We measure the effects of transverse wakefields driven by a relativistic proton bunch in plasma with densities of $2.1\times10^{ 14}$ and \unit[$7.7\times10^{ 14}$]{ electrons/cm$^3$}. We show that these wakefields periodically defocus the proton bunch itself, consistently with the development of the seeded self-modulation process. We show that the defocusing increases both along the bunch and along the plasma by using time resolved and time-integrated measurements of the proton bunch transverse distribution. We evaluate the transverse wakefield amplitudes and show that they exceed their seed value (\unit[$<$15]{ MV/m}) and reach over \unit[300]{ MV/m}. All these results confirm the development of the seeded self-modulation process, a necessary condition for external injection of low energy and acceleration of electrons to multi-GeV energy levels.
Categories: physics.acc-ph