# Experimental observation of a current-driven instability in a neutral electron-positron beam

## TL;DR

This paper reports the first experimental detection of a current-driven instability in a neutral electron-positron beam, revealing strong magnetic fields that persist and are relevant to astrophysical phenomena like gamma-ray bursts.

## Contribution

It provides the first experimental evidence of a current-driven instability in a neutral electron-positron beam, supported by simulations and analytical estimates.

## Key findings

- Detection of magnetic fields ≥ 1 T after beam propagation
- Equipartition parameter ~10^{-3} consistent with astrophysical models
- Magnetic fields persist for thousands of inverse plasma frequencies

## Abstract

We report on the first experimental observation of a current-driven instability developing in a quasi-neutral matter-antimatter beam. Strong magnetic fields ($\geq$ 1 T) are measured, via means of a proton radiography technique, after the propagation of a neutral electron-positron beam through a background electron-ion plasma.The experimentally determined equipartition parameter of $\epsilon_B \approx 10^{-3}$, is typical of values inferred from models of astrophysical gamma-ray bursts, in which the relativistic flows are also expected to be pair dominated. The data, supported by Particle-In-Cell simulations and simple analytical estimates, indicate that these magnetic fields persist in the background plasma for thousands of inverse plasma frequencies. The existence of such long-lived magnetic fields can be related to analog astrophysical systems, such as those prevalent in lepton-dominated jets.