H. Reinholz

Thomson scattering from near-solid density plasmas using soft x-ray free electron lasers

We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode.

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.

Reflectivity in shock wave fronts of xenon

New results for the reflection coefficient of shock-compressed dense xenon plasmas at pressures of 1.6–20 GPa and temperatures around 30 000 K are interpreted. Reflectivities typical of metallic systems are found at high densities. A consistent description of the measured reflectivities is achieved if a finite width of the shock wave front is considered. Several mechanisms to give a microscopic explanation for a finite extension of the shock front are discussed.

Using the Gould–DeWitt scheme to approximate the dynamic collision frequency in a dense electron gas

In order to describe dielectric properties in dense plasmas, a consistent calculation of the collision frequency is required. We present new calculations for an electron gas at parameters which are relevant for warm dense matter. In particular, we focus on the influence of the different approximations for the collision frequency in the Gould–DeWitt scheme. We use the dynamic collision frequency in the Born, Lenard–Balescu and ladder approximation. The inclusion of collisions in a consistent manner modifies, e.g., the dielectric function significantly in the warm dense matter regime.

Quantum-statistical T-matrix approach to line broadening of hydrogen in dense plasmas

The electronic self‐energy ∑e is an important input in a quantum‐statistical theory for spectral line profile calculations. It describes the influence of plasma electrons on bound state properties. In dense plasmas, the effect of strong, i.e. close, electron‐emitter collisions can be considered by three‐particle T‐matrix diagrams. These digrams are approximated with the help of an effective two‐particle T‐matrix, which is obtained from convergent close‐coupling calculations with Debye screening. A comparison with other theories is carried out for the 2p level of hydrogen at kBTu2009=u20091u2009eV and neu2009=u20092⋅1023u2009m−3, and results are given for neu2009=u20091⋅1025u2009m−3.

K-line emission profiles with focus on the self-consistent calculation of plasma polarization

The emitted K α -spectra of moderately ionized titanium radiators in a medium are used to determine plasma temperature and composition in electron heated target regions. A theoretical treatment of spectral line profiles using self-consistent Hartree-Fock and ion sphere model calculations to determine the influence of plasma polarization is applied. We confirm the importance of excited emitter states for line shape modeling.

OPTICAL AND TRANSPORT PROPERTIES IN DENSE PLASMAS COLLISION FREQUENCY FROM BULK TO CLUSTER

The dielectric function of dense plasmas is treated within a many-particle linear response theory beyond the RPA. In the long-wavelength limit, the dynamical collision frequency can be introduced which is expressed in terms of momentum and force auto-correlation functions (ACF). Analytical expressions for the collision frequency are considered for bulk plasmas, and reasonable agreement with MD simulations is found. Different applications such as Thomson scattering, reflectivity, electric and magnetic transport properties are discussed. In particular, experimental results for the static conductivity of inert gas plasmas are now well described. The transition from bulk properties to finite cluster properties is of particular interest. Within semiclassical MD simulations, single-time characteristics as well as two-time correlation functions are evaluated and analyzed. In particular, the Laplace transform of current and force ACFs show typical structures which are interpreted as collective modes of the microplasma. The damping rates of these modes are size dependent. They increase for the transition from small clusters to bulk plasmas.

CORRELATIONS, COLLISION FREQUENCY AND OPTICAL PROPERTIES IN LASER EXCITED CLUSTERS

Within linear response theory, the dielectric and optical properties of a charged particle system are related to equilibrium correlation functions. In particular, the dynamical conductivity and the dynamical collision frequency are expressed in terms of the current-current or force-force correlation function, which can be evaluated analytically using perturbation expansions or numerically by MD simulations. Results are given for bulk material. Furthermore, finite systems such as laser excited clusters are considered. It is shown that the collision frequency is reduced in finite systems. The interaction with the laser field is discussed with respect to the current-current correlation function which changes with time due to the expansion of the laser-irradiated cluster.

Soft X-Ray Thomson scattering in warm dense hydrogen at FLASH

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.

Soft X-Ray Thomson Scattering in Warm Dense Matter at FLASH

We present the attempt to diagnose electron temperature and density of a plasma via Thomson Scattering in the Warm Dense Matter Regimew using soft x-ray Free Electron Laser radiation. A preliminary Self Thomson Scattering experiment has already been conducted. In a current pump-probe experiment, together with an optical heating laser, we will record the temporal evolution of the plasma achieving a resolution of approximately 250fs.