Prof Peter Norreys FInstP;

Abstract:

An intense laser pulse interacting with a near discontinuous plasma vacuum interface causes the plasma surface to perform relativistic oscillations. The reflected laser radiation then contains very high order harmonics of fundamental frequency and - according to current theory - must be bunched in radiation bursts of a few attoseconds duration. Recent experimental results have demonstrated x-ray harmonic radiation extending to 3.3 (3.8 keV, order n > 3200) with the harmonic conversion efficiency scaling as η(n) n -2.5 over the entire observed spectrum ranging from 17 nm to 3.3 . This scaling holds up to a maximum order, nRO 81/2γ 3, where γ is the peak value of the Lorentz factor, above which the harmonic efficiency decreases more rapidly. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h ν > 1 keV is found to be into a cone angle ∼4°, significantly less than that of the incident laser cone (20°). © 2007 IOP Publishing Ltd.

Abstract:

The study of fast electrons generated in laser-matter interactions and their transport through matter is of great interest for the fast ignition concept in the inertial confinement fusion. A typical experimental approach is the interaction of intense laser radiation with foil targets. The transport of the electrons through the foil can be studied with several techniques including X-ray emission measurements and direct measurements on electrons leaving the foil. Some laser-matter interaction features can be monitored using spectroscopy in the visible range. Here we will focus our attention on some of the experimental techniques that have been used in laser-foil experiments at ILIL laboratory in Pisa and in a VULCAN Petawatt experiment at RAL. In particular we will describe high resolution X-ray spectroscopy with a bent crystal, K-alpha flux measurements, direct measurements of the angular distribution and spectrum of the electrons, and spectroscopy of the 3/2ω and 2ω harmonics of the laser radiation.