Prof Peter Norreys FInstP;

Abstract:

The first evidence of x-ray harmonic radiation extending to 3.3, 3.8 keV (order n>3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (η∼n-2.5-n-3) at multi-keV energies. This scaling holds up to a maximum order, nRO∼81/2γ3, where γ is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hν>1keV is found to be into a cone angle ∼4°, significantly less than that of the incident laser cone (20°). © 2007 The American Physical Society.

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°. 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:

Kα 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 75μm for intensities up to 5×1020Wcm-2. The Kα emission and shadowgrams both indicate a larger divergence angle than that reported in the literature at lower intensities. Foils 5μm 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 Kα 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 μm [J. A. Koch, Rev. Sci. Instrum. 74, 2130 (2003)]. Due to the shifting and broadening of the Kα 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.