title: Generation and Beaming of Early Hot Electrons onto the Capsule in Laser-Driven Ignition Hohlraums.
abstract: In hohlraums for inertial confinement fusion (ICF) implosions on the National Ignition Facility, suprathermal hot electrons, generated by laser plasma instabilities early in the laser pulse ("picket") while blowing down the laser entrance hole (LEH) windows, can preheat the capsule fuel. Hard x-ray imaging of a Bi capsule surrogate and of the hohlraum emissions, in conjunction with the measurement of time-resolved bremsstrahlung spectra, allows us to uncover for the first time the directionality of these hot electrons and infer the capsule preheat. Data and Monte Carlo calculations indicate that for most experiments the hot electrons are emitted nearly isotropically from the LEH. However, we have found cases where a significant fraction of the generated electrons are emitted in a collimated beam directly towards the capsule poles, where their local energy deposition is up to 10× higher than the average preheat value and acceptable levels for ICF implosions. The observed "beaming" is consistent with a recently unveiled multibeam stimulated Raman scattering model [P. Michel et al., Phys. Rev. Lett. 115, 055003 (2015)], where laser beams in a cone drive a common plasma wave on axis. Finally, we demonstrate that we can control the amount of generated hot electrons by changing the laser pulse shape and hohlraum plasma.
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: E L
authors.initials: EL
authors.lastname: Dewald
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: F
authors.initials: F
authors.lastname: Hartemann
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: P
authors.initials: P
authors.lastname: Michel
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: J
authors.initials: J
authors.lastname: Milovich
authors.affiliation: Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hohenberger
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: A
authors.initials: A
authors.lastname: Pak
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: O L
authors.initials: OL
authors.lastname: Landen
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: L
authors.initials: L
authors.lastname: Divol
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: H F
authors.initials: HF
authors.lastname: Robey
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: O A
authors.initials: OA
authors.lastname: Hurricane
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: T
authors.initials: T
authors.lastname: Döppner
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: F
authors.initials: F
authors.lastname: Albert
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: B
authors.initials: B
authors.lastname: Bachmann
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: N B
authors.initials: NB
authors.lastname: Meezan
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: MacKinnon
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: D
authors.initials: D
authors.lastname: Callahan
authors.affiliation: Lawrence Livermore National Laboratory (LLNL), P.O. Box 808, Livermore, California 94550, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Edwards