Kinetic characterization of photoionized plasma evolution from FEL pulse interaction with gas jet

The characterization of a photoionized plasma created through the interaction of a short, 50-100 fs, linearly polarized X-ray pulse with an He gas jet is presented by using Monte Carlo (MC) simulations. A kinetic description, which takes into account electron production due to photo-effect, elastic electron scattering on atoms, ionization and excitation of atoms by the secondary electrons, and electron transport across the X-ray beam, is introduced to study relaxation of the electron distribution function (EDF) and evolution of the electron density and mean energy. It is shown that initially an anisotropic monoenergetic EDF forms. Then it relaxes to a monoenergetic isotropic EDF for the case of low-energy X-ray quanta or to a quasi-multi-monoenergetic isotropic EDF for a pulse of high-energy X-ray quanta. This nonequilibrium electron energy distribution remains long after the X-ray pulse terminates and disappears on a ps-time-scale. The electron density distribution in the plane across the X-ray beam is characterized by considerable asymmetry along and across the polarization direction even after the vanishing of the electron energy anisotropy. The results obtained are discussed in the context of future design of experiments on self-Thomson scattering and Thomson scattering of a probe laser beam in a plasma produced by femtosecond FEL pulses.