Direct imaging of shock wave splitting in diamond at Mbar pressure

Understanding the behavior of matter at extreme pressures of the order of a megabar(Mbar)is essential to gain insight into various physical phenomena at macroscales—the formation of planets,young stars,and the cores of super-Earths,and at microscales—damage to ceramic materials and high-pressure plastic transformation and phase transitions in solids.Under dynamic compression of solids up to Mbar pressures,even a solid with high strength exhibits plastic properties,causing the induced shock wave to split in two:an elastic precursor and a plastic shock wave.This phenomenon is described by theoretical models based on indirect measurements of material response.The advent of x-ray free-electron lasers(XFELs)has made it possible to use their ultrashort pulses for direct observations of the propagation of shock waves in solid materials by the method of phase-contrast radiography.However,there is still a lack of comprehensive data for verification of theoretical models of different solids.Here,we present the results of an experiment in which the evolution of the coupled elastic-plastic wave structure in diamond was directly observed and studied with submicrometer spatial resolution,using the unique capabilities of the x-ray free-electron laser(XFEL).The direct measurements allowed,for the first time,the fitting and validation of the 2D failure model for diamond in the range of several Mbar.Our experimental approach opens new possibilities for the direct verification and construction of equations of state of matter in the ultra-high-stress range,which are relevant to solving a variety of problems in high-energy-density physics.

Alignment of solid targets under extreme tight focus conditions generated by an ellipsoidal plasma mirror

The design of ellipsoidal plasma mirrors(EPMs)for the PEARL laser facility is presented.The EPMs achieve a magnification of 0.32 in focal spot size,and the corresponding increase in focused intensity is expected to be about 8.Designing and implementing such focusing optics for short-pulse(<100 fs)systems paves the way for their use in future high-power facilities,where they can be used to achieve intensities beyond 1023W/cm^(2).A retro-imaging-based target alignment system is also described,which is used to align solid targets at the output of the ellispoidal mirrors(with a numerical aperture of 0.75 in this case).

X-Pinches as Broadband Sources of X-Rays for Radiography

Two methods of using the X-pinch as a source of X-ray radiation for radiography of biological objects are presented. X-pinches are found to be a very flexible method for generation of radiation over a wide spectral range and provide a high spatial and temporal resolution.

Clean source of soft X-ray radiation formed in supersonic Ar gas jets by high-contrast femtosecond laser pulses of relativistic intensity

In this work,we optimized a clean,versatile,compact source of soft X-ray radiation(Ex-ray∼3 keV)with an yield per shot up to 7×10^11 photons/shot in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity(Ilas∼10^18-10^19 W/cm^2)with supersonic argon gas jets.Using high-resolution X-ray spectroscopy approaches,the dependence of main characteristics(temperature,density and ionization composition)and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied.The optimal conditions,when the X-ray photon yield reached a maximum value,have been found when the argon plasma has an electron temperature of Te∼185 eV,an electron density of Ne∼7×10^20 cm^-3 and an average charge of Z∼14.In such a plasma,a coefficient of conversion to soft X-ray radiation with energies Ex-ray∼3.1(±0.2)keV reaches 8.57×10^-5,and no processes leading to the acceleration of electrons to MeV energies occur.It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry.We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic,and its intensity linearly depends on the energy of the laser pulse,which was varied in the range of 50-280 mJ.We also found that the yield of X-ray photons can be notably increased by,for example,choosing the optimal laser pulse duration and the inlet pressure of the gas jet.