Mapping the self-generated magnetic fields due to thermal Weibel instability

TL;DR

This study experimentally investigates the electron Weibel instability in plasma, revealing how magnetic fields evolve and self-organize, converting a small fraction of thermal energy into magnetic energy, and validating kinetic theory predictions.

Contribution

First direct measurement of electron Weibel instability evolution with picosecond resolution, demonstrating magnetic field spectrum narrowing and energy conversion in plasma.

Findings

Magnetic field spectra narrow to a dominant mode over time.

Growth rates match kinetic theory predictions.

Up to 1% of thermal energy converts into magnetic energy.

Abstract

Weibel-type instability can self-generate and amplify magnetic fields in both space and laboratory plasmas with temperature anisotropy. The electron Weibel instability has generally proven more challenging to measure than its ion counterpart owing to the much smaller inertia of electrons, resulting in a faster growth rate and smaller characteristic wavelength. Here, we have probed the evolution of the two-dimensional distribution of the magnetic field components and the current density due to electron Weibel instability, in $\rm CO_2$-ionized hydrogen gas (plasma) with picosecond resolution using a relativistic electron beam. We find that the wavenumber spectra of the magnetic fields are initially broad but eventually shrink to a narrow spectrum representing the dominant quasi-single mode. The measured $k$-resolved growth rates of the instability validate kinetic theory. Concurrently, self-organization of microscopic plasma currents is observed to amplify the current modulation magnitude that converts up to $\sim 1\%$ of the plasma thermal energy into magnetic energy.