Prof Peter Norreys FInstP;

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<λp) 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>λp), 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 (cτ λp). 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.

Abstract:

We review the potential to develop sources for neutron scattering science and propose that a merger with the rapidly developing field of inertial fusion energy could provide a major step-change in performance. In stark contrast to developments in synchrotron and laser science, the past 40 years have seen only a factor of 10 increase in neutron source brightness. With the advent of thermonuclear ignition in the laboratory, coupled to innovative approaches in how this may be achieved, we calculate that a neutron source three orders of magnitude more powerful than any existing facility can be envisaged on a 20- to 30-year time scale. Such a leap in source power would transform neutron scattering science.

Abstract:

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2-4keV (ref.1). The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MégaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.