  A long, narrow, relativistic charged particle bunch propagating in plasma is
subject to the self-modulation (SM) instability. We show that SM of a proton
bunch can be seeded by the wakefields driven by a preceding electron bunch. SM
timing reproducibility and control are at the level of a small fraction of the
modulation period. With this seeding method, we independently control the
amplitude of the seed wakefields with the charge of the electron bunch and the
growth rate of SM with the charge of the proton bunch. Seeding leads to larger
growth of the wakefields than in the instability case.
