The rapid progress in high energy, high power, high intensity lasers has tremendously increased the power flux that can be delivered into plasmas. A compelling motivation for that is to drive ion accelerators that are increasingly compact and deliver higher particle currents, making them a suitable ignitor beam for inertial confinement fusion (ICF). Beyond the limits from electrical breakdown in conventional accelerator structures, plasmas can support much larger electric fields (> TV/m) and current densities (e.g., > 1012 protons / ps / (10 µm)2 in some present-day experiments), enabling very short and intense bursts of ions with very high energies. Driving such ion beams requires very high power densities, and such lasers are ideally suited to provide them. The prospect of lasers with increased efficiency, energy, repetition rate and pulse-shape control at the ~ 10 fs level promises laser-driven ion beams with the parameters necessary for a fusion reactor. Specifically, creating the hot spot to ignite suitably compressed D-T fuel requires very high power densities, ~1022 W/cm3, which quantifies the challenge ahead.
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