Multi-bunch plasma wakefield acceleration experiments

Summary form only given. Plasmas can sustain extremely large electric field that can lead to large particle energy gains in short plasma-based accelerators. These fields can be excited or driven by short particle bunches in a scheme known as the plasma wakefield accelerator (PWFA). However, a equally important parameter for application to a future particle accelerator is the efficiency of the energy transfer between the drive bunch and the witness bunch. This efficiency is proportional to the ratio between the accelerating field experienced by the witness bunch and the decelerating field experienced by the drive bunch. A method to enhance this efficiency by a large factor by driving the plasma wake with a train of equally spaced drive bunches. To maximize the energy transfer efficiency the charge contained in each bunch must also increase along the bunch. This method is known as the ramped bunch train (RBT) method. We have recently demonstrated experimentally that trains of sub- picosecond microbunches also separated by sub-picosecond a spacing can be produced. This method uses a mask placed in a region of the electron beam line where the transverse beam size is dominated by the correlated energy spread imposed on the bunch. For these experiments we use a mesh formed by equally spaced wires. The mask spoils the emittance of the electrons that hit its solid parts. These electrons are lost along the beam line, and the spatial mask pattern is transposed onto the bunch as a time pattern. The spacing between the microbunches, their length, as well as the number of microbunches in the train can be adjusted by varying the beam and mask parameters. The charge in each microbunch can in principle also be controlled by limiting the amount of charge transmitted through the mask in the direction perpendicular to the wires. Such trains may also have applications in free-electron lasers. The train of microbunches is a sent into a 1 cm-long capillary discharge plasma. The energy ch- - ange imparted by the plasma to the microbunches is recorded as a function of the plasma density. The plasma density is monitored using time resolved Stark broadening of the hydrogen plasma lines and is varied by adjusting the delay between the capillary discharge and the electron beam. Experimental results will be presented and discussed.