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.