Halo formation and emittance growth of positron beams in plasmas

Abstract

An ultrarelativistic 28.5 GeV, 700-I m-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density ne between 1013 and 5-1014cm-3. Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of 3 in the high emittance plane of the beam 1m downstream from the plasma exit. As ne increases, the formation of a beam halo containing 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 3 and emittance ratio of 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances. © 2008 The American Physical Society.