Ultrafast electron beam imaging of femtosecond laser-induced plasma dynamics

Plasma dynamics in the early stage of laser ablation of a copper target are investigated in real time by making ultrafast electron shadow images and electron deflectometry measurements. These complementary techniques provide both a global view and a local perspective of the associated transient electric field and charge expansion dynamics. The results reveal that the charge cloud above the target surface is composed predominantly of thermally ejected electrons and that it is self-expanding, with a fast front-layer speed exceeding 10⁷ m/s. The average electric field strength of the charge cloud induced by a pump fluence of 2.2 J/cm² is estimated to be ∼ 2.4×10⁵ V/m.