Radiative preheating effect on TNSA protons for Fast Ignition

Ignition through inertial confinement fusion (ICF) has been demonstrated at the National Ignition Facility (NIF) with a record single-shot gain of 4.13. Repeated success has renewed interest in Inertial Fusion Energy (IFE), the concept of a fusion-based reactor for GW-scale electricity production. Such an effort, however, requires implosions at 10 Hz with gains of 50-100 each. While central hot spot ignition was used in recent NIF shots, fast ignition (FI) can potentially yield gains necessary for IFE by separating compression and heating into distinct phases to relax symmetry and reduce driver energy [1]. Proton and electron FI attempts to achieve this by isochorically heating assembled fuel with laser-driven particle beams [2]. The more robust transport and lower divergence of protons are advantageous, although soft X-rays emitted from the Au focusing cone degrade the foil’s rear surface which limits laser-to-proton conversion efficiency. When an intense SP laser (>1018 W/cm²) of several-ps pulse duration irradiates a thin foil (<100 μm), sheath fields are generated at the rear surface, propelling ions to multi-MeV energies—a process called target normal sheath acceleration (TNSA) [3,4]. Since these ions typically originate from sub-micron hydrocarbon layers on the rear, even micron-scale surface density profiles may affect acceleration. Using Particle-in-Cell (PIC) simulations, we examine their impact on TNSA ion spectra and focusing capability in a proton FI scheme [5].