[en] The authors propose to investigate the dynamics of plasmas in the warm dense matter (WDM) regime on ultra-short time scales. Accessible plasma conditions are in the density range of n = 10²⁰ - 10²³ cm^-3 and at moderate temperatures of T = 1 - 20 eV. These plasmas are of importance for laboratory astrophysics, high energy density science and inertial confinement fusion. They are characterized by a coupling parameter of Λ ∼> 1, where electromagnetic interactions are of the same order as the kinetic energy. The high density of the plasma makes it opaque to radiation in the visible range and, as a consequence, UV up to x-ray radiation can be used to probe such systems. Therefore a wide range in the temperature-density plane of WDM is presently unexplored and only the VUV-FEL opens for the first time the opportunity for its detailed investigation. In equilibrium, the macroscopic state of the plasma is completely characterized by its density and temperature. In pump-probe experiments however, the plasma is initially in a nonequilibrium state and relaxes towards equilibrium within the relaxation time τ_R. For t > τ_R, the plasma is in an equilibrium state and expands hydrodynamically on a time scale τ_H. The proposed experiment measures the time-resolved Thomson scattering signal with the VUV-FEL radiation characterizing the plasma in equilibrium and nonequilibrium states. Both regimes are extremely interesting and will provide new insight into the following phenomena: (1) details of nonequilibrium correlations, (2) relaxation phenomena, (3) hydrodynamic expansion, (4) recombination kinetics. The time-resolved Thomson scattering signal is obtained in a pump-probe experiment by varying the delay between pump and probe. The final stage of the relaxation process (t ∼ τ_R) is of special interest since the plasma components (electrons and ion species) can be assumed to be in quasi-equilibrium. This allows for accurate measurements of the electron temperature using the detailed balance relation. For times t ∼< τ_R the scattering spectrum provides also the plasmon damping in nonequilibrium from which information on the formation and decay of collective excitations at short time scales can be obtained. At large time scales (t ∼> τ_H) the hydrodynamic expansion of the plasma sets in. Detailed information on the evolution of the plasma in this regime is available from sophisticated hydrodynamic computer simulations which can be tested with the proposed measurements. With the decreasing plasma density due to the expansion, recombination processes become important and need to be considered as well

[en] A recent paper (Olah et al., 1985) showed that the active areas of HK Lac exhibited sometimes small and sometimes large scale motions. In the case of the Sun it is well known that new spots exhibit large proper motions while the old ones move almost together with the surrounding photosphere. On HK Lac both a newly formed active area and an existing old one showed concurrent rapid motions. A possible interpretation of this phenomenon is that new spots appeared in both the new and the old active areas simultaneously. This time coincidence may be accidental but if spots originate in deeper layers, then spots of common deep origin may appear in remote places of the stellar surface at the same time. Similar phenomena are seen on the Sun when there is simultaneous emergence of new flux in various parts of an extended active region (Zirin, 1983). The distribution of the active areas on the surface of HK Lac during the past eight years is discussed. (author). 3 figs., 10 refs

[en] Laser plasma interaction experiments have been performed using a fs Titanium Sapphire laser. Plasmas have been generated from planar PMMA targets using single laser pulses with 3.3 mJ pulse energy, 50 fs pulse duration at 800 nm wavelength. The electron density distributions of the plasmas in different delay times have been characterized by means of Nomarski Interferometry. Experimental data were compared with hydrodynamic simulation. First results to characterize the plasma density and temperature as a function of space and time are obtained. This work aims to generate plasmas in the warm dense matter (WDM) regime at near solid-density in an ultra-fast laser target interaction process. Plasmas under these conditions can serve as targets to develop x-ray Thomson scattering as a plasma diagnostic tool, e.g., using the VUV free-electron laser (FLASH) at DESY Hamburg

[en] Plasmas at high electron densities of n_e = 10²⁰ - 10²⁶ cm^-3 and moderate temperatures T_e = 1 - 20 eV are important for laboratory astrophysics, high energy density science and inertial confinement fusion. These plasmas are usually referred to as Warm Dense Matter (WDM) and are characterized by a coupling parameter of Λ ∼> 1 where correlations become important. The characterization of such plasmas is still a challenging task due to the lack of direct measurement techniques for temperatures and densities. They propose to measure the Thomson scattering spectrum of vacuum-UV radiation off density fluctuations in the plasma. Collective Thomson scattering provides accurate data for the electron temperature applying first principles. Further, this method takes advantage of the spectral asymmetry resulting from detailed balance and is independent of collisional effects in these dense systems

[en] The scattering of photons in plasmas is an important diagnostic tool. Especially, the region of warm dense matter can be probed by X-ray Thomson scattering. The scattering cross-section is related to the dynamic structure factor. We improve the standard treatment of the scattering on free electrons within the random phase approximation (RPA) by including collisions. The dielectric function is calculated in the Born-Mermin approximation. The inclusion of collisions modifies the dynamic structure factor significantly in the warm dense matter regime. We conclude that a theoretical description beyond the RPA is needed to derive reliable results for plasma parameters from X-ray Thomson scattering experiments. (authors)

[en] 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

[en] The basic idea is to implement Thomson scattering with free electron laser (FEL) radiation at near-solid density plasmas as a diagnostic method which allows the determination of plasma temperatures and densities in the warm dense matter (WDM) regime (free electron density of n_e = 10²¹-10²⁶ cm^-3 with temperatures of several eV). The WDM regime [1] at near-solid density (n_e = 10²¹-10²² cm^-3) is of special interest because, it is where the transition from an ideal plasma to a degenerate, strongly coupled plasma occurs. A systematic understanding of this largely unknown WDM domain is crucial for the modeling and understanding of contemporary plasma experiments, like laser shock-wave or Z-pinch experiments as well as for inertial confinement fusion (ICF) experiments as the plasma evolution follows its path through this domain