Wen-Xi Liang

Triggering and guiding HV discharge in air by filamentation of single and dual fs pulses

The triggering and guiding of the stationary high voltage (HV) discharges at 5-40 kV are demonstrated by using plasma filaments generated by single and dual femtosecond(fs) laser pulses in air. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filamentaion was observed. The amount of free electrons of filaments generated by different pulse configurations was compared by sonography method. The lifetime of filaments is measured by using time-resolved fluorescence spectrum, and the lifetime of filaments generated by dual fs laser pulses was doubled due to the re-ionization by the succeeding pulse. The triggering ability of dual fs laser pulses was demonstrated to be enhanced due to the longer lifetime of filaments.

Effective fast electron acceleration along the target surface

The dependence of angular distributions of fast electrons generated in the interaction of p-polarized femtosecond laser pulses with foil targets on laser intensities is investigated. A novel fast electron beam along the front target surface is observed for high laser intensity. It is found that the electron acceleration along the target surface is more efficient than those in other directions.

Multipeak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of <60°. This multipeak characterization is found to be independent of laser polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Characteristics of 400 nm femtosecond laser filaments in air

The characteristics of the light filaments formed by 400 nm blue femtosecond laser pulses are experimentally investigated. Both pre-focused and free propagation conditions are studied. The diameter, length and electron density of filaments are measured.

Multi-peak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of < 60°. This multi-peak characteristic is found to be independent of the polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Effect of target shape on fast electron emission

Fast electrons emission from the interaction of femtosecond laser pulses with shaped solid targets has been studied. It is found that the angular distributions of the forward fast electrons are closely dependent upon the target shape. The important role played by the electrostatic fields built up near the target surfaces in the confinement of fast electrons is identified. Our two-dimensional particle-in-cell simulations can reproduce the main observations.

Relativistic Laser Acceleration Of Electrons Along Solid Surfaces

Recent experimental and theoretical studies on surface electron emission will be presented. A collimated fast electron beam was observed along the target surface irradiated by intense laser pulses up to 20TW when the laser is incident with large angles such as over 45 degree. Numerical simulations suggest that such an electron beam is formed due to the confinement of the surface quasistatic electric and magnetic fields. Meanwhile, an acceleration process similar to the inverse‐free‐electron‐laser is found to occur and is responsible for the generation of the most energetic electrons. A general formula for electron angular distributions accounting for the quasistatic electric and magnetic fields is given. In certain conditions, quasi‐monoenergetic electron beams are also produced. These results are of interest for potential applications of laser‐produced electron beams and helpful to the undersanding of the cone‐target physics in the fast ignition related experiments.