Showing papers by "Siegfried Glenzer published in 2015"

Ultrabright X-ray laser scattering for dynamic warm dense matter physics

Abstract: Warm dense matter (WDM), which falls in the category between plasmas and condensed matter, is expected to exist in planetary interiors. Now, researchers use an X-ray laser to observe the transition to WDM.

Free-electron X-ray laser measurements of collisional-damped plasmons in isochorically heated warm dense matter.

Abstract: We present the first highly resolved measurements of the plasmon spectrum in an ultrafast heated solid. Multi-keV x-ray photons from the Linac Coherent Light Source have been focused to one micrometer diameter focal spots producing solid density aluminum plasmas with a known electron density of n_{e}=1.8×10^{23} cm^{-3}. Detailed balance is observed through the intensity ratio of up- and down-shifted plasmons in x-ray forward scattering spectra measuring the electron temperature. The plasmon damping is treated by electron-ion collision models beyond the Born approximation to determine the electrical conductivity of warm dense aluminum.

Measurement of charged-particle stopping in warm-dense plasma

Abstract: We measured the stopping of energetic protons in an isochorically heated solid-density Be plasma with an electron temperature of ∼32 eV, corresponding to moderately coupled [(e^{2}/a)/(k_{B}T_{e}+E_{F})∼0.3] and moderately degenerate [k_{B}T_{e}/E_{F}∼2] "warm-dense matter" (WDM) conditions. We present the first high-accuracy measurements of charged-particle energy loss through dense plasma, which shows an increased loss relative to cold matter, consistent with a reduced mean ionization potential. The data agree with stopping models based on an ad hoc treatment of free and bound electrons, as well as the average-atom local-density approximation; this work is the first test of these theories in WDM plasma.

METHOD FOR CHARACTERIZATION OF A SPHERICALLY BENT CRYSTAL FOR K-alpha X-RAY IMAGING OF LASER PLASMAS USING A FOCUSING MONOCHROMATOR GEOMETRY

Abstract: A method is provided for characterizing spectrometric properties (e.g., peak reflectivity, reflection curve width, and Bragg angle offset) of the K.alpha. emission line reflected narrowly off angle of the direct reflection of a bent crystal and in particular of a spherically bent quartz 200 crystal by analyzing the off-angle x-ray emission from a stronger emission line reflected at angles far from normal incidence. The bent quartz crystal can therefore accurately image argon K.alpha. x-rays at near-normal incidence (Bragg angle of approximately 81 degrees). The method is useful for in-situ calibration of instruments employing the crystal as a grating by first operating the crystal as a high throughput focusing monochromator on the Rowland circle at angles far from normal incidence (Bragg angle approximately 68 degrees) to make a reflection curve with the He-like x-rays such as the He-.alpha. emission line observed from a laser-excited plasma.

Ab initio calculation of the ion feature in x-ray Thomson scattering.

Abstract: The spectrum of x-ray Thomson scattering is proportional to the dynamic structure factor. An important contribution is the ion feature which describes elastic scattering of x rays off electrons. We apply an ab initio method for the calculation of the form factor of bound electrons, the slope of the screening cloud of free electrons, and the ion-ion structure factor in warm dense beryllium. With the presented method we can calculate the ion feature from first principles. These results will facilitate a better understanding of x-ray scattering in warm dense matter and an accurate measurement of ion temperatures which would allow determining nonequilibrium conditions, e.g., along shock propagation.

Observation of finite-wavelength screening in high-energy-density matter.

Abstract: A key component for the description of charged particle systems is the screening of the Coulomb interaction between charge carriers. First investigated in the 1920s by Debye and Huckel for electrolytes, charge screening is important for determining the structural and transport properties of matter as diverse as astrophysical and laboratory plasmas, nuclear matter such as quark-gluon plasmas, electrons in solids, planetary cores and charged macromolecules. For systems with negligible dynamics, screening is still mostly described using a Debye–Huckel-type approach. Here, we report the novel observation of a significant departure from the Debye–Huckel-type model in high-energy-density matter by probing laser-driven, shock-compressed plastic with high-energy X-rays. We use spectrally resolved X-ray scattering in a geometry that enables direct investigation of the screening cloud, and demonstrate that the observed elastic scattering amplitude is only well described within a more general approach.

Beyond the gain exponent: Effect of damping, scale length, and speckle length on stimulated scatter.

Abstract: Three-dimensional wave propagation simulations and experiments show that the gain exponent, an often used metric to assess the likelihood of stimulated Brillouin scatter, is insufficient and must be augmented with another parameter, ${N}_{r}$, the ratio of the resonance length, ${L}_{\text{res}}$, to the laser speckle length. The damping rate of ion acoustic waves, $\ensuremath{ u}$, and thus ${L}_{\text{res}}$, which is proportional to $\ensuremath{ u}$, are easily varied with plasma species composition, e.g., by varying the ratio of hydrogen and carbon ions. As ${N}_{r}$ decreases, stimulated Brillouin scattering increases despite the same gain exponent.

Ultrafast electron kinetics in short pulse laser-driven dense hydrogen

Abstract: Dense cryogenic hydrogen is heated by intense femtosecond infrared laser pulses at intensities of 1015-1016 Wcm-2. Three-dimensional particle-in-cell (PIC) simulations predict that this heating is limited to the skin depth, causing an inhomogeneously heated outer shell with a cold core and two prominent temperatures of about 25 and 40 eV for simulated delay times up to +70 fs after the laser pulse maximum. Experimentally, the time-integrated emitted bremsstrahlung in the spectral range of 8-18 nm was corrected for the wavelength-dependent instrument efficiency. The resulting spectrum cannot be fit with a single temperature bremsstrahlung model, and the best fit is obtained using two temperatures of about 13 and 30 eV. The lower temperatures in the experiment can be explained by missing energy-loss channels in the simulations, as well as the inclusion of hot, non- Maxwellian electrons in the temperature calculation. We resolved the time-scale for laser-heating of hydrogen, and PIC results for laser-matter interaction were successfully tested against the experiment data.

X-ray Thomson scattering diagnostics of impact ionization in laser-driven carbon foils

Abstract: We have studied the light–matter interaction of ultra-short, intense optical laser fields with thin carbon foils via particle-in-cell simulations. Especially, the influence of additional impact ionization on the density and temperature of the generated plasma and on the corresponding Thomson scattering spectra was investigated. We predict a pump–probe experiment at the free electron laser FLASH in order to verify the importance of this effect in the laser–matter interaction on ultra-short time scales and to check our predictions quantitatively.

High intensity laser-driven x-ray sources for high energy density science

Abstract: We discuss betatron x-ray radiation from laser wakefield accelerators using femtosecond (blowout regime) and picosecond (self modulated and direct laser acceleration regimes) scale laser pulses. The source will have applications for high energy density science.

Betatron radiation from laser plasma accelerators

Abstract: We present recent experiments on the characterization of Betatron radiation in the blowout regime of laser-wakefield acceleration. We observed Betatron x-rays up to 80 keV, and the characterization of the angular dependence of the x-ray spectrum suggests anisotropic electron trajectories in the plasma. The characterization of the source opens up new possibilities for application experiments.