Optical-field-ionized plasma: a new platform for testing the kinetic theory of plasma instabilities*

With the capability of precisely initializing anisotropic velocity distributions, an optical-field-ionized plasma may be used as a new platform for testing the kinetic theory of plasma instabilities. Here we show that a high-density helium plasma ionized by a circularly polarized laser consists of two pairs of radially counter-streaming beams due to the different ionization potential of the two helium electrons, therefore is unstable to streaming instability and filamentation instability. These instabilities grow on a sub-picosecond (ps) timescale and saturate after ~1 ps as a result of the transverse (perpendicular to the laser propagation direction) phase space diffusion which ultimately terminates the counter streaming of different electron species. The resultant distribution still has a large temperature anisotropy because the plasma remains much colder in the longitudinal direction, which in turn drives a Weibel instability on a ps timescale to further isotropize the plasma. The oscillation frequency and growth rate of these instabilities are measured using Thomson scattering of a probe beam. Particle-in-cell simulations and kinetic theory show good agreement with experiments.