and structures should be based on the initial electron energy.Furthermore,we observed that the variation of the acceleration gradient of the material is different at different initial electron energies.These findings suggest that on-chip accelerators are feasible with the help of these structures and materials.
基金Project supported by the National Natural Science Foundation of China (Grant No. 11975214)。
文摘The dielectric laser accelerator(DLA) is a promising technology for achieving high-gradient acceleration in a compact design. Its advantages include ease of cascading and an energy gain per unit distance which can exceed that of conventional accelerators by two orders of magnitude. This paper establishes rules for efficient particle acceleration using dielectric structures based on basic equations, proposes a design principle for DLA structures with clear physical images and verifies the accuracy of the corresponding formula for energy gain. DLA structures with different specifications, materials and geometric shapes are constructed, and the achievable acceleration gradient is calculated. Our results demonstrate that effective acceleration can be achieved when the electric field sensed by particles in the acceleration cavity has zero frequency,which provides a powerful method for designing such devices. Furthermore, we demonstrate that the simplified formula for calculating energy gain presented in this paper can accurately determine the energy gain of particles during the design of acceleration structures using a dielectric accelerator.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11475027,11765017,11764039,11305132,and 11274255)the Natural Science Foundation of Gansu Province,China(Grant No.17JR5RA076)the Scientific Research Project of Gansu Higher Education,China(Grant No.2016A-005)
文摘The energy and trajectory of the electron, which is irradiated by a high-power laser pulse in a cylindrical plasma channel with a uniform positive charge and a uniform negative current, have been analyzed in terms of a single-electron model of direct laser acceleration. We find that the energy and trajectory of the electron strongly depend on the positive charge density, the negative current density, and the intensity of the laser pulse. The electron can be accelerated significantly only when the positive charge density, the negative current density, and the intensity of the laser pulse are in suitable ranges due to the dephasing rate between the wave and electron motion. Particularly, when their values satisfy a critical condition. the electron can stay in phase with the laser and gain the largest energy from the laser. With the enhancement of the electron energy, strong modulations of the relativistic factor cause a considerable enhancement of the electron transverse oscillations across the channel, which makes the electron trajectory become essentially three-dimensional, even if it is flat at the early stage of the acceleration.
基金This work was supported by the Programs for the National Natural Science Foundation of China(Nos.11975316,11775312,12005305 and 61905287)the Continue Basic Scientific Research Project(Nos.WDJC-2019-02 and BJ20002501).
文摘The supersonic gas-jet target is an important experimental target for laser wakefield acceleration(LWFA),which has great potential for driving novel radiation sources such as betatron radiation and Compton scattering gamma rays.According to different electron acceleration requirements,it is necessary to provide specific supersonic gas jets with different density profiles to generate electron beams with high quality and high repetition rates.In this study,the interference images and density profiles of different gas-jet targets were obtained through a modified Nomarski interference diagnosis system.The relationships between the gas density and back pressure,nozzle structure,and other key parameters were studied.Targets with different characteristics are conducive to meeting the various requirements of LWFA.
基金Project supported by the National Natural Science Foundation of China （Grant Nos 10335020, 10105014 and 10390160）, the National High Technology Inertial Confinement Fusion Foundation of China, and the National Key Basic Research Special Foundation of China （Grant No G1999075200）.
文摘We suggest a scheme of electron acceleration by use of two tightly focused ultra-short intense laser pulses at a IOOTW level. Electroas obtain a preliminary acceleration with a small angular spread by the longitudinal ponderomotive force of the first pulse. They are then injected and further accelerated to hundreds of MeV by the second laser pulse.
