91̽»¨

Abstract:

Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25¡ã. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f -number of the focusing optics is discussed. ? 2007 American Institute of Physics.

Abstract:

K¦Á x-ray emission, extreme ultraviolet emission, and plasma imaging techniques have been used to diagnose energy transport patterns in copper foils ranging in thickness from 5 to 75¦Ìm for intensities up to 5¡Á1020Wcm-2. The K¦Á emission and shadowgrams both indicate a larger divergence angle than that reported in the literature at lower intensities. Foils 5¦Ìm thick show triple-humped plasma expansion patterns at the back and front surfaces. Hybrid code modeling shows that this can be attributed to an increase in the mean energy of the fast electrons emitted at large radii, which only have sufficient energy to form a plasma in such thin targets. ? 2007 The American Physical Society.

Abstract:

The interaction of a 75 J 10 ps, high intensity laser beam with low-mass, solid Cu targets is investigated. Two instruments were fielded as diagnostics of Cu K -shell emission from the targets: a single photon counting spectrometer provided the absolute K¦Á yield [C. Stoeckl, Rev. Sci. Instrum. 75, 3705 (2004)] and a spherically bent Bragg crystal recorded 2D monochromatic images with a spatial resolution of 10 ¦Ìm [J. A. Koch, Rev. Sci. Instrum. 74, 2130 (2003)]. Due to the shifting and broadening of the K¦Á spectral lines with increasing temperature, there is a temperature dependence of the crystal collection efficiency. This affects measurements of the spatial pattern of electron transport, and it provides a temperature diagnostic when cross calibrated against the single photon counting spectrometer. The experimental data showing changing collection efficiency are presented. The results are discussed in light of modeling of the temperature-dependent spectrum of Cu K -shell emission. ? 2007 American Institute of Physics.

Abstract:

The effect of laser-focusing conditions on the evolution of relativistic plasma waves in laser-wakefield accelerators is studied both experimentally and with particle-in-cell simulations. For short focal-length (w0<¦Ëp) interactions, beam breakup prevents stable propagation of the pulse. High field gradients lead to nonlocalized phase injection of electrons, and thus broad energy spread beams. However, for long focal-length geometries (w0>¦Ëp), a single optical filament can capture the majority of the laser energy and self-guide over distances comparable to the dephasing length, even for these short pulses (c¦Ó ¦Ëp). This allows the wakefield to evolve to the correct shape for the production of the monoenergetic electron bunches, as measured in the experiment. ? 2007 The American Physical Society.