Irradiated by femtosecond laser pulses with different energies, opened cone targets behave very differently in the transmission of incident laser pulses. The targets, each with an opening angle of 71° and an open...Irradiated by femtosecond laser pulses with different energies, opened cone targets behave very differently in the transmission of incident laser pulses. The targets, each with an opening angle of 71° and an opening of 5 μm, are fabricated using standard semiconductor technology. When the incident laser energy is low and no pre-plasma is generated on the side walls of the cones, the cone target acts like an optical device to reflect the laser pulse, and 15% of the laser energy can be transmitted through the cones. In contrast, when the incident laser energy is high enough to generate pre-plasmas by the pre-pulse of the main pulse that fills the inner cone, the cone with the plasmas will block the transmission of the laser, which leads to a decrease in laser transmission compared with the low-energy case with no plasma. Simulation results using optical software in the low-energy case, and using the particle-in-cell code in the high-energy case, are primarily in agreement with the experimental results.展开更多
Recently generation of strong magnetic(B)fields has been demonstrated in capacitor coils heated by high power laser pulses[S.Fujioka et al.,Sci.Rep.3,1170(2013)].This paper will present a direct measurement of B field...Recently generation of strong magnetic(B)fields has been demonstrated in capacitor coils heated by high power laser pulses[S.Fujioka et al.,Sci.Rep.3,1170(2013)].This paper will present a direct measurement of B field generated with an open-ended coil target driven by a nanosecond laser pulse using ultrafast proton radiography.The radiographs are analyzed with particle-tracing simulations.The B field at the coil center is inferred to be ~50 T at an irradiance of ~5×10^(14) W·cm^(-2).The B field generation is attributed to the background cold electron flow pointing to the laser focal spot,where a target potential is induced due to the escape of energetic electrons.展开更多
With the advent of ultrashort high intensity laser pulses, laser absorption during the laser–solid interactions has received significant attention over the last two decades since it is related to a variety of applica...With the advent of ultrashort high intensity laser pulses, laser absorption during the laser–solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers,including the hot electron production for fast ignition of fusion targets, table-top bright X-ray and gamma-ray sources,ion acceleration, compact neutron sources, and generally the creation of high energy density matters. Normally, some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses. The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures. Meanwhile, relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena, which are usually not found with nanosecond long pulses. In this paper, we present an overview of the recent progress on the major absorption mechanisms in intense laser–solid interactions, where emphasis is paid to our related theory and simulation studies.展开更多
In this study,we report a laser interferometry experiment for the online-diagnosing of a laserproduced plasma.The laser pulses generating the plasma are ultra-fast(30 femtoseconds),ultraintense(tens of Terawatt) a...In this study,we report a laser interferometry experiment for the online-diagnosing of a laserproduced plasma.The laser pulses generating the plasma are ultra-fast(30 femtoseconds),ultraintense(tens of Terawatt) and are focused on a helium gas jet to generate relativistic electron beams via the laser wakefield acceleration(LWFA) mechanism.A probe laser beam(λ=800 nm) which is split-off the main beam is used to cross the plasma at the time of arrival of the main pulse,allowing online plasma density diagnostics.The interferometer setup is based on the No Marski method in which we used a Fresnel bi-prism where the probe beam interferes with itself after crossing the plasma medium.A high-dynamic range CCD camera is used to record the interference patterns.Based upon the Abel inversion technique,we obtained a 3D density distribution of the plasma density.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
We present laboratory measurement and theoretical analysis of silicon K-shell lines in plasmas produced by Shenguang II laser facility, and discuss the application of line ratios to diagnose the electron density and t...We present laboratory measurement and theoretical analysis of silicon K-shell lines in plasmas produced by Shenguang II laser facility, and discuss the application of line ratios to diagnose the electron density and temperature of laser plasmas.Two types of shots were carried out to interpret silicon plasma spectra under two conditions, and the spectra from 6.6 ?A to6.85 ?A were measured. The radiative-collisional code based on the flexible atomic code(RCF) is used to identify the lines, and it also well simulates the experimental spectra. Satellite lines, which are populated by dielectron capture and large radiative decay rate, influence the spectrum profile significantly. Because of the blending of lines, the traditional G value and R value are not applicable in diagnosing electron temperature and density of plasma. We take the contribution of satellite lines into the calculation of line ratios of He-α lines, and discuss their relations with the electron temperature and density.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
