Measurement of Magnetic-Field Structures in a Laser-Wakefield Accelerator

TL;DR

This paper reports experimental measurements of multi-megagauss magnetic fields in a laser-wakefield accelerator, revealing their rapid generation, correlation with electron production, and providing insights into wakefield evolution.

Contribution

First direct measurement of magnetic fields in a laser-wakefield accelerator using Faraday rotation, linking field growth to plasma wave dynamics.

Findings

Magnetic fields reach multi-megagauss levels

Fields are confined transversely to plasma wavelength

Magnetic field appearance correlates with relativistic electron production

Abstract

Experimental measurements of magnetic fields generated in the cavity of a self-injecting laser-wakefield accelerator are presented. Faraday rotation is used to determine the existence of multi-megagauss fields, constrained to a transverse dimension comparable to the plasma wavelength and several plasma wavelengths longitudinally. The fields are generated rapidly and move with the driving laser. In our experiment, the appearance of the magnetic fields is correlated to the production of relativistic electrons, indicating that they are inherently tied to the growth and wavebreaking of the nonlinear plasma wave. This evolution is confirmed by numerical simulations, showing that these measurements provide insight into the wakefield evolution with high spatial and temporal resolution.