Halo Formation and Emittance Growth of Positron Beams in Plasmas

摘要

An ultrarelativistic 28.5 GeV, 700-μm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n_e between ≈10~(13) and ≈ 5 × 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 ≈ 3 in the high emittance plane of the beam ≈ 1 m downstream from the plasma exit. As n_e 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.