Wakefield acceleration of particles in plasmas is now considered and studied as a possible way to future very high energy. However, the plasma wave that accelerates the particles also decelerates the drive beam that creates the wave. For electron drivers, this imposes a constraint known as the transformer ratio limit, according to which the energy gain of the accelerated (witness) beam cannot be much higher than the driver energy. The limitation arises because of the ratio of electric fields, accelerating to decelerating, which for longitudinally symmetric drivers cannot exceed 2.
Recently it was noticed [1] that the motion of plasma ions can boost the longitudinal electric field by up to 40%, thus increasing the transformer ratio at no cost, but the reasons for the field growth have remained unclear.
In this work we found that transverse breaking of a plasma wave can initiate pumping of the wave energy to the axis. The resulting energy flux can exceed the inevitable energy dissipation that accompanies wavebreaking, leading to an increase in the longitudinal electric field on the axis.

This work was supported by the Russian Science Foundation, Project No. 23-12-00028.

plasma wakefield acceleration,transformer ratio,ion motion