Formation of a novel shaped bunch to enhance transformer ratio in collinear wakefield accelerators

The transformer ratio, characterizing the ratio of maximum accelerating field behind the drive bunch to the maximum decelerating field inside the drive bunch, is one of the key parameters for characterizing the performance of collinear wakefield accelerators. In this paper the use of electron drive bunches possessing a particular temporal profile (the double-triangular bunch) is shown to significantly increase the transformer ratio beyond 2, the limit for a symmetric bunch. The double-triangular bunch is generated using the emittance exchange technique [P. Emma et al., Phys. Rev. ST Accel. Beams 9, 100702 (2006)]. Complete beam simulations have been performed for a collinear wakefield acceleration experiment planned at the Argonne Wakefield Accelerator facility including the effects of the physical emittance increase of the bunch in the wakefield device. A transformer ratio of 6.5 and gradient of 20 MV/m is expected in this proof-of-principle experiment for a 3 nC double-triangular bunch traversing a 10 cm long, 200 GHz quartz based dielectric wakefield accelerator structure.