Electron Reflection in Monte Carlo Simulations with the code GEANT

Target preheat is a significant concern in inertial confinement fusion experiments because, when the temperature of the target becomes too large, the areal density (density times radius) required for ignition cannot be attained. The majority of preheat comes from fast electrons produced when the laser interacts with the target. When these energetic electrons are slowed by collisions, they release highenergy x rays. The energy profile of these x rays is thus a very useful diagnostic of the preheat problem. This energy profile has been obtained from Monte Carlo simulations using the code GEANT. The GEANT code simulates electron transport in the target ending either with the electron's loss of all kinetic energy through collisions or with its escape. In reality, only a negligible number of electrons escape before the target becomes charged enough to reflect the remaining electrons back into the target. A FORTRAN program was written to model the specular reflection of these electrons back into the target. The results of this program are then used as input for another GEANT run with the reflected electrons. This allows for more accurate simulation of electron transport and of the profile of the x rays generated. Nevertheless, it has been found that electron reflection is not a major factor in CH coated Cu spheres. Introduction Target preheat is one of the major obstacles to achieving ignition in ICF experiments. Because the fast electrons produced by the laser-target interaction heat the DT fuel in the target before it has been compressed, they prevent the compression of the target to the requisite areal density. The areal density of the fuel has to be large enough for the energy in the alpha particles (helium ions) produced by the initial fusion reactions to be re-deposited in the DT core. The laser pulse itself can only provide the energy for fusion reactions in the central hot core; the main fusion reactions will result from the