Fast ignition with laser-driven ion beams: Progress on ignitor beam development based on a new relativistic laser-plasma regime

Summary form only given. 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. In this presentation, the requirements for ICF are summarized, including why the fuel must be compressed [1], and the specific requirements for “fast ignition” (FI) [2] are highlighted [3]. FI is “isochoric” ignition: fuel compressed by one driver is ignited by a separate driver that makes a “hot spot” very quickly (isochorically) within the fuel at the time of peak compression, ≈ 400 g/cm3 and ≈ 3 g/cm² for inertial fusion energy (IFE) D-T capsules. “Hot spot” ignition requires heating (to ~ 10 keV) a fuel volume of dimension equal to the range of the 3.5 MeV alpha particle fusion product (~ 20 μm), in order to initiate a propagating burn. FI concepts based on laser driv- n ion beams have been developed for protons (e.g., see Refs. [4,5]) and carbon ions [6,7,8]. This talk summarizes the FI concepts, but concentrates on the progress in developing laser-driven C-ion beams, and the new physics regime that is accessed in the process [9]. Significant understanding of the physics underlying laser-driven generation of intense ion beams has been gained over the last decade. In this presentation we summarize the physics and the "taxonomy" of several relevant ion-acceleration mechanisms [10]. At Los Alamos, based on that understanding, applied through large-scale laser-plasma simulations with the VPIC code [11], we have made significant and steady progress in the generation of such beams, in turn validating our understanding and modeling methods. Our experiments rely on specialized targets and on the unique capabilities at the LANL Trident laser facility [12], such as its ultrahigh laser-pulse contrast [13] and near diffraction limited far-field laser profile. This enables us to accelerate C6+ ions up to 1 GeV, as well as protons and deuterons to > 150 MeV via Breakout-Afterburner (BOA) ion acceleration mechanism [14], which operates in a novel regime of laser-plasmas: the relativistically transparent regime [9]. The experiments are diagnosed with a unique and powerful array of diagnostics, including novel angularly resolved ion spectrometers [15] and optical diagnostics [16], which are used to diagnose the waveform of single-shot high-energy beams (including the incoming Trident laser pulse) with ~ 50 fs resolution, and > 60 dB single-shot dynamic range with a new diagnostic called iFOX-SRSI. Based on this progress, we have been able to: 1) use the beams that Trident can generate for near term applications, such a intense neutron beam generation [17] and 2) ascertain the laser-pulse and laser-target characteristics that would yield a suitable ignitor ion beam for FI. Specifically, we present the VPIC simulation that yields such a su