The AWAKE Proton-driven Plasma Wakefield Acceleration Experiment at CERN

We are planning an experiment at CERN to accelerate externally injected electrons $e^-$ on the wake driven by a long, self-modulated proton $p^+$ bunch. In the plan the $12cm$-long bunch from the SPS with $10^{11}$ $p^+$ experiences a two-stream transverse instability that modulates the bunch radius at the plasma wake period. The bunch is focused to $200\mu m$ into a plasma with density in the $10^{14}-10^{15}cm^{-3}$ range. The modulation instability is seeded by co-propagating with the $p^+$ bunch a short laser pulse that ionizes a gas or vapor. The modulation resonantly drives wakefields to large amplitude. The low energy $e^-$ ($\approx5-20MeV$) produced by a rf-photoinjector gun are injected after the instability has saturated, $\approx3-5m$ into the plasma and is accelerated to the $GeV$ energy range. The $e^-$ energy spectrum is measured by a large energy acceptance magnetic spectrometer. Bunch modulation diagnostics such as time resolved OTR and electro-optic measurements are also included. The general plans for the experiment as well as the latest developments will be presented.