Prof Peter Norreys FInstP;

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.

Abstract:

In recent fast ignitor research, neutron measurements have become increasingly important not only to understand the ultraintense laser-plasma interaction physics associated with ion acceleration and energy transport processes in dense plasmas but also the characterization of the plasma temperature in integrated experiments, as summarized in this paper. New technologies that are relevant to the next-generation integrated fast ignition experiments are also reviewed. These will become increasingly important in the next few years as second-generation multikilojoule petawatt facilities come online and the detection environment becomes increasingly hostile, particularly if, as anticipated, the generated neutron fluxes begin to approach energy breakeven conditions.

Abstract:

We report the observation of monoenergetic electron beams (dE/E < 5%) produced by the interaction of a 12TW, 40fs laser pulse with underdense plasma, in contrast to all previous experiments, which produced energy spreads ~100%. © 2005 Optical Society of America.