Modeling Collisional Blooming and Straggling of the Electron Beam in the Fast Ignition Scenario

The motion and distribution of electrons in the plasma in the fast-ignition scenario are defined by three equations that describe the spread in the direction of motion (straggling), the spread in the direction perpendicular to the motion (blooming), and the amount of energy lost as the electron travels through the plasma. A program was written that models the distribution of the electrons in the plasma and tracks their energy deposition. This model treats the beam of electrons as many parallel beams of infinitesimal width that move in a straight line. When the blooming or straggling exceeds a certain value, each beam of electrons is split into multiple beams of different weights. The model was found to require exponentially more time and memory for greater degrees of accuracy and to be sensitive to small adjustments in the splitting algorithm. The results of this model applied to a test problem were found to be very similar to the analytic predictions, with errors ranging from ~2% to 11%. Introduction In conventional inertial confinement fusion, ignition occurs when one of the fusion products (alpha particles) created in a central hot-spot are stopped in the high density, cold fuel, causing a propagating burn. A proposed alternative ignition method, known as “fast ignition”, is to heat a part of the high density, cold fuel with a beam of relativistic electrons created by focusing a high-intensity laser into the target. The transport of these electrons is currently carried out with a straight-line model in which the electrons do not deviate from straight-line trajectories. In reality, the electrons undergo