# Restoring betatron phase coherence in a beam-loaded laser-wakefield accelerator

## TL;DR

This paper demonstrates that controlling injected charge and acceleration gradients in laser wakefield acceleration reduces energy spread and beam divergence, enhancing phase space density by restoring betatron phase coherence.

## Contribution

It introduces a method to independently control injection and acceleration to minimize energy spread and chromatic decoherence in LWFA.

## Key findings

- Reduced energy spread with controlled injection and gradients
- Decreased beam divergence observed
- Constant beam radius confirmed via betatron radiation spectroscopy

## Abstract

Matched beam loading in laser wakefield acceleration (LWFA), characterizing the state of flattening of the acceleration electric field along the bunch, leads to the minimization of energy spread at high bunch charges. Here, we demonstrate by independently controlling injected charge and acceleration gradients, using the self-truncated ionization injection scheme, that minimal energy spread coincides with a reduction of the normalized beam divergence. With the simultaneous confirmation of a constant beam radius at the plasma exit, deduced from betatron radiation spectroscopy, we attribute this effect to the reduction of chromatic betatron decoherence. Thus, beam loaded LWFA enables highest longitudinal and transverse phase space densities.