The role of electron–ion equilibration on the size scaling of laser direct-drive implosions

The impact of electron–ion equilibration on the target-size scaling of direct-drive laser-fusion implosions is investigated. It is found that, with increasing scale, enhanced equilibration degrades implosion performance by reducing laser-drive efficiency during the acceleration phase and suppressing ion temperatures during deceleration, thereby diminishing the ion-temperature enhancement associated with thermal conduction at larger scales. By extending a steady-state spherical ablation model to include finite ion-temperature effects, we show that both the ablation pressure and mass ablation rate scale as (1+Ti/ZTe)−1. One-dimensional simulations are used to validate the ablation-model predictions and to assess the effects of equilibration during the deceleration phase. Because larger implosions experience stronger electron–ion equilibration, these results suggest a corresponding reduction in laser-drive efficiency during acceleration and degraded implosion performance during deceleration with increasing target size.