We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at 13.5nm wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of 2x10;{14}Wcm;{2} . The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about 38eV , supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.

We discuss a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) 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 diagnosis of such plasmas is a longstanding issue which is addressed here using a pumpeprobe scattering experiment to reveal the collective electron plasma mode (plasmon) using the highbrilliance radiation to probe the plasma. Distinctive scattering features allow one to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature are determined from scattering off the plasmon mode.

Laser interaction with a nanobrush target plasma is investigated at the SILEX-I laser facility [X. F. Wei et al., J. Phys. Conf. Ser. 112, 032010 (2008)] with a laser of intensity 7.9×1018 W/cm2. Highly collimated fast electron beams with yields of more than three times higher than that from the planar target can be produced. Two-dimensional particle-in-cell simulation confirms that a layered surface structure can increase the efficiency of laser energy absorption, and the resulting fast electrons are tightly collimated and guided by the plasma layers to a cross section of about the laser spot size.

Hot electrons generated by short-pulse-laser interaction with nanolayered target (NT) are investigated using two-dimensional particle-in-cell simulation. Compared to the planar target, the NT leads to more efficient conversion of laser energy to the kinetic energy of the accelerated electrons. However, the energy absorption by the NT decreases at both too-low and too-high laser intensities. At lower laser intensities it is because of the weaker electric and magnetic fields generated by the hot-electron jets and smaller relativistic skin depth. At higher laser intensities it is because of the damage or destruction of the layered structure by the laser field. On the other hand, the dependence of the conversion efficiency and hot-electron number on the duration of the (short) laser pulse and the nanolayer length is weak. Control of the hot-electron characteristics by tailoring the parameters of the laser and the NT is discussed.

We report on soft x-ray scattering experiments on cryogenic hydrogen and simple metal samples. As a source of intense, ultrashort soft x-ray pulses we have used free-electron laser radiation at 92 eV photon energy from FLASH at DESY, Hamburg. X-ray pulses with energies up to 150 µJ and durations 15-50 fs provide interaction with the sample leading simultaneously to plasma formation and scattering. Experiments exploiting both 1 of these interactions have been carried out, using the same experimental setup. Firstly, recording of soft x-ray inelastic scattering from near-solid density hydrogen plasmas at few electron volt temperatures confirms the feasibility of this diagnostics technique.Secondly, the soft x-ray excitation of few electron volt solid-density plasmas in bulk metal samples could be studied by recording soft x-ray line and continuum emission integrated over emission times from fs to ns. PACS: 52.25.Os, 52.70.La, 41.60.Cr Keywords: free-electron laser, soft x-ray spectroscopy, inelastic x-ray scattering, stronglycoupled plasmas, ultrafast processes, pump-probe experiments 2