  Ionization injection in a plasma wakefield accelerator was investigated
experimentally using two lithium plasma sources of different lengths. The
ionization of the helium gas, used to confine the lithium, injects electrons in
the wake. After acceleration, these injected electrons were observed as a
distinct group from the drive beam on the energy spectrometer. They typically
have a charge of tens of pC, an energy spread of a few GeV, and a maximum
energy of up to 30 GeV. The emittance of this group of electrons can be many
times smaller than the initial emittance of the drive beam. The energy scaling
for the trapped charge from one plasma length to the other is consistent with
the blowout theory of the plasma wakefield.

Accelerator Physics;High Energy Physics - Experiment;Plasma Physics