  The AWAKE experiment relies on the self-modulation instability of a long
proton bunch to effectively drive wakefields and accelerate an electron bunch
to GeV-level energies. During the first experimental run (2016-2018) the
instability was made phase reproducible by means of a seeding process: a short
laser pulse co-propagates within the proton bunch in a rubidium vapor. Thus,
the fast creation of plasma and the onset of beam-plasma interaction within the
bunch drives seed wakefields. However, this seeding method leaves the front of
the bunch not modulated. The bunch front could self-modulate in a second,
preformed plasma and drive wakefields that would interfere with those driven by
the (already self-modulated) back of the bunch and with the acceleration
process. We present studies of the seeded the self-modulation (SSM) of a long
proton bunch using a short electron bunch. The short seed bunch is placed ahead
of the proton bunch leading to self-modulation of the entire bunch. Numerical
simulations show that this method have other advantages when compared to the
ionization front method. We discuss the requirements for the electron bunch
parameters (charge, emittance, transverse size at the focal point, length), to
effectively seed the self-modulation process. We also present preliminary
experimental studies on the electron bunch seed wakefields generation.