The plasma behavior of filamentary barrier discharges in helium is simulated using a twodimensional(2D) particle-in-cell/Monte Carlo model. Four different phases have been suggested in terms of the development of th...The plasma behavior of filamentary barrier discharges in helium is simulated using a twodimensional(2D) particle-in-cell/Monte Carlo model. Four different phases have been suggested in terms of the development of the discharge: the Townsend phase; the space-charge dominated phase; the formation of the cathode layer, and the extinguishing phase. The spatialtemporal evolution of the particle densities, velocities of the charged particles, electric fields, and surface charges has been demonstrated. Our simulation provides insights into the underlying mechanism of the discharge and explains many dynamical behaviors of dielectric barrier discharge(DBD) filaments.展开更多
The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order ...The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.展开更多
Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy...Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-Ⅱ laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.展开更多
As a promising new way to generate a controllable strong magnetic field, laser-driven magnetic coils have attracted interest in many research fields. In 2013, a kilotesla level magnetic field was achieved at the Gekk...As a promising new way to generate a controllable strong magnetic field, laser-driven magnetic coils have attracted interest in many research fields. In 2013, a kilotesla level magnetic field was achieved at the Gekko XⅡ laser facility with a capacitor–coil target. A similar approach has been adopted in a number of laboratories, with a variety of targets of different shapes. The peak strength of the magnetic field varies from a few tesla to kilotesla, with different spatiotemporal ranges. The differences are determined by the target geometry and the parameters of the incident laser. Here we present a review of the results of recent experimental studies of laser-driven magnetic field generation, as well as a discussion of the diagnostic techniques required for such rapidly changing magnetic fields. As an extension of the magnetic field generation, some applications are discussed.展开更多
The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar res...The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings.In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense,and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.展开更多
A review is presented on our recent Vlasov-Fokker-Planck(VFP)simulation code development and applications for high-power laser-plasma interactions.Numerical schemes are described for solving the kinetic VFP equation w...A review is presented on our recent Vlasov-Fokker-Planck(VFP)simulation code development and applications for high-power laser-plasma interactions.Numerical schemes are described for solving the kinetic VFP equation with both electronelectron and electron-ion collisions in one-spatial and two-velocity(1D2V)coordinates.They are based on the positive and flux conservation method and the finite volume method,and these twomethods can insure the particle number conservation.Our simulation code can deal with problems in high-power laser/beam-plasma interactions,where highly non-Maxwellian electron distribution functions usually develop and the widely-used perturbation theories with the weak anisotropy assumption of the electron distribution function are no longer in point.We present some new results on three typical problems:firstly the plasma current generation in strong direct current electric fields beyond Spitzer-H¨arm’s transport theory,secondly the inverse bremsstrahlung absorption at high laser intensity beyond Langdon’s theory,and thirdly the heat transport with steep temperature and/or density gradients in laser-produced plasma.Finally,numerical parameters,performance,the particle number conservation,and the energy conservation in these simulations are provided.展开更多
At Shanghai Jiao Tong University(SJTU) we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wake...At Shanghai Jiao Tong University(SJTU) we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration(LWFA) scheme, multi-hundred Me V electron beams of reasonable quality are generated using 20–40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.展开更多
