Dynamics of thin metal foils irradiated by moderate-contrast high-intensity laser beamsThe modeling of petawatt laser-generated hot electrons in mass-limited solid-foil-target interactions at "relativistic" laser intensities is presented using copper targets and parameters motivated by recent experiments at the Rutherford Appleton Laboratory Petawatt and 100-TW facilities ͓Theobald et al., Phys. Plasmas 13, 043102 ͑2006͔͒. Electron refluxing allows a unique determination of the laser-electron conversion efficiency and a test with simulations. Good agreement between experiments and simulations is found for conversion efficiencies of 10%.

We have performed experiments using Callisto, the Vulcan 100 TW and the Vulcan Petawatt high intensity lasers to understand the characteristics of high energy, Kα x-ray sources and to implement workable radiography solutions at 20-100 keV. Our measurements show that the Kα size from a simple foil target is larger than 60 µm, far larger than the experiment resolution requirement. The total Kα yield is independent of target thicknesses verifying that refluxing plays a major role in photon generation. Smaller radiating volumes emit brighter Kα radiation. 1-D radiography experiments using small-edge-on foils resolved 10 µm features with high contrast.We tested a variety of small volume 2-D point sources such as cones, wires, and embedded wires, measuring photon yields and comparing our measurements with predictions from hybrid-PIC LSP simulations. In addition to high-energy, high-resolution backlighters, future experiments will also need imaging detectors and diagnostic tools that are workable in the 20-100 keV energy range. An initial look at some of these detector issues is also presented.

A hot, T e ~ 2-to 3-keV surface plasma was observed in the interaction of a 0.7-ps petawatt laser beam with solid copper-foil targets at intensities >10 20 W/cm 2 . Copper K-shell spectra were measured in the range of 8 to 9 keV using a single-photon-counting x-ray CCD camera. In addition to K α and K β inner-shell lines, the emission contained the Cu He α and Ly α lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than 3 × 10 18 W/cm 2 , only the K α and K β inner-shell emissions are detected. Measurements of the absolute K α yield as a function of the laser intensity are in agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume.2

The heating of solid targets irradiated by 5 10 20 Wcm ÿ2 , 0.8 ps, 1:05 m wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to 5 keV with an axial temperature gradient of 0:6 m scale length. Images of Ni Ly show the hot region has 25 m diameter. These data are consistent with collisional particle-in-cell simulations using preformed plasma density profiles from hydrodynamic modeling which show that the >100 G bar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer.