Nanoscale femtosecond imaging of transient hot solid density plasmas with elemental and charge state sensitivity using resonant coherent diffraction

Abstract

Here we propose to exploit the low energy bandwidth, small wavelength and penetration power of ultrashort pulses from XFELs for resonant Small Angle Scattering (SAXS) on plasma structures in laser excited plasmas. Small angle scattering allows to detect nanoscale density fluctuations in forward scattering direction. Typically, the SAXS signal from laser excited plasmas is expected to be dominated by the free electron distribution. We propose that the ionic scattering signal becomes visible when the X-ray energy is in resonance with an electron transition between two bound states (Resonant coherent X-ray diffraction, RCXD). In this case the scattering cross-section dramatically increases so that the signal of X-ray scattering from ions silhouettes against the free electron scattering background which allows to measure the opacity and derived quantities with high spatial and temporal resolution, being fundamentally limited only by the X-ray wavelength and timing. Deriving quantities such as ion spatial distribution, charge state distribution and plasma temperature with such high spatial and temporal resolution will make a vast number of processes in shortpulse laser-solid interaction accessible for direct experimental observation e.g. hole-boring and shock propagation, filamentation and instability dynamics, electron transport, heating and ultrafast ionization dynamics.

Building an Optical Free-Electron Laser in the Traveling-Wave Thomson-Scattering Geometry

“…The shorter wavelength of an Ångström TWTS OFEL allows to penetrate through thicker samples and resolve smaller structures while providing femtosecond scale temporal resolution due to its short pulse duration following from the requirement of kiloampere currents. This allows for example to determine the structure of biological specimen which can not be grown in large crystals [78] or to analyze the ultrafast dynamics of explosively driven materials at nanometer length scales [79,80]. This example gives an outlook on the compactness of hard Xray OFEL setups achievable with TWTS.…”

Simulations of ultrafast x–ray laser experiments

“…39 They can create plasma instabilities, 40 ionize and heat the target bulk, 41 generate strong magnetic fields, or drive shocks inside the target. These phenomena can potentially be studied with high spatial resolution of a few nanometers and temporal resolutions of a few femtoseconds using x-ray lasers 42,43 We briefly outline an experiment that we plan to simulate with the tools described above. The experiment would be carried out at the HED instrument at the European XFEL.…”

Nanometer-scale characterization of laser-driven compression, shocks, and phase transitions, by x-ray scattering using free electron lasers

“…When the Fourier SAXS and the resonant variant RCXD promise a wide range of plasma properties to become accessible with nanometer and femtosecond resolution. This includes the mode structure of the opacity, ion distribution and -with phase reconstruction techniques reconstructing the real and imaginary part of the optical corrections independently -also the plasma electron energy distribution [57] since the ratio depends on the energy distribution. RCXD offers the prospect of direct in situ experimental tests of ionization models and ion dynamics in kinetic plasma codes, or analytic approximations.…”

“…In this paper we expand upon the initial concepts presented in Refs [56], [57], and demonstrate the feasibility of performing SAXS on high-power laser-driven samples to extract nm-scale information, in two experiments performed at LCLS. In the following sections, we survey some of the laser-plasma processes of specific interest, we review the relevant scattering theory in plasmas, we elucidate some specific observables found in the X-ray scattering distribution, and we present the application of SAXS to two examples, namely short pulse laser ablation and long-pulse laser compression.…”

“…A great deal of information then can already be extracted directly from the X-ray scattering data, when characteristic features can be identified in reciprocal space with predictive plasma simulation and modeling. As shown in Refs [56], [57] these can include processes such as plasma ablation and laser hole-boring, surface instabilities, electron transport instabilities, filamentation, and ionization dynamics, all of which occur on the spatial scale of micron to few nm in relativistic laser-matter interactions. Nanometer scale phenomena are also important also in non-relativistic laser experiments such as shock compression and high pressure phase transitions.…”