Direct observations of plasma wakefield caused by a train of linac bunches

A plasma wakefield accelerator (PEFA) is one of the plasma-based-type accelerators that show promise to produce ultra-high accelerating gradients. In the PWFA, a high-intensity relativistic driving bunch excites a large amplitude plasma wave which, in turn, accelerates a low-intensity trailing bunch. The PWFA has to attain a high transformer ratio in order to be a real accelerator, which is the ratio between the energy reached by the trailing bunch and the energy lost from the driving bunch. One method proposed to attain the high ratio is to shape the longitudinal distribution of the particles inside the driving bunch as triangular. Because this idea is technically difficult to realize, an alternative method is proposed, which uses a train of driving pulses with a triangular envelope. Here, a plasma wakefield caused by a train of 14 MeV linac bunches was detected by a coaxial diode detector. The pulse period was 350 psec while the duration of the pulse train was 6μsec. The time evolution of the plasma wave was explained by a simple linear model. Damping time of the plasma wave was derived from the experiments