Shen Bai-Fei

Widely Tunable Femtosecond Soliton Pulse Generation by Using Soliton-Frequency Shift in a Photonic Crystal Fibre

Femtosecond Raman soliton generation, tunable from 800 to 1044 nm, has been theoretically investigated for a photonic crystal fibre pumped by a 200-fs pulse. A highly nonlinear photonic crystal fibre with a length of only 57.7 cm and a nonlinear coefficient of 0.075 (Wm)−1 is used to achieved such a broadband. It is found that the spectral bandwidth increases with the input peak power. In particular, it is also found that the output wavelengths of the resulting sub-40 fs Raman solitons can also be tuned effectively by varying the initial pulse chirp. There exists an optimal positive chirp which maximizes the bandwidth, corresponding to the formation of only one long-wavelength Raman soliton.

Further Acceleration of MeV Electrons by a Relativistic Laser Pulse

With the development of photocathode rf electron gun, electrons with high-brightness and mono-energy can be obtained easily. By numerically solving the relativistic equations of motion of an electron generated from this facility in laser fields modelled by a circular polarized Gaussian laser pulse, we find the electron can obtain high energy gain from the laser pulse. The corresponding acceleration distance for this electron driven by the ascending part of the laser pulse is much longer than the Rayleigh length, and the light amplitude experienced on the electron is very weak when the laser pulse overtakes the electron. The electron is accelerated effectively and the deceleration can be neglected. For intensities around 10(19) W(.)mu m(2)/cm(2), an electrons energy gain near 0.1 GeV can be realized when its initial energy is 4.5 MeV, and the final velocity of the energetic electron is parallel with the propagation axis. The energy gain can be up to 1 GeV if the intensity is about 10(21) W(.)mu m(2)/cm(2). The final energy gain of the electron as a function of its initial conditions and the parameters of the laser beam has also been discussed.

Proton Acceleration with Nano-Scale Micro-Structured Target by Circularly Polarized Laser Pulse

Two dimensional particle-in-cell simulations are taken to study the interaction of a circularly polarized laser pulse with a nano-scale micro-structured target. The protons which are doped in the rear side of the target experience the electrostatic fields caused by both the radiation pressure driven shock and the target normal sheath at the rear side of the target. A quasimonoenergetic proton bunch with central energy of about 11 MeV and energy spread of Δe/e about 0.18 is achieved by using a 3.45 × 1019 W/cm2, 66fs laser pulse. A comparison with the case of linearly polarized laser pulse and the same target condition is considered.

Demonstration of X-ray Thomson Scattering on Shenguang-II Laser Facility

X-ray Thomson scattering technique for diagnosing dense plasma was demonstrated on Shenguang-II laser facility. Laser plasma x-ray source of titanium He-α lines (~4.75 keV), generated by laser beam (1.5 kJ/527 nm/2 ns) heated titanium thin foil, was used as x-ray probe beam. The x-ray probe was then scattered by cold CH foam column of 1 g/cm3 density. The scattered radiation at 90° was diffracted by polyethylene terephthalate (PET) crystal and recorded on x-ray charge-coupled device. Well-defined scattering spectra were obtained with good signal to noise ratio.

Self-Generated Magnetic Fields in Plasmas

The self-generated magnetic field induced by an ultraintense short laser pulse propagating in a uniform underdense plasma is investigated. The magnetic fields are strong and associated with the plasma wave wake field. Some properties of dc magnetic field in two extreme laser pulse regime are described.

The propagation and deposition of fast muons in dense DT mixture

The propagation of the fast muon population mainly due to collisional effect in a dense deuterium-tritium (DT for short) mixture is investigated and analysed within the framework of the relativistic Fokker-Planck equation. Without the approximation that the muons propagate straightly in the DT mixture, the muon penetration length, the straggling length, and the mean transverse dispersion radius are calculated for different initial energies, and especially for different densities of the densely compressed DT mixture in our suggested muon-driven fast ignition (FI). Unlike laser-driven FI requiring super-high temperature, muons can catalyze DT fusion at lower temperatures and may generate an ignition sparkle before the self-heating fusion follows. Our calculation is important for the feasibility and the experimental study of muon-driven FI.

The Electron Trajectory in a Relativistic Femtosecond Laser Pulse

In this report, we start from Lagrange equation and analyze theoretically the electron dynamics in electromagnetic field. By solving the relativistic government equations of electron, the trajectories of an electron in plane laser pulse, focused laser pulse have been given for different initial conditions. The electron trajectory is determined by its initial momentum, the amplitude, spot size and polarization of the laser pulse. The optimum initial momentum of the electron for LSS (laser synchrotron source) is obtained. Linear polarized laser is more advantaged than circular polarized laser for generating harmonic radiation.

Few-cycle relativistic solitons in plasmas

A set of exact one-dimensional solutions to coupled nonlinear equations describing the propagation of a relativistic ultrashort circularly polarized laser pulse in a cold collisionless and bounded plasma where electrons have an initial velocity in the laser propagating direction is presented. The solutions investigated here are in the form of quickly moving envelop solitons at a propagation velocity comparable to the light speed. The features of solitons in both underdense and overdense plasmas with electrons having different given initial velocities in the laser propagating direction are described. It is found that the amplitude of solitons is larger and soliton width shorter in plasmas where electrons have a larger initial velocity. In overdense plasmas, soliton duration is shorter, the amplitude higher than that in underdense plasmas where electrons have the same initial velocity.

RADIATIVE COOLING INSTABILITY IN LARGE SCALE LINE-SHAPED LASER PLASMA

In the experiment of generating laser plasma in a line-focus system we found that the x-ray emission nonuniformity was increased rapidly after the pumping laser has been turned off. Our study indicates that radiative cooling instability is one possible reason for this x-ray emission nonuniformity.