Ionization-induced electron injection in laser wakefield accelerators, which was recently proposed to lower the laser intensity threshold for electron trapping into the wake wave, has the drawback of generating electr...Ionization-induced electron injection in laser wakefield accelerators, which was recently proposed to lower the laser intensity threshold for electron trapping into the wake wave, has the drawback of generating electron beams with large and continuous energy spreads, severely limiting their future applications. Complex target designs based on separating the electron trapping and acceleration stages were proposed as the only way for getting small energy-spread electron beams. Here, based on the self-truncated ionization-injection concept which requires the use of unmatched laser–plasma parameters and by using tens of TW laser pulses focused onto a gas jet of helium mixed with low concentrations of nitrogen, we demonstrate single-stage laser wakefield acceleration of multi-hundred MeV electron bunches with energy spreads of a few percent. The experimental results are verified by PIC simulations.展开更多
It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The pr...It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such a kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses is formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from modulated asymmetric laser fields.展开更多
The proton spectral and angular distributions simultaneously within the target normal direction and laser propagation direction by using an angle-resolved proton energy spectrometer are studied.For the protons generat...The proton spectral and angular distributions simultaneously within the target normal direction and laser propagation direction by using an angle-resolved proton energy spectrometer are studied.For the protons generated in the interactions of 100 fs,800nm laser pulses with aluminum foil targets,the deviations of proton beam centers of different energies from the target normal direction towards the laser propagation direction are different.This is probably because of the toroidal magnetic fields generated at the rear target surface,which deflect protons transversely.As a result,protons in low energy range have large deviation angles,protons in middle energy range have the smallest deviation angles,while protons in high energy tail have large deviation angles.展开更多
基金supported by National Natural Science Foundation of China(Nos.10925421,10735050,10974250,10935002)
文摘Irradiated by femtosecond laser pulses with different energies, opened cone targets behave very differently in the transmission of incident laser pulses. The targets, each with an opening angle of 71° and an opening of 5 μm, are fabricated using standard semiconductor technology. When the incident laser energy is low and no pre-plasma is generated on the side walls of the cones, the cone target acts like an optical device to reflect the laser pulse, and 15% of the laser energy can be transmitted through the cones. In contrast, when the incident laser energy is high enough to generate pre-plasmas by the pre-pulse of the main pulse that fills the inner cone, the cone with the plasmas will block the transmission of the laser, which leads to a decrease in laser transmission compared with the low-energy case with no plasma. Simulation results using optical software in the low-energy case, and using the particle-in-cell code in the high-energy case, are primarily in agreement with the experimental results.
基金supported by the National Basic Research Program of China(Grant No.2013CBA01501)the National Nature Science Foundation of China(Grant Nos.11135012,11520101003 and 11375262)the National High Technology Research and Development Program of China.
文摘Recently generation of strong magnetic(B)fields has been demonstrated in capacitor coils heated by high power laser pulses[S.Fujioka et al.,Sci.Rep.3,1170(2013)].This paper will present a direct measurement of B field generated with an open-ended coil target driven by a nanosecond laser pulse using ultrafast proton radiography.The radiographs are analyzed with particle-tracing simulations.The B field at the coil center is inferred to be ~50 T at an irradiance of ~5×10^(14) W·cm^(-2).The B field generation is attributed to the background cold electron flow pointing to the laser focal spot,where a target potential is induced due to the escape of energetic electrons.
基金Project supported by the National Basic Research Program of China(Grant No.2013CBA01504)the National Natural Science Foundation of China(Grant Nos.11421064,11129503,11374209,and 11374210)
文摘With the advent of ultrashort high intensity laser pulses, laser absorption during the laser–solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers,including the hot electron production for fast ignition of fusion targets, table-top bright X-ray and gamma-ray sources,ion acceleration, compact neutron sources, and generally the creation of high energy density matters. Normally, some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses. The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures. Meanwhile, relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena, which are usually not found with nanosecond long pulses. In this paper, we present an overview of the recent progress on the major absorption mechanisms in intense laser–solid interactions, where emphasis is paid to our related theory and simulation studies.
