# Controlling injection using a magnetic field to produce sub-femtosecond bunches in the laser wakefield accelerator

## TL;DR

This paper demonstrates that applying an external magnetic field can precisely control electron injection in laser wakefield accelerators, enabling the production of ultra-short, sub-femtosecond electron bunches with high peak currents.

## Contribution

The study introduces a novel method of using a magnetic field to manipulate electron injection timing and duration in plasma wakefield acceleration.

## Key findings

- Magnetic field suppresses electron injection by creating a density hole.
- Controlled injection allows generation of sub-femtosecond electron bunches.
- Peak currents of a few kilo-Amperes achieved with ~10 Tesla magnetic field.

## Abstract

It is shown that electron injection into a laser-driven plasma bubble can be manipulated by applying an external magnetic field in the presence of a plasma density gradient. The down-ramp of the density-tailored plasma locally reduces the plasma wave phase velocity, which triggers injection. The longitudinal magnetic field dynamically induces an expanding hole in the electron density distribution at the rear of the wake bubble, which reduces the peak electron velocity in its vicinity. Electron injection is suppressed when the electron velocity drops below the phase velocity, which depends on the size of the density hole. This enables the start and end of electron injection to be independently controlled, which allows generation of sub-femtosecond electron bunches with peak currents of a few kilo-Ampere, for an applied magnetic field of $\sim 10$ Tesla.