Beam current from downramp injection in electron-driven plasma wakefields

TL;DR

This paper investigates the stability and properties of density-downramp injection in electron-driven plasma wakefield accelerators, revealing key dependencies of injected current on driver and plasma parameters for optimizing acceleration efficiency.

Contribution

It demonstrates that the injected electron bunch current primarily depends on a combined driver-plasma parameter, independent of downramp length, enhancing understanding of injection control.

Findings

Injected current depends mainly on a combined driver and plasma parameter.

The stability of the wake remains until driver electrons reach near-zero energy.

Injected current is unaffected by the downramp length if trapping conditions are met.

Abstract

We study the stability of plasma wake wave and the properties of density-downramp injection in an electron-driven plasma accelerator. In this accelerator type, a short high-current electron bunch (generated by a conventional accelerator or a laser-wakefield acceleration stage) drives a strongly nonlinear plasma wake wave (blowout), and accelerated electrons are injected into it using a sharp density transition which leads to the elongation of the wake. The accelerating structure remains highly stable until the moment some electrons of the driver reach almost zero energy, which corresponds to the best interaction length for optimal driver-to-plasma energy transfer efficiency. For a particular driver, this efficiency can be optimized by choosing appropriate plasma density. Studying the dependence of the current of the injected bunch on driver and plasma parameters, we show that it does not depend on the density downramp length as long as the condition for trapping is satisfied. Most importantly, we find that the current of the injected bunch primarily depends on just one parameter which combines both the properties of the driver (its current and duration) and the plasma density.