文摘In this study,we report a laser interferometry experiment for the online-diagnosing of a laserproduced plasma.The laser pulses generating the plasma are ultra-fast(30 femtoseconds),ultraintense(tens of Terawatt) and are focused on a helium gas jet to generate relativistic electron beams via the laser wakefield acceleration(LWFA) mechanism.A probe laser beam(λ=800 nm) which is split-off the main beam is used to cross the plasma at the time of arrival of the main pulse,allowing online plasma density diagnostics.The interferometer setup is based on the No Marski method in which we used a Fresnel bi-prism where the probe beam interferes with itself after crossing the plasma medium.A high-dynamic range CCD camera is used to record the interference patterns.Based upon the Abel inversion technique,we obtained a 3D density distribution of the plasma density.
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supported by the Science Challenge Project(No.TZ2016005)the National Basic Research Program of China(No.2013CBA01503)+1 种基金the National Natural Science Foundation of China(Nos.10905004,11220101002,and11622323)the Fundamental Research Funds for the Central Universities
文摘We present laboratory measurement and theoretical analysis of silicon K-shell lines in plasmas produced by Shenguang II laser facility, and discuss the application of line ratios to diagnose the electron density and temperature of laser plasmas.Two types of shots were carried out to interpret silicon plasma spectra under two conditions, and the spectra from 6.6 ?A to6.85 ?A were measured. The radiative-collisional code based on the flexible atomic code(RCF) is used to identify the lines, and it also well simulates the experimental spectra. Satellite lines, which are populated by dielectron capture and large radiative decay rate, influence the spectrum profile significantly. Because of the blending of lines, the traditional G value and R value are not applicable in diagnosing electron temperature and density of plasma. We take the contribution of satellite lines into the calculation of line ratios of He-α lines, and discuss their relations with the electron temperature and density.
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sponsored by National Natural Science Foundation of China under Grant Nos.11505044,11405042 and 11421064the Natural Science Foundation of Hebei Province under Grant No.A2016201066+1 种基金the Research Foundation of Education Bureau of Hebei province under Grant No.BJ2016006the Midwest Universities Comprehensive Strength Promotion Project
文摘The plasma behavior of filamentary barrier discharges in helium is simulated using a twodimensional(2D) particle-in-cell/Monte Carlo model. Four different phases have been suggested in terms of the development of the discharge: the Townsend phase; the space-charge dominated phase; the formation of the cathode layer, and the extinguishing phase. The spatialtemporal evolution of the particle densities, velocities of the charged particles, electric fields, and surface charges has been demonstrated. Our simulation provides insights into the underlying mechanism of the discharge and explains many dynamical behaviors of dielectric barrier discharge(DBD) filaments.
基金supported by the National Key Technology R&D Program of the Ministry of Science and Technology,China(Grant No.2012BAC23B00)the National Natural Science Foundation of China(Grants No.11404335)
文摘The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.
基金supported by the Science Challenge Project (No. TZ2016005)the National Basic Program of China (No. 2013CBA01501/03)+2 种基金the National Natural Science Foundation of China (Nos. 11503041, 11522326, 11622323, and 11573040)the Strategic Priority Research Program of the Chinese Academy of Sciences (Nos. XDB16010200 and XDB07030300)the Project Funded by China Postdoctoral Science Foundation (No. 2015M571124)
文摘Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-Ⅱ laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.
基金supported in part by the Science Challenge Project(No.TZ2016005)the CAS-JSPS Joint Research Program(External Cooperation Program of the BIC,Chinese Academy of Sciences,No.112111KYSB20160015)+1 种基金the National Natural Science Foundation of China(Nos.11520101003,11535001 and11861121001)the Strategic Priority Research Program of the Chinese Academy of Sciences(Nos.XDB16010200 and XDB07030300)
文摘As a promising new way to generate a controllable strong magnetic field, laser-driven magnetic coils have attracted interest in many research fields. In 2013, a kilotesla level magnetic field was achieved at the Gekko XⅡ laser facility with a capacitor–coil target. A similar approach has been adopted in a number of laboratories, with a variety of targets of different shapes. The peak strength of the magnetic field varies from a few tesla to kilotesla, with different spatiotemporal ranges. The differences are determined by the target geometry and the parameters of the incident laser. Here we present a review of the results of recent experimental studies of laser-driven magnetic field generation, as well as a discussion of the diagnostic techniques required for such rapidly changing magnetic fields. As an extension of the magnetic field generation, some applications are discussed.
