A multibunch plasma wakefield acceleration experiment at the Brookhaven National Laboratory

Plasma-based particle accelerators have made remarkable progress in the recent years. In particular the doubling of the energy of 42 GeV trailing electrons has been demonstrated in a bean-driven plasma wakefield accelerator (PWFA) at the SLAC National Accelerator Laboratory [I. Blumenfeld et al, Nature 445, 741-744 (15 February 2007)]. The next significant step for a PWFA is to demonstrate the acceleration of a witness bunch with a narrow energy spread. We have recently demonstrated experimentally a simple mask method to generate a train of drive bunches followed by a witness bunch with variable spacing in the picosecond range [P. Muggli et al., Phys. Rev. Lett. 101, 054801 (2008)]. The train of drive bunches resonantly drives the wakefield, while the witness bunch is accelerated. The number of drive bunches, and therefore the accelerating wakefield amplitude can be selected. In a multi-drive-bunch configuration the witness bunch energy can be multiplied by a factor larger than two, and the transformer ratio and the transfer efficiency can in principle be maximized. The plasma is generated by a H₂-filled capillary discharge. The plasma density measured and time-resolved using Stark broadening of the H_alpha atomic line. It is adjusted to the resonance by varying the delay between the capillary discharge time and the arrival time of the electron train. The experimental set-up, and the plasma discharge and diagnostics will be presented, and the latest experimental results will be discussed.