# Probing Ultrafast Magnetic-Field Generation by Current Filamentation Instability in Femtosecond Relativistic Laser-Matter Interactions

## TL;DR

This study demonstrates the ultrafast generation of strong magnetic fields during femtosecond laser interactions with solid targets, using relativistic electron probes and simulations to reveal their origin from a Weibel instability.

## Contribution

It provides the first experimental measurement of femtosecond-scale magnetic fields generated by current filamentation instability in laser-matter interactions.

## Key findings

- Measured a line-integrated magnetic field of 2.70 ± 0.39 kT μm.
- Simulations show the fields originate from a Weibel-type filamentation instability.
- Fields grow rapidly, affecting electron beam angular distribution.

## Abstract

We present experimental measurements of the femtosecond time-scale generation of strong magnetic-field fluctuations during the interaction of ultrashort, moderately relativistic laser pulses with solid targets. These fields were probed using low-emittance, highly relativistic electron bunches from a laser wakefield accelerator, and a line-integrated $B$-field of $2.70 \pm 0.39\,\rm kT\,\mu m$ was measured. Three-dimensional, fully relativistic particle-in-cell simulations indicate that such fluctuations originate from a Weibel-type current filamentation instability developing at submicron scales around the irradiated target surface, and that they grow to amplitudes strong enough to broaden the angular distribution of the probe electron bunch a few tens of femtoseconds after the laser pulse maximum. Our results highlight the potential of wakefield-accelerated electron beams for ultrafast probing of relativistic laser-driven phenomena.