Thermalization of Optical-Field Ionized Helium Plasmas*

We experimentally study the thermalization process of a non-Maxwellian helium plasma produced by a femtosecond laser pulse. Such a unique plasma has drawn theoretical and experimental attentions for its role in producing hot and cold plasmas and its tendency to induce high level of instabilities and plasma turbulences. However, the role of plasma instabilities in thermalization process has never been experimentally addressed. In this experiment, we utilized various Thomson scattering techniques and geometries focusing on different evolution stages: (1) Polarization-dependent initial distributions have been inferred from collinear Thomson scattering of a second-harmonic probing pulse generated from a KDP crystal. (2) A 90-degree Thomson scattering system with changeable wavelengths has been set up to probe plasma modes induced by streaming and filament instabilities due the strongly anisotropic initial plasma distribution. (3) Time-resolved scattering spectra have been fitted using the theory of Thomson scattering for Maxwellian plasmas. The combined results show the thermalization process of an OFI helium plasma, which involves the development of plasma instabilities followed by electron-electron collisions.