Self-magnetization of CO₂-produced plasmas by electron Weibel instability

Weibel-type instability can self-generate and amplify magnetic fields in plasmas with anisotropic velocity distribution. Thermal Weibel instability driven by temperature anisotropy of a stationary plasma, as originally proposed by E. S. Weibel, has proven challenging to measure because of the difficulty in preparing such a distribution. Here we show that by using an ultrashort but intense CO₂ laser to ionize hydrogen gas, one can prepare a plasma with tri-Maxwellian velocity distribution that is perfectly suitable for studying thermal electron Weibel instability. The onset, growth and damping of the magnetic fields are captured by a picosecond-long, relativistic electron probe bunch from a linear accelerator. We find that the magnetic fields start growing with a broad two-dimensional wavenumber spectrum, but as the instability grows, the spectrum shrinks to a quasi-single mode in both directions perpendicular to the probe direction. The k-resolved growth rates of the instability deduced agree with kinetic theory. It is also observed that Weibel instability amplifies the magnetic fields and converts up to ~1% of the plasma thermal energy into magnetic energy, which supports the hypothesis of spontaneous magnetization of collisionless astrophysical plasmas by Weibel instability.