Energy relaxation in dense, strongly coupled two-temperature plasmas

A quantum kinetic approach for the energy relaxation in strongly coupled plasmas with different electron and ion temperatures is presented. Based on the density operator formalism, we derive a balance equation for the energies of electrons and ions connecting kinetic, correlation, and exchange energies with a quite general expression for the electron-ion energy-transfer rate. The latter is given in terms of the correlation function of density fluctuations which allows for a derivation of increasingly realistic approximation schemes including a coupled-mode expression. The equilibration of the contributions of the total energy including the species temperatures in dense hydrogen and beryllium relevant for inertial confinement fusion is investigated as an example

Energy relaxation study for warm dense matter experiments

We present a quantum kinetic description of temperature relaxation in warm dense matter. Starting from a general balance equation, we show how kinetic, correlation, and exchange energies of electrons and ions are exchanged during the equilibration towards equilibrium. Different approximations for the electron-ion energy transfer rates are discussed. The approach is finally applied to heated solids and shock-compressed matter as investigated in recent warm dense matter experiments to estimate the minimum time needed between pump and probe pulses when investigating equilibrium properties. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Energy and temperature relaxation described by nonequilibrium green's functions

A quantum kinetic approach is presented to investigate the energy relaxation of dense strongly coupled two-temperature plasmas. We derive a balance equation for the mean total energy of a plasma species including a quite general expression for the transfer rate. An approximation scheme is used leading to an expression of the transfer rates for systems with coupled modes relevant for the warm dense matter regime. The theory is then applied to dense beryllium plasmas under conditions such as realized in recent experiments. Special attention is paid to the influence of correlation and quantum effects on the relaxation process

Equilibration dynamics and conductivity of warm dense hydrogen

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility(FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a < 300-fs short x-rayburst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variabletime delay. We show that the initial molecular structure dissociates within (0.9 ± 0.2) ps, allowing us to inferthe energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization modelsin radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculatethe static structure factor. A conductivity model for partially ionized plasma is validated by two-temperaturedensity-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Ourresults provide important insights and the needed experimental data on transport properties of dense plasmas

Probing near-solid density plasmas using soft x-ray scattering

X-ray scattering using highly brilliant x-ray free-electron laser (FEL) radiation provides new access to probe free-electron density, temperature and ionization in near-solid density plasmas. First experiments at the soft x-ray FEL FLASH at DESY, Hamburg, show the capabilities of this technique. The ultrashort FEL pulses in particular can probe equilibration phenomena occurring after excitation of the plasma using ultrashort optical laser pumping. We have investigated liquid hydrogen and find that the interaction of very intense soft x-ray FEL radiation alone heats the sample volume. As the plasma establishes, photons from the same pulse undergo scattering, thus probing the transient, warm dense matter state. We find a free-electron density of (2.6 ± 0.2) × 1020 cm-3 and an electron temperature of 14 ± 3.5 eV. In pump-probe experiments, using intense optical laser pulses to generate more extreme states of matter, this interaction of the probe pulse has to be considered in the interpretation of scattering data. In this paper, we present details of the experimental setup at FLASH and the diagnostic methods used to quantitatively analyse the data. © 2010 IOP Publishing Ltd

Turning solid aluminium transparent by intense soft X-ray photoionization

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm 2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. © 2009 Macmillan Publishers Limited. All rights reserved