Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s→2p transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.
Modelling plasma-based seeded soft X-ray lasers from the creation of the plasma to the propagation of a femtosecond high-order harmonic (HOH) seed throughout several millimetres of inhomogeneous plasma is a complex challenge. Different spatio-temporal scales from the hydrodynamic evolution of the plasma (hundreds of micrometres and nanoseconds) to the propagation of pulses through the plasma (millimetres and tens of picoseconds), electron collisions (picoseconds or even shorter) and the evolution of the envelope of the seeded HOH (tens of femtoseconds) must be tackled in order to fully understand these systems. In this paper, we will present the multi-scale computational paradigm that we have used to perform a full ab initio simulation of a dense, Ni-like Krypton plasma amplifier of soft X-rays. Results of the modelling and expected future applications will also be shown.
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