Description
Electron beams from a wakefield accelerator (LWFA) are typically assumed to be short, comparable to the plasma wavelength. Under certain conditions, as the electrons start to exit the accelerator, the plasma wavelength increases and the electrons are moved into the decelerating phase of the wakefield. At this point, the electrons start to lose energy, and a fraction of them are separated and lost from the beam, a process we call as "electron shedding."
Using femtosecond relativistic electron microscopy, we imaged the evolution in the longitudinal current of the beam as it exits the accelerator. It was seen that an initially short electron beam lengthens to > 40 times the plasma period in the accelerating stage before losing electrons on the way.
Electron shedding results from a significant redistribution of energy in the beam which results in a lower useful charge and contributes to the formation of dark current. Simulations predict nearly 20 % of the beam energy can be lost at across donwramp reducing the efficiency of an LWFA and limiting the available energy for subsequent beam-driven wakefield acceleration stages.