An empirical target discharging model relevant to hot-electron preheat in direct-drive implosions on OMEGA

Charging of inertial confinement fusion (ICF) targets generates a potential well that traps energetic electrons within the target. Trapped electrons can preheat the fuel, raise the adiabat, degrade compression and perhaps have an effect on achieving the high areal densities (ρR) required for ignition and gain. The decay time of this potential is thus an important parameter for any calculations of preheat. A nonlinear model of electrical discharging of ICF capsules has been developed and is presented here. The empirical model, which captures the essential dynamics of the target voltage decay, incorporates previous charged-particle spectroscopic and radiographic measurements of the fields surrounding the target. On the basis of this model, it is shown that the decay time is weakly dependent on the initial voltage of the target. In addition, it is shown that currents through the target support fiber aid target discharging. Implications of these findings for inertial fusion energy targets without support fibers are discussed. (paper)