Awake: Plasma wakefield acceleration experiments with cern proton bunches

Summary form only given. Relativistic proton bunches routinely produced at CERN can carry many kJ of energy. They are therefore the only wakefield drivers available today that can potentially accelerate electrons or positrons to the TeV energy scale in a single plasma stage¹. Unlike in simulations, real proton bunches are long (σ_z~10cm) and not suitable to directly drive large amplitude wakefields that are driven by short bunches (σ_z~λ_pe) in dense plasmas (E_z~λ_pe^-1/2~n_e^1/2). It was recently proposed² that these long bunches can be transformed into a train of short bunches with a period ~λ_pe≪σ_z through the self-modulation instability (SMI). This train can then resonantly drive wakefields to large amplitudes. Externally injected electrons can be accelerated to high energies.The AWAKE3 experiment was recently approved at CERN to study plasma wakefields driven by self-modulated proton bunches and accelerate electrons to the GeV level. The experiment will use the σz~10cm, 400GeV proton bunch produced by the SPS with 1-3x1011particles. The bunch will be focused to ~200μm near the entrance of a 10m-long, photo-ionized plasma with a density in the 1014-1015cm-3 range4. The SMI will be seeded by the relativistic ionization front created by an ionizing laser pulse. Simulations show that the SMI saturates in 3-5m and GV/m level wakefields are driven over the remaining plasma length. The first phase of the experiment will focus on the detection and study of the proton bunch self-modulation5. In a later phase, 10-20MeV long (a few λpe) electron bunches will be injected in the wakefields. They can be side-injected once the SMI has saturated6 or injected directly on the axis, at the plasma entrance. In both cases the GeV energy range is reached with a narrow final energy - pread. Injection of an ultra-short LWFA-produced electron bunch is also considered7. At the same time various options for producing short, high charge proton bunches suitable for single bunch wakefields excitation are explored. Options for plasma sources that can be extended to the 100´s of meters in length are investigated. Application of the electron bunches that could be produced using this acceleration scheme are also considered, including electron/proton scattering experiments.