Transformer Ratio Enhancement Using a Ramped Bunch Train in a Collinear Wakefield Accelerator

We present a practical method for achieving a transformer ratio (R) greater than 2 with any collinear wakefield accelerator i.e. with either plasma or structure based wakefield accelerators. It is known that the transformer ratio cannot generally be greater than 2 for a symmetric drive bunch in a collinear wakefield accelerator. However, using a ramped bunch train (RBT) where a train of n electron drive bunches, with increasing ('ramping') charge, one can achieve R = 2n after the bunch train. We believe this method is feasible from an engineering standpoint. We describe a proof of principle experiment using an disk-loaded waveguide, of frequency 13.65 GHz, driven by a RBT of 4 electron bunches. We expect to achieve R > 6 using 4 electron bunches. Details of the simulation and experimental design are presented.