Yutong Li

Combined proton acceleration from foil targets by ultraintense short laser pulses

Proton emission from solid foil targets irradiated by relativistically intense femtosecond laser pulses is studied experimentally. Broad plateaus in energy spectra are measured from micron-thick targets when the incident laser pulses have relatively low intensity contrasts. It is proposed that such proton spectra can be attributed to the combined processes of laser-driven collisionless shock acceleration and target normal sheath acceleration. Simple analytic estimation and two-dimensional particle-in-cell simulations are performed, which support our interpretation. The obtained plateau-shape spectrum may also serve as an effective tool to diagnose the plasma state and verify the ion acceleration mechanisms in laser-solid interactions.

Bow shocks formed by a high-speed laser-driven plasma cloud interacting with a cylinder obstacle*

A bow shock is formed in the interaction of a high-speed laser-driven plasma cloud with a cylinder obstacle. Its temporal and spatial structures are observed by shadowgraphy and interferometry. The width of the shock transition region is ~ 50 μm, comparable to the ion–ion collision mean free path, which indicates that collision is dominated in the shock probably. The Mach-number of the ablating plasma cloud is ~ 15 at first, and decreases with time resulting in a changing shock structure. A two-dimension hydrodynamics code, USim, is used to simulate the interaction process. The simulated shocks can well reproduce the observed.

Demonstration of laser-produced neutron diagnostic by radiative capture gamma-rays

We report a new scenario of the time-of-flight technique in which fast neutrons and delayed gamma-ray signals were both recorded in a millisecond time window in harsh environments induced by high-intensity lasers. The delayed gamma signals, arriving far later than the original fast neutron and often being ignored previously, were identified to be the results of radiative captures of thermalized neutrons. The linear correlation between the gamma photon number and the fast neutron yield shows that these delayed gamma events can be employed for neutron diagnosis. This method can reduce the detecting efficiency dropping problem caused by prompt high-flux gamma radiation and provides a new way for neutron diagnosing in high-intensity laser-target interaction experiments.

Note: A Laue crystal imager for high energy quasi-monochromatic x-ray

A newly designed transmission type x-ray Laue imager for tens of keV hard x-rays is reported. Compared with the traditional reflection type x-ray crystal imager, the transmission geometry produces a much better image quality for high energy hard x-rays. This system was assessed via a calibration experiment performed at the SPring8 synchrotron radiation facility. With a Ta x-ray fluorescer, the mono-energetic x-ray at 70 keV from the synchrotron radiation was converted to an isotropically emitted Ta K-shell source at 57.5 keV and 65 keV. A tungsten pinhole array was employed as the test object, and clear images of the pinholes with a magnification of ∼5 were acquired. These images exhibited superior quality in the dispersion plane. As an extension of this work, a slit-free full-spectral Laue imager is proposed for high resolution hard x-ray imaging.

An angular-resolved multi-channel Thomson parabola spectrometer for laser-driven ion measurement

A multi-channel Thomson parabola spectrometer was designed and employed to diagnose ion beams driven by intense laser pulses. Angular-resolved energy spectra for different ion species can be measured in a single shot. It contains parallel dipole magnets and wedged electrodes to fit ion dispersion of different charge-to-mass ratios. The diameter and separation of the entrance pinhole channels were designed properly to provide sufficient resolution and avoid overlapping of dispersed ion beams. To obtain a precise energy spectral resolving, three-dimensional distributions of the electric and magnetic fields were simulated. Experimental measurement of energy-dependent angular distributions of target normal sheath accelerated protons and deuterons was demonstrated. This novel compact design provides a comprehensive characterization for ion beams.

Collimated ultrabright gamma rays from electron wiggling along a petawatt laser-irradiated wire in the QED regime

W.-M. Wang, 2, 3 Z.-M. Sheng, 4, 5, 6 P. Gibbon, 8 L.-M. Chen, 5 Y.-T. Li, 5, 9 and J. Zhang 5 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100190, China SUPA, Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom Beijing Advanced Innovation Center for Imaging Technology, Department of Physics, Capital Normal University, Beijing 100048, China Key Laboratory for Laser Plasmas (MoE) and School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China Forschungzentrum Jülich GmbH, Institute for Advanced Simulation, Jülich Supercomputing Centre, D-52425 Jülich, Germany Centre for Mathematical Plasma Astrophysics, Katholieke Universiteit Leuven, 3000 Leuven, Belgium School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China (Dated: November 23, 2017)Significance Even though bright X-rays below mega-electron volt photon energy can be obtained from X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate collimated bright gamma-ray beams over 10 mega-electron volts. We propose a scheme to efficiently generate such beams from submicron wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. With significant quantum electrodynamics effects existing even with petawatt lasers, our full 3D simulations show that directional gamma rays can be generated with thousand-fold higher brilliance and thousand-fold higher photon energy than those from synchrotron radiation facilities. In addition, the photon yield efficiency approaches 10%, 100,000-fold higher than those typical from betatron radiation and Compton scattering based on laser-wakefield accelerators. Even though high-quality X- and gamma rays with photon energy below mega-electron volt (MeV) are available from large-scale X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate bright gamma rays over 10 MeV. Recently, gamma rays with energies up to the MeV level were observed in Compton scattering experiments based on laser wakefield accelerators, but the yield efficiency was as low as 10−6, owing to low charge of the electron beam. Here, we propose a scheme to efficiently generate gamma rays of hundreds of MeV from submicrometer wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. The wiggling is caused by the quasistatic electric and magnetic fields induced around the wire surface, and these are so high that even quantum electrodynamics (QED) effects become significant for gamma-ray generation, although the driving lasers are only at the petawatt level. Our full 3D simulations show that directional, ultrabright gamma rays are generated, containing 1012 photons between 5 and 500 MeV within a 10-fs duration. The brilliance, up to 1027 photons s−1u2009mrad−2u2009mm−2 per 0.1% bandwidth at an average photon energy of 20 MeV, is second only to X-ray free electron lasers, while the photon energy is 3 orders of magnitude higher than the latter. In addition, the gamma ray yield efficiency approaches 10%—that is, 5 orders of magnitude higher than the Compton scattering based on laser wakefield accelerators. Such high-energy, ultrabright, femtosecond-duration gamma rays may find applications in nuclear photonics, radiotherapy, and laboratory astrophysics.

Ultrafast pulsed magnetic fields generated by a femtosecond laser

An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52u2009T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52u2009T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.

Data used in the manuscript “Cherenkov radiation-based optical fibre diagnostics of fast electrons generated in intense laser-plasma interactions”

STFC: Access time: experiment (App. No. 16110035) nNewton China-UK joint research grant: on laser-ion acceleration and novel Terahertz radiation nEPSRC grant EP/K022415/1 nNational Basic Research Program of China (Grants No. 2013CBA01501) nNational Nature Science Foundation of China (Grants No. 11520101003) nNational Nature Science Foundation of China (Grants No. 11535001) nStrategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB16010200) nStrategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07030300) nNational Postdoctoral Program for Innovative Talents (Grants No. BX201600106)

Theoretic study on strong terahertz radiation from laser-driven gas plasma

We investigated powerful terahertz (THz) radiation from laser-driven gas plasma by theory and particle-in-cell (PIC) simulation. We proposed a model to describe THz radiation emission from a net current in plasma. We found that the well-known two-color scheme can be extended to the second laser at any even harmonic of the main laser. To strengthen the THz radiation, we proposed mid-infrared laser scheme, picosecond laser scheme, and asymmetric laser scheme. Recently, we proposed an approach to generate tunable, circularly-polarized, narrow-band THz radiation.