  A unified model of electron penetration into linear plasma wakefields is
formulated and studied. The optimum angle for side injection of electrons is
found. At smaller angles, all electrons are reflected radially. At larger
angles, electrons enter the wakefield with superfluous transverse momentum that
is unfavorable for trapping. Separation of incident electrons into penetrated
and reflected fractions occur in the outer region of the wakefield at some
"reflection" radius that depends on the electron energy.
