Abstract
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 μm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.
Highlights
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory
We report on the integral fast ignition experiments with the super-penetration scheme, which are performed by injecting a short pulse ultra-intense laser (UIL) into a spherically compressed target
In joint shots, the short pulse (1.5 ps in full width at half maximum (FWHM)) LFEX laser was injected in the equatorial plane at different times around the peak compression (2.6 ns), generating a forward moving fast electron beam
Summary
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. Further optimization of the experiments has been proposed by creating a collimated electron beam with magnetic fields produced either externally[18,19] or using an engineering target[20,21] Another unique solution to improve the coupling efficiency is the super-penetration scheme[22,23,24], proposed based on the original idea of FI, where two successive short pulse UILs are injected into the pre-compressed fuel core: the first UIL producing a low-density plasma channel through the coronal region, while the second, guided by the preformed channel, acting as an ignitor pulse to generate the fast electrons. Recent experiments on OMEGA have demonstrated the formation of a plasma channel up to overcritical density under fast-ignition-relevant conditions[29] and observed collimated fast electrons on the channel axis[30], rather promising to achieve fast ignition with this super-penetration scheme
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