Simulations of two-bunch Plasma Wake Field Accelerator on FACET*

Previous experiments on FFTB at SLAC demonstrated that short electron bunches can produce accelerating gradient of 50 GeV/m over one meter[1]. These experiments provided the science case for the new FACET facility which will have 23 GeV high current beams. Two-bunch PWFA experiment has a second bunch appropriately loaded into the wake of the first bunch so that the second bunch maintains a narrow energy spread. Simulation results show that in possible two bunch scenarios the first bunch (with less current than that in the FFTB case) still can generate a meter long plasma column via field ionization with a density around $5\times10^{16}cm^{-3}$ if a gas with lower ionization threshold is used. The trailing beam can gain $\sim$10 GeV with a very narrow energy spread. The energy gain can be increased to 25 GeV by using a pre-ionized plasma . The possibility of using a partially ionized pre-plasma instead of the fully pre-ionized plasma is also discussed.\\[4pt] [1] I. Blumenfeld et al., Nature 445, 741 (2007).