title: Effect of a laser prepulse on a narrow-cone ejection of MeV electrons from a gas jet irradiated by an ultrashort laser pulse.
abstract: Spatial and energy distributions of energetic electrons produced by an ultrashort, intense laser pulse with a short focal length optical system (Ti:sapphire, 12 TW, 50 fs, lambda=790 nm, f/3.5) in a He gas jet are measured. They are shown to depend strongly on the contrast ratio and shape of the laser prepulse. The wave breaking of the plasma waves at the front of the shock wave formed by a proper laser prepulse is found to make a narrow-cone (0.1pi mm mrad) electron injection. These electrons are further accelerated by the plasma wake field generated by the laser pulse up to tens of MeV forming a Maxwell-like energy distribution. In the case of nonmonotonic prepulse, hydrodynamic instability at the shock front leads to a broader, spotted spatial distribution. The numerical analysis based on a two-dimensional (2D) hydrodynamic (for the laser prepulse) and 2D particle-in-cell (PIC) simulation justifies the mechanism of electron acceleration. The PIC calculation predicts that electrons with energy from 10 to 40 MeV form a bunch with a pulse duration of about 40 fs.
authors.affiliation: Nuclear Engineering Research Laboratory, Graduate School of Engineering, University of Tokyo, 22-2 Shirane-shirakata, Tokai, Naka, Ibaraki, 319-1188, Japan. hosokai@tokai.t.u-tokyo.ac.jp
authors.forename: Tomonao
authors.initials: T
authors.lastname: Hosokai
authors.forename: Kenichi
authors.initials: K
authors.lastname: Kinoshita
authors.forename: Alexei
authors.initials: A
authors.lastname: Zhidkov
authors.forename: Kei
authors.initials: K
authors.lastname: Nakamura
authors.forename: Takahiro
authors.initials: T
authors.lastname: Watanabe
authors.forename: Toru
authors.initials: T
authors.lastname: Ueda
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Kotaki
authors.forename: Masaki
authors.initials: M
authors.lastname: Kando
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Nakajima
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Uesaka