Prof Peter Norreys FInstP;

Abstract:

During the last few years laser-driven plasma accelerators have been shown to generate quasi-monoenergetic electron beams with energies up to several hundred MeV. Extending the output energy of laser-driven plasma accelerators to the GeV range requires operation at plasma densities an order of magnitude lower, i.e. 10(18) cm(-3), and increasing the distance over which acceleration is maintained from a few millimetres to a few tens of millimetres. One approach for achieving this is to guide the driving laser pulse in the plasma channel formed in a gas-filled capillary discharge waveguide. We present transverse interferometric measurements of the evolution of the plasma channel formed and compare these measurements with models of the capillary discharge. We describe in detail experiments performed at Lawrence Berkeley National Laboratory and at Rutherford Appleton Laboratory in which plasma accelerators were driven within this type of waveguide to generate quasi-monoenergetic electron beams with energies up to I GeV.

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.