Resolving Ultrafast Heating of Dense Cryogenic Hydrogen (original) (raw)
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Frontiers in Physics
Angularly resolved X-ray scattering measurements from fs-laser heated hydrogen have been used to determine the equilibration of electron and ion temperatures in the warm dense matter regime. The relaxation of rapidly heated cryogenic hydrogen is visualized using 5.5 keV X-ray pulses from the Linac Coherent Light (LCLS) source in a 1 Hz repetition rate pump-probe setting. We demonstrate that the electron-ion energy transfer is faster than quasi-classical Landau-Spitzer models that use ad hoc cutoffs in the Coulomb logarithm.
Equilibration dynamics and conductivity of warm dense hydrogen
Phys. Rev. E, 2014
We performed the first experiment on warm dense hydrogen with sub-picosecond time resolution at the XUV freeelectron laser facility (FLASH) at DESY (Hamburg). Ultra-fast impulsive electron heating is initiated by a ≤ 300 fs short x-ray burst of 92 eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial molecular structure dissociates within 0.9 ps, allowing to infer the energy transfer rate between electron and ions. We evaluate Saha and Thomas-Fermi ionization models in the equation of state used in radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculate the static structure factor. Hydrodynamics simulations based on a conductivity model for partially ionized plasma are validated by two-temperature density functional theory coupled to molecular dynamics simulations, and agree with the experimental data. Our results provide important insights and the needed experimental data on transport properties of dense plasmas.
Transient ionization potential depression in nonthermal dense plasmas at high x-ray intensity
Physical review, 2021
The advent of x-ray free-electron lasers (XFELs), which provide intense ultrashort x-ray pulses, has brought a new way of creating and analyzing hot and warm dense plasmas in the laboratory. Because of the ultrashort pulse duration, the XFEL-produced plasma will be out of equilibrium at the beginning and even the electronic subsystem may not reach thermal equilibrium while interacting with a femtosecond timescale pulse. In the dense plasma, the ionization potential depression (IPD) induced by the plasma environment plays a crucial role for understanding and modeling microscopic dynamical processes. However, all theoretical approaches for IPD have been based on local thermal equilibrium (LTE) and it has been controversial to use LTE IPD models for the nonthermal situation. In this work, we propose a non-LTE (NLTE) approach to calculate the IPD effect by combining a quantum-mechanical electronicstructure calculation and a classical molecular dynamics simulation. This hybrid approach enables us to investigate the time evolution of ionization potentials and IPDs during and after the interaction with XFEL pulses, without the limitation of the LTE assumption. In our NLTE approach, the transient IPD values are presented as distributions evolving with time, which cannot be captured by conventional LTE-based models. The time-integrated ionization potential values are in good agreement with benchmark experimental data on solid-density aluminum plasma and other theoretical predictions based on LTE. The present work is promising to provide critical insights into nonequilibrium dynamics of dense plasma formation and thermalization induced by XFEL pulses.
Observation of Ultrafast Nonequilibrium Collective Dynamics in Warm Dense Hydrogen
Physical Review Letters, 2010
We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2:8 Â 10 20 cm À3. For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions.
A broadband laser plasma x-ray source for application in ultrafast chemical structure dynamics
A plasma source free from characteristic emission lines is described, based on laser irradiation of a water jet in a helium atmosphere. Various key aspects of the laser interaction are presented along with practical characterization of the observed isotropic ϳ4-10 keV x-ray emissions, measurements of which indicate subpicosecond duration. Observations are consistent with a vacuum heating plasma mechanism at the helium-water interface and indicate strong potential for in-house ultrafast chemical structure dynamics application when coupled to contemporary detector developments.
Soft X-Ray Thomson scattering in warm dense hydrogen at FLASH
Proceedings of SPIE - The International Society for Optical Engineering, 2009
We present collective Thomson scattering with soft x-ray free electron laser radiation as a method to track the evolution of warm dense matter plasmas with ~200 fs time resolution. In a pump-probe scheme an 800 nm laser heats a 20 μm hydrogen droplet to the plasma state. After a variable time delay in the order of ps the plasma is probed by an x-ray ultra violet (XUV) pulse which scatters from the target and is recorded spectrally. Alternatively, in a self-Thomson scattering experiment, a single XUV pulse heats the target while a portion of its photons are being scattered probing the target. From such inelastic x-ray scattering spectra free electron temperature and density can be inferred giving insight on relaxation time scales in plasmas as well as the equation of state. We prove the feasibility of this method in the XUV range utilizing the free electron laser facility in Hamburg, FLASH. We recorded Thomson scattering spectra for hydrogen plasma, both in the self-scattering and in the pump-probe mode using optical laser heating. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/11/2013 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 7451 74510D-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/11/2013 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 7451 74510D-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/11/2013 Terms of Use: http://spiedl.org/terms
Probing ultrafast laser plasma processes inside solids with resonant small angle X-ray scattering
arXiv: Plasma Physics, 2021
Extreme states of matter exist throughout the universe e.g. inside planetary cores, stars or astrophysical jets. Such conditions are generated in the laboratory in the interaction of powerful lasers with solids, and their evolution can be probed with femtosecond precision using ultra-short X-ray pulses to study laboratory astrophysics, laser-fusion research or compact particle acceleration. X-ray scattering (SAXS) patterns and their asymmetries occurring at X-ray energies of atomic bound-bound transitions contain information on the volumetric nanoscopic distribution of density, ionization and temperature. Buried heavy ion structures in high intensity laser irradiated solids expand on the nanometer scale following heat diffusion, and are heated to more than 2 million Kelvin. These experiments demonstrate resonant SAXS with the aim to better characterize dynamic processes in extreme laboratory plasmas.
Time-resolved x-ray emission spectra from optically ionized helium and neon plasmas
Physical Review E, 1998
The interaction of high-power, subpicosecond laser pulses with gas targets is expected to produce highly nonequilibrium plasmas whose parameters are controlled by the laser wavelength and polarization. We investigate such plasmas by measuring time-resolved x-ray-emission spectra in highly ionized helium and neon plasmas produced by high-power optical ionization. Electron temperatures are observed to increase with increasing laser wavelength and with variation of the laser polarization from linear to circular. These results are in qualitative agreement with current models for production of tunnel-ionized laser plasmas. Limited quantitative agreement, however, reflects the complexity of the optical ionization process and suggests the important role rapid cooling processes can play in these plasmas. Emission spectra are combined with time-dependent kinetic simulations to assess prospects for x-ray lasers pumped by rapid electron-ion recombination. ͓S1063-651X͑97͒10812-1͔