Ultrafast optical field–ionized gases—A laboratory platform for studying kinetic plasma instabilities

Abstract

Kinetic instabilities arising from anisotropic electron velocity distributions are ubiquitous in ionospheric, cosmic, and terrestrial plasmas, yet there are only a handful of experiments that purport to validate their theory. It is known that optical field ionization of atoms using ultrashort laser pulses can generate plasmas with known anisotropic electron velocity distributions. Here, we show that following the ionization but before collisions thermalize the electrons, the plasma undergoes two-stream, filamentation, and Weibel instabilities that isotropize the electron distributions. The polarization-dependent frequency and growth rates of these kinetic instabilities, measured using Thomson scattering of a probe laser, agree well with the kinetic theory and simulations. Thus, we have demonstrated an easily deployable laboratory platform for studying kinetic instabilities in plasmas.

Probing ultrafast magnetic-field generation by current filamentation instability in femtosecond relativistic laser-matter interactions

“…The Weibel-type current filamentation instability (CFI) [1,2] has been extensively investigated in past decades owing to its recognized importance in an increasing variety of plasma environments. Induced by temperature anisotropies or relative drifts between the plasma constituents [3][4][5][6], it gives rise to kinetic-scale, current filaments surrounded by toroidal magnetic fields, through which the charged particles are progressively isotropized [3,7,8]. This instability is widely thought to underpin the physics of relativistic outflows in powerful astrophysical objects (e.g., gamma-ray bursts, pulsar winds, active galactic nuclei), especially as the source of the collisionless shock waves held responsible for generating nonthermal high-energy particles and radiations [9][10][11][12][13].…”

Measurements of the growth and saturation of electron Weibel instability in optical-field ionized plasmas

“…This suggests that the plasma anisotropy plasma has dropped to a small value (A < 1) when the signal become detectable in the experiment if one assumes k ∼ k m . As previously mentioned, such a rapid drop is attributed to precursor instabilities such as streaming and current filamentation instabilities as well as collisions [37].…”

“…In addition to the large anisotropy, the initial transverse EVD of the OFI helium plasma consists of two concentric rings in the momentum space, as shown and measured in a recent experiment [36]. In such a plasma there follows a hierarchy of kinetic instabilities that begins with largely electrostatic two-stream and the oblique current filamentation instabilities which have been measured with 100 fs resolution using Thomson scattering [37]. These instabilities not only reduce the plasma anisotropy rapidly from initially > 100 to ∼ 10 in just one ps but also lead to approximately bi-Maxwellian plasma electrons with T ⊥ ≈ 500 eV and T ≈ 40 eV as observed in previous PIC simulations [38].…”

Conservation of angular momentum in second harmonic generation from under-dense plasmas

“…Axial magnetic field can depolarize the radiated 2𝜔 emission via the Faraday effect. Second, laser-ionized plasmas are known to have non-thermal electron velocity distributions and are susceptible to plasma kinetic instabilities [39,40]. The axial helicoidal magnetic field induced by electron filamentation/Weibel instability can cause depolarization of the 2𝜔 radiation and plasma emission.…”