Fusion Science and Technology / Volume 49 / Number 3 / April 2006 / Pages 316-326
Technical Paper / Fast Ignition
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.