Four-dimensional imaging of the initial stage of fast evolving plasmas

Using an ultrafast electron probe capable of four-dimensional diagnosis, the initial stage of fast evolving plasmas produced by a 1014 W/cm2 laser irradiation of a metal target was investigated in real time with picosecond time resolution. The associated strong transient electric field was identified to have two components, which either focus or defocus the probe electron beam. The effects of this field on the probe electron beam can be reproduced by a self-expanding charge cloud containing about 5x107 suprathermal electrons with the outermost layer expanding at an average speed of 1.2x107 m/s.