Regimes of interaction between charges particle bunches and plasmas

Summary form only given. The interaction of relativistic charged particle bunches with plasmas can be characterized by comparing the longitudinal (σ<;sub>z<;/sub>) and transverse (σ<;sub>r<;/sub>) bunch size to the cold plasma collisionless skindepth c/ω<;sub>pe<;/sub> or equivalently to the corresponding wavenumber k<;sub>pe<;/sub>=1/(c/ω<;sub>pe<;/sub>).Plasma wakefields are most effectively driven by short (kpeσz<;1) and small bunches (kpeσr<;1). In this case the interaction occurs only over one plasma period and few instabilities exist in this plasma wakefield accelerator (PWFA) regime. PWFA experiments in the nonlinear regime have shown acceleration of 42GeV trailing electrons by 42GeV in only 85cm of plasma1. Experiments with positron bunches have also shown energy gain2. When the bunch is large (kpeσr>1) the plasma return current flows through the bunch. The bunch is subject to the current filamentation instability (CFI), a limit case of the Weibel instability. The CFI was recently been observed for the first time3. The CFI could explain the generation of magnetic fields in the interstellar medium by conversion of particle flows kinetic energy into magnetic energy. When the bunch is long (kpeσz>1, kpeσr<;1), the bunch is subject to a transverse two-stream instability recently identified4, the self-modulation instability (SMI). The SMI transforms the long bunch into a train of short bunches with period ~2π/kpe. Each short bunch is now in the PWFA regime and the train can resonantly drive wakefields to large amplitudes. This process is the basis for the PWFA experiment driven by long (~10cm) proton bunches, known as AWAKE5, which was recently approved at CERN. Experiments aiming at studying the physics of SMI with electron and positron bunches are also underway at SLACFACET6. The gen- ration of seed wakefields for the SMI was recently observed experimentally7. The demonstration of resonant wakefields excitation by a preformed bunch train was also demonstrated experimentally8. The physics of these different regimes will be presented and illustrated with the most salient experimental results.