Demonstration of efficient relativistic electron acceleration by surface plasmonics with sequential target processing using high repetition lasers

For high repetition ultrahigh-intensity laser system, automatic alignment of structured target is key to achieving consistent particle acceleration and plasma heating. In this work, we demonstrate efficient electron acceleration with two sequential steps of laser processing using a high repetition rate, 30-fs ultrahigh-intensity laser. The first pulse does laser machining and creates a steep cylindrical crater on the surface of a flat stainless-steel target. The crater is formed by the hydrodynamic expansion of the heated surface and by spallation of the inner, deeper material by nonthermal relativistic electrons. The crater shape is well controlled and reproducible with 200 μm width and 350 μm depth. The second pulse irradiates deeply inside the crater and interacts with the crater wall, efficiently accelerating electrons via surface plasmonic, without need for target realignment. The laser absorption efficiency increases from 32.5 to 97.5% by the process.