Pair creation and energy coupling of multi-MeV electrons and positrons to the compressed central hot spot of a DT capsule and their effects on the fusion reactions.

In the fast ignition (FI) approach to DT fusion, it is necessary to rapidly deliver 10kJ or more of extra thermal energy to the central hot spot of the DT capsule at maximum areal density, so that the DT temperature can far exceed the Lawson criterion. Past and current approches of FI mostly involve injecting short-pulse electron or ion beams. However, due to the large current required, external charged particle beams face major difficulty in focussing their energy onto the central hot spot. We have investigated an alternative concept of creating multi-MeV electron-positron pairs at the center of the compressed DT plasma using gamma-rays, which avoids the large current problem. The e+e- pairs are created at maximum compression using short-pulse gamma-rays irradiating high-Z dopants imbedded in the DT. We will present detailed Geant4 simulations and kinetic calculations to study pair creation and their energy coupling to DT ions. The goal is to.determine the optimal energy and number of gamma-rays needed to deliver 10kJ of short-pulse e+e- energy to the central DT hot spot, and their net effects on the fusion reactions. We will further discuss the implications of our results for fast ignition..