Prof Peter Norreys FInstP;

Abstract:

The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived, and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high-repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved. © 2006 The American Physical Society.

Abstract:

Electron energy transport experiments conducted on the Vulcan 100 TW laser facility with large area foil targets are described. For plastic targets it is shown, by the plasma expansion observed in shadowgrams taken after the interaction, that there is a transition between the collimated electron flow previously reported at the 10 TW power level to an annular electron flow pattern with a 20° divergence angle for peak powers of 68 TW. Intermediate powers show that both the central collimated flow pattern and the surrounding annular-shaped heated region can co-exist. The measurements are consistent with the Davies rigid beam model for fast electron flow (Davies 2003 Phys. Rev. E 68 056404) and LSP modelling provides additional insight into the observed results. © 2006 IOP Publishing Ltd.

Abstract:

A reentrant cone concept for efficient heating of high-density plasmas has been studied as an advanced fast ignition scheme. The roles of the reentrant cone, as indicated by particle-in-cell (PIC) code simulations and confirmed by basic experiments, are reviewed, particularly the efficient collection and guidance of the laser light into the cone tip and the direction of the energetic electrons into the high-density region. It has been shown that the energetic electrons converge to the tip of the cone as a result of the surface electron flow guided by self-generated quasi-static magnetic fields and electrostatic sheath fields. As a result, the energetic electron density at the tip is locally greater than the case of using an open geometry such as a normal flat foil target. Using these advantageous properties of the reentrant cone, efficient fast heating of imploded high-density plasmas has been demonstrated in integrated fast ignition experiments. A hybrid PIC code (LSP) has been used to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas. The simulation results are in reasonable agreement with the integrated experiments. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-electromagnetic force impeding energetic electrons.

Abstract:

This paper reviews the important schemes that have been investigated thus far in fast ignition research. Integral experiments for fast ignition research have been conducted utilizing various schemes: (a) double-pulse experiments with two 100-ps pulses injected to a compressed core, (b) gold cone-guided implosion with 100-TW laser pulse heating, and (c) imploded core heated by both a 100-TW and petawatt laser pulses through gold cones. Reviewing these results, several important issues were raised for further development of fast ignition research. The imploded core heated by a petawatt laser through a gold cone showed a 103 D-D neutron increase compared to the one with only the CD shell implosion.