Lateral propagation of fast electrons at the laser-irradiated target surfaces

Lateral propagation of fast electrons at the target surfaces irradiated by femtosecond intense laser pulses is measured by kα x-ray imaging technique when a preplasma is presented. An annular halo surrounding a bright spot is observed in the x-ray images when the scale length of the electron density is large. For an incidence angle of 70° the x-ray images show a non-symmetrical distribution peaked to the laser propagation direction. The x-ray photons in the halo are mainly excited by the fast electrons that flow in the preplasma when their paths intersect the high density regions near the target surface.