Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

TL;DR

This paper proposes smooth current profiles for electron bunches to improve transformer ratios in beam-driven wakefield acceleration, using laser shaping techniques and linac designs for practical implementation.

Contribution

It introduces novel smooth current distributions that enhance transformer ratios and details a laser-shaping method for their generation at photoinjectors.

Findings

Smooth current profiles can significantly increase transformer ratios.

Laser shaping can produce the proposed distributions directly from a photoinjector.

Potential for practical implementation in dielectric accelerators.

Abstract

Collinear high-gradient ${\cal O} (GV/m)$ beam-driven wakefield methods for charged-particle acceleration could be critical to the realization of compact, cost-efficient, accelerators, e.g., in support of TeV-scale lepton colliders or multiple-user free-electron laser facilities. To make these options viable, the high accelerating fields need to be complemented with large transformer ratios $>2$, a parameter characterizing the efficiency of the energy transfer between a wakefield-exciting "drive" bunch to an accelerated "witness" bunch. While several potential current distributions have been discussed, their practical realization appears challenging due to their often discontinuous nature. In this paper we propose several alternative current profiles which are smooth which also lead to enhanced transformer ratios. We especially explore a laser-shaping method capable of generating one the suggested distributions directly out of a photoinjector and discuss a linac concept that could possible drive a dielectric accelerator.