基金jointly supported by the National Natural Science Foundation of China (Nos. 11121504, 11074297, 11274152)the CAS project of KJCX2-YWT01the National Basic Research Program of China (No. 2007CB815101)
文摘The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings.In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense,and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.
基金This work was supported by the National Natural Science Foundation of China(Grants No.11075105,10947108)the National Basic Research Program of China(Grant No.2009GB105002)One of the authors(S.M.W.)wishes to thank Professor P.Mulser of Technische Universitat Darmstadt and Professor M.Murakami of Osaka University for fruitful discussions and suggestions and acknowledges support from the Alexander von Humboldt Foundation.H.Xu acknowledges support from the Natural Science Foundation of Shandong Province(Grand No.Q2008A05).
文摘A review is presented on our recent Vlasov-Fokker-Planck(VFP)simulation code development and applications for high-power laser-plasma interactions.Numerical schemes are described for solving the kinetic VFP equation with both electronelectron and electron-ion collisions in one-spatial and two-velocity(1D2V)coordinates.They are based on the positive and flux conservation method and the finite volume method,and these twomethods can insure the particle number conservation.Our simulation code can deal with problems in high-power laser/beam-plasma interactions,where highly non-Maxwellian electron distribution functions usually develop and the widely-used perturbation theories with the weak anisotropy assumption of the electron distribution function are no longer in point.We present some new results on three typical problems:firstly the plasma current generation in strong direct current electric fields beyond Spitzer-H¨arm’s transport theory,secondly the inverse bremsstrahlung absorption at high laser intensity beyond Langdon’s theory,and thirdly the heat transport with steep temperature and/or density gradients in laser-produced plasma.Finally,numerical parameters,performance,the particle number conservation,and the energy conservation in these simulations are provided.
基金Supported by 973 National Basic Research Program of China(2013CBA01504)Natural Science Foundation of China NSFC(11121504,11334013,11175119,11374209)
文摘At Shanghai Jiao Tong University(SJTU) we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration(LWFA) scheme, multi-hundred Me V electron beams of reasonable quality are generated using 20–40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.
基金supported by the National ‘973’ Program of China under Grant No.2013CBA01504supported by Shanghai Supercomputer Center and the center for high performance computing at Shanghai Jiao Tong University
文摘Ionization-induced electron injection in laser wakefield accelerators, which was recently proposed to lower the laser intensity threshold for electron trapping into the wake wave, has the drawback of generating electron beams with large and continuous energy spreads, severely limiting their future applications. Complex target designs based on separating the electron trapping and acceleration stages were proposed as the only way for getting small energy-spread electron beams. Here, based on the self-truncated ionization-injection concept which requires the use of unmatched laser–plasma parameters and by using tens of TW laser pulses focused onto a gas jet of helium mixed with low concentrations of nitrogen, we demonstrate single-stage laser wakefield acceleration of multi-hundred MeV electron bunches with energy spreads of a few percent. The experimental results are verified by PIC simulations.
基金supported by the National Science Foundation of China (Grant No. 11121504, 11374209, 11374210,and 11375261)
文摘It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such a kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses is formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from modulated asymmetric laser fields.
基金supported by the National Basic Research Program of China(Grant No.2013CBA01501)the National Natural Science Foundation of China(Grant Nos.11375262 and 11135012)
文摘The proton spectral and angular distributions simultaneously within the target normal direction and laser propagation direction by using an angle-resolved proton energy spectrometer are studied.For the protons generated in the interactions of 100 fs,800nm laser pulses with aluminum foil targets,the deviations of proton beam centers of different energies from the target normal direction towards the laser propagation direction are different.This is probably because of the toroidal magnetic fields generated at the rear target surface,which deflect protons transversely.As a result,protons in low energy range have large deviation angles,protons in middle energy range have the smallest deviation angles,while protons in high energy tail have large deviation angles.