title: Halo formation and emittance growth of positron beams in plasmas.
abstract: An ultrarelativistic 28.5 GeV, 700-microm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n(e) between approximately equal to 10(13) and approximately equal to 5 x 10(14) cm(-3). Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of approximately equal to 3 in the high emittance plane of the beam approximately equal to 1 m downstream from the plasma exit. As n(e) increases, the formation of a beam halo containing approximately 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of approximately 3 and emittance ratio of approximately 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.
authors.affiliation: University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: P
authors.initials: P
authors.lastname: Muggli
authors.forename: B E
authors.initials: BE
authors.lastname: Blue
authors.forename: C E
authors.initials: CE
authors.lastname: Clayton
authors.forename: F J
authors.initials: FJ
authors.lastname: Decker
authors.forename: M J
authors.initials: MJ
authors.lastname: Hogan
authors.forename: C
authors.initials: C
authors.lastname: Huang
authors.forename: C
authors.initials: C
authors.lastname: Joshi
authors.forename: T C
authors.initials: TC
authors.lastname: Katsouleas
authors.forename: W
authors.initials: W
authors.lastname: Lu
authors.forename: W B
authors.initials: WB
authors.lastname: Mori
authors.forename: C L
authors.initials: CL
authors.lastname: O'Connell
authors.forename: R H
authors.initials: RH
authors.lastname: Siemann
authors.forename: D
authors.initials: D
authors.lastname: Walz
authors.forename: M
authors.initials: M
authors.lastname: Zhou