Effect of prepulse on fast electron lateral transport at the target surface irradiated by intense femtosecond laser pulses

The effects of preplasma on lateral fast electron transport at front target surface irradiated by ultraintense (>10 18 W/cm 2 ) laser pulses are experimentally investigated. A spherically bent quartz crystal is used to record two-dimensional spatially resolved K α x-ray emission resulting from fast electron transport. A large (∼ 600µm in diameter) annular K a halo structure surrounding a central spot is observed when a preplasma is presented. Furthermore, the halo size increases with the preplasma scale length, and it finally vanishes when the scale length is sufficiently large. Moreover, an obvious reduction of the K α yield measured by a single photon counting charge-coupled device (CCD) is observed for a large preplasma scale length. Specially designed step-like target is used to identify the possible electron transport mechanisms resulting in the experimental observations. It is believed that the halo of the K α x-ray emission is mainly generated by the out-going fast electrons laterally diffused in the self-generated magnetic and electrostatic fields in the preplasma. This understanding is supported by the simulated fast electron trajectories in specified magnetic and electrostatic fields using a two-dimensional numerical model.