Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

High Mach number blast waves were created by focusing a laser pulse on a solid pin, surrounded by nitrogen or xenon gas. In xenon, the initial shock is strongly radiative, sending out a supersonic radiative heat wave far ahead of itself. The shock propagates into the heated gas, diminishing in strength as it goes. The radiative heat wave also slows, and when its Mach number drops to two with respect to the downstream plasma, the heat wave drives a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame; the heat wave becomes subsonic behind the second shock. For some time both shocks are observed simultaneously. Eventually the initial shock diminishes in strength so much that it can longer be observed, but the second shock continues to propagate long after this time. This sequence of events is a new phenomenon that has not previously been discussed in the literature. Numerical simulation clarifies the origin of the second shock, and its position is consistent with an analytical estimate. © 2006 American Institute of Physics.

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.