title: Seeding of self-modulation instability of a long electron bunch in a plasma.
abstract: We demonstrate experimentally that a relativistic electron bunch shaped with a sharp rising edge drives plasma wakefields with one to seven periods along the bunch as the plasma density is increased. The plasma density is varied in the 10(15)-10(17)  cm(-3) range. The wakefields generation is observed after the plasma as a periodic modulation of the correlated energy spectrum of the incoming bunch. We choose a low bunch charge of 50 pC for optimum visibility of the modulation at all plasma densities. The longitudinal wakefields creating the modulation are in the MV/m range and are indirect evidence of the generation of transverse wakefields that can seed the self-modulation instability, although the instability does not grow significantly over the short plasma length (2 cm). We show that the seeding provides a phase reference for the wakefields, a necessary condition for the deterministic external injection of a witness bunch in an accelerator. This electron work supports the concept of similar experiments in the future, e.g., SMI experiments using long bunches of relativistic protons.
authors.affiliation: University of Southern California, Los Angeles, California 90089, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Fang
authors.affiliation: Stanford Linear Accelerator Center, Stanford, California 94309, USA.
authors.forename: V E
authors.initials: VE
authors.lastname: Yakimenko
authors.affiliation: Brookhaven National Laboratory, Upton, Long Island, New York 11973, USA.
authors.forename: M
authors.initials: M
authors.lastname: Babzien
authors.affiliation: Brookhaven National Laboratory, Upton, Long Island, New York 11973, USA.
authors.forename: M
authors.initials: M
authors.lastname: Fedurin
authors.affiliation: Brookhaven National Laboratory, Upton, Long Island, New York 11973, USA.
authors.forename: K P
authors.initials: KP
authors.lastname: Kusche
authors.affiliation: Brookhaven National Laboratory, Upton, Long Island, New York 11973, USA.
authors.forename: R
authors.initials: R
authors.lastname: Malone
authors.affiliation: GoLP/Instituto de Plasmas e Fusão Nuclear Instituto Superior Técnico (IST), Lisbon, Portugal.
authors.forename: J
authors.initials: J
authors.lastname: Vieira
authors.affiliation: University of California, Los Angeles, California 90095, USA.
authors.forename: W B
authors.initials: WB
authors.lastname: Mori
authors.affiliation: University of Southern California, Los Angeles, California 90089, USA and Max Planck Institute for Physics, Munich, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Muggli