Measuring the Self-modulation Instability of Electron and Positron Bunches in Plasmas

The self-modulation instability (SMI) can be used to transform a long, charged particle bunch into a train of periodically spaced shorter bunches. The SMI occurs in a plasma when the plasma wake period is much shorter than the bunch length. The train of short bunches can then resonantly drive wakefields to much larger amplitude that the long bunch can. The SMI will be used in the AWAKE experiment at CERN, where the wakefields will be driven by a high-energy (400GeV) proton bunch. However, most of the SMI physics can be tested with the electron and positron bunches available at SLAC-FACET. In this case, the bunch is ~10 plasma wavelengths long, but can drive wakefields in the GV/m range. FACET has a meter-long plasma and is well equipped in terms of diagnostic for SMI detection: optical transition radiation for transverse bunch profile measurements, coherent transition radiation interferometry for radial modulation period measurements and energy spectrometer for energy loss and gain measurement of the drive bunch particles. The latest experimental results will be presented.