Initializing anisotropic electron velocity distribution functions in optical-field ionized plasmas

Optical-field ionization (OFI) is often used to produce plasmas for myriad laboratory applications. OFI of gases is known to generate electrons that have anisotropic distributions. In this paper we show that at the time of plasma formation the electron velocity distribution functions can be controlled by changing the wavelength and polarization of the pump laser and ionization state of the plasma. Thomson scattering is used to measure the distinct electron velocity distributions of helium plasmas produced by linearly and circularly polarized laser pulses within a few inverse plasma periods after the plasma formation. In both cases, non-thermal and highly anisotropic initial electron distributions are observed that are consistent with expectations from OFI of both He electrons.